Cumrun Vafa: String Theory
物理与宇宙学音乐与艺术数学生物与进化哲学与宗教
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theorystringphysicsquantumdongravitysymmetryideasspacemechanicssaiddimensiondimensionalmathparticledimensionsuniverserealitydoesnfour
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🎙️ 完整对话（3292 条）
Lex Fridman (00:00.000)
The following is a conversation with Kamran Valfa,
以下是与 Kamran Valfa 的对话，
Lex Fridman (00:02.700)
a theoretical physicist at Harvard
哈佛大学理论物理学家
Lex Fridman (00:04.740)
specializing in string theory.
专门研究弦理论。
Lex Fridman (00:07.140)
He is the winner of the 2017 Breakthrough Prize
他是2017年突破奖获得者
Lex Fridman (00:10.940)
in Fundamental Physics,
在基础物理学中，
Cumrun Vafa (00:12.300)
which is the most lucrative academic prize in the world.
这是世界上最赚钱的学术奖项。
Lex Fridman (00:15.740)
Quick mention of our sponsors,
快速提及我们的赞助商，
Cumrun Vafa (00:17.380)
Headspace, Jordan Harmer's show,
乔丹·哈默 (Jordan Harmer) 的《Headspace》节目，
Lex Fridman (00:20.140)
Squarespace, and Allform.
Squarespace 和 Allform。
Cumrun Vafa (00:22.860)
Check them out in the description to support this podcast.
在说明中查看它们以支持此播客。
Lex Fridman (00:25.980)
As a side note, let me say that string theory
作为旁注，让我说一下弦理论
Cumrun Vafa (00:28.020)
is a theory of quantum gravity
是一种量子引力理论
Lex Fridman (00:29.940)
that unifies quantum mechanics and general relativity.
统一了量子力学和广义相对论。
Cumrun Vafa (00:33.260)
It says that quarks, electrons, and all other particles
它说夸克、电子和所有其他粒子
Lex Fridman (00:36.660)
are made up of much tinier strings of vibrating energy.
由更细小的振动能量串组成。
Cumrun Vafa (00:40.300)
They vibrate in 10 or more dimensions,
它们在 10 个或更多维度上振动，
Lex Fridman (00:42.920)
depending on the flavor of the theory.
取决于理论的风格。
Cumrun Vafa (00:45.020)
Different vibrating patterns result in different particles.
不同的振动模式会产生不同的颗粒。
Lex Fridman (00:48.620)
From its origins, for a long time,
从诞生之日起，长期以来，
Cumrun Vafa (00:50.660)
string theory was seen as too good not to be true,
弦理论被认为好得令人难以置信，
Lex Fridman (00:54.680)
but has recently fallen out of favor
Cumrun Vafa (00:56.380)
in the physics community,
Lex Fridman (00:57.660)
partly because over the past 40 years,
Cumrun Vafa (01:00.180)
it has not been able to make any novel predictions
Lex Fridman (01:03.100)
that could then be validated through experiment.
Cumrun Vafa (01:06.260)
Nevertheless, to this day,
Lex Fridman (01:08.260)
it remains one of our best candidates
Cumrun Vafa (01:10.420)
for a theory of everything,
Lex Fridman (01:12.140)
or a theory that unifies the laws of physics.
Cumrun Vafa (01:15.540)
Let me mention that a similar story happened
Lex Fridman (01:18.000)
with neural networks
Cumrun Vafa (01:18.980)
in the field of artificial intelligence,
Lex Fridman (01:21.060)
where it fell out of favor
Cumrun Vafa (01:22.580)
after decades of promise and research,
Lex Fridman (01:24.820)
but found success again in the past decade
Cumrun Vafa (01:28.000)
as part of the deep learning revolution.
Lex Fridman (01:30.340)
So I think it pays to keep an open mind,
Cumrun Vafa (01:33.140)
since we don't know which of the ideas in physics
Lex Fridman (01:36.020)
may be brought back decades later
Lex Fridman (01:38.100)
and be found to solve the biggest mysteries
Lex Fridman (01:40.640)
in theoretical physics.
Cumrun Vafa (01:42.380)
String theory still has that promise.
Lex Fridman (01:45.980)
This is the Lex Friedman podcast,
Lex Fridman (01:47.740)
and here's my conversation with Kamran Wafa.
Lex Fridman (01:51.860)
What is the difference between mathematics
Lex Fridman (01:54.580)
and physics?
Lex Fridman (01:55.700)
Well, that's a difficult question,
Cumrun Vafa (01:57.460)
because in many ways,
Lex Fridman (01:58.420)
math and physics are unified in many ways.
Lex Fridman (02:01.640)
So to distinguish them is not an easy task.
Lex Fridman (02:04.480)
I would say that perhaps the goals
Cumrun Vafa (02:06.420)
of math and physics are different.
Lex Fridman (02:09.640)
Math does not care to describe reality, physics does.
Cumrun Vafa (02:14.420)
That's the major difference.
Lex Fridman (02:16.220)
But a lot of the thoughts, processes, and so on,
Cumrun Vafa (02:19.380)
which goes to understanding the nature and reality,
Lex Fridman (02:22.780)
are the same things that mathematicians do.
Lex Fridman (02:24.680)
So in many ways, they are similar.
Lex Fridman (02:27.220)
Mathematicians care about deductive reasoning,
Lex Fridman (02:32.680)
and physicists or physics in general,
Lex Fridman (02:35.500)
we care less about that.
Cumrun Vafa (02:37.700)
We care more about interconnection of ideas,
Lex Fridman (02:40.980)
about how ideas support each other,
Cumrun Vafa (02:42.900)
or if there's a puzzle, discord between ideas.
Lex Fridman (02:46.220)
That's more interesting for us.
Lex Fridman (02:48.220)
And part of the reason is that we have learned in physics
Lex Fridman (02:50.540)
that the ideas are not sequential.
Lex Fridman (02:53.300)
And if we think that there's one idea
Lex Fridman (02:54.920)
which is more important,
Lex Fridman (02:56.020)
and we start with there and go to the next idea,
Lex Fridman (02:58.180)
and next one, and deduce things from that,
Cumrun Vafa (02:59.780)
like mathematicians do,
Lex Fridman (03:01.580)
we have learned that the third or fourth thing
Cumrun Vafa (03:03.740)
we deduce from that principle
Lex Fridman (03:05.580)
turns out later on to be the actual principle.
Lex Fridman (03:08.380)
And from a different perspective,
Lex Fridman (03:10.900)
starting from there leads to new ideas,
Cumrun Vafa (03:12.740)
which the original one didn't lead to,
Lex Fridman (03:14.940)
and that's the beginning of a new revolution in science.
Lex Fridman (03:18.140)
So this kind of thing we have seen again and again
Lex Fridman (03:20.220)
in the history of science,
Cumrun Vafa (03:21.080)
we have learned to not like deductive reasoning
Lex Fridman (03:24.100)
because that gives us a bad starting point,
Cumrun Vafa (03:27.180)
to think that we actually have the original thought process
Lex Fridman (03:30.260)
should be viewed as the primary thought,
Lex Fridman (03:32.220)
and all these are deductions,
Lex Fridman (03:33.920)
like the way mathematicians sometimes do.
Lex Fridman (03:35.580)
So in physics, we have learned to be skeptical
Lex Fridman (03:37.980)
of that way of thinking.
Cumrun Vafa (03:38.940)
We have to be a bit open to the possibility
Lex Fridman (03:41.220)
that what we thought is a deduction of a hypothesis
Cumrun Vafa (03:44.260)
is actually the reason that's true is the opposite.
Lex Fridman (03:47.080)
And so we reverse the order.
Lex Fridman (03:48.940)
And so this switching back and forth between ideas
Lex Fridman (03:52.660)
makes us more fluid about deductive fashion.
Cumrun Vafa (03:56.620)
Of course, it sometimes gives a wrong impression
Lex Fridman (03:59.460)
like physicists don't care about rigor.
Cumrun Vafa (04:00.980)
They just say random things.
Lex Fridman (04:03.380)
They are willing to say things that are not backed
Cumrun Vafa (04:05.900)
by the logical reasoning.
Lex Fridman (04:07.780)
That's not true at all.
Lex Fridman (04:09.220)
So despite this fluidity
Lex Fridman (04:12.620)
in saying which one is a primary thought,
Cumrun Vafa (04:14.980)
we are very careful about trying to understand
Lex Fridman (04:17.100)
what we have really understood in terms of relationship
Cumrun Vafa (04:19.900)
between ideas.
Lex Fridman (04:21.140)
So that's an important ingredient.
Lex Fridman (04:24.000)
And in fact, solid math, being behind physics
Lex Fridman (04:27.340)
is I think one of the attractive features
Cumrun Vafa (04:30.740)
of a physical law.
Lex Fridman (04:32.220)
So we look for beautiful math underpinning it.
Cumrun Vafa (04:35.260)
Can we dig into that process of starting from one place
Lex Fridman (04:39.740)
and then ending up at like the fourth step
Lex Fridman (04:43.060)
and realizing all along that the place you started at
Lex Fridman (04:46.120)
was wrong?
Lex Fridman (04:47.000)
So is that happened when there's a discrepancy
Lex Fridman (04:50.540)
between what the math says
Lex Fridman (04:53.140)
and what the physical world shows?
Lex Fridman (04:54.940)
Is that how you then can go back
Lex Fridman (04:56.860)
and do the revolutionary idea
Lex Fridman (04:59.660)
for different starting place altogether?
Cumrun Vafa (05:02.060)
Perhaps I give an example to see how it goes.
Lex Fridman (05:04.800)
And in fact, the historical example is Newton's work
Cumrun Vafa (05:08.820)
on classical mechanics.
Lex Fridman (05:10.140)
So Newton formulated the laws of mechanics,
Cumrun Vafa (05:14.320)
the force F equals to MA and his other laws,
Lex Fridman (05:17.860)
and they look very simple, elegant, and so forth.
Cumrun Vafa (05:20.980)
Later, when we studied more examples of mechanics
Lex Fridman (05:25.860)
and other similar things, physicists came up with the idea
Cumrun Vafa (05:28.980)
that the notion of potential is interesting.
Lex Fridman (05:31.420)
Potential was an abstract idea, which kind of came,
Cumrun Vafa (05:33.820)
you could take its gradient and relate it to the force.
Lex Fridman (05:37.000)
So you don't really need it a priori,
Lex Fridman (05:38.720)
but it solved, helped some thoughts.
Lex Fridman (05:41.200)
And then later, Euler and Lagrange reformulated
Cumrun Vafa (05:45.940)
Newtonian mechanics in a totally different way
Lex Fridman (05:49.220)
in the following fashion.
Cumrun Vafa (05:50.340)
They said, if you take,
Lex Fridman (05:51.580)
if you wanna know where a particle at this point
Lex Fridman (05:53.580)
and at this time, how does it get to this point
Lex Fridman (05:55.980)
at the later time, is the following.
Cumrun Vafa (05:58.940)
You take all possible paths connecting this particle
Lex Fridman (06:01.620)
from going from the initial point to the final point,
Lex Fridman (06:04.660)
and you compute the action.
Lex Fridman (06:07.140)
And what is an action?
Cumrun Vafa (06:08.380)
Action is the integral over time
Lex Fridman (06:10.780)
of the kinetic term of the particle minus its potential.
Lex Fridman (06:15.020)
So you take this integral,
Lex Fridman (06:16.780)
and each path will give you some quantity.
Lex Fridman (06:19.100)
And the path it actually takes, the physical path,
Lex Fridman (06:23.060)
is the one which minimizes this integral or this action.
Cumrun Vafa (06:26.580)
Now, this sounded like a backward step from Newton's.
Lex Fridman (06:29.900)
Newton's formula seemed very simple.
Cumrun Vafa (06:32.720)
F equals to ma, and you can write F is minus
Lex Fridman (06:35.180)
the gradient of the potential.
Lex Fridman (06:36.860)
So why would anybody start formulating such a simple thing
Lex Fridman (06:40.020)
in terms of this complicated looking principle?
Cumrun Vafa (06:43.220)
You have to study the space of all paths and all things
Lex Fridman (06:46.740)
and find the minimum, and then you get the same equation.
Lex Fridman (06:48.740)
So what's the point?
Lex Fridman (06:50.020)
So Euler and Lagrange's formulation of Newton,
Cumrun Vafa (06:52.460)
which was kind of recasting in this language,
Lex Fridman (06:56.440)
is just a consequence of Newton's law.
Cumrun Vafa (06:58.060)
F equals to ma gives you the same fact
Lex Fridman (06:59.820)
that this path is a minimum action.
Cumrun Vafa (07:02.220)
Now, what we learned later, last century,
Lex Fridman (07:05.420)
was that when we deal with quantum mechanics,
Cumrun Vafa (07:08.520)
Newton's law is only an average correct.
Lex Fridman (07:12.960)
And the particle going from one to the other
Cumrun Vafa (07:15.820)
doesn't take exactly one path.
Lex Fridman (07:17.880)
It takes all the paths with the amplitude,
Cumrun Vafa (07:21.780)
which is proportional to the exponential
Lex Fridman (07:23.660)
of the action times an imaginary number, i.
Lex Fridman (07:26.940)
And so this fact turned out to be the reformulation
Lex Fridman (07:29.940)
of quantum mechanics.
Cumrun Vafa (07:30.780)
We should start there as the basis of the new law,
Lex Fridman (07:33.500)
which is quantum mechanics, and Newton is only
Cumrun Vafa (07:35.660)
an approximation on the average correct.
Lex Fridman (07:37.980)
And when you say amplitude, you mean probability?
Cumrun Vafa (07:40.460)
Yes, the amplitude means if you sum up all these paths
Lex Fridman (07:43.180)
with exponential i times the action,
Cumrun Vafa (07:45.100)
if you sum this up, you get the number, complex number.
Lex Fridman (07:48.380)
You square the norm of this complex number,
Cumrun Vafa (07:50.480)
gives you a probability to go from one to the other.
Lex Fridman (07:52.740)
Is there ways in which mathematics can lead us astray
Lex Fridman (07:57.820)
when we use it as a tool to understand the physical world?
Lex Fridman (08:01.500)
Yes, I would say that mathematics can lead us astray
Cumrun Vafa (08:04.580)
as much as old physical ideas can lead us astray.
Lex Fridman (08:08.260)
So if you get stuck in something,
Cumrun Vafa (08:11.700)
then you can easily fool yourself
Lex Fridman (08:13.300)
that just like the thought process,
Cumrun Vafa (08:15.540)
we have to free ourselves of that.
Lex Fridman (08:17.300)
Sometimes math does that role, like say,
Cumrun Vafa (08:19.060)
oh, this is such a beautiful math.
Lex Fridman (08:20.580)
I definitely want to use it somewhere.
Lex Fridman (08:22.020)
And so you just get carried away
Lex Fridman (08:23.820)
and you just get maybe carried too far away.
Lex Fridman (08:25.760)
So that is certainly true, but I wouldn't say
Lex Fridman (08:28.200)
it's more dangerous than old physical ideas.
Cumrun Vafa (08:30.620)
To me, new math ideas is as much potential
Lex Fridman (08:34.420)
to lead us astray as old physical ideas,
Cumrun Vafa (08:36.380)
which could be long held principles of physics.
Lex Fridman (08:38.780)
So I'm just saying that we should keep an open mind
Cumrun Vafa (08:41.860)
about the role the math plays,
Lex Fridman (08:43.900)
not to be antagonistic towards it
Lex Fridman (08:46.020)
and not to over, over welcoming it.
Lex Fridman (08:48.980)
We should just be open to possibilities.
Lex Fridman (08:51.060)
What about looking at a particular characteristics
Lex Fridman (08:53.660)
of both physical ideas and mathematical ideas,
Lex Fridman (08:55.700)
which is beauty?
Lex Fridman (08:56.820)
You think beauty leads us astray, meaning,
Lex Fridman (09:00.060)
and you offline showed me a really nice puzzle
Lex Fridman (09:03.180)
that illustrates this idea a little bit.
Cumrun Vafa (09:06.380)
Now, maybe you can speak to that or another example
Lex Fridman (09:09.020)
where beauty makes it tempting for us to assume
Cumrun Vafa (09:13.540)
that the law and the theory we found
Lex Fridman (09:16.360)
is actually one that perfectly describes reality.
Cumrun Vafa (09:19.740)
I think that beauty does not lead us astray
Lex Fridman (09:22.820)
because I feel that beauty is a requirement
Cumrun Vafa (09:25.920)
for principles of physics.
Lex Fridman (09:27.740)
So beauty is a fundamental in the universe?
Cumrun Vafa (09:29.580)
I think beauty is fundamental.
Lex Fridman (09:30.900)
At least that's the way many of us view it.
Cumrun Vafa (09:32.740)
It's not emergent.
Lex Fridman (09:33.940)
It's not emergent.
Cumrun Vafa (09:35.140)
I think Hardy is the mathematician who said
Lex Fridman (09:37.740)
that there's no permanent place for ugly mathematics.
Lex Fridman (09:40.820)
And so I think the same is true in physics
Lex Fridman (09:42.940)
that if we find the principle which looks ugly,
Cumrun Vafa (09:47.900)
we are not going to be, that's not the end stage.
Lex Fridman (09:49.660)
So therefore beauty is going to lead us somewhere.
Cumrun Vafa (09:52.040)
Now, it doesn't mean beauty is enough.
Lex Fridman (09:54.840)
It doesn't mean if you just have beauty,
Cumrun Vafa (09:56.500)
if I just look at something is beautiful, then I'm fine.
Lex Fridman (09:58.980)
No, that's not the case.
Cumrun Vafa (10:00.260)
Beauty is certainly a criteria that every good
Lex Fridman (10:03.460)
physical theory should pass.
Cumrun Vafa (10:04.500)
That's at least the view we have.
Lex Fridman (10:06.500)
Why do we have this view?
Cumrun Vafa (10:08.260)
That's a good question.
Lex Fridman (10:09.760)
It is partly, you could say, based on experience
Cumrun Vafa (10:13.340)
of science over centuries, partly is philosophical view
Lex Fridman (10:17.060)
of what reality is or should be.
Lex Fridman (10:20.080)
And in principle, it could have been ugly
Lex Fridman (10:23.780)
and we might have had to deal with it,
Lex Fridman (10:25.540)
but we have gotten maybe confident through examples
Lex Fridman (10:29.520)
in the history of science to look for beauty.
Lex Fridman (10:32.460)
And our sense of beauty seems to incorporate
Lex Fridman (10:34.300)
a lot of things that are essential for us
Cumrun Vafa (10:36.040)
to solve some difficult problems like symmetry.
Lex Fridman (10:37.980)
We find symmetry beautiful
Lex Fridman (10:39.340)
and the breaking of symmetry beautiful.
Lex Fridman (10:41.260)
Somehow symmetry is a fundamental part
Cumrun Vafa (10:45.420)
of how we conceive of beauty at all layers of reality,
Lex Fridman (10:50.020)
which is interesting.
Cumrun Vafa (10:51.220)
Like in both the visual space, like the way we look at art,
Lex Fridman (10:55.220)
we look at each other as human beings,
Cumrun Vafa (10:57.140)
the way we look at creatures in the biological space,
Lex Fridman (10:59.780)
the way we look at chemistry,
Lex Fridman (11:01.220)
and then into the physics world as the work you do.
Lex Fridman (11:04.420)
It's kind of interesting.
Cumrun Vafa (11:05.420)
It makes you wonder like,
Lex Fridman (11:08.500)
which one is the chicken or the egg?
Cumrun Vafa (11:10.020)
Is symmetry the chicken and our conception of beauty
Lex Fridman (11:13.100)
the egg or the other way around?
Cumrun Vafa (11:15.340)
Or somehow the fact that the symmetry is part of reality,
Lex Fridman (11:20.900)
it somehow creates a brain that then is able to perceive it.
Cumrun Vafa (11:24.220)
Or maybe this is just because we,
Lex Fridman (11:27.060)
maybe it's so obvious, it's almost trivial,
Cumrun Vafa (11:32.300)
that symmetry, of course,
Lex Fridman (11:33.540)
will be part of every kind of universe that's possible.
Lex Fridman (11:37.940)
And then any kind of organism that's able to observe
Lex Fridman (11:40.980)
that universe is going to appreciate symmetry.
Cumrun Vafa (11:44.340)
Well, these are good questions.
Lex Fridman (11:46.060)
We don't have a deep understanding
Cumrun Vafa (11:47.380)
of why we get attracted to symmetry.
Lex Fridman (11:49.740)
Why do laws of nature seem to have symmetries underlying
Cumrun Vafa (11:54.500)
them and the reasoning or the examples of whether,
Lex Fridman (11:57.900)
if there wasn't symmetry,
Cumrun Vafa (11:58.980)
we would have understood it or not.
Lex Fridman (12:00.740)
We could have said that, yeah, if there were, you know,
Cumrun Vafa (12:02.820)
things which didn't look that great,
Lex Fridman (12:04.060)
we could understand them.
Cumrun Vafa (12:04.900)
For example, we know that symmetries get broken
Lex Fridman (12:08.060)
and we have appreciated nature
Cumrun Vafa (12:10.300)
in the broken symmetry phase as well.
Lex Fridman (12:12.420)
The world we live in has many things
Cumrun Vafa (12:14.300)
which do not look symmetric,
Lex Fridman (12:16.140)
but even those have underlying symmetry
Cumrun Vafa (12:18.940)
when you look at it more deeply.
Lex Fridman (12:20.640)
So we have gotten maybe spoiled perhaps
Cumrun Vafa (12:23.080)
by the appearance of symmetry all over the place.
Lex Fridman (12:25.940)
And we look for it.
Lex Fridman (12:26.980)
And I think this is perhaps related to a sense of aesthetics
Lex Fridman (12:31.940)
that scientists have.
Lex Fridman (12:33.340)
And we don't usually talk about it among scientists.
Lex Fridman (12:36.780)
In fact, it's kind of a philosophical view
Cumrun Vafa (12:39.540)
of why do we look for simplicity or beauty or so forth.
Lex Fridman (12:43.240)
And I think in a sense, scientists are a lot
Cumrun Vafa (12:47.300)
like philosophers.
Lex Fridman (12:48.900)
Sometimes I think, especially modern science
Cumrun Vafa (12:51.180)
seems to shun philosophers and philosophical views.
Lex Fridman (12:54.880)
And I think at their peril, I think in my view,
Cumrun Vafa (12:58.220)
science owes a lot to philosophy.
Lex Fridman (13:01.340)
And in my view, many scientists, in fact,
Cumrun Vafa (13:04.940)
probably all good scientists
Lex Fridman (13:06.180)
are perhaps amateur philosophers.
Cumrun Vafa (13:08.840)
They may not state that they are philosophers
Lex Fridman (13:11.020)
or they may not like to be labeled philosophers,
Lex Fridman (13:13.400)
but in many ways what they do
Lex Fridman (13:14.740)
is like what is philosophical takes of things.
Cumrun Vafa (13:18.640)
Looking for simplicity or symmetry
Lex Fridman (13:20.340)
is an example of that in my opinion, or seeing patterns.
Cumrun Vafa (13:23.640)
You see, for example, another example of the symmetry
Lex Fridman (13:26.660)
is like how you come up with new ideas in science.
Cumrun Vafa (13:29.420)
You see, for example, an idea A
Lex Fridman (13:31.900)
is connected with an idea B.
Cumrun Vafa (13:33.660)
Okay, so you study this connection very deeply.
Lex Fridman (13:36.780)
And then you find the cousin of an idea A,
Cumrun Vafa (13:39.300)
let me call it A prime.
Lex Fridman (13:41.140)
And then you immediately look for B prime.
Cumrun Vafa (13:44.060)
If A is like B and if there's an A prime,
Lex Fridman (13:46.260)
then you look for B prime.
Lex Fridman (13:47.340)
Why?
Lex Fridman (13:48.180)
Well, it completes the picture.
Lex Fridman (13:50.920)
Why?
Lex Fridman (13:51.760)
Well, it's philosophically appealing
Cumrun Vafa (13:53.180)
to have more balance in terms of that.
Lex Fridman (13:55.300)
And then you look for B prime and lo and behold,
Cumrun Vafa (13:57.180)
you find this other phenomenon,
Lex Fridman (13:58.580)
which is a physical phenomenon, which you call B prime.
Lex Fridman (14:01.160)
So this kind of thinking motivates
Lex Fridman (14:03.580)
asking questions and looking for things.
Lex Fridman (14:05.500)
And it has guided scientists, I think, through many centuries
Lex Fridman (14:08.580)
and I think it continues to do so today.
Lex Fridman (14:10.860)
And I think if you look at the long arc of history,
Lex Fridman (14:12.940)
I suspect that the things that will be remembered
Cumrun Vafa (14:16.420)
is the philosophical flavor of the ideas of physics
Lex Fridman (14:21.460)
and chemistry and computer science and mathematics.
Cumrun Vafa (14:24.540)
Like, I think the actual details
Lex Fridman (14:29.300)
will be shown to be incomplete or maybe wrong,
Lex Fridman (14:33.220)
but the philosophical intuitions
Lex Fridman (14:34.960)
will carry through much longer.
Cumrun Vafa (14:36.880)
There's a sense in which, if it's true,
Lex Fridman (14:39.380)
that we haven't figured out most of how things work,
Cumrun Vafa (14:43.140)
currently, that it'll all be shown as wrong and silly.
Lex Fridman (14:47.140)
It'd almost be a historical artifact.
Lex Fridman (14:49.900)
But the human spirit, whatever,
Lex Fridman (14:52.660)
like the longing to understand,
Cumrun Vafa (14:55.620)
the way we perceive the world, the way we conceive of it,
Lex Fridman (14:59.040)
of our place in the world, those ideas will carry on.
Cumrun Vafa (15:02.540)
I completely agree.
Lex Fridman (15:03.380)
In fact, I believe that almost,
Cumrun Vafa (15:05.580)
well, I believe that none of the principles
Lex Fridman (15:08.220)
or laws of physics we know today are exactly correct.
Cumrun Vafa (15:11.580)
All of them are approximations to something.
Lex Fridman (15:13.700)
They are better than the previous versions that we had,
Lex Fridman (15:15.660)
but none of them are exactly correct,
Lex Fridman (15:17.740)
and none of them are gonna stand forever.
Lex Fridman (15:19.940)
So I agree that that's the process we are heading,
Lex Fridman (15:22.780)
we are improving.
Lex Fridman (15:24.100)
And yes, indeed, the thought process
Lex Fridman (15:26.380)
and that philosophical take is common.
Lex Fridman (15:28.800)
So when we look at older scientists,
Lex Fridman (15:33.420)
or maybe even all the way back to Greek philosophers
Lex Fridman (15:36.020)
and the things that the way they thought and so on,
Lex Fridman (15:38.140)
almost everything they said about nature was incorrect.
Lex Fridman (15:42.140)
But the way they thought about it
Lex Fridman (15:43.900)
and many things that they were thinking
Cumrun Vafa (15:45.860)
is still valid today.
Lex Fridman (15:46.940)
For example, they thought about symmetry breaking.
Cumrun Vafa (15:50.060)
They were trying to explain the following.
Lex Fridman (15:51.960)
This is a beautiful example, I think.
Cumrun Vafa (15:53.660)
They had figured out that the Earth is round,
Lex Fridman (15:55.900)
and they said, okay, Earth is round.
Cumrun Vafa (15:57.420)
They have seen the length of the shadow of a meter stick,
Lex Fridman (16:01.300)
and they have seen that if you go
Cumrun Vafa (16:02.500)
from the equator upwards north,
Lex Fridman (16:04.540)
they find that depending on how far away you are,
Cumrun Vafa (16:06.460)
that the length of the shadow changes.
Lex Fridman (16:07.900)
And from that, they had even measured
Cumrun Vafa (16:09.900)
the radius of the Earth to good accuracy.
Lex Fridman (16:12.260)
That's brilliant, by the way, the fact that they did that.
Cumrun Vafa (16:14.540)
Very brilliant, very brilliant.
Lex Fridman (16:15.660)
So these Greek philosophers are very smart.
Lex Fridman (16:17.700)
And so they had taken it to the next step.
Lex Fridman (16:20.640)
They asked, okay, so the Earth is round,
Lex Fridman (16:23.100)
why doesn't it move?
Lex Fridman (16:25.140)
They thought it doesn't move.
Cumrun Vafa (16:26.260)
They were looking around, nothing seemed to move.
Lex Fridman (16:28.540)
So they said, okay, we have to have a good explanation.
Cumrun Vafa (16:31.420)
It wasn't enough for them to be there.
Lex Fridman (16:33.300)
So they really wanna deeply understand that fact.
Lex Fridman (16:36.060)
And they come up with a symmetry argument.
Lex Fridman (16:38.660)
And the symmetry argument was,
Cumrun Vafa (16:40.300)
oh, if the Earth is a spherical,
Lex Fridman (16:43.460)
it must be at the center of the universe for sure.
Lex Fridman (16:45.500)
So they said the Earth is at the center of the universe.
Lex Fridman (16:47.380)
That makes sense.
Lex Fridman (16:48.220)
And they said, if the Earth is going to move,
Lex Fridman (16:50.700)
which direction does it pick?
Cumrun Vafa (16:52.180)
Any direction it picks, it breaks that spherical symmetry
Lex Fridman (16:54.820)
because you have to pick a direction.
Lex Fridman (16:57.060)
And that's not good because it's not symmetrical anymore.
Lex Fridman (16:59.320)
So therefore, the Earth decides to sit put
Cumrun Vafa (17:01.940)
because it would break the symmetry.
Lex Fridman (17:03.940)
So they had the incorrect science.
Cumrun Vafa (17:05.760)
They thought Earth doesn't move.
Lex Fridman (17:07.260)
But they had this beautiful idea
Cumrun Vafa (17:08.660)
that symmetry might explain it.
Lex Fridman (17:11.180)
But they were even smarter than that.
Cumrun Vafa (17:12.620)
Aristotle didn't agree with this argument.
Lex Fridman (17:15.940)
He said, why do you think symmetry prevents it from moving?
Lex Fridman (17:18.420)
Because the preferred position?
Lex Fridman (17:19.920)
Not so.
Cumrun Vafa (17:21.340)
He gave an example.
Lex Fridman (17:22.300)
He said, suppose you are a person
Lex Fridman (17:26.100)
and we put you at the center of a circle
Lex Fridman (17:29.380)
and we spread food around you on a circle around you,
Cumrun Vafa (17:32.880)
loaves of bread, let's say.
Lex Fridman (17:35.040)
And we say, okay, stay at the center of the circle forever.
Cumrun Vafa (17:39.080)
Are you going to do that
Lex Fridman (17:39.980)
just because it's a symmetric point?
Cumrun Vafa (17:43.220)
No, you are going to get hungry.
Lex Fridman (17:44.640)
You're going to move towards one of those loaves of bread,
Cumrun Vafa (17:46.940)
despite the fact that it breaks the symmetry.
Lex Fridman (17:49.460)
So from this way, he tried to argue
Cumrun Vafa (17:51.340)
being at the symmetric point
Lex Fridman (17:52.640)
may not be the preferred thing to do.
Lex Fridman (17:55.360)
And this idea of spontaneous symmetry breaking
Lex Fridman (17:57.560)
is something we just use today
Cumrun Vafa (17:59.640)
to describe many physical phenomena.
Lex Fridman (18:01.580)
So spontaneous symmetry breaking
Cumrun Vafa (18:03.600)
is the feature that we now use.
Lex Fridman (18:04.940)
But this idea was there thousands of years ago,
Lex Fridman (18:08.040)
but applied incorrectly to the physical world,
Lex Fridman (18:11.040)
but now we are using it.
Lex Fridman (18:12.000)
So these ideas are coming back in different forms.
Lex Fridman (18:14.740)
So I agree very much that the thought process
Cumrun Vafa (18:17.360)
is more important and these ideas are more interesting
Lex Fridman (18:20.000)
than the actual applications that people may find today.
Cumrun Vafa (18:23.160)
Did they use the language of symmetry
Lex Fridman (18:24.640)
and the symmetry breaking and spontaneous symmetry breaking?
Cumrun Vafa (18:26.840)
That's really interesting.
Lex Fridman (18:28.200)
Because I could see a conception of the universe
Cumrun Vafa (18:32.500)
that kind of tends towards perfect symmetry
Lex Fridman (18:35.200)
and is stuck there, not stuck there,
Lex Fridman (18:38.540)
but achieves that optimal and stays there.
Lex Fridman (18:42.100)
The idea that you would spontaneously
Cumrun Vafa (18:43.800)
break out of symmetry, like have these perturbations,
Lex Fridman (18:47.920)
like jump out of symmetry and back,
Cumrun Vafa (18:51.160)
that's a really difficult idea to load into your head.
Lex Fridman (18:55.320)
Like where does that come from?
Lex Fridman (18:57.320)
And then the idea that you may not be
Lex Fridman (18:59.940)
at the center of the universe.
Cumrun Vafa (19:02.420)
That is a really tough idea.
Lex Fridman (19:04.920)
Right, so symmetry sometimes is an explanation
Cumrun Vafa (19:07.320)
of being at the symmetric point.
Lex Fridman (19:08.920)
It's sometimes a simple explanation of many things.
Cumrun Vafa (19:10.920)
Like if you have a bowl, a circular bowl,
Lex Fridman (19:15.520)
then the bottom of it is the lowest point.
Lex Fridman (19:18.120)
So if you put a pebble or something,
Lex Fridman (19:19.760)
it will slide down and go there at the bottom
Lex Fridman (19:21.560)
and stays there at the symmetric point
Lex Fridman (19:23.720)
because it's the preferred point, the lowest energy point.
Lex Fridman (19:26.400)
But if that same symmetric circular bowl that you had
Lex Fridman (19:29.260)
had a bump on the bottom, the bottom might not be
Cumrun Vafa (19:33.600)
at the center, it might be on a circle on the table,
Lex Fridman (19:36.920)
in which case the pebble would not end up at the center,
Cumrun Vafa (19:39.320)
it would be the lower energy point.
Lex Fridman (19:40.880)
Symmetrical, but it breaks the symmetry
Cumrun Vafa (19:43.000)
once it takes a point on that circle.
Lex Fridman (19:45.200)
So we can have symmetry reasoning for where things end up
Cumrun Vafa (19:48.920)
or symmetry breakings, like this example would suggest.
Lex Fridman (19:52.740)
We talked about beauty.
Cumrun Vafa (19:54.500)
I find geometry to be beautiful.
Lex Fridman (19:56.640)
You have a few examples that are geometric
Lex Fridman (1:00:02.500)
but the version that ended up being related
Lex Fridman (1:00:04.780)
to having particles like electron,
Lex Fridman (1:00:06.820)
what we call fermions, needed 10 dimensions,
Lex Fridman (1:00:09.340)
what we call super string.
Lex Fridman (1:00:12.220)
Now, why super?
Lex Fridman (1:00:13.060)
Why the word super?
Cumrun Vafa (1:00:13.900)
It turns out this version of the string,
Lex Fridman (1:00:17.340)
which had fermions, had an extra symmetry,
Cumrun Vafa (1:00:21.340)
which we call supersymmetry.
Lex Fridman (1:00:23.740)
This is a symmetry between a particle and another particle
Cumrun Vafa (1:00:27.940)
with exactly the same properties,
Lex Fridman (1:00:29.620)
same mass, same charge, et cetera.
Cumrun Vafa (1:00:31.660)
The only difference is that one of them
Lex Fridman (1:00:33.060)
has a little different spin than the other one.
Lex Fridman (1:00:35.820)
And one of them is a boson, one of them is a fermion
Lex Fridman (1:00:38.780)
because of that shift of spin.
Cumrun Vafa (1:00:41.060)
Otherwise, they're identical.
Lex Fridman (1:00:42.100)
So there was this symmetry.
Cumrun Vafa (1:00:43.560)
String theory had this symmetry.
Lex Fridman (1:00:45.520)
In fact, supersymmetry was discovered
Cumrun Vafa (1:00:48.340)
through string theory, theoretically.
Lex Fridman (1:00:51.540)
So theoretically, the first place that this was observed
Cumrun Vafa (1:00:53.860)
when you were describing these fermionic strings.
Lex Fridman (1:00:57.700)
So that was the beginning of the study of supersymmetry
Cumrun Vafa (1:01:00.220)
was via string theory.
Lex Fridman (1:01:02.180)
And then it had remarkable properties
Cumrun Vafa (1:01:05.020)
that the symmetry meant and so forth
Lex Fridman (1:01:07.220)
that people began studying supersymmetry after that.
Lex Fridman (1:01:10.680)
And that was a kind of a tangent direction
Lex Fridman (1:01:13.580)
at the beginning for string theory.
Lex Fridman (1:01:15.720)
But people in particle physics started also thinking,
Lex Fridman (1:01:17.940)
oh, supersymmetry is great.
Cumrun Vafa (1:01:19.040)
Let's see if we can have supersymmetry
Lex Fridman (1:01:21.060)
in particle physics and so forth.
Cumrun Vafa (1:01:22.380)
Forget about strings.
Lex Fridman (1:01:23.260)
And they developed on a different track as well.
Cumrun Vafa (1:01:25.700)
Supersymmetry in different models
Lex Fridman (1:01:27.660)
became a subject on its own right,
Lex Fridman (1:01:29.100)
understanding supersymmetry and what does this mean?
Lex Fridman (1:01:32.260)
Because it unified bosons and fermion,
Cumrun Vafa (1:01:34.100)
unified some ideas together.
Lex Fridman (1:01:36.040)
So photon is a boson, electron is a fermion.
Lex Fridman (1:01:39.140)
Could things like that be somehow related?
Lex Fridman (1:01:41.540)
It was a kind of a natural kind of a question
Cumrun Vafa (1:01:43.660)
to try to kind of unify
Lex Fridman (1:01:44.940)
because in physics, we love unification.
Cumrun Vafa (1:01:48.100)
Now, gradually, string theory was beginning
Lex Fridman (1:01:50.220)
to show signs of unification.
Cumrun Vafa (1:01:51.700)
It had graviton, but people found that you also have
Lex Fridman (1:01:54.820)
things like photons in them,
Cumrun Vafa (1:01:56.500)
different excitations of string behave like photons,
Lex Fridman (1:01:59.100)
another one behaves like electron.
Lex Fridman (1:02:01.100)
So a single string was unifying all these particles
Lex Fridman (1:02:04.740)
into one object.
Cumrun Vafa (1:02:06.340)
That's remarkable.
Lex Fridman (1:02:08.580)
It's in 10 dimensions though.
Cumrun Vafa (1:02:10.580)
It is not our universe
Lex Fridman (1:02:11.740)
because we live in three plus one dimension.
Lex Fridman (1:02:13.500)
How could that be possibly true?
Lex Fridman (1:02:15.580)
So this was a conundrum.
Cumrun Vafa (1:02:18.220)
It was elegant, it was beautiful,
Lex Fridman (1:02:19.940)
but it was very specific
Cumrun Vafa (1:02:21.940)
about which dimension you're getting,
Lex Fridman (1:02:23.600)
which structure you're getting.
Cumrun Vafa (1:02:25.060)
It wasn't saying, oh, you just put D equals to four,
Lex Fridman (1:02:27.660)
you'll get your space time dimension that you want.
Cumrun Vafa (1:02:29.540)
No, it didn't like that.
Lex Fridman (1:02:30.900)
It said, I want 10 dimensions and that's the way it is.
Lex Fridman (1:02:34.300)
So it was very specific.
Lex Fridman (1:02:35.700)
Now, so people try to reconcile this
Cumrun Vafa (1:02:37.660)
by the idea that, you know,
Lex Fridman (1:02:39.340)
maybe these extra dimensions are tiny.
Lex Fridman (1:02:41.980)
So if you take three macroscopic spatial dimensions
Lex Fridman (1:02:45.260)
on one time and six extra tiny spatial dimensions,
Cumrun Vafa (1:02:49.460)
like tiny spheres or tiny circles,
Lex Fridman (1:02:51.760)
then it avoids contradiction with manifest fact
Cumrun Vafa (1:02:55.980)
that we haven't seen extra dimensions in experiments today.
Lex Fridman (1:02:59.740)
So that was a way to avoid conflict.
Cumrun Vafa (1:03:03.060)
Now, this was a way to avoid conflict,
Lex Fridman (1:03:05.900)
but it was not observed in experiments.
Lex Fridman (1:03:09.300)
A string observed in experiments?
Lex Fridman (1:03:10.580)
No, because it's so small.
Lex Fridman (1:03:12.980)
So it's beginning to sound a little bit funny.
Lex Fridman (1:03:16.020)
Similar feeling to the way perhaps Dirac had felt
Cumrun Vafa (1:03:19.620)
about this positron plus or minus, you know,
Lex Fridman (1:03:21.900)
it was beginning to sound a little bit like,
Cumrun Vafa (1:03:24.180)
oh yeah, not only I have to have 10 dimension,
Lex Fridman (1:03:25.900)
but I have to have this, I have to also this.
Lex Fridman (1:03:28.660)
And so conservative physicists would say,
Lex Fridman (1:03:31.140)
hmm, you know, I haven't seen these experiments.
Cumrun Vafa (1:03:34.340)
I don't know if they are really there.
Lex Fridman (1:03:35.860)
Are you pulling my leg?
Lex Fridman (1:03:37.740)
Do you want me to imagine things that are not there?
Lex Fridman (1:03:40.480)
So this was an attitude of some physicists
Cumrun Vafa (1:03:42.900)
towards string theory, despite the fact
Lex Fridman (1:03:45.380)
that the puzzle of gravity and quantum mechanics
Cumrun Vafa (1:03:47.620)
merging together work, but still was this skepticism.
Lex Fridman (1:03:50.900)
You're putting all these things that you want me
Cumrun Vafa (1:03:52.540)
to imagine there, these extra dimensions
Lex Fridman (1:03:54.220)
that I cannot see, aha, aha.
Lex Fridman (1:03:56.020)
And you want me to believe that string
Lex Fridman (1:03:57.260)
that you have not even seen the experiments are real,
Lex Fridman (1:03:59.060)
aha, okay, what else do you want me to believe?
Lex Fridman (1:04:01.140)
So this kind of beginning to sound a little funny.
Cumrun Vafa (1:04:03.380)
Now, I will pass forward a little bit further.
Lex Fridman (1:04:08.820)
A few decades later, when string theory became
Cumrun Vafa (1:04:11.300)
the mainstream of efforts to unify the forces
Lex Fridman (1:04:13.980)
and particles together, we learned
Cumrun Vafa (1:04:16.500)
that these extra dimensions actually solved problems.
Lex Fridman (1:04:20.780)
They weren't a nuisance the way they originally appeared.
Cumrun Vafa (1:04:24.380)
First of all, the properties of these extra dimensions
Lex Fridman (1:04:28.020)
reflected the number of particles we got in four dimensions.
Cumrun Vafa (1:04:31.460)
If you took these six dimensions to have like five holes
Lex Fridman (1:04:34.280)
or four holes, change the number of particles
Cumrun Vafa (1:04:37.020)
that you see in four dimensional space time,
Lex Fridman (1:04:39.460)
you get one electron and one muon if you had this,
Lex Fridman (1:04:42.060)
but if you did the other J shape, you get something else.
Lex Fridman (1:04:44.580)
So geometrically, you could get different kinds of physics.
Lex Fridman (1:04:47.700)
So it was kind of a mirroring of geometry by physics
Lex Fridman (1:04:51.860)
down in the macroscopic space.
Lex Fridman (1:04:53.400)
So these extra dimension were becoming useful.
Lex Fridman (1:04:56.780)
Fine, but we didn't need the extra dimension
Cumrun Vafa (1:04:58.820)
to just write an electron in three dimensions,
Lex Fridman (1:05:00.620)
we did rewrote it, so what?
Lex Fridman (1:05:02.820)
Was there any other puzzle?
Lex Fridman (1:05:04.260)
Yes, there were, Hawking.
Cumrun Vafa (1:05:07.140)
Hawking had been studying black holes in mid 70s
Lex Fridman (1:05:10.900)
following the work of Bekenstein,
Cumrun Vafa (1:05:12.840)
who had predicted that black holes have entropy.
Lex Fridman (1:05:17.940)
So Bekenstein had tried to attach the entropy
Cumrun Vafa (1:05:20.260)
to the black hole.
Lex Fridman (1:05:21.540)
If you throw something into the black hole,
Cumrun Vafa (1:05:23.860)
the entropy seems to go down
Lex Fridman (1:05:25.320)
because you had something entropy outside the black hole
Lex Fridman (1:05:28.020)
and you throw it, black hole was unique,
Lex Fridman (1:05:30.740)
so the entropy did not have any, black hole had no entropy.
Lex Fridman (1:05:33.500)
So the entropy seemed to go down.
Lex Fridman (1:05:35.980)
And so that's against the laws of thermodynamics.
Lex Fridman (1:05:38.040)
So Bekenstein was trying to say, no, no,
Lex Fridman (1:05:40.040)
therefore black hole must have an entropy.
Lex Fridman (1:05:42.140)
So he was trying to understand that he found that
Lex Fridman (1:05:43.860)
if you assign entropy to be proportional
Cumrun Vafa (1:05:47.180)
to the area of the black hole, it seems to work.
Lex Fridman (1:05:49.220)
And then Hawking found not only that's correct,
Cumrun Vafa (1:05:52.640)
he found the correct proportionality factor
Lex Fridman (1:05:54.940)
of a one quarter of the area and Planck units
Cumrun Vafa (1:05:57.060)
is the correct amount of entropy.
Lex Fridman (1:05:59.400)
And he gave an argument using
Cumrun Vafa (1:06:01.100)
quantum semi classical arguments,
Lex Fridman (1:06:03.300)
which means basically using a little bit
Cumrun Vafa (1:06:05.660)
of a quantum mechanics,
Lex Fridman (1:06:06.880)
because he didn't have the full quantum mechanics
Cumrun Vafa (1:06:09.020)
of string theory, he could do some aspects
Lex Fridman (1:06:11.060)
of approximate quantum arguments.
Lex Fridman (1:06:12.940)
So he heuristic quantum arguments led
Lex Fridman (1:06:14.860)
to this entropy formula.
Lex Fridman (1:06:17.400)
But then he didn't answer the following question.
Lex Fridman (1:06:20.780)
He was getting a big entropy for the black hole,
Cumrun Vafa (1:06:23.380)
the black hole with the size of the horizon
Lex Fridman (1:06:25.020)
of a black hole is huge, has a huge amount of entropy.
Lex Fridman (1:06:27.700)
What are the microstates of this entropy?
Lex Fridman (1:06:29.660)
When you say, for example, the gas is entropy,
Cumrun Vafa (1:06:32.260)
you count where the atoms are,
Lex Fridman (1:06:33.780)
you count this bucket or that bucket,
Cumrun Vafa (1:06:35.540)
there's an information about there and so on, you count them.
Lex Fridman (1:06:38.660)
For the black hole, the way Hawking was thinking,
Cumrun Vafa (1:06:40.820)
there was no degree of freedom, you throw them in,
Lex Fridman (1:06:43.180)
and there was just one solution.
Lex Fridman (1:06:44.500)
So where are these entropy?
Lex Fridman (1:06:46.700)
What are these microscopic states?
Cumrun Vafa (1:06:50.140)
They were hidden somewhere.
Lex Fridman (1:06:51.940)
So later in string theory,
Cumrun Vafa (1:06:54.340)
the work that we did with my colleague Strominger,
Lex Fridman (1:06:57.620)
in particular showed that these ingredients
Cumrun Vafa (1:07:00.500)
in string theory of black hole arise
Lex Fridman (1:07:04.340)
from the extra dimensions.
Lex Fridman (1:07:06.080)
So the degrees of freedom are hidden
Lex Fridman (1:07:08.300)
in terms of things like strings,
Cumrun Vafa (1:07:10.020)
wrapping these extra circles in these hidden dimensions.
Lex Fridman (1:07:13.660)
And then we started counting how many ways
Cumrun Vafa (1:07:16.020)
like the strings can wrap around this circle
Lex Fridman (1:07:18.180)
and the extra dimension or that circle
Lex Fridman (1:07:19.960)
and counted the microscopic degrees of freedom.
Lex Fridman (1:07:22.460)
And lo and behold, we got the microscopic degrees
Cumrun Vafa (1:07:24.780)
of freedom that Hawking was predicting four dimensions.
Lex Fridman (1:07:27.760)
So the extra dimensions became useful
Cumrun Vafa (1:07:30.180)
for resolving a puzzle in four dimensions.
Lex Fridman (1:07:32.820)
The puzzle was where are the degrees of freedom
Lex Fridman (1:07:35.340)
of the black hole hidden?
Lex Fridman (1:07:36.680)
The answer, hidden in the extra dimensions.
Cumrun Vafa (1:07:39.300)
The tiny extra dimensions.
Lex Fridman (1:07:41.020)
So then by this time, it was beginning to,
Cumrun Vafa (1:07:43.820)
we see aspects that extra dimensions
Lex Fridman (1:07:46.160)
are useful for many things.
Cumrun Vafa (1:07:47.340)
It's not a nuisance.
Lex Fridman (1:07:48.720)
It wasn't to be kind of, you know, be ashamed of.
Cumrun Vafa (1:07:51.180)
It was actually in the welcome features.
Lex Fridman (1:07:53.540)
New feature, nevertheless.
Lex Fridman (1:07:54.900)
How do you intuit the 10 dimensional world?
Lex Fridman (1:07:59.580)
So yes, it's a feature for describing certain phenomena
Cumrun Vafa (1:08:03.140)
like the entropy in black holes,
Lex Fridman (1:08:06.380)
but what you said that to you a theory becomes real
Cumrun Vafa (1:08:14.020)
or becomes powerful when you can connect it
Lex Fridman (1:08:16.380)
to some deep intuition.
Lex Fridman (1:08:18.200)
So how do we intuit 10 dimensions?
Lex Fridman (1:08:20.580)
Yes, so I will explain how some of the analogies work.
Cumrun Vafa (1:08:24.820)
First of all, we do a lot of analogies.
Lex Fridman (1:08:28.740)
And by analogies, we build intuition.
Lex Fridman (1:08:31.020)
So I will start with this example.
Lex Fridman (1:08:33.260)
I will try to explain that if we are in 10 dimensional space,
Cumrun Vafa (1:08:37.500)
if we have a seven dimensional plane
Lex Fridman (1:08:40.260)
and eight dimensional plane,
Cumrun Vafa (1:08:42.860)
we ask typically in what space do they intersect each other
Lex Fridman (1:08:45.580)
in what dimension?
Cumrun Vafa (1:08:46.720)
That might sound like,
Lex Fridman (1:08:48.040)
how do you possibly give an answer to this?
Lex Fridman (1:08:50.540)
So we start with lower dimensions.
Lex Fridman (1:08:52.440)
We start with two dimensions.
Cumrun Vafa (1:08:53.620)
We say, if you have one dimension and a point,
Lex Fridman (1:08:56.540)
do they intersect typically on a plane?
Cumrun Vafa (1:08:58.600)
The answer is no.
Lex Fridman (1:08:59.440)
So a line one dimensional, a point zero dimension
Cumrun Vafa (1:09:02.980)
on a two dimensional plane, they don't typically meet.
Lex Fridman (1:09:05.780)
But if you have a one dimensional line and another line,
Cumrun Vafa (1:09:08.540)
which is one plus one on a plane,
Lex Fridman (1:09:10.540)
they typically intersect at a point.
Cumrun Vafa (1:09:13.620)
Typically means if you're not parallel,
Lex Fridman (1:09:15.140)
typically they intersect at a point.
Lex Fridman (1:09:17.100)
So one plus one is two and in two dimension,
Lex Fridman (1:09:20.620)
they intersect at the zero dimensional point.
Lex Fridman (1:09:23.020)
So you see two dimension, one and one, two,
Lex Fridman (1:09:25.740)
two minus two is zero.
Lex Fridman (1:09:26.900)
So you get point out of intersection.
Lex Fridman (1:09:29.860)
Let's go to three dimension.
Cumrun Vafa (1:09:31.860)
You have a plane, two dimensional plane and a point.
Lex Fridman (1:09:33.860)
Do they intersect?
Cumrun Vafa (1:09:34.700)
No, two and zero.
Lex Fridman (1:09:37.840)
How about the plane and a line?
Cumrun Vafa (1:09:39.420)
A plane is two dimensional and a line is one.
Lex Fridman (1:09:41.420)
Two plus one is three.
Cumrun Vafa (1:09:42.980)
In three dimension, a plane and a line meet at points,
Lex Fridman (1:09:47.000)
which is zero dimensional.
Cumrun Vafa (1:09:47.900)
Three minus three is zero.
Lex Fridman (1:09:49.700)
Okay, so plane and a line intersect
Cumrun Vafa (1:09:52.920)
at a point in three dimension.
Lex Fridman (1:09:54.060)
How about the plane and a plane in 3D?
Cumrun Vafa (1:09:56.020)
Well, plane is two and this is two.
Lex Fridman (1:09:57.660)
Two plus two is four.
Cumrun Vafa (1:09:59.160)
In 3D, four minus three is one.
Lex Fridman (1:10:01.040)
They intersect on a one dimensional line.
Cumrun Vafa (1:10:03.100)
Okay, we're beginning to see the pattern.
Lex Fridman (1:10:04.540)
Okay, now come to the question.
Cumrun Vafa (1:10:06.060)
We're in 10 dimension.
Lex Fridman (1:10:06.900)
Now we have the intuition.
Cumrun Vafa (1:10:08.100)
We have a seven dimensional plane
Lex Fridman (1:10:09.380)
and eight dimensional plane in 10 dimension.
Cumrun Vafa (1:10:11.540)
They intersect on a plane.
Lex Fridman (1:10:13.140)
What's the dimension?
Cumrun Vafa (1:10:14.020)
Well, seven plus eight is 15 minus 10 is five.
Lex Fridman (1:10:16.860)
We draw the same picture as two planes
Lex Fridman (1:10:20.320)
and we write seven dimension, eight dimension,
Lex Fridman (1:10:22.520)
but we have gotten the intuition
Cumrun Vafa (1:10:23.940)
from the lower dimensional one.
Lex Fridman (1:10:25.300)
What to expect?
Cumrun Vafa (1:10:26.980)
It doesn't scare us anymore.
Lex Fridman (1:10:28.660)
So we draw this picture.
Cumrun Vafa (1:10:30.220)
We cannot see all the seven dimensions
Lex Fridman (1:10:32.700)
by looking at this two dimensional visualization of it,
Lex Fridman (1:10:36.220)
but it has all the features we want.
Lex Fridman (1:10:38.380)
It has, so we draw this picture.
Cumrun Vafa (1:10:39.900)
It says seven, seven,
Lex Fridman (1:10:40.840)
and they meet at the five dimensional plane.
Cumrun Vafa (1:10:43.660)
It says five.
Lex Fridman (1:10:44.640)
So we have built this intuition.
Cumrun Vafa (1:10:46.540)
Now, this is an example of how we come up with intuition.
Lex Fridman (1:10:51.860)
Let me give you more examples of it
Cumrun Vafa (1:10:53.260)
because I think this will show you
Lex Fridman (1:10:54.860)
that people have to come up with intuitions to visualize it.
Cumrun Vafa (1:10:57.900)
Otherwise, we will be a little bit lost.
Lex Fridman (1:11:00.860)
So what you just described is kind of
Cumrun Vafa (1:11:02.940)
in these high dimensional spaces,
Lex Fridman (1:11:04.280)
focus on the meeting place of two planes
Cumrun Vafa (1:11:08.780)
in high dimensional spaces.
Lex Fridman (1:11:10.220)
Exactly, how the planes meet, for example,
Cumrun Vafa (1:11:12.220)
what's the dimension of their intersection and so on.
Lex Fridman (1:11:14.760)
So how do we come up with intuition?
Cumrun Vafa (1:11:16.620)
We borrow examples from lower dimensions,
Lex Fridman (1:11:19.580)
build up intuition and draw the same pictures
Cumrun Vafa (1:11:21.940)
as if we are talking about 10 dimensions,
Lex Fridman (1:11:24.780)
but we are drawing the same as a two dimensional plane
Cumrun Vafa (1:11:26.860)
because we cannot do any better.
Lex Fridman (1:11:28.260)
But our words change, but not our pictures.
Lex Fridman (1:11:32.540)
So your sense is we can have a deep understanding
Lex Fridman (1:11:35.820)
of reality by looking at its slices,
Cumrun Vafa (1:11:39.220)
at lower dimensional slices.
Lex Fridman (1:11:40.460)
Exactly, exactly.
Lex Fridman (1:11:41.780)
And this brings me to the next example I wanna mention,
Lex Fridman (1:11:45.200)
which is sphere.
Lex Fridman (1:11:46.440)
Let's think about how do we think about the sphere?
Lex Fridman (1:11:48.580)
Well, the sphere is a sphere, the round nice thing,
Lex Fridman (1:11:51.460)
but sphere has a circular symmetry.
Lex Fridman (1:11:53.520)
Now, I can describe the sphere in the following way.
Cumrun Vafa (1:11:57.880)
I can describe it by an interval,
Lex Fridman (1:12:01.520)
which is thinking about this going from the north
Cumrun Vafa (1:12:04.440)
of the sphere to the south.
Lex Fridman (1:12:06.360)
And at each point, I have a circle attached to it.
Lex Fridman (1:12:09.480)
So you can think about the sphere as a line
Lex Fridman (1:12:11.440)
with a circle attached with each point,
Cumrun Vafa (1:12:13.920)
the circle shrinks to a point at end points
Lex Fridman (1:12:17.520)
of the interval.
Lex Fridman (1:12:18.360)
So I can say, oh, one way to think about the sphere
Lex Fridman (1:12:21.920)
is an interval where at each point on that interval,
Cumrun Vafa (1:12:25.300)
there's another circle I'm not drawing.
Lex Fridman (1:12:27.280)
But if you like, you can just draw it.
Cumrun Vafa (1:12:29.280)
Say, okay, I won't draw it.
Lex Fridman (1:12:30.200)
So from now on, there's this mnemonic.
Cumrun Vafa (1:12:32.520)
I draw an interval when I wanna talk about the sphere
Lex Fridman (1:12:34.640)
and you remember that the end points of the interval
Cumrun Vafa (1:12:37.240)
mean a strong circle, that's all.
Lex Fridman (1:12:39.320)
And they say, yeah, I see, that's a sphere, good.
Cumrun Vafa (1:12:41.280)
Now, we wanna talk about the product of two spheres.
Lex Fridman (1:12:44.300)
That's four dimensional, how can I visualize it?
Cumrun Vafa (1:12:47.040)
Easy, you just take an interval and another interval,
Lex Fridman (1:12:50.900)
that's just gonna be a square.
Lex Fridman (1:12:54.160)
A square is a four dimensional space, yeah, why is that?
Lex Fridman (1:12:57.600)
Well, at each point on the square, there's two circles,
Cumrun Vafa (1:13:02.160)
one for each of those directions you drew.
Lex Fridman (1:13:04.840)
And when you get to the boundaries of each direction,
Cumrun Vafa (1:13:07.080)
one of the circles shrink on each edge of that square.
Lex Fridman (1:13:09.920)
And when you get to the corners of the square,
Cumrun Vafa (1:13:11.680)
all both circles shrink.
Lex Fridman (1:13:13.660)
This is a sphere times a sphere, I have defined interval.
Cumrun Vafa (1:13:17.480)
I just described for you a four dimensional space.
Lex Fridman (1:13:20.240)
Do you want a six dimensional space?
Cumrun Vafa (1:13:21.640)
No problem, take a corner of a room.
Lex Fridman (1:13:25.780)
In fact, if you want to have a sphere times a sphere
Cumrun Vafa (1:13:28.160)
times a sphere times a sphere, take a cube.
Lex Fridman (1:13:32.360)
A cube is a rendition of this six dimensional space,
Cumrun Vafa (1:13:36.800)
two sphere times another sphere times another sphere,
Lex Fridman (1:13:39.320)
where three of the circles I'm not drawing for you.
Cumrun Vafa (1:13:41.860)
For each one of those directions, there's another circle.
Lex Fridman (1:13:43.720)
But each time you get to the boundary of the cube,
Cumrun Vafa (1:13:45.880)
one circle shrinks.
Lex Fridman (1:13:47.080)
When the boundaries meet, two circles shrinks.
Cumrun Vafa (1:13:48.760)
When three boundaries meet, all the three circles shrink.
Lex Fridman (1:13:51.960)
So I just give you a picture.
Cumrun Vafa (1:13:53.320)
Now, mathematicians come up with amazing things.
Lex Fridman (1:13:55.900)
Like, you know what, I want to take a point in space
Lex Fridman (1:13:58.080)
and blow it up.
Lex Fridman (1:13:59.400)
You know, these concepts like topology and geometry,
Lex Fridman (1:14:01.920)
complicated, how do you do?
Lex Fridman (1:14:03.520)
In this picture, it's very easy.
Cumrun Vafa (1:14:05.360)
Blow it up in this picture means the following.
Lex Fridman (1:14:07.840)
You think about this cube, you go to the corner
Lex Fridman (1:14:10.280)
and you chop off a corner.
Lex Fridman (1:14:12.640)
Chopping off the corner replaces the point.
Cumrun Vafa (1:14:15.160)
Yeah.
Lex Fridman (1:14:16.000)
Replace the point by a triangle.
Cumrun Vafa (1:14:17.160)
Yes.
Lex Fridman (1:14:18.000)
So you're blowing up a point and then this triangle
Cumrun Vafa (1:14:19.760)
is what they call P2, projective two space.
Lex Fridman (1:14:22.320)
But these pictures are very physical and you feel it.
Cumrun Vafa (1:14:25.040)
There's nothing amazing.
Lex Fridman (1:14:26.280)
I'm not talking about six dimension.
Cumrun Vafa (1:14:28.040)
Four plus six is 10, the dimension of string theory.
Lex Fridman (1:14:30.520)
So we can visualize it, no problem.
Cumrun Vafa (1:14:32.280)
Okay, so that's building the intuition
Lex Fridman (1:14:34.280)
to a complicated world of string theory.
Cumrun Vafa (1:14:36.860)
Nevertheless, these objects are really small.
Lex Fridman (1:14:39.940)
And just like you said, experimental validation
Cumrun Vafa (1:14:42.520)
is very difficult because the objects are way smaller
Lex Fridman (1:14:45.360)
than anything that we currently have the tools
Lex Fridman (1:14:48.320)
and accelerators and so on to reveal through experiment.
Lex Fridman (1:14:53.760)
So there's a kind of skepticism
Cumrun Vafa (1:14:56.280)
that's not just about the nature of the theory
Lex Fridman (1:14:59.340)
because of the 10 dimensions, as you've explained,
Lex Fridman (1:15:01.800)
but in that we can't experimentally validate it
Lex Fridman (1:15:05.000)
and it doesn't necessarily, to date,
Cumrun Vafa (1:15:07.480)
maybe you can correct me,
Lex Fridman (1:15:09.000)
predict something fundamentally new.
Lex Fridman (1:15:12.040)
So it's beautiful as an explaining theory,
Lex Fridman (1:15:16.080)
which means that it's very possible
Cumrun Vafa (1:15:18.200)
that it is a fundamental theory
Lex Fridman (1:15:19.880)
that describes reality and unifies the laws,
Lex Fridman (1:15:22.580)
but there's still a kind of skepticism.
Lex Fridman (1:15:25.500)
And me, from sort of an outside observer perspective,
Cumrun Vafa (1:15:30.900)
have been observing a little bit of a growing cynicism
Lex Fridman (1:15:34.320)
about string theory in the recent few years.
Lex Fridman (1:15:37.300)
Can you describe the cynicism about,
Lex Fridman (1:15:40.080)
sort of by cynicism I mean a cynicism
Cumrun Vafa (1:15:42.760)
about the hope for this theory
Lex Fridman (1:15:46.440)
of pushing theoretical physics forward?
Cumrun Vafa (1:15:49.840)
Yes.
Lex Fridman (1:15:50.800)
Can you do describe why this is cynicism
Lex Fridman (1:15:53.960)
and how do we reverse that trend?
Lex Fridman (1:15:56.040)
Yes, first of all, the criticism for string theory
Cumrun Vafa (1:16:01.520)
is healthy in a sense that in science
Lex Fridman (1:16:04.600)
we have to have different viewpoints and that's good.
Lex Fridman (1:16:07.260)
So I welcome criticism and the reason for criticism
Lex Fridman (1:16:12.440)
and I think that is a valid reason
Cumrun Vafa (1:16:13.880)
is that there has been zero experimental evidence
Lex Fridman (1:16:15.880)
for string theory.
Cumrun Vafa (1:16:17.120)
That is no experiment has been done
Lex Fridman (1:16:20.300)
to show that there's this loop of energy moving around.
Lex Fridman (1:16:24.080)
And so that's a valid objection and valid worry.
Lex Fridman (1:16:28.520)
And if I were to say, you know what,
Cumrun Vafa (1:16:30.240)
string theory can never be verified
Lex Fridman (1:16:31.800)
or experimentally checked, that's the way it is,
Cumrun Vafa (1:16:34.480)
they would have every right to say
Lex Fridman (1:16:36.280)
what you're talking about is not science.
Cumrun Vafa (1:16:37.840)
Because in science we will have to have
Lex Fridman (1:16:39.740)
experimental consequences and checks.
Cumrun Vafa (1:16:42.360)
The difference between string theory
Lex Fridman (1:16:44.600)
and something which is not scientific
Cumrun Vafa (1:16:45.780)
is that string theory has predictions.
Lex Fridman (1:16:47.660)
The problem is that the predictions we have today
Cumrun Vafa (1:16:49.660)
of string theory is hard to access by experiments
Lex Fridman (1:16:52.960)
available with the energies we can achieve
Cumrun Vafa (1:16:55.220)
with the colliders today.
Lex Fridman (1:16:56.680)
It doesn't mean there's a problem with string theory,
Cumrun Vafa (1:16:58.360)
it just means technologically we're not that far ahead.
Lex Fridman (1:17:01.640)
Now, we can have two attitudes.
Cumrun Vafa (1:17:04.320)
You say, well, if that's the case, why are you studying
Lex Fridman (1:17:06.780)
this subject?
Cumrun Vafa (1:17:07.620)
Because you can't do experiment today.
Lex Fridman (1:17:09.440)
Now, this is becoming a little bit more like mathematics
Cumrun Vafa (1:17:12.480)
in that sense.
Lex Fridman (1:17:13.320)
You say, well, I want to learn,
Cumrun Vafa (1:17:15.400)
I want to know how the nature works
Lex Fridman (1:17:16.680)
even though I cannot prove it today
Cumrun Vafa (1:17:18.520)
that this is it because of experiments.
Lex Fridman (1:17:21.060)
That should not prevent my mind not to think about it.
Lex Fridman (1:17:24.240)
So that's the attitude many string theorists follow,
Lex Fridman (1:17:26.260)
that it should be like this.
Cumrun Vafa (1:17:28.220)
Now, so that's an answer to the criticism,
Lex Fridman (1:17:30.940)
but there's actually a better answer to the criticism,
Cumrun Vafa (1:17:33.000)
I would say.
Lex Fridman (1:17:34.120)
We don't have experimental evidence for string theory,
Lex Fridman (1:17:37.440)
but we have theoretical evidence for string theory.
Lex Fridman (1:17:39.320)
And what do I mean by theoretical evidence
Lex Fridman (1:17:41.640)
for string theory?
Lex Fridman (1:17:42.920)
String theory has connected different parts
Cumrun Vafa (1:17:45.260)
of physics together.
Lex Fridman (1:17:47.520)
It didn't have to.
Cumrun Vafa (1:17:49.680)
It has brought connections between part of physics,
Lex Fridman (1:17:52.160)
although suppose you're just interested
Cumrun Vafa (1:17:53.600)
in particle physics.
Lex Fridman (1:17:54.760)
Suppose you're not even interested in gravity at all.
Cumrun Vafa (1:17:57.880)
It turns out there are properties
Lex Fridman (1:17:59.720)
of certain particle physics models
Cumrun Vafa (1:18:02.360)
that string theory has been able to solve using gravity,
Lex Fridman (1:18:06.080)
using ideas from string theory,
Cumrun Vafa (1:18:08.200)
ideas known as holography,
Lex Fridman (1:18:10.680)
which is relating something which has to do with particles
Cumrun Vafa (1:18:13.160)
to something having to do with gravity.
Lex Fridman (1:18:15.600)
Why did it have to be this rich?
Cumrun Vafa (1:18:17.760)
The subject is very rich.
Lex Fridman (1:18:20.240)
It's not something we were smart enough to develop.
Cumrun Vafa (1:18:23.000)
It came at us.
Lex Fridman (1:18:23.840)
As I explained to you,
Cumrun Vafa (1:18:24.660)
the development of string theory
Lex Fridman (1:18:25.720)
came from accidental discovery.
Cumrun Vafa (1:18:28.080)
It wasn't because we were smart enough
Lex Fridman (1:18:29.760)
to come up with the idea,
Cumrun Vafa (1:18:30.840)
oh yeah, string of course has gravity in it.
Lex Fridman (1:18:32.360)
No, it was accidental discovery.
Lex Fridman (1:18:34.480)
So some people say it's not fair to say
Lex Fridman (1:18:36.160)
we have no evidence for string theory.
Cumrun Vafa (1:18:38.120)
Graviton, gravity is an evidence for string theory.
Lex Fridman (1:18:41.280)
It's predicted by string theory.
Cumrun Vafa (1:18:43.280)
We didn't put it by hand, we got it.
Lex Fridman (1:18:45.840)
So there's a qualitative check.
Cumrun Vafa (1:18:47.920)
Okay, gravity is a prediction of string theory.
Lex Fridman (1:18:51.080)
It's a postdiction because we know gravity existed.
Lex Fridman (1:18:53.640)
But still, logically it is a prediction
Lex Fridman (1:18:56.760)
because really we didn't know it had the graviton
Cumrun Vafa (1:19:00.000)
that we later learned that, oh, that's the same as gravity.
Lex Fridman (1:19:02.760)
So literally that's the way it was discovered.
Cumrun Vafa (1:19:04.480)
It wasn't put in by hand.
Lex Fridman (1:19:06.280)
So there are many things like that,
Cumrun Vafa (1:19:08.480)
that there are different facets of physics,
Lex Fridman (1:19:11.480)
like questions in condensed matter physics,
Cumrun Vafa (1:19:13.400)
questions of particle physics,
Lex Fridman (1:19:15.280)
questions about this and that have come together
Cumrun Vafa (1:19:18.060)
to find beautiful answers by using ideas
Lex Fridman (1:19:21.320)
from string theory at the same time
Cumrun Vafa (1:19:24.320)
as a lot of new math has emerged.
Lex Fridman (1:19:27.080)
That's an aspect which I wouldn't emphasize
Cumrun Vafa (1:19:29.660)
as evidence to physicists necessarily,
Lex Fridman (1:19:31.920)
because they would say, okay, great, you got some math,
Lex Fridman (1:19:33.860)
but what's it do with reality?
Lex Fridman (1:19:35.580)
But as I explained, many of the physical principles
Cumrun Vafa (1:19:38.680)
we know of have beautiful math underpinning them.
Lex Fridman (1:19:41.700)
So it certainly leads further confidence
Cumrun Vafa (1:19:45.200)
that we may not be going astray,
Lex Fridman (1:19:46.820)
even though that's not the full proof as we know.
Lex Fridman (1:19:49.200)
So there are these aspects that give further evidence
Lex Fridman (1:19:52.280)
for string theory, connections between each other,
Cumrun Vafa (1:19:55.300)
connection with the real world,
Lex Fridman (1:19:56.260)
but then there are other things that come about
Lex Fridman (1:19:58.480)
and I can try to give examples of that.
Lex Fridman (1:20:01.120)
So these are further evidences
Lex Fridman (1:20:03.000)
and these are certain predictions of string theory.
Lex Fridman (1:20:05.840)
They are not as detailed as we want,
Lex Fridman (1:20:08.880)
but there are still predictions.
Lex Fridman (1:20:11.760)
Why is the dimension of space and time three plus one?
Cumrun Vafa (1:20:16.800)
Say, I don't know, just deal with it, three plus one.
Lex Fridman (1:20:20.160)
But in physics, we want to know why.
Cumrun Vafa (1:20:23.320)
Well, take a random dimension from one to infinity.
Lex Fridman (1:20:26.520)
What's your random dimension?
Cumrun Vafa (1:20:28.740)
A random dimension from one to infinity would not be four.
Lex Fridman (1:20:33.320)
Eight would most likely be a humongous number,
Cumrun Vafa (1:20:35.640)
if not infinity.
Lex Fridman (1:20:36.480)
I mean, there's no, if you choose any reasonable distribution
Cumrun Vafa (1:20:39.640)
which goes from one to infinity,
Lex Fridman (1:20:41.340)
three or four would not be your pick.
Cumrun Vafa (1:20:44.080)
The fact that we are in three or four dimension
Lex Fridman (1:20:45.900)
is already strange.
Cumrun Vafa (1:20:48.540)
The fact that strings are sorry,
Lex Fridman (1:20:49.880)
I cannot go beyond 10 or maybe 11 or something.
Cumrun Vafa (1:20:52.720)
The fact that there's this upper bound,
Lex Fridman (1:20:54.620)
the range is not from one to infinity,
Cumrun Vafa (1:20:56.280)
it's from one to 10 or 11 or whatnot,
Lex Fridman (1:20:58.720)
it already brings a natural prior.
Cumrun Vafa (1:21:00.720)
Oh yeah, three or four is just on the average.
Lex Fridman (1:21:03.200)
If you pick some of the compactification,
Cumrun Vafa (1:21:05.120)
then it could easily be that.
Lex Fridman (1:21:06.040)
So in other words, it makes it much more possible
Cumrun Vafa (1:21:08.880)
that it could be three of our universe.
Lex Fridman (1:21:11.080)
So the fact that the dimension already is so small,
Cumrun Vafa (1:21:14.240)
it should be surprising.
Lex Fridman (1:21:16.000)
We don't ask that question.
Cumrun Vafa (1:21:17.440)
We should be surprised because we could have conceived
Lex Fridman (1:21:20.360)
of universes with our pre dimension.
Lex Fridman (1:21:22.440)
Why is it that we have such a small dimension?
Lex Fridman (1:21:24.280)
That's number one.
Lex Fridman (1:21:25.520)
So good theory of the universe should give you
Lex Fridman (1:21:28.760)
an intuition of the why it's four or three plus one.
Lex Fridman (1:21:33.600)
And it's not obvious that it should be.
Lex Fridman (1:21:36.120)
That should be explained.
Cumrun Vafa (1:21:37.200)
We take that as an assumption,
Lex Fridman (1:21:40.520)
but that's a thing that should be explained.
Cumrun Vafa (1:21:43.360)
Yeah, so we haven't explained that in string theory.
Lex Fridman (1:21:45.040)
Actually, I did write a model within string theory
Cumrun Vafa (1:21:47.440)
to try to describe why we end up
Lex Fridman (1:21:49.040)
with three plus one space time dimensions,
Cumrun Vafa (1:21:52.520)
which are big compared to the rest of them.
Lex Fridman (1:21:54.720)
And even though this has not been,
Cumrun Vafa (1:21:57.000)
the technical difficulties to prove it is still not there,
Lex Fridman (1:22:00.240)
but I will explain the idea because the idea connects
Cumrun Vafa (1:22:02.640)
to some other piece of elegant math,
Lex Fridman (1:22:05.360)
which is the following.
Cumrun Vafa (1:22:06.280)
Consider a universe made of a box, three dimensional box.
Lex Fridman (1:22:11.480)
Or in fact, if we start in string theory,
Cumrun Vafa (1:22:13.280)
nine dimensional box,
Lex Fridman (1:22:14.280)
because we have nine spatial dimension on one time.
Lex Fridman (1:22:17.200)
So imagine a nine dimensional box.
Lex Fridman (1:22:20.240)
So we should imagine the box of a typical size of the string,
Cumrun Vafa (1:22:23.880)
which is small.
Lex Fridman (1:22:25.600)
So the universe would naturally start
Cumrun Vafa (1:22:27.640)
with a very tiny nine dimensional box.
Lex Fridman (1:22:30.520)
What do strings do?
Cumrun Vafa (1:22:31.400)
Well, strings go around the box
Lex Fridman (1:22:34.000)
and move around and vibrate and all that,
Lex Fridman (1:22:35.360)
but also they can wrap around one side of the box
Lex Fridman (1:22:38.600)
to the other because I'm imagining a box
Cumrun Vafa (1:22:41.400)
with periodic boundary conditions.
Lex Fridman (1:22:42.760)
So what we call the torus.
Lex Fridman (1:22:44.600)
So the string can go from one side to the other.
Lex Fridman (1:22:46.960)
This is what we call a winding string.
Cumrun Vafa (1:22:48.520)
The string can wind around the box.
Lex Fridman (1:22:50.400)
Now, suppose you have, you've now evolved the universe.
Cumrun Vafa (1:22:54.200)
Because there's energy, the universe starts to expand.
Lex Fridman (1:22:57.640)
But it doesn't expand too far.
Lex Fridman (1:23:00.240)
Why is it?
Lex Fridman (1:23:01.320)
Well, because there are these strings
Cumrun Vafa (1:23:03.800)
which are wrapped around
Lex Fridman (1:23:04.720)
from one side of the wall to the other.
Cumrun Vafa (1:23:07.120)
When the universe, the walls of the universe are growing,
Lex Fridman (1:23:10.440)
it is stretching the string
Lex Fridman (1:23:11.960)
and the strings are becoming very, very massive.
Lex Fridman (1:23:15.040)
So it becomes difficult to expand.
Cumrun Vafa (1:23:16.840)
It kind of puts a halt on it.
Lex Fridman (1:23:18.320)
In order to not put a halt,
Cumrun Vafa (1:23:19.880)
a string which is going this way
Lex Fridman (1:23:21.280)
and a string which is going that way
Cumrun Vafa (1:23:22.720)
should intersect each other
Lex Fridman (1:23:25.400)
and disconnect each other and unwind.
Lex Fridman (1:23:27.840)
So a string which winds this way
Lex Fridman (1:23:29.640)
and the string which finds the opposite way
Cumrun Vafa (1:23:31.720)
should find each other to reconnect
Lex Fridman (1:23:35.400)
and this way disappear.
Lex Fridman (1:23:37.080)
So if they find each other and they disappear.
Lex Fridman (1:23:40.560)
But how can strings find each other?
Cumrun Vafa (1:23:42.120)
Well, the string moves and another string moves.
Lex Fridman (1:23:45.480)
A string is one dimensional, one plus one is two
Lex Fridman (1:23:48.600)
and one plus one is two and two plus two is four.
Lex Fridman (1:23:52.280)
In four dimensional space time, they will find each other.
Cumrun Vafa (1:23:55.600)
In a higher dimensional space time,
Lex Fridman (1:23:57.360)
they typically miss each other.
Cumrun Vafa (1:23:59.400)
Oh, interesting.
Lex Fridman (1:24:00.240)
So if the dimension were too big,
Cumrun Vafa (1:24:01.800)
they would miss each other,
Lex Fridman (1:24:02.800)
they wouldn't be able to expand.
Lex Fridman (1:24:04.480)
So in order to expand, they have to find each other
Lex Fridman (1:24:06.760)
and three of them can find each other
Lex Fridman (1:24:08.920)
and those can expand and the other one will be stuck.
Lex Fridman (1:24:10.960)
So that explains why within string theory,
Cumrun Vafa (1:24:13.200)
these particular dimensions are really big
Lex Fridman (1:24:15.520)
and full of exciting stuff.
Cumrun Vafa (1:24:16.680)
That could be an explanation.
Lex Fridman (1:24:17.800)
That's a model we suggested with my colleague Brandenberger.
Lex Fridman (1:24:21.800)
But it turns out to be related to a deep piece of math.
Lex Fridman (1:24:23.880)
You see, for mathematicians,
Cumrun Vafa (1:24:26.800)
manifolds of dimension bigger than four are simple.
Lex Fridman (1:24:31.360)
Four dimension is the hardest dimension for math,
Cumrun Vafa (1:24:34.960)
it turns out.
Lex Fridman (1:24:35.800)
And it turns out the reason it's difficult is the following.
Cumrun Vafa (1:24:38.680)
It turns out that in higher dimension,
Lex Fridman (1:24:41.200)
you use what's called surgery in mathematical terminology,
Cumrun Vafa (1:24:45.200)
where you use these two dimensional tubes
Lex Fridman (1:24:48.200)
to maneuver them off of each other.
Lex Fridman (1:24:50.440)
So you have two plus two becoming four.
Lex Fridman (1:24:53.160)
In higher than four dimension,
Cumrun Vafa (1:24:54.400)
you can pass them through each other
Lex Fridman (1:24:56.240)
without them intersecting.
Cumrun Vafa (1:24:57.920)
In four dimension, two plus two
Lex Fridman (1:25:01.440)
doesn't allow you to pass them through each other.
Lex Fridman (1:25:03.120)
So the same techniques that work in higher dimension
Lex Fridman (1:25:05.280)
don't work in four dimension because two plus two is four.
Cumrun Vafa (1:25:08.280)
The same reasoning I was just telling you
Lex Fridman (1:25:10.160)
about strings finding each other in four
Cumrun Vafa (1:25:12.760)
ends up to be the reason why four is much more complicated
Lex Fridman (1:25:16.120)
to classify for mathematicians as well.
Lex Fridman (1:25:18.480)
So there might be these things.
Lex Fridman (1:25:20.280)
So I cannot say that this is the reason
Cumrun Vafa (1:25:22.840)
that string theory is giving you three plus one,
Lex Fridman (1:25:25.080)
but it could be a model for it.
Lex Fridman (1:25:26.440)
And so there are these kinds of ideas
Lex Fridman (1:25:28.320)
that could underlie why we have three extra dimensions
Cumrun Vafa (1:25:31.440)
which are large and the rest of them are small.
Lex Fridman (1:25:32.840)
But absolutely, we have to have a good reason.
Cumrun Vafa (1:25:35.000)
We cannot leave it like that.
Lex Fridman (1:25:36.080)
Can I ask a tricky human question?
Lex Fridman (1:25:38.320)
So you are one of the seminal figures in string theory.
Lex Fridman (1:25:42.480)
You got the Breakthrough Prize.
Cumrun Vafa (1:25:44.000)
You've worked with Edward Witten.
Lex Fridman (1:25:45.840)
There's no Nobel Prize that has been given on string theory.
Cumrun Vafa (1:25:51.400)
Credit assignment is tricky in science.
Lex Fridman (1:25:54.080)
It makes you quite sad, especially big, like LIGO,
Cumrun Vafa (1:25:57.800)
big experimental projects when so many incredible people
Lex Fridman (1:26:01.440)
have been involved and yet the Nobel Prize is annoying
Cumrun Vafa (1:26:04.640)
in that it's only given to three people.
Lex Fridman (1:26:06.640)
Who do you think gets the Nobel Prize
Lex Fridman (1:26:08.760)
for string theory at first?
Lex Fridman (1:26:12.280)
If it turns out that it, if not in full, then in part,
Cumrun Vafa (1:26:19.400)
is a good model of the way the physics of the universe works.
Lex Fridman (1:26:24.920)
Who are the key figures?
Cumrun Vafa (1:26:26.360)
Maybe let's put Nobel Prize aside.
Lex Fridman (1:26:28.680)
Who are the key figures?
Cumrun Vafa (1:26:29.520)
Okay, I like the second version of the question.
Lex Fridman (1:26:31.480)
Because I think to try to give a prize to one person
Cumrun Vafa (1:26:34.600)
in string theory doesn't do justice to the diversity
Lex Fridman (1:26:37.120)
of the subject.
Cumrun Vafa (1:26:37.960)
That to me is.
Lex Fridman (1:26:38.800)
So there was quite a lot of incredible people
Cumrun Vafa (1:26:41.040)
in the history of string theory.
Lex Fridman (1:26:41.880)
Quite a lot of people.
Cumrun Vafa (1:26:43.200)
I mean, starting with Veneziano,
Lex Fridman (1:26:44.520)
who wasn't talking about strings.
Cumrun Vafa (1:26:46.120)
I mean, he wrote down the beginning of the strings.
Lex Fridman (1:26:48.440)
We cannot ignore that for sure.
Lex Fridman (1:26:50.000)
And so you start with that and you go on
Lex Fridman (1:26:52.440)
with various other figures and so on.
Lex Fridman (1:26:54.080)
So there are different epochs in string theory
Lex Fridman (1:26:56.280)
and different people have been pushing it.
Lex Fridman (1:26:57.840)
And so for example, the early epoch,
Lex Fridman (1:26:59.760)
we just told you people like Veneziano,
Lex Fridman (1:27:02.640)
and Nambu, and the Susskind, and others were pushing it.
Lex Fridman (1:27:05.480)
Green and Schwarz were pushing it and so forth.
Lex Fridman (1:27:07.840)
So this was, or Scherck and so on.
Lex Fridman (1:27:09.640)
So these were the initial periods of pioneers,
Cumrun Vafa (1:27:13.040)
I would say, of string theory.
Lex Fridman (1:27:14.600)
And then there were the mid 80s that Edward Witten
Cumrun Vafa (1:27:18.760)
was the major proponent of string theory.
Lex Fridman (1:27:20.600)
And he really changed the landscape of string theory
Cumrun Vafa (1:27:23.760)
in terms of what people do and how we view it.
Lex Fridman (1:27:26.080)
And I think his efforts brought a lot of attention
Cumrun Vafa (1:27:29.240)
to the community of string theory.
Lex Fridman (1:27:31.560)
To the community about high energy community
Cumrun Vafa (1:27:34.640)
to focus on this effort as the correct theory
Lex Fridman (1:27:37.080)
of unification of forces.
Lex Fridman (1:27:38.480)
So he brought a lot of research as well as, of course,
Lex Fridman (1:27:41.120)
the first rate work he himself did to this area.
Lex Fridman (1:27:44.440)
So that's in mid 80s and onwards,
Lex Fridman (1:27:45.960)
and also in mid 90s where he was one of the proponents
Cumrun Vafa (1:27:49.040)
of the duality revolution in string theory.
Lex Fridman (1:27:51.640)
And with that came a lot of these other ideas
Cumrun Vafa (1:27:54.240)
that led to breakthroughs involving, for example,
Lex Fridman (1:27:58.400)
the example I told you about black holes and holography,
Lex Fridman (1:28:00.920)
and the work that was later done by Maldacena
Lex Fridman (1:28:03.880)
about the properties of duality between particle physics
Lex Fridman (1:28:06.600)
and quantum gravity and the deeper connections
Lex Fridman (1:28:10.160)
of holography, and it continues.
Lex Fridman (1:28:12.920)
And there are many people within this range,
Lex Fridman (1:28:15.320)
which I haven't even mentioned.
Cumrun Vafa (1:28:16.600)
They have done fantastic important things.
Lex Fridman (1:28:20.080)
How it gets recognized, I think, is secondary,
Cumrun Vafa (1:28:22.640)
in my opinion, than the appreciation
Lex Fridman (1:28:25.480)
that the effort is collective.
Cumrun Vafa (1:28:27.680)
That, in fact, that to me is the more important part
Lex Fridman (1:28:30.400)
of science that gets forgotten.
Cumrun Vafa (1:28:32.200)
For some reason, humanity likes heroes,
Lex Fridman (1:28:35.160)
and science is no exception.
Cumrun Vafa (1:28:36.400)
We like heroes, but I personally try to avoid that trap.
Lex Fridman (1:28:40.480)
I feel, in my work, most of my work is with colleagues.
Cumrun Vafa (1:28:44.840)
I have much more collaborations than sole author papers,
Lex Fridman (1:28:49.240)
and I enjoy it, and I think that that's, to me,
Cumrun Vafa (1:28:51.880)
one of the most satisfying aspects of science
Lex Fridman (1:28:54.320)
is to interact and learn and debate ideas with colleagues
Cumrun Vafa (1:28:59.240)
because that influx of ideas enriches it,
Lex Fridman (1:29:02.040)
and that's why I find it interesting.
Cumrun Vafa (1:29:05.680)
To me, science, if I was on an island,
Lex Fridman (1:29:08.240)
and if I was developing string theory by myself
Lex Fridman (1:29:10.360)
and had nothing to do with anybody,
Lex Fridman (1:29:11.800)
it would be much less satisfying, in my opinion.
Cumrun Vafa (1:29:14.120)
Even if I could take credit I did it,
Lex Fridman (1:29:17.120)
it won't be as satisfying.
Cumrun Vafa (1:29:18.320)
Sitting alone with a big metal drinking champagne, no.
Lex Fridman (1:29:22.640)
I think, to me, the collective work is more exciting,
Lex Fridman (1:29:25.720)
and you mentioned my getting the breakthrough.
Lex Fridman (1:29:28.280)
When I was getting it, I made sure to mention
Cumrun Vafa (1:29:30.480)
that it is because of the joint work
Lex Fridman (1:29:32.280)
that I've done with colleagues.
Cumrun Vafa (1:29:33.760)
At that time, it was around 180 or so collaborators,
Lex Fridman (1:29:36.800)
and I acknowledged them in the webpage for them.
Cumrun Vafa (1:29:39.720)
I write all of their names
Lex Fridman (1:29:41.120)
and the collaborations that led to this.
Cumrun Vafa (1:29:42.640)
So, to me, science is fun when it's collaboration,
Lex Fridman (1:29:46.520)
and yes, there are more important
Lex Fridman (1:29:48.720)
and less important figures, as in any field,
Lex Fridman (1:29:51.200)
and that's true, that's true in string theory as well,
Lex Fridman (1:29:53.280)
but I think that I would like to view this
Lex Fridman (1:29:55.880)
as a collective effort.
Cumrun Vafa (1:29:56.920)
So, setting the heroes aside,
Lex Fridman (1:30:00.480)
the Nobel Prize is a celebration of,
Cumrun Vafa (1:30:04.200)
what's the right way to put it,
Lex Fridman (1:30:05.720)
that this idea turned out to be right.
Cumrun Vafa (1:30:08.200)
So, like, you look at Einstein
Lex Fridman (1:30:11.560)
didn't believe in black holes,
Lex Fridman (1:30:13.720)
and then black holes got their Nobel Prize.
Lex Fridman (1:30:17.800)
Do you think string theory will get its Nobel Prize,
Lex Fridman (1:30:22.040)
Nobel Prizes, if you were to bet money?
Lex Fridman (1:30:25.400)
If this was an investment meeting
Lex Fridman (1:30:27.800)
and we had to bet all our money,
Lex Fridman (1:30:29.960)
do you think he gets the Nobel Prizes?
Cumrun Vafa (1:30:31.960)
I think it's possible that none of the living physicists
Lex Fridman (1:30:34.520)
will get the Nobel Prize in string theory,
Lex Fridman (1:30:35.960)
but somebody will.
Lex Fridman (1:30:37.080)
Because, unfortunately, the technology available today
Cumrun Vafa (1:30:41.560)
is not very encouraging
Lex Fridman (1:30:43.040)
in terms of seeing directly evidence for string theory.
Lex Fridman (1:30:46.280)
Do you think it ultimately boils down to
Lex Fridman (1:30:48.280)
the Nobel Prize will be given
Lex Fridman (1:30:49.600)
when there is some direct or indirect evidence?
Lex Fridman (1:30:53.240)
There would be, but I think that part of this
Cumrun Vafa (1:30:55.960)
breakthrough prize was precisely the appreciation
Lex Fridman (1:30:58.760)
that when we have sufficient evidence,
Cumrun Vafa (1:31:01.440)
theoretical as it is, not experiment,
Lex Fridman (1:31:04.160)
because of this technology lag,
Cumrun Vafa (1:31:06.200)
you appreciate what you think is the correct path.
Lex Fridman (1:31:08.920)
So, there are many people who have been recognized precisely
Cumrun Vafa (1:31:12.520)
because they may not be around
Lex Fridman (1:31:14.000)
when it actually gets experimented,
Cumrun Vafa (1:31:16.120)
even though they discovered it.
Lex Fridman (1:31:17.840)
So, there are many things like that
Cumrun Vafa (1:31:19.840)
that's going on in science.
Lex Fridman (1:31:21.480)
So, I think that I would want to attach less significance
Cumrun Vafa (1:31:25.600)
to the recognitions of people.
Lex Fridman (1:31:28.240)
And I have a second review on this,
Cumrun Vafa (1:31:31.080)
which is there are people who look at these works
Lex Fridman (1:31:35.560)
that people have done and put them together
Lex Fridman (1:31:37.480)
and make the next big breakthrough.
Lex Fridman (1:31:39.960)
And they get identified with, perhaps rightly,
Cumrun Vafa (1:31:43.800)
with many of these new visions.
Lex Fridman (1:31:47.960)
But they are on the shoulders of these little scientists.
Cumrun Vafa (1:31:51.360)
Which don't get any recognition.
Lex Fridman (1:31:54.040)
You know, yeah, you did this little work.
Cumrun Vafa (1:31:55.480)
Oh yeah, you did this little work.
Lex Fridman (1:31:56.920)
Oh yeah, yeah, five of you.
Cumrun Vafa (1:31:57.760)
Oh yeah, these showed this pattern.
Lex Fridman (1:31:59.280)
And then somebody else, it's not fair.
Cumrun Vafa (1:32:01.800)
To me, those little guys, which kind of like,
Lex Fridman (1:32:05.720)
like seem to do the little calculation here,
Cumrun Vafa (1:32:07.440)
a little thing there, which doesn't rise to the occasion
Lex Fridman (1:32:10.120)
of this grandiose kind of thing,
Cumrun Vafa (1:32:11.800)
doesn't make it to the New York Times headlines and so on,
Lex Fridman (1:32:15.120)
deserve a lot of recognition.
Lex Fridman (1:32:17.080)
And I think they don't get enough.
Lex Fridman (1:32:18.400)
I would say that there should be this Nobel prize
Cumrun Vafa (1:32:20.960)
for, you know, they have these Doctors Without Borders,
Lex Fridman (1:32:23.920)
they're a huge group.
Cumrun Vafa (1:32:24.760)
They should do a similar thing.
Lex Fridman (1:32:25.600)
And these String Theors Without Borders kind of,
Cumrun Vafa (1:32:27.800)
everybody is doing a lot of work.
Lex Fridman (1:32:29.320)
And I think that I would like to see that effort recognized.
Cumrun Vafa (1:32:32.880)
I think in the long arc of history,
Lex Fridman (1:32:35.480)
we're all little guys and girls
Cumrun Vafa (1:32:38.080)
standing on the shoulders of each other.
Lex Fridman (1:32:40.640)
I mean, it's all going to look tiny in retrospect.
Cumrun Vafa (1:32:44.560)
If we celebrate, the New York Times,
Lex Fridman (1:32:46.960)
you know, as a newspaper,
Cumrun Vafa (1:32:51.040)
or the idea of a newspaper in a few centuries from now
Lex Fridman (1:32:55.000)
will be long forgotten.
Cumrun Vafa (1:32:56.200)
Yes, I agree with that.
Lex Fridman (1:32:57.680)
Especially in the context of String Theory,
Cumrun Vafa (1:32:59.000)
we should have a very long term view.
Lex Fridman (1:33:00.840)
Yes, exactly.
Cumrun Vafa (1:33:01.960)
Just as a tiny tangent, we mentioned Edward Witten.
Lex Fridman (1:33:05.360)
And he, in a bunch of walks of life for me as an outsider,
Cumrun Vafa (1:33:09.800)
comes up as a person who is widely considered as like
Lex Fridman (1:33:14.200)
one of the most brilliant people in the history of physics,
Cumrun Vafa (1:33:17.920)
just as a powerhouse of a human,
Lex Fridman (1:33:21.520)
like the exceptional places that a human mind can rise to.
Cumrun Vafa (1:33:27.280)
Yes.
Lex Fridman (1:33:28.120)
You've gotten the chance to work with him.
Lex Fridman (1:33:29.720)
What's he like?
Lex Fridman (1:33:30.560)
Yes, more than that.
Cumrun Vafa (1:33:31.400)
He was my advisor, PhD advisor.
Lex Fridman (1:33:34.240)
So I got to know him very well
Lex Fridman (1:33:35.920)
and I benefited from his insights.
Lex Fridman (1:33:37.480)
In fact, what you said about him is accurate.
Cumrun Vafa (1:33:40.080)
He is not only brilliant,
Lex Fridman (1:33:42.280)
but he is also multifaceted in terms of the impact
Cumrun Vafa (1:33:46.560)
he has had in not only physics, but also mathematics.
Lex Fridman (1:33:49.520)
He has gotten the Fields Medal
Cumrun Vafa (1:33:50.880)
because of his work in mathematics.
Lex Fridman (1:33:52.440)
And rightly so, he has used his knowledge of physics
Cumrun Vafa (1:33:58.240)
in a way which impacted deep ideas in modern mathematics.
Lex Fridman (1:34:01.640)
And that's an example of the power of these ideas
Cumrun Vafa (1:34:05.960)
in modern high energy physics and string theory,
Lex Fridman (1:34:08.680)
the applicability of it to modern mathematics.
Lex Fridman (1:34:11.600)
So he's quite an exceptional individual.
Lex Fridman (1:34:16.360)
We don't come across such people a lot in history.
Lex Fridman (1:34:19.560)
So I think, yes, indeed,
Lex Fridman (1:34:20.680)
he's one of the rare figures in this history of subject.
Cumrun Vafa (1:34:24.120)
He has had great impact on a lot of aspects
Lex Fridman (1:34:26.480)
of not just string theory,
Cumrun Vafa (1:34:27.320)
a lot of different areas in physics,
Lex Fridman (1:34:29.360)
and also, yes, in mathematics as well.
Lex Fridman (1:34:32.720)
So I think what you said about him is accurate.
Lex Fridman (1:34:34.960)
I had the pleasure of interacting with him as a student
Lex Fridman (1:34:37.560)
and later on as colleagues writing papers together
Lex Fridman (1:34:40.680)
and so on.
Lex Fridman (1:34:41.520)
What impact did he have on your life?
Lex Fridman (1:34:43.160)
Like what have you learned from him?
Cumrun Vafa (1:34:46.040)
If you were to look at the trajectory of your mind
Lex Fridman (1:34:48.200)
of the way you approach science and physics and mathematics,
Lex Fridman (1:34:51.520)
how did he perturb that trajectory in a way?
Lex Fridman (1:34:54.360)
Yes, he did actually.
Lex Fridman (1:34:55.600)
So I can explain because when I was a student,
Lex Fridman (1:34:57.680)
I had the biggest impact by him,
Cumrun Vafa (1:35:01.000)
clearly as a grad student at Princeton.
Lex Fridman (1:35:02.400)
So I think that was a time where I was a little bit confused
Cumrun Vafa (1:35:06.640)
about the relation between math and physics.
Lex Fridman (1:35:08.760)
I got a double major in mathematics and physics
Cumrun Vafa (1:35:11.280)
at MIT because I really enjoyed both.
Lex Fridman (1:35:14.600)
And I liked the elegance and the rigor of mathematics.
Lex Fridman (1:35:18.280)
And I liked the power of ideas in physics
Lex Fridman (1:35:21.160)
and its applicability to reality
Lex Fridman (1:35:22.760)
and what it teaches about the real world around us.
Lex Fridman (1:35:26.360)
But I saw this tension between rigorous thinking
Cumrun Vafa (1:35:30.200)
in mathematics and lack thereof in physics.
Lex Fridman (1:35:33.440)
And this troubled me to no end.
Cumrun Vafa (1:35:36.240)
I was troubled by that.
Lex Fridman (1:35:38.000)
So I was at crossroads when I decided
Cumrun Vafa (1:35:40.680)
to go to graduate school in physics
Lex Fridman (1:35:42.200)
because I did not like some of the lack of rigors
Cumrun Vafa (1:35:44.800)
I was seeing in physics.
Lex Fridman (1:35:47.120)
On the other hand, to me, mathematics,
Cumrun Vafa (1:35:48.840)
even though it was rigorous,
Lex Fridman (1:35:49.920)
I didn't see the point of it.
Cumrun Vafa (1:35:53.040)
In other words, the math theorem by itself could be beautiful
Lex Fridman (1:35:57.400)
but I really wanted more than that.
Cumrun Vafa (1:35:58.760)
I wanted to say, okay, what does it teach us
Lex Fridman (1:36:00.320)
about something else, something more than just math?
Lex Fridman (1:36:02.880)
So I wasn't that enamored with just math
Lex Fridman (1:36:05.240)
but physics was a little bit bothersome.
Cumrun Vafa (1:36:07.320)
Nevertheless, I decided to go to physics
Lex Fridman (1:36:08.960)
and I decided to go to Princeton
Lex Fridman (1:36:10.840)
and I started working with Edward Witten
Lex Fridman (1:36:13.280)
as my thesis advisor.
Lex Fridman (1:36:15.920)
And at that time I was trying to put physics
Lex Fridman (1:36:20.480)
in rigorous mathematical terms.
Cumrun Vafa (1:36:22.400)
I took quantum field theory.
Lex Fridman (1:36:23.840)
I tried to make rigorous out of it and so on.
Lex Fridman (1:36:26.800)
And no matter how hard I was trying,
Lex Fridman (1:36:29.080)
I was not being able to do that.
Lex Fridman (1:36:31.760)
And I was falling behind from my classes.
Lex Fridman (1:36:33.760)
I was not learning much physics
Lex Fridman (1:36:35.640)
and I was not making it rigorous.
Lex Fridman (1:36:37.000)
And to me, it was this dichotomy between math and physics.
Lex Fridman (1:36:40.600)
What am I doing?
Lex Fridman (1:36:41.440)
I like math but this is not exactly this.
Cumrun Vafa (1:36:45.560)
There comes Edward Witten as my advisor
Lex Fridman (1:36:47.480)
and I see him in action thinking about math and physics.
Cumrun Vafa (1:36:52.040)
He was amazing in math.
Lex Fridman (1:36:53.640)
He knew all about the math.
Cumrun Vafa (1:36:54.880)
It was no problem with him.
Lex Fridman (1:36:56.240)
But he thought about physics in a way
Cumrun Vafa (1:36:58.560)
which did not find this tension between the two.
Lex Fridman (1:37:02.600)
It was much more harmonious.
Cumrun Vafa (1:37:04.040)
For him, he would draw the Feynman diagrams
Lex Fridman (1:37:06.480)
but he wouldn't view it as a formalism.
Cumrun Vafa (1:37:08.880)
He was viewed, oh yeah, the particle goes over there
Lex Fridman (1:37:10.560)
and this is what's going on.
Lex Fridman (1:37:11.480)
And so wait, you're thinking really,
Lex Fridman (1:37:13.240)
is this particle, this is really electron going there?
Cumrun Vafa (1:37:15.760)
Oh, yeah, yeah.
Lex Fridman (1:37:16.600)
It's not the form or the result perturbation.
Cumrun Vafa (1:37:18.880)
No, no, no.
Lex Fridman (1:37:19.920)
You just feel like the electron.
Cumrun Vafa (1:37:21.160)
You're moving with this guy and do that and so on.
Lex Fridman (1:37:23.040)
And you're thinking invariantly about physics
Cumrun Vafa (1:37:24.880)
or the way he thought about relativity.
Lex Fridman (1:37:27.760)
Like I was thinking about this momentum system.
Cumrun Vafa (1:37:29.840)
He was thinking invariantly about physics,
Lex Fridman (1:37:31.480)
just like the way you think about invariant concepts
Lex Fridman (1:37:34.000)
and relativity, which don't depend on the frame of reference.
Lex Fridman (1:37:36.520)
He was thinking about the physics in invariant ways,
Cumrun Vafa (1:37:39.840)
the way that doesn't, that gives you a bigger perspective.
Lex Fridman (1:37:42.920)
So this gradually helped me appreciate
Cumrun Vafa (1:37:46.480)
that interconnections between ideas and physics
Lex Fridman (1:37:50.200)
replaces mathematical rigor.
Cumrun Vafa (1:37:53.880)
That the different facets reinforce each other.
Lex Fridman (1:37:56.360)
They say, oh, I cannot rigorously define
Lex Fridman (1:37:58.560)
what I mean by this,
Lex Fridman (1:37:59.400)
but this thing connects with this other physics I've seen
Lex Fridman (1:38:01.640)
and this other thing.
Lex Fridman (1:38:02.960)
And they together form an elegant story.
Lex Fridman (1:38:06.360)
And that replaced for me what I believed as a solidness,
Lex Fridman (1:38:09.760)
which I found in math as a rigor, solid.
Cumrun Vafa (1:38:13.000)
I found that replaced the rigor and solidness in physics.
Lex Fridman (1:38:16.160)
So I found, okay, that's the way you can hang onto.
Cumrun Vafa (1:38:19.200)
It is not wishy washy.
Lex Fridman (1:38:20.320)
It's not like somebody is just not being able to prove it,
Cumrun Vafa (1:38:23.200)
just making up a story.
Lex Fridman (1:38:24.720)
It was more than that.
Lex Fridman (1:38:25.960)
And it was no tension with mathematics.
Lex Fridman (1:38:28.480)
In fact, mathematics was helping it, like friends.
Lex Fridman (1:38:31.720)
And so much more harmonious and gives insights to physics.
Lex Fridman (1:38:34.800)
So that's, I think, one of the main things I learned
Cumrun Vafa (1:38:36.600)
from interactions with Witten.
Lex Fridman (1:38:38.640)
And I think that now perhaps I have taken that
Cumrun Vafa (1:38:42.120)
to a far extreme.
Lex Fridman (1:38:43.440)
Maybe he wouldn't go this far as I have.
Cumrun Vafa (1:38:45.080)
Namely, I use physics to define new mathematics
Lex Fridman (1:38:48.560)
in a way which would be far less rigorous
Cumrun Vafa (1:38:50.920)
than a physicist might necessarily believe,
Lex Fridman (1:38:53.360)
because I take the physical intuition,
Cumrun Vafa (1:38:55.680)
perhaps literally in many ways that could teach us about.
Lex Fridman (1:38:58.880)
So now I've gained so much confidence
Cumrun Vafa (1:39:01.120)
in physical intuition that I make bold statements
Lex Fridman (1:39:03.680)
that sometimes takes math friends off guard.
Lex Fridman (1:39:08.400)
So an example of it is mirror symmetry.
Lex Fridman (1:39:10.840)
So we were studying these compactification
Cumrun Vafa (1:39:14.360)
of string geometries.
Lex Fridman (1:39:15.760)
This is after my PhD now.
Cumrun Vafa (1:39:17.040)
I've, by the time I come to Harvard,
Lex Fridman (1:39:19.520)
we're studying these aspects of string compactification
Cumrun Vafa (1:39:21.640)
on these complicated manifolds,
Lex Fridman (1:39:23.440)
six dimensional spaces called Kalabial manifolds,
Cumrun Vafa (1:39:26.320)
very complicated.
Lex Fridman (1:39:28.040)
And I noticed with a couple other colleagues
Cumrun Vafa (1:39:31.120)
that there was a symmetry in physics suggested
Lex Fridman (1:39:35.200)
between different Kalabials.
Cumrun Vafa (1:39:36.600)
It suggested that you couldn't actually compute
Lex Fridman (1:39:40.360)
the Euler characteristic of a Kalabia.
Cumrun Vafa (1:39:42.560)
Euler characteristic is counting the number of points
Lex Fridman (1:39:45.520)
minus the number of edges plus the number of faces minus.
Lex Fridman (1:39:48.400)
So you can count the alternating sequence
Lex Fridman (1:39:50.480)
of properties of a space,
Cumrun Vafa (1:39:51.800)
which is a topological property of a space.
Lex Fridman (1:39:54.680)
So Euler characteristics of the Kalabia
Cumrun Vafa (1:39:56.680)
was a property of the space.
Lex Fridman (1:39:57.880)
And so we noticed that from the physics formalism,
Cumrun Vafa (1:40:01.560)
if string moves in a Kalabia,
Lex Fridman (1:40:03.520)
you cannot distinguish,
Cumrun Vafa (1:40:05.480)
we cannot compute the Euler characteristic.
Lex Fridman (1:40:07.360)
You can only compute the absolute value of it.
Cumrun Vafa (1:40:10.280)
Now this bothered us
Lex Fridman (1:40:11.280)
because how could you not compute the actual sign
Lex Fridman (1:40:15.640)
unless the both sides were the same?
Lex Fridman (1:40:18.880)
So I conjectured maybe for every Kalabia
Cumrun Vafa (1:40:21.120)
with Euler characteristics positive,
Lex Fridman (1:40:22.320)
there's one with negative.
Cumrun Vafa (1:40:23.600)
I told this to my colleague Yao
Lex Fridman (1:40:25.240)
who's namesake is Kalabia,
Cumrun Vafa (1:40:30.680)
that I'm making this conjecture.
Lex Fridman (1:40:31.880)
Is it possible that for every Kalabia,
Lex Fridman (1:40:33.600)
there's one with the opposite Euler characteristic?
Lex Fridman (1:40:36.560)
Sounds not reasonable.
Lex Fridman (1:40:37.840)
I said, why?
Lex Fridman (1:40:38.680)
He said, well, we know more Kalabias
Cumrun Vafa (1:40:40.320)
with negative Euler characteristics than positive.
Lex Fridman (1:40:44.520)
I said, but physics says we cannot distinguish them.
Cumrun Vafa (1:40:46.520)
At least I don't see how.
Lex Fridman (1:40:47.840)
So we conjectured that for every Kalabia
Cumrun Vafa (1:40:50.440)
with one sign, there's the other one,
Lex Fridman (1:40:51.720)
despite the mathematical evidence,
Cumrun Vafa (1:40:54.080)
despite the mathematical evidence,
Lex Fridman (1:40:55.680)
despite the expert telling us it's not the right idea.
Cumrun Vafa (1:40:59.320)
If a few years later, this symmetry, mirror symmetry
Lex Fridman (1:41:02.040)
between the sign with the opposite sign
Cumrun Vafa (1:41:04.520)
was later confirmed by mathematicians.
Lex Fridman (1:41:06.960)
So this is actually the opposite view.
Cumrun Vafa (1:41:09.160)
That is physics is so sure about it
Lex Fridman (1:41:11.560)
that you're going against the mathematical wisdom,
Cumrun Vafa (1:41:13.720)
telling them they better look for it.
Lex Fridman (1:41:15.400)
So taking the physical intuition literally
Lex Fridman (1:41:19.360)
and then having that drive the mathematics.
Lex Fridman (1:41:22.120)
Exactly.
Lex Fridman (1:41:22.960)
And now we are so confident about many such examples
Lex Fridman (1:41:26.160)
that has affected modern mathematics in ways like this,
Cumrun Vafa (1:41:30.320)
that we are much more confident
Lex Fridman (1:41:31.600)
about our understanding of what string theory is.
Cumrun Vafa (1:41:33.920)
These are another aspects,
Lex Fridman (1:41:35.080)
other aspects of why we feel string theory is correct.
Cumrun Vafa (1:41:37.600)
It's doing these kinds of things.
Lex Fridman (1:41:39.880)
I've been hearing your talk quite a bit
Cumrun Vafa (1:41:41.640)
about string theory, landscape and the swamp land.
Lex Fridman (1:41:46.000)
What the heck are those two concepts?
Cumrun Vafa (1:41:47.840)
Okay, very good question.
Lex Fridman (1:41:48.840)
So let's go back to what I was describing about Feynman.
Cumrun Vafa (1:41:51.920)
Feynman was trying to do these diagrams for graviton
Lex Fridman (1:41:55.800)
and electrons and all that.
Cumrun Vafa (1:41:57.320)
He found that he's getting infinities he cannot resolve.
Lex Fridman (1:42:01.240)
Okay, the natural conclusion is that field theories
Lex Fridman (1:42:04.080)
and gravity and quantum theory don't go together
Lex Fridman (1:42:06.600)
and you cannot have it.
Lex Fridman (1:42:08.640)
So in other words, field theories and gravity
Lex Fridman (1:42:11.320)
are inconsistent with quantum mechanics, period.
Cumrun Vafa (1:42:14.240)
String theory came up with examples
Lex Fridman (1:42:18.400)
but didn't address the question more broadly
Cumrun Vafa (1:42:20.800)
that is it true that every field theory
Lex Fridman (1:42:23.120)
can be coupled to gravity in a quantum mechanical way?
Cumrun Vafa (1:42:27.400)
It turns out that Feynman was essentially right.
Lex Fridman (1:42:30.640)
Almost all particle physics theories,
Cumrun Vafa (1:42:33.200)
no matter what you add to it,
Lex Fridman (1:42:35.520)
when you put gravity in it, doesn't work.
Cumrun Vafa (1:42:38.320)
Only rare exceptions work.
Lex Fridman (1:42:41.440)
So string theory are those rare exceptions.
Lex Fridman (1:42:44.160)
So therefore the general principle
Lex Fridman (1:42:46.040)
that Feynman found was correct.
Cumrun Vafa (1:42:47.800)
Quantum field theory and gravity and quantum mechanics
Lex Fridman (1:42:50.320)
don't go together except for Joule's exceptional cases.
Cumrun Vafa (1:42:54.760)
There are exceptional cases.
Lex Fridman (1:42:56.400)
Okay, the total vastness of quantum field theories
Cumrun Vafa (1:43:00.200)
that are there we call the set of quantum field theories,
Lex Fridman (1:43:04.320)
possible things.
Lex Fridman (1:43:05.320)
Which ones can be consistently coupled to gravity?
Lex Fridman (1:43:09.920)
We call that subspace the landscape.
Cumrun Vafa (1:43:13.160)
The rest of them we call the swampland.
Lex Fridman (1:43:16.200)
It doesn't mean they are bad quantum field theories,
Cumrun Vafa (1:43:18.040)
they are perfectly fine.
Lex Fridman (1:43:19.960)
But when you couple them to gravity,
Cumrun Vafa (1:43:21.880)
they don't make sense, unfortunately.
Lex Fridman (1:43:24.200)
And it turns out that the ratio of them,
Cumrun Vafa (1:43:27.160)
the number of theories which are consistent with gravity
Lex Fridman (1:43:29.720)
to the ones without,
Cumrun Vafa (1:43:31.320)
the ratio of the area of the landscape
Lex Fridman (1:43:33.960)
to the swampland, in other words, is measure zero.
Lex Fridman (1:43:37.880)
So the swampland's infinitely large?
Lex Fridman (1:43:40.240)
The swampland's infinitely large.
Lex Fridman (1:43:41.640)
So let me give you one example.
Lex Fridman (1:43:43.200)
Take a theory in four dimension with matter
Cumrun Vafa (1:43:46.480)
with maximum amount of supersymmetry.
Lex Fridman (1:43:48.880)
Can you get, it turns out a theory in four dimension
Cumrun Vafa (1:43:51.680)
with maximum amount of supersymmetry
Lex Fridman (1:43:53.800)
is characterized just with one thing, a group.
Lex Fridman (1:43:56.660)
What we call the gauge group.
Lex Fridman (1:43:58.360)
Once you pick a group, you have to find the theory.
Lex Fridman (1:44:01.500)
Okay, so does every group make sense?
Lex Fridman (1:44:04.120)
Yeah.
Cumrun Vafa (1:44:05.280)
As far as quantum field theory, every group makes sense.
Lex Fridman (1:44:07.460)
There are infinitely many groups,
Cumrun Vafa (1:44:08.560)
there are infinitely many quantum field theories.
Lex Fridman (1:44:10.680)
But it turns out there are only finite number of them
Cumrun Vafa (1:44:13.760)
which are consistent with gravity out of that same list.
Lex Fridman (1:44:16.840)
So you can take any group but only finite number of them,
Cumrun Vafa (1:44:19.420)
the ones who's, what we call the rank of the group,
Lex Fridman (1:44:22.680)
the ones whose rank is less than 23.
Cumrun Vafa (1:44:26.200)
Any one bigger than rank 23 belongs to the swampland.
Lex Fridman (1:44:29.720)
There are infinitely many of them.
Cumrun Vafa (1:44:31.200)
They're beautiful field theories,
Lex Fridman (1:44:33.100)
but not when you include gravity.
Lex Fridman (1:44:35.600)
So then this becomes a hopeful thing.
Lex Fridman (1:44:37.760)
So in other words, in our universe, we have gravity.
Cumrun Vafa (1:44:41.920)
Therefore, we are part of that jewel subset.
Lex Fridman (1:44:44.660)
Now, is this jewel subset small or large?
Cumrun Vafa (1:44:49.480)
Yeah.
Lex Fridman (1:44:50.320)
It turns out that subset is humongous,
Lex Fridman (1:44:54.480)
but we believe still finite.
Lex Fridman (1:44:57.400)
The set of possibilities is infinite,
Lex Fridman (1:44:59.400)
but the set of consistent ones,
Lex Fridman (1:45:02.280)
I mean, the set of quantum field theories are infinite,
Lex Fridman (1:45:04.280)
but the consistent ones are finite, but humongous.
Lex Fridman (1:45:08.000)
The fact that they're humongous
Cumrun Vafa (1:45:10.080)
is the problem we are facing in string theory,
Lex Fridman (1:45:12.280)
because we do not know which one of these possibilities
Cumrun Vafa (1:45:16.560)
the universe we live in.
Lex Fridman (1:45:18.160)
If we knew, we could make more specific predictions
Cumrun Vafa (1:45:20.380)
about our universe.
Lex Fridman (1:45:21.400)
We don't know.
Lex Fridman (1:45:22.360)
And that is one of the challenges when string theory,
Lex Fridman (1:45:24.520)
which point on the landscape,
Lex Fridman (1:45:26.040)
which corner of this landscape do we live in?
Lex Fridman (1:45:28.660)
We don't know.
Lex Fridman (1:45:30.120)
So what do we do?
Lex Fridman (1:45:31.780)
Well, there are principles that are beginning to emerge.
Lex Fridman (1:45:35.680)
So I will give you one example of it.
Lex Fridman (1:45:38.040)
You look at the patterns of what you're getting
Cumrun Vafa (1:45:40.720)
in terms of these good ones,
Lex Fridman (1:45:41.940)
the ones which are in the landscape
Cumrun Vafa (1:45:43.400)
compared to the ones which are not.
Lex Fridman (1:45:45.540)
You find certain patterns.
Cumrun Vafa (1:45:46.620)
I'll give you one pattern.
Lex Fridman (1:45:49.280)
You find in all the ones that you get from string theory,
Cumrun Vafa (1:45:52.700)
gravitational force is always there,
Lex Fridman (1:45:55.560)
but it's always, always the weakest force.
Cumrun Vafa (1:46:00.360)
However, you could easily imagine field theories
Lex Fridman (1:46:03.800)
for which gravity is not the weakest force.
Cumrun Vafa (1:46:05.720)
For example, take our universe.
Lex Fridman (1:46:08.960)
If you take mass of the electron,
Cumrun Vafa (1:46:10.680)
if you increase the mass of electron by a huge factor,
Lex Fridman (1:46:14.080)
the gravitational attraction of the electrons
Cumrun Vafa (1:46:16.060)
will be bigger than the electric repulsion
Lex Fridman (1:46:17.840)
between two electrons.
Lex Fridman (1:46:19.440)
And the gravity will be stronger.
Lex Fridman (1:46:20.920)
That's all.
Cumrun Vafa (1:46:22.780)
It happens that it's not the case in our universe
Lex Fridman (1:46:25.060)
because electron is very tiny in mass compared to that.
Cumrun Vafa (1:46:28.620)
Just like our universe, gravity is the weakest force.
Lex Fridman (1:46:31.920)
We find in all these other ones,
Cumrun Vafa (1:46:33.840)
which are part of the good ones,
Lex Fridman (1:46:36.180)
the gravity is the weakest force.
Cumrun Vafa (1:46:37.960)
This is called the weak gravity conjecture.
Lex Fridman (1:46:40.720)
We conjecture that all the points in the landscape
Cumrun Vafa (1:46:43.660)
have this property.
Lex Fridman (1:46:45.920)
Our universe being just an example of it.
Lex Fridman (1:46:47.680)
So there are these qualitative features
Lex Fridman (1:46:49.520)
that we are beginning to see.
Lex Fridman (1:46:50.920)
But how do we argue for this?
Lex Fridman (1:46:52.320)
Just by looking patterns?
Lex Fridman (1:46:53.960)
Just by looking string theory as this?
Lex Fridman (1:46:55.760)
No, that's not enough.
Cumrun Vafa (1:46:58.080)
We need more reason, more better reasoning.
Lex Fridman (1:47:00.320)
And it turns out there is.
Cumrun Vafa (1:47:01.840)
The reasoning for this turns out to be studying black holes.
Lex Fridman (1:47:05.040)
Ideas of black holes turn out to put certain restrictions
Cumrun Vafa (1:47:09.500)
of what a good quantum filter should be.
Lex Fridman (1:47:12.040)
It turns out using black hole,
Cumrun Vafa (1:47:14.480)
the fact that the black holes evaporate,
Lex Fridman (1:47:17.700)
the fact that the black holes evaporate
Cumrun Vafa (1:47:20.120)
gives you a way to check the relation
Lex Fridman (1:47:23.560)
between the mass and the charge of elementary particle.
Cumrun Vafa (1:47:25.880)
Because what you can do, you can take a charged particle
Lex Fridman (1:47:28.480)
and throw it into a charged black hole
Lex Fridman (1:47:30.520)
and wait it to evaporate.
Lex Fridman (1:47:32.040)
And by looking at the properties of evaporation,
Cumrun Vafa (1:47:34.440)
you find that if it cannot evaporate particles
Lex Fridman (1:47:37.800)
whose mass is less than their charge,
Cumrun Vafa (1:47:39.500)
then it will never evaporate.
Lex Fridman (1:47:40.800)
You will be stuck.
Lex Fridman (1:47:42.120)
And so the possibility of a black hole evaporation
Lex Fridman (1:47:44.520)
forces you to have particles whose mass
Cumrun Vafa (1:47:47.180)
is sufficiently small so that the gravity is weaker.
Lex Fridman (1:47:50.480)
So you connect this fact to the other fact.
Lex Fridman (1:47:52.920)
So we begin to find different facts
Lex Fridman (1:47:55.040)
that reinforce each other.
Lex Fridman (1:47:56.320)
So different parts of the physics reinforce each other.
Lex Fridman (1:47:59.360)
And once they all kind of come together,
Cumrun Vafa (1:48:02.360)
you believe that you're getting the principle correct.
Lex Fridman (1:48:04.240)
So weak gravity conjecture
Cumrun Vafa (1:48:05.560)
is one of the principles we believe in
Lex Fridman (1:48:07.480)
as a necessity of these conditions.
Lex Fridman (1:48:09.760)
So these are the predictions string theory are making.
Lex Fridman (1:48:12.280)
Is that enough?
Cumrun Vafa (1:48:13.120)
Well, it's qualitative.
Lex Fridman (1:48:14.680)
It's a semi quantity.
Cumrun Vafa (1:48:16.200)
It's just the mass of the electron
Lex Fridman (1:48:17.600)
should be less than some number.
Lex Fridman (1:48:19.560)
But that number is, if I call that number one,
Lex Fridman (1:48:23.040)
the mass of the electron
Cumrun Vafa (1:48:23.880)
turns out to be 10 to the minus 20 actually.
Lex Fridman (1:48:25.640)
So it's much less than one.
Cumrun Vafa (1:48:26.800)
It's not one.
Lex Fridman (1:48:28.000)
But on the other hand,
Cumrun Vafa (1:48:30.240)
there's a similar reasoning for a big black hole
Lex Fridman (1:48:32.760)
in our universe.
Lex Fridman (1:48:34.020)
And if that evaporation should take place,
Lex Fridman (1:48:36.460)
gives you another restriction,
Cumrun Vafa (1:48:37.520)
tells you the mass of the electron
Lex Fridman (1:48:39.300)
is bigger than 10 to the,
Cumrun Vafa (1:48:41.320)
now in this case, bigger than something.
Lex Fridman (1:48:43.240)
It shows bigger than 10 to the minus 30 in the Planck unit.
Lex Fridman (1:48:45.760)
So you find, huh,
Lex Fridman (1:48:47.420)
the mass of the electron should be less than one,
Lex Fridman (1:48:49.400)
but bigger than 10 to the minus 30.
Lex Fridman (1:48:51.320)
In our universe,
Cumrun Vafa (1:48:52.160)
the mass of the electron is 10 to the minus 20.
Lex Fridman (1:48:54.440)
Okay, now this kind of you could call postiction,
Lex Fridman (1:48:57.040)
but I would say it follows from principles
Lex Fridman (1:48:59.140)
that we now understand from string theory, first principle.
Lex Fridman (1:49:01.920)
So we are making, beginning to make
Lex Fridman (1:49:04.400)
these kinds of predictions,
Cumrun Vafa (1:49:05.840)
which are very much connected to aspects of particle physics
Lex Fridman (1:49:09.900)
that we didn't think are related to gravity.
Cumrun Vafa (1:49:12.260)
We thought, just take any electron mass you want.
Lex Fridman (1:49:14.960)
What's the problem?
Cumrun Vafa (1:49:15.800)
It has a problem with gravity.
Lex Fridman (1:49:17.380)
And so that conjecture
Cumrun Vafa (1:49:20.040)
has also a happy consequence
Lex Fridman (1:49:22.480)
that it explains that our universe,
Cumrun Vafa (1:49:24.700)
like why the heck is gravity so weak as a force
Lex Fridman (1:49:28.640)
and that's not only an accident, but almost a necessity
Lex Fridman (1:49:32.360)
if these forces are to coexist effectively?
Lex Fridman (1:49:35.360)
Exactly, so that's the reinforcement
Cumrun Vafa (1:49:38.280)
of what we know in our universe,
Lex Fridman (1:49:40.760)
but we are finding that as a general principle.
Lex Fridman (1:49:43.300)
So we want to know what aspects of our universe
Lex Fridman (1:49:46.400)
is forced on us,
Cumrun Vafa (1:49:47.920)
like the weak gravity conjecture and other aspects.
Lex Fridman (1:49:50.740)
How much of them do we understand?
Lex Fridman (1:49:52.760)
Can we have particles lighter than neutrinos?
Lex Fridman (1:49:54.800)
Or maybe that's not possible.
Cumrun Vafa (1:49:56.380)
You see the neutrino mass,
Lex Fridman (1:49:57.440)
it turns out to be related to dark energy
Cumrun Vafa (1:49:59.600)
in a mysterious way.
Lex Fridman (1:50:01.640)
Naively, there's no relation between dark energy
Lex Fridman (1:50:04.400)
and the mass of a particle.
Lex Fridman (1:50:06.520)
We have found arguments
Cumrun Vafa (1:50:07.560)
from within the swampland kind of ideas,
Lex Fridman (1:50:10.040)
why it has to be related.
Lex Fridman (1:50:12.640)
And so there are beginning to be these connections
Lex Fridman (1:50:15.280)
between graph consistency of quantum gravity
Lex Fridman (1:50:17.920)
and aspects of our universe gradually being sharpened.
Lex Fridman (1:50:22.300)
But we are still far from a precise quantitative prediction
Cumrun Vafa (1:50:25.200)
like we have to have such and such, but that's the hope,
Lex Fridman (1:50:27.880)
that we are going in that direction.
Cumrun Vafa (1:50:29.520)
Coming up with the theory of everything
Lex Fridman (1:50:31.000)
that unifies general relativity and quantum field theory
Cumrun Vafa (1:50:34.200)
is one of the big dreams of human civilization.
Lex Fridman (1:50:39.880)
Us descendants of apes wondering about how this world works.
Lex Fridman (1:50:43.400)
So a lot of people dream.
Lex Fridman (1:50:46.100)
What are your thoughts about sort of other out there ideas,
Lex Fridman (1:50:50.920)
theories of everything or unifying theories?
Lex Fridman (1:50:56.120)
So there's a quantum loop gravity.
Cumrun Vafa (1:50:59.920)
There's also more sort of like a friend of mine,
Lex Fridman (1:51:03.160)
Eric Weinstein beginning to propose
Cumrun Vafa (1:51:05.600)
something called geometric unity.
Lex Fridman (1:51:07.600)
So these kinds of attempts,
Cumrun Vafa (1:51:09.080)
whether it's through mathematical physics
Lex Fridman (1:51:10.800)
or through other avenues,
Cumrun Vafa (1:51:12.520)
or with Stephen Wolfram,
Lex Fridman (1:51:13.800)
a more computational view of the universe.
Cumrun Vafa (1:51:16.160)
Again, in his case, it's these hyper graphs
Lex Fridman (1:51:18.820)
that are very tiny objects as well.
Cumrun Vafa (1:51:21.520)
Similarly, a string theory
Lex Fridman (1:51:23.680)
and trying to grapple with this world.
Lex Fridman (1:51:25.800)
What do you think?
Lex Fridman (1:51:26.880)
Is there any of these theories that are compelling to you,
Cumrun Vafa (1:51:30.200)
that are interesting that may turn out to be true
Lex Fridman (1:51:33.520)
or at least may turn out to contain ideas that are useful?
Cumrun Vafa (1:51:36.240)
Yes, I think the latter.
Lex Fridman (1:51:37.320)
I would say that the containing ideas that are true
Cumrun Vafa (1:51:40.760)
is my opinion was what some of these ideas might be.
Lex Fridman (1:51:43.600)
For example, loop quantum gravity
Cumrun Vafa (1:51:45.720)
is to me not a complete theory of gravity in any sense,
Lex Fridman (1:51:47.920)
but they have some nuggets of truth in them.
Lex Fridman (1:51:50.320)
And typically what I expect to happen,
Lex Fridman (1:51:53.000)
and I have seen examples of this within string theory,
Cumrun Vafa (1:51:55.720)
aspects which we didn't think are part of string theory
Lex Fridman (1:51:57.960)
come to be part of it.
Cumrun Vafa (1:51:58.840)
For example, I'll give you one example.
Lex Fridman (1:52:00.840)
String was believed to be 10 dimensional.
Lex Fridman (1:52:03.320)
And then there was this 11 dimensional super gravity.
Lex Fridman (1:52:05.960)
Nobody know what the heck is that?
Lex Fridman (1:52:08.120)
Why are we getting 11 dimensional super gravity
Lex Fridman (1:52:10.040)
whereas string is saying it should be 10 dimensional?
Cumrun Vafa (1:52:11.720)
11 was the maximum dimension you can have a super gravity,
Lex Fridman (1:52:14.880)
but string was saying, sorry, we're 10 dimensional.
Lex Fridman (1:52:17.980)
So for a while we thought that theory is wrong
Lex Fridman (1:52:20.520)
because how could it be?
Cumrun Vafa (1:52:21.420)
Because string theory is definitely a theory of everything.
Lex Fridman (1:52:23.380)
We later learned that one of the circles
Cumrun Vafa (1:52:25.440)
of string theory itself was tiny,
Lex Fridman (1:52:28.520)
that we had not appreciated that fact.
Lex Fridman (1:52:30.200)
And we discovered by doing thought experiments
Lex Fridman (1:52:32.200)
of string theory that there's gotta be an extra circle
Lex Fridman (1:52:35.000)
and that circle is connected
Lex Fridman (1:52:36.420)
to an 11 dimensional perspective.
Lex Fridman (1:52:38.360)
And that's what later on got called M theory.
Lex Fridman (1:52:40.720)
So there are these kinds of things
Cumrun Vafa (1:52:43.240)
that we do not know what exactly string theory is.
Lex Fridman (1:52:45.880)
We're still learning.
Lex Fridman (1:52:47.380)
So we do not have a final formulation of string theory.
Lex Fridman (1:52:50.520)
It's very well could be the different facets
Cumrun Vafa (1:52:52.380)
of different ideas come together
Lex Fridman (1:52:53.940)
like loop quantum gravity or whatnot,
Lex Fridman (1:52:55.320)
but I wouldn't put them on par.
Lex Fridman (1:52:56.840)
Namely, loop quantum gravity is a scatter of ideas
Cumrun Vafa (1:53:01.080)
about what happens to space when they get very tiny.
Lex Fridman (1:53:03.800)
For example, you replace things by discrete data
Lex Fridman (1:53:06.480)
and try to quantize it and so on.
Lex Fridman (1:53:08.720)
And it sounds like a natural idea to quantize space.
Cumrun Vafa (1:53:13.560)
If you were naively trying to do quantum space,
Lex Fridman (1:53:15.200)
you might think about trying to take points
Lex Fridman (1:53:17.280)
and put them together in some discrete fashion
Lex Fridman (1:53:20.160)
in some way that is reminiscent of loop quantum gravity.
Cumrun Vafa (1:53:24.760)
String theory is more subtle than that.
Lex Fridman (1:53:27.000)
For example, I will just give you an example.
Lex Fridman (1:53:29.140)
And this is the kind of thing that we didn't put in by hand,
Lex Fridman (1:53:31.200)
we got it out.
Lex Fridman (1:53:32.440)
And so it's more subtle than,
Lex Fridman (1:53:33.840)
so what happens if you squeeze the space
Lex Fridman (1:53:35.840)
to be smaller and smaller?
Lex Fridman (1:53:37.780)
Well, you think that after a certain distance,
Cumrun Vafa (1:53:41.040)
the notion of distance should break down.
Lex Fridman (1:53:43.800)
You know, when you go smaller than Planck scale,
Cumrun Vafa (1:53:47.680)
should break down.
Lex Fridman (1:53:48.700)
What happens in string theory?
Cumrun Vafa (1:53:50.640)
We do not know the full answer to that,
Lex Fridman (1:53:52.160)
but we know the following.
Cumrun Vafa (1:53:53.120)
Namely, if you take a space
Lex Fridman (1:53:55.120)
and bring it smaller and smaller,
Cumrun Vafa (1:53:56.560)
if the box gets smaller than the Planck scale
Lex Fridman (1:53:58.560)
by a factor of 10,
Cumrun Vafa (1:54:00.500)
it is equivalent by the duality transformation
Lex Fridman (1:54:04.000)
to a space which is 10 times bigger.
Lex Fridman (1:54:05.760)
So there's a symmetry called T duality,
Lex Fridman (1:54:10.080)
which takes L to one over L.
Cumrun Vafa (1:54:12.480)
Well, L is measured in Planck units,
Lex Fridman (1:54:14.420)
or more precisely string units.
Cumrun Vafa (1:54:16.280)
This inversion is a very subtle effect.
Lex Fridman (1:54:20.520)
And I would not have been,
Cumrun Vafa (1:54:21.680)
or any physicist would not have been able to design a theory
Lex Fridman (1:54:23.760)
which has this property,
Cumrun Vafa (1:54:25.060)
that when you make the space smaller,
Lex Fridman (1:54:27.160)
it is as if you're making it bigger.
Cumrun Vafa (1:54:29.440)
That means there is no experiment you can do
Lex Fridman (1:54:32.480)
to distinguish the size of the space.
Cumrun Vafa (1:54:34.860)
This is remarkable.
Lex Fridman (1:54:35.740)
For example, Einstein would have said,
Cumrun Vafa (1:54:37.800)
of course I can't measure the size of the space.
Lex Fridman (1:54:39.400)
What do I do?
Cumrun Vafa (1:54:40.240)
Well, I take a flashlight,
Lex Fridman (1:54:41.400)
I send the light around,
Cumrun Vafa (1:54:43.020)
measure how long it takes for the light
Lex Fridman (1:54:44.340)
to go around the space,
Lex Fridman (1:54:45.320)
and bring back and find the radius
Lex Fridman (1:54:47.040)
or circumference of the universe.
Lex Fridman (1:54:48.800)
What's the problem?
Lex Fridman (1:54:50.880)
I said, well, suppose you do that,
Lex Fridman (1:54:52.120)
and you shrink it.
Lex Fridman (1:54:52.960)
He said, well, it gets smaller and smaller.
Lex Fridman (1:54:54.000)
So what?
Lex Fridman (1:54:54.840)
I said, well, it turns out in string theory,
Cumrun Vafa (1:54:56.840)
there are two different kinds of photons.
Lex Fridman (1:55:00.480)
One photon measures one over L,
Cumrun Vafa (1:55:02.360)
the other one measures L.
Lex Fridman (1:55:03.560)
And so this duality reformulates.
Lex Fridman (1:55:07.520)
And when the space gets smaller,
Lex Fridman (1:55:08.840)
it says, oh no, you better use the bigger perspective
Cumrun Vafa (1:55:10.720)
because the smaller one is harder to deal with.
Lex Fridman (1:55:13.020)
So you do this one.
Lex Fridman (1:55:13.860)
So these examples of loop quantum gravity
Lex Fridman (1:55:16.160)
have none of these features.
Cumrun Vafa (1:55:17.240)
These features that I'm telling you about,
Lex Fridman (1:55:18.640)
we have learned from string theory.
Lex Fridman (1:55:20.200)
But they nevertheless have some of these ideas
Lex Fridman (1:55:22.040)
like topological gravity aspects
Cumrun Vafa (1:55:24.500)
are emphasized in the context of loop quantum gravity
Lex Fridman (1:55:28.040)
in some form.
Lex Fridman (1:55:28.860)
And so these ideas might be there in some kernel,
Lex Fridman (1:55:31.280)
in some corners of string theory.
Cumrun Vafa (1:55:32.400)
In fact, I wrote a paper about topological string theory
Lex Fridman (1:55:35.360)
and some connections with potentially loop quantum gravity,
Cumrun Vafa (1:55:38.240)
which could be part of that.
Lex Fridman (1:55:39.240)
So there are little facets of connections.
Cumrun Vafa (1:55:41.640)
I wouldn't say they're complete,
Lex Fridman (1:55:43.840)
but I would say most probably what will happen
Cumrun Vafa (1:55:46.120)
to some of these ideas, the good ones at least,
Lex Fridman (1:55:48.480)
they will be absorbed to string theory,
Cumrun Vafa (1:55:50.720)
if they are correct.
Lex Fridman (1:55:51.840)
Let me ask a crazy out there question.
Lex Fridman (1:55:54.360)
Can physics help us understand life?
Lex Fridman (1:55:59.360)
So we spoke so confidently about the laws of physics
Cumrun Vafa (1:56:06.180)
being able to explain reality.
Lex Fridman (1:56:07.700)
But, and we even said words like theory of everything,
Cumrun Vafa (1:56:11.900)
implying that the word everything
Lex Fridman (1:56:13.620)
is actually describing everything.
Cumrun Vafa (1:56:15.820)
Is it possible that the four laws we've been talking about
Lex Fridman (1:56:20.540)
are actually missing,
Cumrun Vafa (1:56:22.080)
they are accurate in describing what they're describing,
Lex Fridman (1:56:24.840)
but they're missing the description
Cumrun Vafa (1:56:26.260)
of a lot of other things,
Lex Fridman (1:56:27.900)
like emergence of life
Lex Fridman (1:56:31.700)
and emergence of perhaps consciousness.
Lex Fridman (1:56:35.100)
So is there, do you ever think about this kind of stuff
Cumrun Vafa (1:56:39.280)
where we would need to understand extra physics
Lex Fridman (1:56:44.480)
to try to explain the emergence of these complex pockets
Cumrun Vafa (1:56:51.160)
of interesting weird stuff that we call life
Lex Fridman (1:56:54.280)
and consciousness in this big homogeneous universe
Lex Fridman (1:56:58.140)
that's mostly boring and nothing is happening yet?
Lex Fridman (1:57:00.420)
So first of all, we don't claim that string theory
Cumrun Vafa (1:57:03.580)
is the theory of everything in the sense that
Lex Fridman (1:57:05.780)
we know enough what this theory is.
Cumrun Vafa (1:57:07.700)
We don't know enough about string theory itself,
Lex Fridman (1:57:09.260)
we are learning it.
Lex Fridman (1:57:10.100)
So I wouldn't say, okay, give me whatever,
Lex Fridman (1:57:12.020)
I will tell you how it works, no.
Cumrun Vafa (1:57:14.400)
However, I would say by definition,
Lex Fridman (1:57:16.500)
by definition to me physics is checking all reality.
Cumrun Vafa (1:57:20.720)
Any form of reality, I call it physics,
Lex Fridman (1:57:22.580)
that's my definition.
Cumrun Vafa (1:57:23.420)
I mean, I may not know a lot of it,
Lex Fridman (1:57:25.580)
like maybe the origin of life and so on,
Cumrun Vafa (1:57:27.860)
maybe a piece of that,
Lex Fridman (1:57:29.300)
but I would call that as part of physics.
Cumrun Vafa (1:57:30.940)
To me, reality is what we're after.
Lex Fridman (1:57:33.560)
I don't claim I know everything about reality.
Cumrun Vafa (1:57:35.720)
I don't claim string theory necessarily has the tools
Lex Fridman (1:57:38.660)
right now to describe all the reality either,
Lex Fridman (1:57:41.300)
but we are learning what it is.
Lex Fridman (1:57:42.380)
So I would say that I would not put a border to say,
Cumrun Vafa (1:57:44.860)
no, from this point onwards, it's not my territory,
Lex Fridman (1:57:47.140)
it's somebody else's.
Lex Fridman (1:57:48.420)
But whether we need new ideas in string theory
Lex Fridman (1:57:50.860)
to describe other reality features, for sure I believe,
Cumrun Vafa (1:57:53.980)
as I mentioned, I don't believe any of the laws
Lex Fridman (1:57:57.100)
we know today is final.
Lex Fridman (1:57:58.140)
So therefore, yes, we will need new ideas.
Lex Fridman (1:58:00.860)
This is a very tricky thing for us to understand
Lex Fridman (1:58:05.340)
and be precise about.
Lex Fridman (1:58:08.100)
But just because you understand the physics
Cumrun Vafa (1:58:12.780)
doesn't necessarily mean that you understand
Lex Fridman (1:58:17.040)
the emergence of chemistry, biology, life,
Cumrun Vafa (1:58:21.800)
intelligence, consciousness.
Lex Fridman (1:58:23.840)
So those are built, it's like you might understand
Cumrun Vafa (1:58:27.780)
the way bricks work, but to understand what it means
Lex Fridman (1:58:32.860)
to have a happy family, you don't get from the bricks.
Lex Fridman (1:58:37.620)
So directly, in theory you could,
Lex Fridman (1:58:42.300)
if you ran the universe over again,
Lex Fridman (1:58:44.780)
but just understanding the rules of the universe
Lex Fridman (1:58:47.460)
doesn't necessarily give you a sense
Cumrun Vafa (1:58:49.900)
of the weird, beautiful things that emerge.
Lex Fridman (1:58:52.260)
Right, no, so let me describe what you just said.
Lex Fridman (1:58:55.380)
So there are two questions.
Lex Fridman (1:58:56.220)
One is whether or not the techniques I use
Cumrun Vafa (1:58:58.540)
in let's say quantum field theory and so on
Lex Fridman (1:59:00.620)
will describe how the society works.
Cumrun Vafa (1:59:02.420)
Yes.
Lex Fridman (1:59:03.260)
Okay, that's far different scales of questions
Cumrun Vafa (1:59:06.500)
that we're asking here.
Lex Fridman (1:59:08.260)
The question is, is there a change of,
Cumrun Vafa (1:59:10.540)
is there a new law which takes over
Lex Fridman (1:59:12.940)
that cannot be connected to the older laws
Lex Fridman (1:59:15.420)
that we know, or more fundamental laws that we know?
Lex Fridman (1:59:18.100)
Do you need new laws to describe it?
Cumrun Vafa (1:59:20.340)
I don't think that's necessarily the case
Lex Fridman (1:59:21.980)
in many of these phenomena like chemistry
Cumrun Vafa (1:59:23.660)
or so on you mentioned.
Lex Fridman (1:59:25.240)
So we do expect in principle chemistry
Cumrun Vafa (1:59:27.700)
can be described by quantum mechanics.
Lex Fridman (1:59:29.620)
We don't think there's gonna be a magical thing,
Lex Fridman (1:59:31.620)
but chemistry is complicated.
Lex Fridman (1:59:32.980)
Yeah, indeed, there are rules of chemistry
Cumrun Vafa (1:59:34.840)
that chemists have put down which has not been explained yet
Lex Fridman (1:59:37.820)
using quantum mechanics.
Cumrun Vafa (1:59:39.460)
Do I believe that they will be something
Lex Fridman (1:59:41.100)
described by quantum mechanics?
Cumrun Vafa (1:59:42.160)
Yes, I do.
Lex Fridman (1:59:43.000)
I don't think they are going to be sitting there
Cumrun Vafa (1:59:44.940)
in this just forever, but maybe it's too complicated
Lex Fridman (1:59:47.060)
and maybe we'll wait for very powerful quantum computers
Cumrun Vafa (1:59:50.340)
or whatnot to solve those problems.
Lex Fridman (1:59:51.620)
I don't know.
Lex Fridman (1:59:52.620)
But I don't think in that context
Lex Fridman (1:59:54.760)
we have new principles to be added to fix those.
Lex Fridman (1:59:57.980)
So I'm perfectly fine in the intermediate situation
Lex Fridman (20:01.880)
in nature in your book.
Lex Fridman (20:04.500)
How can geometry in ancient times or today
Lex Fridman (20:06.840)
be used to understand reality?
Lex Fridman (20:09.520)
And maybe how do you think about geometry
Lex Fridman (20:12.600)
as a distinct tool in mathematics and physics?
Cumrun Vafa (20:17.540)
Yes, geometry is my favorite part of math as well.
Lex Fridman (20:19.960)
And Greeks were enamored by geometry.
Cumrun Vafa (20:22.480)
They tried to describe physical reality using geometry
Lex Fridman (20:25.680)
and principles of geometry and symmetry.
Cumrun Vafa (20:27.980)
Platonic solids, the five solids they had discovered
Lex Fridman (20:31.240)
had these beautiful solids.
Cumrun Vafa (20:33.200)
They thought it must be good for some reality.
Lex Fridman (20:35.560)
There must be explaining something.
Cumrun Vafa (20:37.000)
They attached one to air, one to fire and so forth.
Lex Fridman (20:40.720)
They tried to give physical reality to symmetric objects.
Cumrun Vafa (20:45.360)
These symmetric objects are symmetries of rotation
Lex Fridman (20:48.600)
and discrete symmetry groups we call today
Cumrun Vafa (20:50.760)
of rotation group in three dimensions.
Lex Fridman (20:53.400)
Now, we know now, we kind of laugh at the way
Cumrun Vafa (20:56.240)
they were trying to connect that symmetry
Lex Fridman (20:57.960)
to the laws of the realities of physics.
Lex Fridman (21:02.040)
But actually it turns out in modern days,
Lex Fridman (21:05.840)
we use symmetries in not too far away
Cumrun Vafa (21:09.880)
exactly in these kinds of thoughts processes
Lex Fridman (21:12.440)
in the following way.
Cumrun Vafa (21:14.120)
In the context of string theory,
Lex Fridman (21:16.420)
which is the field light study,
Cumrun Vafa (21:18.640)
we have these extra dimensions.
Lex Fridman (21:20.840)
And these extra dimensions are compact tiny spaces typically
Lex Fridman (21:24.040)
but they have different shapes and sizes.
Lex Fridman (21:27.260)
We have learned that if these extra shapes and sizes
Cumrun Vafa (21:30.200)
have symmetries, which are related
Lex Fridman (21:32.400)
to the same rotation symmetries
Cumrun Vafa (21:34.160)
that the Greek we're talking about,
Lex Fridman (21:36.200)
if they enjoy those discrete symmetries
Lex Fridman (21:38.560)
and if you take that symmetry and caution the space by it,
Lex Fridman (21:41.760)
in other words, identify points under these symmetries,
Cumrun Vafa (21:44.540)
you get properties of that space at the singular points
Lex Fridman (21:48.520)
which force emanates from them.
Lex Fridman (21:51.360)
What forces?
Lex Fridman (21:52.240)
Forces like the ones we have seen in nature today,
Cumrun Vafa (21:54.960)
like electric forces, like strong forces, like weak forces.
Lex Fridman (21:59.080)
So these same principles that were driving them
Cumrun Vafa (22:02.880)
to connect geometry and symmetries to nature
Lex Fridman (22:06.600)
is driving today's physics,
Cumrun Vafa (22:10.440)
now much more modern ideas, but nevertheless,
Lex Fridman (22:13.920)
the symmetries connecting geometry to physics.
Cumrun Vafa (22:17.040)
In fact, often sometimes we ask the following question,
Lex Fridman (22:20.560)
suppose I want to get this particular physical reality,
Cumrun Vafa (22:24.680)
I wanna have this particles with these forces and so on,
Lex Fridman (22:27.420)
what do I do?
Cumrun Vafa (22:28.800)
It turns out that you can geometrically design
Lex Fridman (22:31.480)
the space to give you that.
Cumrun Vafa (22:33.160)
You say, oh, I put the sphere here, I will do this,
Lex Fridman (22:35.400)
I will shrink them.
Lex Fridman (22:36.680)
So if you have two spheres touching each other
Lex Fridman (22:39.540)
and shrinking to zero size, that gives you strong forces.
Cumrun Vafa (22:43.900)
If you have one of them, it gives you the weak forces.
Lex Fridman (22:45.720)
If you have this, you get that.
Lex Fridman (22:46.840)
And if you want to unify forces, do the other thing.
Lex Fridman (22:49.040)
So these geometrical translation of physics
Cumrun Vafa (22:52.840)
is one of my favorite things that we have discovered
Lex Fridman (22:54.880)
in modern physics and the context of string theory.
Cumrun Vafa (22:57.600)
The sad thing is when you go into multiple dimensions
Lex Fridman (22:59.840)
and we'll talk about it is we start to lose our capacity
Cumrun Vafa (23:05.040)
to visually intuit the world we're discussing.
Lex Fridman (23:09.560)
And then we go into the realm of mathematics
Lex Fridman (23:11.680)
and we'll lose that.
Lex Fridman (23:12.760)
Unfortunately, our brains are such that we're limited.
Lex Fridman (23:15.720)
But before we go into that mysterious, beautiful world,
Lex Fridman (23:19.880)
let's take a small step back.
Lex Fridman (23:21.440)
And you also in your book have this kind of
Lex Fridman (23:24.760)
through the space of puzzles, through the space of ideas,
Cumrun Vafa (23:27.280)
have a brief history of physics, of physical ideas.
Lex Fridman (23:32.000)
Now, we talked about Newtonian mechanics leading all
Cumrun Vafa (23:35.680)
through different Lagrangian, Hamiltonian mechanics.
Lex Fridman (23:38.960)
Can you describe some of the key ideas
Lex Fridman (23:41.160)
in the history of physics?
Lex Fridman (23:42.840)
Maybe lingering on each from electromagnetism to relativity
Cumrun Vafa (23:46.960)
to quantum mechanics and to today,
Lex Fridman (23:49.640)
as we'll talk about with quantum gravity and string theory.
Cumrun Vafa (23:52.760)
Sure, so I mentioned the classical mechanics
Lex Fridman (23:55.880)
and the Euler Lagrangian formulation.
Cumrun Vafa (23:59.440)
One of the next important milestones for physics
Lex Fridman (24:03.200)
were the discoveries of laws of electricity and magnetism.
Lex Fridman (24:07.240)
So Maxwell put the discoveries all together
Lex Fridman (24:10.520)
in the context of what we call the Maxwell's equations.
Lex Fridman (24:13.600)
And he noticed that when he put these discoveries
Lex Fridman (24:16.880)
that Faraday's and others had made about electric
Lex Fridman (24:20.560)
and magnetic phenomena in terms of mathematical equations,
Lex Fridman (24:23.640)
it didn't quite work.
Cumrun Vafa (24:25.240)
There was a mathematical inconsistency.
Lex Fridman (24:27.800)
Now, one could have had two attitudes.
Lex Fridman (24:31.240)
One would say, okay, who cares about math?
Lex Fridman (24:32.760)
I'm doing nature, electric force, magnetic force,
Cumrun Vafa (24:35.400)
math I don't care about.
Lex Fridman (24:36.840)
But it bothered him.
Cumrun Vafa (24:37.920)
It was inconsistent.
Lex Fridman (24:39.000)
The equations he were writing, the two equations
Cumrun Vafa (24:40.760)
he had written down did not agree with each other.
Lex Fridman (24:43.320)
And this bothered him, but he figured out,
Cumrun Vafa (24:45.280)
if you add this jiggle, this equation
Lex Fridman (24:47.360)
by adding one little term there, it works.
Cumrun Vafa (24:50.040)
At least it's consistent.
Lex Fridman (24:51.520)
What is the motivation for that term?
Cumrun Vafa (24:53.400)
He said, I don't know.
Lex Fridman (24:54.480)
Have we seen it in experiments?
Cumrun Vafa (24:56.040)
No.
Lex Fridman (24:57.200)
Why did you add it?
Cumrun Vafa (24:58.040)
Well, because of mathematical consistency.
Lex Fridman (24:59.800)
So he said, okay, math forced him to do this term.
Cumrun Vafa (25:04.520)
He added this term, which we now today call the Maxwell term.
Lex Fridman (25:08.200)
And once he added that term, his equations were nice,
Cumrun Vafa (25:11.280)
differential equations, mathematically consistent,
Lex Fridman (25:13.520)
beautiful, but he also found the new physical phenomena.
Cumrun Vafa (25:17.120)
He found that because of that term,
Lex Fridman (25:19.000)
he could now get electric and magnetic waves
Cumrun Vafa (25:22.520)
moving through space at a speed that he could calculate.
Lex Fridman (25:27.280)
So he calculated the speed of the wave
Lex Fridman (25:29.560)
and lo and behold, he found it's the same
Lex Fridman (25:31.160)
as the speed of light, which puzzled him
Cumrun Vafa (25:33.440)
because he didn't think light had anything
Lex Fridman (25:35.240)
to do with electricity and magnetism.
Lex Fridman (25:37.760)
But then he was courageous enough to say,
Lex Fridman (25:39.400)
well, maybe light is nothing
Lex Fridman (25:40.920)
but these electric and magnetic fields moving around.
Lex Fridman (25:44.520)
And he wasn't alive to see the verification
Cumrun Vafa (25:48.920)
of that prediction and indeed it was true.
Lex Fridman (25:50.400)
So this mathematical inconsistency,
Cumrun Vafa (25:53.440)
which we could say this mathematical beauty drove him
Lex Fridman (25:58.240)
to this physical, very important connection
Cumrun Vafa (26:02.320)
between light and electric and magnetic phenomena,
Lex Fridman (26:05.080)
which was later confirmed.
Lex Fridman (26:07.520)
So then physics progresses and it comes to Einstein.
Lex Fridman (26:11.080)
Einstein looks at Maxwell's equation,
Cumrun Vafa (26:13.480)
says, beautiful, these are nice equation,
Lex Fridman (26:15.120)
except we get one speed light.
Lex Fridman (26:18.640)
Who measures this light speed?
Lex Fridman (26:20.480)
And he asked the question, are you moving?
Lex Fridman (26:23.480)
Are you not moving?
Lex Fridman (26:24.320)
If you move, the speed of light changes,
Lex Fridman (26:25.760)
but Maxwell's equation has no hint
Lex Fridman (26:27.720)
of different speeds of light.
Cumrun Vafa (26:29.360)
It doesn't say, oh, only if you're not moving,
Lex Fridman (26:31.680)
you get the speed, it's just you always get the speed.
Lex Fridman (26:33.600)
So Einstein was very puzzled and he was daring enough
Lex Fridman (26:37.320)
to say, well, you know, maybe everybody gets
Cumrun Vafa (26:39.120)
the same speed for light.
Lex Fridman (26:40.960)
And that motivated his theory of special relativity.
Lex Fridman (26:44.360)
And this is an interesting example
Lex Fridman (26:45.720)
because the idea was motivated from physics,
Cumrun Vafa (26:47.760)
from Maxwell's equations, from the fact
Lex Fridman (26:50.080)
that people try to measure the properties of ether,
Cumrun Vafa (26:56.720)
which was supposed to be the medium
Lex Fridman (26:58.760)
in which the light travels through.
Lex Fridman (27:00.720)
And the idea was that only in that medium,
Lex Fridman (27:03.920)
the speed of, if you're at risk with respect
Cumrun Vafa (27:06.520)
to the ether, the speed, the speed of light,
Lex Fridman (27:08.560)
then if you're moving, the speed changes
Lex Fridman (27:10.480)
and people did not discover it.
Lex Fridman (27:11.920)
Michelson and Morley's experiment showed there's no ether.
Lex Fridman (27:15.040)
So then Einstein was courageous enough to say,
Lex Fridman (27:17.560)
you know, light is the same speed for everybody,
Cumrun Vafa (27:20.120)
regardless of whether you're moving or not.
Lex Fridman (27:22.440)
And the interesting thing is about special theory
Cumrun Vafa (27:25.760)
of relativity is that the math underpinning it
Lex Fridman (27:29.880)
is very simple.
Cumrun Vafa (27:31.320)
It's a linear algebra, nothing terribly deep.
Lex Fridman (27:35.640)
You can teach it at a high school level, if not earlier.
Cumrun Vafa (27:39.560)
Okay, does that mean Einstein's special relativity
Lex Fridman (27:42.560)
is boring?
Cumrun Vafa (27:43.400)
Not at all.
Lex Fridman (27:44.560)
So this is an example where simple math, you know,
Cumrun Vafa (27:47.280)
linear algebra leads to deep physics.
Lex Fridman (27:50.880)
Einstein's theory of special relativity.
Cumrun Vafa (27:53.080)
Motivated by this inconsistency that Maxwell's equation
Lex Fridman (27:56.640)
would suggest for the speed of light,
Cumrun Vafa (27:58.080)
depending on who observes it.
Lex Fridman (27:59.080)
What's the most daring idea there,
Lex Fridman (28:00.720)
that the speed of light could be the same everywhere?
Lex Fridman (28:03.880)
That's the basic, that's the guts of it.
Cumrun Vafa (28:05.720)
That's the core of Einstein's theory.
Lex Fridman (28:07.080)
That statement underlies the whole thing.
Cumrun Vafa (28:09.480)
Speed of light is the same for everybody.
Lex Fridman (28:11.040)
It's hard to swallow and it doesn't sound right.
Cumrun Vafa (28:13.360)
It sounds completely wrong on the face of it.
Lex Fridman (28:16.320)
And it took Einstein to make this daring statement.
Cumrun Vafa (28:19.960)
It would be laughing in some sense.
Lex Fridman (28:22.560)
How could anybody make this possibly ridiculous claim?
Lex Fridman (28:26.520)
And it turned out to be true.
Lex Fridman (28:27.720)
How does that make you feel?
Cumrun Vafa (28:28.880)
Because it still sounds ridiculous.
Lex Fridman (28:31.400)
It sounds ridiculous until you learn
Cumrun Vafa (28:33.080)
that our intuition is at fault
Lex Fridman (28:34.960)
about the way we conceive of space and time.
Cumrun Vafa (28:37.560)
The way we think about space and time is wrong
Lex Fridman (28:40.120)
because we think about the nature of time as absolute.
Lex Fridman (28:43.360)
And part of it is because we live in a situation
Lex Fridman (28:46.720)
where we don't go with very high speeds.
Cumrun Vafa (28:49.400)
There are speeds that are small
Lex Fridman (28:50.480)
compared to the speed of light.
Lex Fridman (28:52.000)
And therefore the phenomena we observe
Lex Fridman (28:54.760)
does not distinguish the relativity of time.
Cumrun Vafa (28:57.440)
The time also depends on who measures it.
Lex Fridman (28:59.200)
There's no absolute time.
Cumrun Vafa (29:00.800)
When you say it's noon today and now,
Lex Fridman (29:02.880)
it depends on who's measuring it.
Lex Fridman (29:04.360)
And not everybody would agree with that statement.
Lex Fridman (29:07.000)
And to see that you would have to have fast observer
Cumrun Vafa (29:10.520)
moving speeds close to the speed of light.
Lex Fridman (29:12.720)
So this shows that our intuition is at fault.
Lex Fridman (29:15.760)
And a lot of the discoveries in physics
Lex Fridman (29:19.320)
precisely is getting rid of the wrong old intuition.
Lex Fridman (29:23.640)
And it is funny because we get rid of it,
Lex Fridman (29:25.960)
but it's always lingers in us in some form.
Cumrun Vafa (29:28.200)
Like even when I'm describing it,
Lex Fridman (29:30.000)
I feel like a little bit like, isn't it funny?
Cumrun Vafa (29:32.960)
As you're just feeling the same way.
Lex Fridman (29:34.480)
It is, it is.
Lex Fridman (29:35.800)
But we kind of replace it by an intuition.
Lex Fridman (29:40.600)
And actually there's a very beautiful example of this,
Lex Fridman (29:43.440)
how physicists do this, try to replace their intuition.
Lex Fridman (29:46.040)
And I think this is one of my favorite examples
Cumrun Vafa (29:48.400)
about how physicists develop intuition.
Lex Fridman (29:52.160)
It goes to the work of Galileo.
Cumrun Vafa (29:54.960)
So, again, let's go back to Greek philosophers
Lex Fridman (29:58.600)
or maybe Aristotle in this case.
Cumrun Vafa (2:00:01.780)
to have rules of thumb or principles that chemists have found
Lex Fridman (2:00:04.860)
which are working, which are not founded
Cumrun Vafa (2:00:06.940)
on the basis of quantum mechanical laws, which does the job.
Lex Fridman (2:00:10.420)
Similarly, as biologists do not found everything
Cumrun Vafa (2:00:13.320)
in terms of chemistry, but they think,
Lex Fridman (2:00:15.460)
there's no reason why chemistry cannot.
Cumrun Vafa (2:00:16.740)
They don't think necessarily they're doing something
Lex Fridman (2:00:18.780)
amazingly not possible with chemistry.
Cumrun Vafa (2:00:20.980)
Coming back to your question,
Lex Fridman (2:00:22.220)
does consciousness, for example, bring this new ingredient?
Cumrun Vafa (2:00:26.180)
If indeed it needs a new ingredient,
Lex Fridman (2:00:28.260)
I will call that new ingredient part of physical law.
Cumrun Vafa (2:00:30.540)
We have to understand it.
Lex Fridman (2:00:31.700)
To me that, so I wouldn't put a line to say,
Cumrun Vafa (2:00:34.500)
okay, from this point onwards, it's disconnected.
Lex Fridman (2:00:37.380)
It's fully disconnected from string theory or whatever.
Cumrun Vafa (2:00:39.520)
We have to do something else.
Lex Fridman (2:00:41.100)
It's not a line.
Lex Fridman (2:00:42.340)
What I'm referring to is can physics of a few centuries
Lex Fridman (2:00:45.800)
from now that doesn't understand consciousness
Cumrun Vafa (2:00:48.220)
be much bigger than the physics of today,
Lex Fridman (2:00:51.620)
where the textbook grows?
Cumrun Vafa (2:00:53.980)
It definitely will.
Lex Fridman (2:00:54.940)
I would say, it will grow.
Cumrun Vafa (2:00:55.980)
I don't know if it grows because of consciousness
Lex Fridman (2:00:58.820)
being part of it or we have different view of consciousness.
Cumrun Vafa (2:01:01.220)
I do not know where the consciousness will fit.
Lex Fridman (2:01:03.780)
It's gonna be hard for me to guess.
Cumrun Vafa (2:01:07.540)
I mean, I can make random guesses now
Lex Fridman (2:01:09.420)
which probably most likely is wrong,
Lex Fridman (2:01:11.420)
but let me just do just for the sake of discussion.
Lex Fridman (2:01:14.740)
I could say, brain could be their quantum computer,
Cumrun Vafa (2:01:18.180)
classical computer.
Lex Fridman (2:01:19.020)
Their arguments against this being a quantum thing,
Lex Fridman (2:01:20.820)
so it's probably classical, and if it's classical,
Lex Fridman (2:01:22.980)
it could be like what we are doing in machine learning,
Cumrun Vafa (2:01:24.660)
slightly more fancy and so on.
Lex Fridman (2:01:26.220)
Okay, people can go to this argument to no end
Lex Fridman (2:01:28.540)
and to some whether consciousness exists or not,
Lex Fridman (2:01:30.120)
or life, does it have any meaning?
Cumrun Vafa (2:01:32.320)
Or is there a phase transition where you can say,
Lex Fridman (2:01:34.780)
does electron have a life or not?
Lex Fridman (2:01:36.700)
At what level does a particle become life?
Lex Fridman (2:01:39.140)
Maybe there's no definite definition of life
Cumrun Vafa (2:01:41.300)
in that same way that, we cannot say electron,
Lex Fridman (2:01:43.860)
if you, I like this example quite a bit.
Cumrun Vafa (2:01:48.860)
We distinguish between liquid and a gas phase,
Lex Fridman (2:01:51.160)
like water is liquid or vapor is gas,
Lex Fridman (2:01:53.860)
and we say they're different.
Lex Fridman (2:01:54.700)
You can distinguish them.
Cumrun Vafa (2:01:55.740)
Actually, that's not true.
Lex Fridman (2:01:57.340)
It's not true because we know from physics
Cumrun Vafa (2:01:59.860)
that you can change temperatures and pressure
Lex Fridman (2:02:01.960)
to go from liquid to the gas
Cumrun Vafa (2:02:03.660)
without making any phase transition.
Lex Fridman (2:02:05.700)
So there is no point that you can say this was a liquid
Lex Fridman (2:02:08.460)
and this was a gas.
Lex Fridman (2:02:10.180)
You can continuously change the parameters
Cumrun Vafa (2:02:12.060)
to go from one to the other.
Lex Fridman (2:02:13.820)
So at the end, it's very different looking.
Cumrun Vafa (2:02:15.860)
Like, I know that water is different from vapor,
Lex Fridman (2:02:18.060)
but there's no precise point this happens.
Cumrun Vafa (2:02:21.260)
I feel many of these things that we think,
Lex Fridman (2:02:24.020)
like consciousness, clearly dead person
Cumrun Vafa (2:02:25.940)
is not conscious and the other one is.
Lex Fridman (2:02:27.220)
So there's a difference like water and vapor,
Lex Fridman (2:02:30.480)
but there's no point you could say that this is conscious.
Lex Fridman (2:02:32.860)
There's no sharp transition.
Lex Fridman (2:02:34.180)
So it could very well be that what we call heuristically
Lex Fridman (2:02:38.420)
in daily life, consciousness is similar,
Cumrun Vafa (2:02:41.020)
or life is similar to that.
Lex Fridman (2:02:43.140)
I don't know if it's like that or not.
Cumrun Vafa (2:02:44.500)
I'm just hypothesizing it's possible.
Lex Fridman (2:02:46.540)
Like there's no.
Cumrun Vafa (2:02:48.260)
There's no discrete phases.
Lex Fridman (2:02:49.380)
There's no discrete phase transition like that.
Cumrun Vafa (2:02:51.060)
Yeah, yeah, but there might be concepts of temperature
Lex Fridman (2:02:56.580)
and pressure that we need to understand
Cumrun Vafa (2:02:59.340)
to describe what the head consciousness in life is
Lex Fridman (2:03:02.740)
that we're totally missing.
Cumrun Vafa (2:03:04.900)
I think that's not a useless question.
Lex Fridman (2:03:07.420)
Even those questions,
Cumrun Vafa (2:03:08.780)
they is back to our original discussion of philosophy.
Lex Fridman (2:03:11.740)
I would say consciousness and free will, for example,
Cumrun Vafa (2:03:15.220)
are topics that are very much so
Lex Fridman (2:03:18.900)
in the realm of philosophy currently.
Cumrun Vafa (2:03:20.860)
Yes.
Lex Fridman (2:03:21.700)
But I don't think they will always be.
Cumrun Vafa (2:03:22.980)
I agree with you.
Lex Fridman (2:03:23.940)
I agree with you.
Lex Fridman (2:03:24.760)
And I think I'm fine with some topics
Lex Fridman (2:03:27.060)
being part of a different realm than physics today
Cumrun Vafa (2:03:29.820)
because we don't have the right tools,
Lex Fridman (2:03:32.180)
just like biology was.
Cumrun Vafa (2:03:33.360)
I mean, before we had DNA and all that genetics
Lex Fridman (2:03:35.620)
and all that gradually began to take hold.
Cumrun Vafa (2:03:37.660)
I mean, when people were beginning phase experiments
Lex Fridman (2:03:42.100)
with biology and chemistry and so on,
Cumrun Vafa (2:03:44.180)
gradually they came together.
Lex Fridman (2:03:46.040)
So it wasn't like together.
Lex Fridman (2:03:47.300)
So yeah, I'd be perfectly understanding of a situation
Lex Fridman (2:03:49.940)
where we don't have the tools.
Lex Fridman (2:03:51.540)
So do these experiments that you think
Lex Fridman (2:03:53.340)
as defines a conscious in different form
Lex Fridman (2:03:55.180)
and gradually we'll build it and connect it.
Lex Fridman (2:03:57.220)
And yes, we might discover new principles of nature
Cumrun Vafa (2:03:59.660)
that we didn't know.
Lex Fridman (2:04:01.100)
I don't know, but I would say that if they are,
Cumrun Vafa (2:04:03.420)
they will be deeply connected with the else.
Lex Fridman (2:04:04.700)
We have seen in physics,
Cumrun Vafa (2:04:06.980)
we don't have things in isolation.
Lex Fridman (2:04:08.300)
You cannot compartmentalize,
Cumrun Vafa (2:04:10.820)
this is gravity, this is electricity, this is that.
Lex Fridman (2:04:13.300)
We have learned they all talk to each other.
Cumrun Vafa (2:04:15.220)
There's no way to make them in one corner and don't talk.
Lex Fridman (2:04:19.060)
So the same thing with anything, anything which is real.
Lex Fridman (2:04:21.300)
So consciousness is real.
Lex Fridman (2:04:22.380)
So therefore we have to connect it to everything else.
Lex Fridman (2:04:25.200)
So to me, once you connect it,
Lex Fridman (2:04:26.640)
you cannot say it's not reality.
Lex Fridman (2:04:27.860)
And once it's reality, it's physics.
Lex Fridman (2:04:29.860)
I call it physics.
Cumrun Vafa (2:04:30.700)
It may not be the physics I know today, for sure it's not,
Lex Fridman (2:04:32.980)
but I would be surprised if there's disconnected realities
Cumrun Vafa (2:04:37.300)
that you cannot imagine them as part of the same soup.
Lex Fridman (2:04:41.260)
So I guess God doesn't have a biology or chemistry textbook
Lex Fridman (2:04:45.180)
and mostly, or maybe he or she reads it for fun,
Lex Fridman (2:04:49.460)
biology and chemistry,
Lex Fridman (2:04:50.500)
but when you're trying to get some work done,
Lex Fridman (2:04:52.100)
it'll be going to the physics textbook.
Cumrun Vafa (2:04:54.500)
Okay, what advice, let's put on your wise visionary hat.
Lex Fridman (2:04:59.500)
What advice do you have for young people today?
Cumrun Vafa (2:05:03.860)
You've dedicated your book actually to your kids,
Lex Fridman (2:05:08.060)
to your family.
Lex Fridman (2:05:09.620)
What advice would you give to them?
Lex Fridman (2:05:11.500)
What advice would you give to young people today
Cumrun Vafa (2:05:13.880)
thinking about their career, thinking about life,
Lex Fridman (2:05:16.780)
of how to live successful life, how to live a good life?
Cumrun Vafa (2:05:19.820)
Yes, yes, I have three sons.
Lex Fridman (2:05:23.020)
And in fact, to them, I have tried not to give
Cumrun Vafa (2:05:26.900)
too much advice.
Lex Fridman (2:05:28.280)
So even though I've tried to kind of not give advice,
Cumrun Vafa (2:05:31.420)
maybe indirectly it has been some impact.
Lex Fridman (2:05:33.920)
My oldest one is doing biophysics, for example,
Lex Fridman (2:05:36.020)
and the second one is doing machine learning
Lex Fridman (2:05:38.320)
and the third one is doing theoretical computer science.
Lex Fridman (2:05:40.620)
So there are these facets of interest
Lex Fridman (2:05:42.500)
which are not too far from my area,
Lex Fridman (2:05:44.660)
but I have not tried to impact them in that way,
Lex Fridman (2:05:47.980)
but they have followed their own interests.
Lex Fridman (2:05:49.980)
And I think that's the advice I would give
Lex Fridman (2:05:51.860)
to any young person, follow your own interests
Lex Fridman (2:05:54.980)
and let that take you wherever it takes you.
Lex Fridman (2:05:58.580)
And this I did in my own case that I was planning
Cumrun Vafa (2:06:03.380)
to study economics and electrical engineering
Lex Fridman (2:06:06.220)
when I started at MIT.
Lex Fridman (2:06:08.100)
And I discovered that I'm more passionate
Lex Fridman (2:06:10.620)
about math and physics.
Lex Fridman (2:06:11.700)
And at that time I didn't feel math and physics
Lex Fridman (2:06:14.020)
would make a good career.
Lex Fridman (2:06:15.860)
And so I was kind of hesitant to go in that direction,
Lex Fridman (2:06:18.540)
but I did because I kind of felt that
Cumrun Vafa (2:06:20.560)
that's what I'm driven to do.
Lex Fridman (2:06:22.580)
So I don't regret it, I'm lucky in the sense
Cumrun Vafa (2:06:26.260)
that society supports people like me
Lex Fridman (2:06:28.340)
who are doing these abstract stuff,
Cumrun Vafa (2:06:29.900)
which may or may not be experimentally verified
Lex Fridman (2:06:32.540)
even let alone applied to the technology in our lifetimes.
Cumrun Vafa (2:06:36.380)
I'm lucky I'm doing that.
Lex Fridman (2:06:37.660)
And I feel that if people follow their interests,
Cumrun Vafa (2:06:41.300)
they will find the niche that they're good at.
Lex Fridman (2:06:43.960)
And this coincidence of hopefully their interests
Lex Fridman (2:06:48.020)
and abilities are kind of aligned,
Lex Fridman (2:06:51.860)
at least some extent to be able to drive them
Cumrun Vafa (2:06:54.420)
to something which is successful.
Lex Fridman (2:06:56.260)
And not to be driven by things like,
Cumrun Vafa (2:06:58.540)
this doesn't make a good career,
Lex Fridman (2:07:00.020)
or this doesn't do that, and my parents expect that,
Lex Fridman (2:07:02.400)
or what about this?
Lex Fridman (2:07:03.240)
And I think ultimately you have to live with yourself
Lex Fridman (2:07:06.140)
and you only have one life and it's short, very short.
Lex Fridman (2:07:08.660)
I can tell you I'm getting there.
Lex Fridman (2:07:10.580)
So I know it's short.
Lex Fridman (2:07:11.500)
So you really want not to do things
Cumrun Vafa (2:07:14.980)
that you don't want to do.
Lex Fridman (2:07:15.900)
So I think following an interest
Cumrun Vafa (2:07:17.340)
is my strongest advice to young people.
Lex Fridman (2:07:19.300)
Yeah, it's scary when your interest
Cumrun Vafa (2:07:22.180)
doesn't directly map to a career of the past or of today.
Lex Fridman (2:07:26.620)
So you're almost anticipating future careers
Cumrun Vafa (2:07:28.700)
that could be created.
Lex Fridman (2:07:29.640)
It's scary.
Lex Fridman (2:07:32.320)
But yeah, there's something to that,
Lex Fridman (2:07:34.180)
especially when the interest and the ability align,
Cumrun Vafa (2:07:36.820)
that you will pave a path,
Lex Fridman (2:07:39.620)
that will find a way to make money,
Cumrun Vafa (2:07:41.220)
especially in this society,
Lex Fridman (2:07:42.660)
in a capitalistic United States society.
Cumrun Vafa (2:07:46.220)
It feels like ability and passion paves the way.
Lex Fridman (2:07:52.260)
Yes.
Cumrun Vafa (2:07:54.500)
At the very least, you can sell funny T shirts.
Lex Fridman (2:07:56.940)
Yes.
Cumrun Vafa (2:07:57.780)
You've mentioned life is short.
Lex Fridman (2:08:00.820)
Do you think about your mortality?
Lex Fridman (2:08:04.140)
Are you afraid of death?
Lex Fridman (2:08:05.900)
I don't think about my mortality.
Cumrun Vafa (2:08:09.540)
I think that I don't think about my death.
Lex Fridman (2:08:12.460)
I don't think about death in general too much.
Cumrun Vafa (2:08:14.700)
First of all, it's something that I can't do much about,
Lex Fridman (2:08:16.820)
and I think it's something
Cumrun Vafa (2:08:18.180)
that it doesn't drive my everyday action.
Lex Fridman (2:08:21.640)
It is natural to expect
Cumrun Vafa (2:08:23.300)
that it's somewhat like the time reversal situation.
Lex Fridman (2:08:25.940)
So we believe that we have this approximate symmetry
Cumrun Vafa (2:08:27.940)
in nature, time reversal.
Lex Fridman (2:08:29.440)
Going forward, we die.
Cumrun Vafa (2:08:30.500)
Going backwards, we get born.
Lex Fridman (2:08:32.300)
So what was it to get born?
Cumrun Vafa (2:08:35.060)
It wasn't such a good or bad feeling.
Lex Fridman (2:08:37.340)
I have no feeling of it.
Lex Fridman (2:08:38.780)
So who knows what the death will feel like,
Lex Fridman (2:08:42.140)
the moment of death or whatnot.
Lex Fridman (2:08:43.460)
So I don't know.
Lex Fridman (2:08:44.660)
It is not known,
Lex Fridman (2:08:45.500)
but in what form do we exist before or after?
Lex Fridman (2:08:50.160)
Again, it's something that it's partly philosophical maybe.
Cumrun Vafa (2:08:53.740)
I like how you draw comfort from symmetry.
Lex Fridman (2:08:55.900)
It does seem that there is something asymmetric here,
Cumrun Vafa (2:08:58.660)
a breaking of symmetry,
Lex Fridman (2:08:59.700)
because there's something to the creative force
Cumrun Vafa (2:09:05.940)
of the human spirit that goes only one way.
Lex Fridman (2:09:09.580)
Right.
Cumrun Vafa (2:09:10.400)
That it seems the finiteness of life
Lex Fridman (2:09:13.140)
is the thing that drives the creativity.
Lex Fridman (2:09:15.940)
And so it does seem that at least the contemplation
Lex Fridman (2:09:21.540)
of the finiteness of life, of mortality,
Cumrun Vafa (2:09:24.580)
is the thing that helps you get your stuff together.
Lex Fridman (2:09:27.100)
Yes, I think that's true,
Lex Fridman (2:09:28.140)
but actually I have a different perspective
Lex Fridman (2:09:29.560)
on that a little bit.
Cumrun Vafa (2:09:30.400)
Yes.
Lex Fridman (2:09:31.240)
Namely, suppose I told you you're immortal.
Cumrun Vafa (2:09:34.820)
Yes.
Lex Fridman (2:09:37.120)
I think your life will be totally boring after that,
Cumrun Vafa (2:09:39.780)
because you will not,
Lex Fridman (2:09:41.460)
I think part of the reason we have enjoyment in life
Cumrun Vafa (2:09:45.960)
is the finiteness of it.
Lex Fridman (2:09:47.280)
Yes.
Lex Fridman (2:09:48.120)
And so I think mortality might be a blessing,
Lex Fridman (2:09:52.180)
and immortality may not.
Lex Fridman (2:09:54.000)
So I think that we value things
Lex Fridman (2:09:55.960)
because we have that finite life.
Cumrun Vafa (2:09:58.080)
We appreciate things.
Lex Fridman (2:09:59.300)
We want to do this.
Cumrun Vafa (2:10:00.140)
We want to do that.
Lex Fridman (2:10:00.960)
We have motivation.
Cumrun Vafa (2:10:01.800)
If I told you, you know, you have infinite life.
Lex Fridman (2:10:03.080)
Oh, I don't, I don't need to do this today.
Cumrun Vafa (2:10:04.700)
I have another billion or trillion or infinite life.
Lex Fridman (2:10:08.520)
So why do I do now?
Cumrun Vafa (2:10:10.160)
There is no motivation.
Lex Fridman (2:10:11.840)
A lot of the things that we do
Cumrun Vafa (2:10:13.600)
are driven by that finiteness of these resources.
Lex Fridman (2:10:16.960)
So I think it is a blessing in disguise.
Cumrun Vafa (2:10:20.100)
I don't regret it that we have more finite life.
Lex Fridman (2:10:23.380)
And I think that the process of being part of this thing,
Cumrun Vafa (2:10:31.040)
that, you know, the reality,
Lex Fridman (2:10:33.080)
to me, part of what attracts me to science
Cumrun Vafa (2:10:36.400)
is to connect to that immortality kind of,
Lex Fridman (2:10:39.700)
namely the laws, the reality beyond us.
Cumrun Vafa (2:10:43.940)
To me, I'm resigned to the fact that not only me,
Lex Fridman (2:10:47.900)
everybody's going to die.
Lex Fridman (2:10:49.780)
So this is a little bit of a consolation.
Lex Fridman (2:10:51.940)
None of us are going to be around.
Lex Fridman (2:10:53.860)
So therefore, okay,
Lex Fridman (2:10:55.220)
and none of the people before me are around.
Lex Fridman (2:10:57.380)
So therefore, yeah, okay,
Lex Fridman (2:10:58.620)
this is something everybody goes through.
Lex Fridman (2:10:59.940)
So taking that minuscule version of,
Lex Fridman (2:11:03.420)
okay, how tiny we are and how short time it is and so on,
Cumrun Vafa (2:11:07.520)
to connect to the deeper truth beyond us,
Lex Fridman (2:11:10.140)
the reality beyond us,
Cumrun Vafa (2:11:11.900)
is what sense of, quote unquote, immortality I would get.
Lex Fridman (2:11:16.820)
Namely, at least I can hang on
Cumrun Vafa (2:11:18.780)
to this little piece of truth,
Lex Fridman (2:11:20.700)
even though I know, I know it's not complete.
Cumrun Vafa (2:11:23.180)
I know it's going to be imperfect.
Lex Fridman (2:11:25.780)
I know it's going to change and it's going to be improved.
Lex Fridman (2:11:28.420)
But having a little bit deeper insight
Lex Fridman (2:11:30.580)
than just the naive thing around us,
Cumrun Vafa (2:11:32.700)
little earth here and little galaxy and so on,
Lex Fridman (2:11:35.240)
makes me feel a little bit more pleasure to live this life.
Lex Fridman (2:11:40.140)
So I think that's the way I view my role as a scientist.
Lex Fridman (2:11:43.180)
Yeah, the scarcity of this life helps us appreciate
Cumrun Vafa (2:11:48.380)
the beauty of the immortal,
Lex Fridman (2:11:50.180)
the universal truths of that physics present us.
Lex Fridman (2:11:53.700)
And maybe one day physics will have something to say
Lex Fridman (2:11:58.900)
about that beauty in itself,
Cumrun Vafa (2:12:03.380)
explaining why the heck it's so beautiful
Lex Fridman (2:12:06.540)
to appreciate the laws of physics,
Lex Fridman (2:12:08.280)
and yet why it's so tragic that we would die so quickly.
Lex Fridman (2:12:14.660)
Yes, we die so quickly.
Lex Fridman (2:12:16.160)
So that can be a bit longer, that's for sure.
Lex Fridman (2:12:18.100)
It would be very nice.
Cumrun Vafa (2:12:19.220)
Maybe physics will help out.
Lex Fridman (2:12:20.820)
Well, Kamran, it was an incredible conversation.
Cumrun Vafa (2:12:23.880)
Thank you so much once again
Lex Fridman (2:12:25.120)
for painting a beautiful picture of the history of physics.
Lex Fridman (2:12:28.340)
And it kind of presents a hopeful view
Lex Fridman (2:12:32.420)
of the future of physics.
Lex Fridman (2:12:33.500)
So I really, really appreciate that.
Lex Fridman (2:12:35.780)
It's a huge honor that you would talk to me
Lex Fridman (2:12:37.380)
and waste all your valuable time with me.
Lex Fridman (2:12:39.300)
I really appreciate it.
Cumrun Vafa (2:12:40.140)
Thanks, Lex.
Lex Fridman (2:12:40.960)
It was a pleasure, and I loved talking with you.
Lex Fridman (2:12:42.740)
And this is wonderful set of discussions.
Lex Fridman (2:12:44.500)
I really enjoyed my time with this discussion.
Cumrun Vafa (2:12:46.460)
Thank you.
Lex Fridman (2:12:47.940)
Thanks for listening to this conversation
Cumrun Vafa (2:12:49.420)
with Kamran Vafa.
Lex Fridman (2:12:50.580)
And thank you to Headspace, Jordan Harmerjee Show,
Cumrun Vafa (2:12:54.220)
Squarespace, and Allform.
Lex Fridman (2:12:56.620)
Check them out in the description to support this podcast.
Lex Fridman (2:13:00.180)
And now, let me leave you with some words
Lex Fridman (2:13:02.620)
from the great Richard Feynman.
Cumrun Vafa (2:13:04.960)
"'Physics isn't the most important thing.
Lex Fridman (2:13:07.660)
"'Love is.'"
Cumrun Vafa (2:13:08.660)
Thank you for listening, and hope to see you next time.
Lex Fridman (30:00.520)
Now, again, let's make a criticism.
Cumrun Vafa (30:02.560)
He thought that the heavier objects fall faster
Lex Fridman (30:05.880)
than the lighter objects.
Cumrun Vafa (30:07.000)
Makes sense.
Lex Fridman (30:07.880)
It kind of makes sense.
Lex Fridman (30:08.840)
And people say about the feather and so on,
Lex Fridman (30:10.960)
but that's because of the air resistance.
Lex Fridman (30:12.680)
But you might think like,
Lex Fridman (30:13.640)
if you have a heavy stone and a light pebble,
Cumrun Vafa (30:17.000)
the heavy one will fall first.
Lex Fridman (30:18.480)
If you don't do any experiments,
Cumrun Vafa (30:20.280)
that's the first gut reaction.
Lex Fridman (30:21.480)
I would say everybody would say that's the natural thing.
Cumrun Vafa (30:24.080)
Galileo did not believe this.
Lex Fridman (30:25.560)
And he kind of did the experiment.
Cumrun Vafa (30:29.880)
Famously it said he went on the top of Pisa Tower
Lex Fridman (30:32.640)
and he dropped these heavy and light stones
Lex Fridman (30:34.680)
and they fell at the same time
Lex Fridman (30:35.800)
when he dropped it at the same time from the same height.
Cumrun Vafa (30:39.000)
Okay, good.
Lex Fridman (30:39.920)
So he said, I'm done.
Cumrun Vafa (30:41.400)
I've showed that the heavy and lighter objects
Lex Fridman (30:43.640)
fall at the same time.
Cumrun Vafa (30:44.480)
I did the experiment.
Lex Fridman (30:45.840)
Scientists at that time did not accept it.
Lex Fridman (30:49.440)
Why was that?
Lex Fridman (30:50.760)
Because at that time, science was not just experimental.
Cumrun Vafa (30:54.240)
The experiment was not enough.
Lex Fridman (30:56.720)
They didn't think that they have to soil their hands
Cumrun Vafa (30:59.560)
in doing experiments to get to the reality.
Lex Fridman (31:01.840)
They said, why is it the case?
Lex Fridman (31:03.320)
Why?
Lex Fridman (31:04.160)
So Galileo had to come up with an explanation
Cumrun Vafa (31:06.360)
of why heavier and lighter objects fall at the same rate.
Lex Fridman (31:09.520)
This is the way he convinced them using symmetry.
Cumrun Vafa (31:13.240)
He said, suppose you have three bricks,
Lex Fridman (31:16.120)
the same shape, the same size, same mass, everything.
Lex Fridman (31:21.120)
And we hold these three bricks at the same height
Lex Fridman (31:24.880)
and drop them.
Lex Fridman (31:27.080)
Which one will fall to the ground first?
Lex Fridman (31:29.960)
Everybody said, of course, we know it's symmetry
Cumrun Vafa (31:32.080)
tells you they're all the same shape,
Lex Fridman (31:33.840)
same size, same height.
Cumrun Vafa (31:35.240)
Of course, they fall at the same time.
Lex Fridman (31:36.800)
Yeah, we know that.
Cumrun Vafa (31:37.640)
Next, next.
Lex Fridman (31:38.680)
It's trivial.
Cumrun Vafa (31:39.880)
He said, okay, what if we move these bricks around
Lex Fridman (31:42.000)
with the same height?
Lex Fridman (31:42.840)
Does it change the time they hit the ground?
Lex Fridman (31:45.160)
They said, if it's the same height,
Cumrun Vafa (31:46.320)
again, by the symmetry principle,
Lex Fridman (31:47.800)
because the height translation horizontal
Cumrun Vafa (31:49.360)
translates to the symmetry, no, it doesn't matter.
Lex Fridman (31:52.200)
They all fall at the same rate.
Cumrun Vafa (31:53.560)
Good.
Lex Fridman (31:54.400)
Does it matter how close I bring them together?
Cumrun Vafa (31:55.840)
No, it doesn't.
Lex Fridman (31:56.920)
Okay, suppose I make the two bricks touch
Lex Fridman (31:59.040)
and then let them go.
Lex Fridman (31:59.880)
Do they fall at the same rate?
Cumrun Vafa (32:01.120)
Yes, they do.
Lex Fridman (32:02.560)
But then he said, well, the two bricks that touch
Cumrun Vafa (32:04.920)
are twice more mass than this other brick.
Lex Fridman (32:07.160)
And you just agreed that they fall at the same rate.
Cumrun Vafa (32:09.640)
They say, yeah, yeah, we just agreed.
Lex Fridman (32:10.760)
That's right, that's great.
Cumrun Vafa (32:12.440)
Yes.
Lex Fridman (32:13.280)
So he deconfused them by the symmetry reasoning.
Lex Fridman (32:15.440)
So this way of repackaging some intuition,
Lex Fridman (32:18.320)
a different type of intuition.
Cumrun Vafa (32:19.840)
When the intuitions clash,
Lex Fridman (32:21.920)
then you side on the, you replace the intuition.
Cumrun Vafa (32:24.920)
That's brilliant.
Lex Fridman (32:26.960)
In some of these more difficult physical ideas,
Cumrun Vafa (32:31.280)
physics ideas in the 20th century and the 21st century,
Lex Fridman (32:34.200)
it starts becoming more and more difficult
Cumrun Vafa (32:36.120)
to then replace the intuition.
Lex Fridman (32:38.320)
What does the world look like
Lex Fridman (32:39.840)
for an object traveling close to the speed of light?
Lex Fridman (32:42.840)
You start to think about the edges
Cumrun Vafa (32:44.960)
of supermassive black holes,
Lex Fridman (32:47.480)
and you start to think like, what's that look like?
Cumrun Vafa (32:51.000)
Or I've been into gravitational waves recently.
Lex Fridman (32:55.720)
It's like when the fabric of space time
Cumrun Vafa (32:58.040)
is being morphed by gravity,
Lex Fridman (33:01.400)
like what's that actually feel like?
Lex Fridman (33:03.400)
If I'm riding a gravitational wave, what's that feel like?
Lex Fridman (33:09.080)
I mean, I think some of those are more sort of hippy,
Cumrun Vafa (33:12.160)
not useful intuitions to have,
Lex Fridman (33:15.800)
but if you're an actual physicist
Cumrun Vafa (33:18.720)
or whatever the particular discipline is,
Lex Fridman (33:20.560)
I wonder if it's possible to meditate,
Cumrun Vafa (33:23.520)
to sort of escape through thinking,
Lex Fridman (33:27.520)
prolong thinking and meditation on a world,
Cumrun Vafa (33:31.720)
like live in a visualized world that's not like our own
Lex Fridman (33:35.280)
in order to understand a phenomenon deeply.
Lex Fridman (33:38.080)
So like replace the intuition,
Lex Fridman (33:41.440)
like through rigorous meditation on the idea
Cumrun Vafa (33:44.560)
in order to conceive of it.
Lex Fridman (33:46.360)
I mean, if we talk about multiple dimensions,
Cumrun Vafa (33:48.720)
I wonder if there's a way to escape
Lex Fridman (33:51.600)
with a three dimensional world in our mind
Cumrun Vafa (33:53.920)
in order to then start to reason about it.
Lex Fridman (33:56.280)
It's, the more I talk to topologists,
Cumrun Vafa (34:01.200)
the more they seem to not operate at all
Lex Fridman (34:04.280)
in the visual space.
Cumrun Vafa (34:05.720)
They really trust the mathematics,
Lex Fridman (34:07.840)
like which is really annoying to me because topology
Lex Fridman (34:10.600)
and differential geometry feels like it has a lot
Lex Fridman (34:15.480)
of potential for beautiful pictures.
Cumrun Vafa (34:17.480)
Yes, I think they do.
Lex Fridman (34:18.640)
Actually, I would not be able to do my research
Cumrun Vafa (34:23.280)
if I don't have an intuitive feel about geometry.
Lex Fridman (34:26.160)
And we'll get to it as you mentioned before
Cumrun Vafa (34:29.640)
that how, for example, in strength theory,
Lex Fridman (34:32.080)
you deal with these extra dimensions.
Lex Fridman (34:33.440)
And I'll be very happy to describe how we do it
Lex Fridman (34:35.480)
because without intuition, we will not get anywhere.
Lex Fridman (34:37.640)
And I don't think you can just rely on formalism.
Lex Fridman (34:40.440)
I don't.
Cumrun Vafa (34:41.360)
I don't think any physicist just relies on formalism.
Lex Fridman (34:44.120)
That's not physics.
Cumrun Vafa (34:45.000)
That's not understanding.
Lex Fridman (34:46.680)
So we have to intuit it.
Lex Fridman (34:48.080)
And that's crucial.
Lex Fridman (34:49.280)
And there are steps of doing it.
Lex Fridman (34:50.760)
And we learned it might not be trivial,
Lex Fridman (34:52.720)
but we learn how to do it.
Cumrun Vafa (34:53.840)
Similar to what this Galileo picture I just told you,
Lex Fridman (34:56.520)
you have to build these gradually.
Lex Fridman (34:59.400)
But you have to connect the bricks.
Lex Fridman (35:02.000)
Exactly, you have to connect the bricks, literally.
Lex Fridman (35:04.760)
So yeah, so then, so going back to your question
Lex Fridman (35:07.720)
about the path of the history of the science.
Lex Fridman (35:10.040)
So I was saying about the electricity and magnetism
Lex Fridman (35:12.400)
and the special relativity where simple idea
Cumrun Vafa (35:14.560)
led to special relativity.
Lex Fridman (35:16.400)
But then he went further thinking about acceleration
Cumrun Vafa (35:20.040)
in the context of relativity.
Lex Fridman (35:21.760)
And he came up with general relativity
Cumrun Vafa (35:23.840)
where he talked about the fabric of space time
Lex Fridman (35:26.000)
being curved and so forth and matter
Cumrun Vafa (35:28.800)
affecting the curvature of the space and time.
Lex Fridman (35:32.080)
So this gradually became a connection
Cumrun Vafa (35:36.960)
between geometry and physics.
Lex Fridman (35:38.560)
Namely, he replaced Newton's gravitational force
Cumrun Vafa (35:43.160)
with a very geometrical, beautiful picture.
Lex Fridman (35:46.000)
It's much more elegant than Newton's,
Lex Fridman (35:47.520)
but much more complicated mathematically.
Lex Fridman (35:49.960)
So when we say it's simpler,
Cumrun Vafa (35:52.760)
we mean in some form it's simpler,
Lex Fridman (35:55.000)
but not in pragmatic terms of equation solving.
Cumrun Vafa (35:57.800)
The equations are much harder to solve
Lex Fridman (35:59.800)
in Einstein's theory.
Lex Fridman (36:01.360)
And in fact, so much harder that Einstein himself
Lex Fridman (36:03.920)
couldn't solve many of the cases.
Cumrun Vafa (36:06.000)
He thought, for example, you couldn't solve the equation
Lex Fridman (36:07.920)
for a spherical symmetric matter,
Cumrun Vafa (36:10.760)
like if you had a symmetric sun,
Lex Fridman (36:12.960)
he didn't think you can actually solve his equation for that.
Lex Fridman (36:15.680)
And a year after he said that it was solved by Schwarzschild.
Lex Fridman (36:19.320)
So it was that hard
Cumrun Vafa (36:21.120)
that he didn't think it's gonna be that easy.
Lex Fridman (36:22.880)
So yeah, deformism is hard.
Lex Fridman (36:24.960)
But the contrast between the special relativity
Lex Fridman (36:27.400)
and general relativity is very interesting
Cumrun Vafa (36:29.040)
because one of them has almost trivial math
Lex Fridman (36:31.520)
and the other one has super complicated math.
Cumrun Vafa (36:34.520)
Both are physically amazingly important.
Lex Fridman (36:37.600)
And so we have learned that, you know,
Cumrun Vafa (36:40.080)
the physics may or may not require complicated math.
Lex Fridman (36:44.840)
We should not shy from using complicated math
Cumrun Vafa (36:47.560)
like Einstein did.
Lex Fridman (36:48.760)
Nobody, Einstein wouldn't say,
Cumrun Vafa (36:49.920)
I'm not gonna touch this math because it's too much,
Lex Fridman (36:52.040)
you know, tensors or, you know, curvature
Lex Fridman (36:54.680)
and I don't like the four dimensional space time
Lex Fridman (36:56.320)
because I can't see four dimension.
Cumrun Vafa (36:57.960)
He wasn't doing that.
Lex Fridman (36:59.080)
He was willing to abstract from that
Cumrun Vafa (37:01.480)
because physics drove him in that direction.
Lex Fridman (37:03.600)
But his motivation was physics.
Cumrun Vafa (37:05.400)
Physics pushed him.
Lex Fridman (37:06.520)
Just like Newton pushed to develop calculus
Cumrun Vafa (37:09.960)
because physics pushed him that he didn't have the tools.
Lex Fridman (37:12.480)
So he had to develop the tools
Cumrun Vafa (37:14.120)
to answer his physics questions.
Lex Fridman (37:16.000)
So his motivation was physics again.
Lex Fridman (37:18.720)
So to me, those are examples which show
Lex Fridman (37:20.760)
that math and physics have this symbiotic relationship
Cumrun Vafa (37:24.560)
which kind of reinforce each other.
Lex Fridman (37:26.880)
Here I'm using, I'm giving you examples of both of them,
Cumrun Vafa (37:30.080)
namely Newton's work led to development
Lex Fridman (37:32.520)
of mathematics, calculus.
Lex Fridman (37:34.520)
And in the case of Einstein, he didn't develop
Lex Fridman (37:36.720)
Riemannian geometry, he just used them.
Lex Fridman (37:38.760)
So it goes both ways and in the context of modern physics,
Lex Fridman (37:42.160)
we see that again and again, it goes both ways.
Cumrun Vafa (37:44.520)
Let me ask a ridiculous question.
Lex Fridman (37:46.920)
You know, you talk about your favorite soccer player,
Cumrun Vafa (37:48.880)
the bar, I'll ask the same question about Einstein's ideas
Lex Fridman (37:52.400)
which is, which one do you think
Lex Fridman (37:54.640)
is the biggest leap of genius?
Lex Fridman (37:56.520)
Is it the E equals MC squared?
Lex Fridman (37:59.960)
Is it Brownian motion?
Lex Fridman (38:01.560)
Is it special relativity, is it general relativity?
Cumrun Vafa (38:05.200)
Which of the famous set of papers he's written in 1905
Lex Fridman (38:09.760)
and in general, his work was the biggest leap of genius?
Cumrun Vafa (38:13.760)
In my opinion, it's special relativity.
Lex Fridman (38:16.280)
The idea that speed of light is the same for everybody
Cumrun Vafa (38:19.120)
is the beginning of everything he did.
Lex Fridman (38:20.520)
The beginning is the seed.
Cumrun Vafa (38:21.360)
The beginning.
Lex Fridman (38:22.200)
Once you embrace that weirdness,
Cumrun Vafa (38:24.400)
all the weirdness, all the rest.
Lex Fridman (38:25.640)
I would say that's, even though he says
Cumrun Vafa (38:27.800)
the most beautiful moment for him,
Lex Fridman (38:29.720)
he says that is when he realized that if you fall
Cumrun Vafa (38:31.840)
in an elevator, you don't know if you're falling
Lex Fridman (38:33.720)
or whether you're in the falling elevator
Cumrun Vafa (38:36.440)
or whether you're next to the earth, gravitational.
Lex Fridman (38:39.120)
That to him was his aha moment,
Cumrun Vafa (38:41.800)
which inertial mass and gravitational mass
Lex Fridman (38:43.640)
being identical geometrically and so forth
Cumrun Vafa (38:46.480)
as part of the theory, not because of, you know,
Lex Fridman (38:49.520)
some funny coincidence.
Cumrun Vafa (38:52.280)
That's for him, but I feel from outside at least,
Lex Fridman (38:54.400)
it feels like the speed of light being the same
Cumrun Vafa (38:56.880)
is the really aha moment.
Lex Fridman (38:59.200)
The general relativity to you is not
Cumrun Vafa (39:02.160)
like the conception of space time.
Lex Fridman (39:04.960)
In a sense, the conception of space time
Cumrun Vafa (39:06.600)
already was part of the special relativity
Lex Fridman (39:08.600)
when you talk about length contraction.
Lex Fridman (39:10.920)
So general relativity takes that to the next step,
Lex Fridman (39:13.040)
but beginning of it was already space,
Cumrun Vafa (39:15.560)
length contracts, time dilates.
Lex Fridman (39:17.560)
So once you talk about those, then yeah,
Cumrun Vafa (39:19.080)
you can dilate more or less different places
Lex Fridman (39:20.760)
than its curvature.
Lex Fridman (39:21.960)
So you don't have a choice.
Lex Fridman (39:22.920)
So it kind of started just with that same simple thought.
Cumrun Vafa (39:26.480)
Speed of light is the same for all.
Lex Fridman (39:28.680)
Where does quantum mechanics come into view?
Cumrun Vafa (39:32.000)
Exactly, so this is the next step.
Lex Fridman (39:33.520)
So Einstein's, you know, developed general relativity
Lex Fridman (39:36.800)
and he's beginning to develop the foundation
Lex Fridman (39:38.440)
of quantum mechanics at the same time,
Cumrun Vafa (39:39.920)
the photoelectric effects and others.
Lex Fridman (39:42.240)
And so quantum mechanics overtakes, in fact,
Cumrun Vafa (39:45.680)
Einstein in many ways because he doesn't like
Lex Fridman (39:47.560)
the probabilistic interpretation of quantum mechanics
Lex Fridman (39:50.320)
and the formulas that's emerging,
Lex Fridman (39:52.320)
but fits his march on and try to, for example,
Cumrun Vafa (39:56.280)
combine Einstein's theory of relativity
Lex Fridman (39:59.840)
with quantum mechanics.
Lex Fridman (40:01.040)
So Dirac takes special relativity,
Lex Fridman (40:04.040)
tries to see how is it compatible with quantum mechanics.
Lex Fridman (40:07.800)
Can we pause and briefly say what is quantum mechanics?
Lex Fridman (40:10.520)
Oh yes, sure.
Lex Fridman (40:11.360)
So quantum mechanics, so I discussed briefly
Lex Fridman (40:14.760)
when I talked about the connection
Cumrun Vafa (40:16.440)
between Newtonian mechanics
Lex Fridman (40:18.520)
and the Euler Lagrange reformulation
Cumrun Vafa (40:20.760)
of the Newtonian mechanics and interpretation
Lex Fridman (40:23.120)
of this Euler Lagrange formulas in terms of the paths
Cumrun Vafa (40:27.240)
that the particle take.
Lex Fridman (40:28.560)
So when we say a particle goes from here to here,
Cumrun Vafa (40:31.440)
we usually think it classically follows
Lex Fridman (40:34.480)
a specific trajectory, but actually in quantum mechanics,
Cumrun Vafa (40:38.520)
it follows every trajectory with different probabilities.
Lex Fridman (40:42.960)
And so there's this fuzziness.
Cumrun Vafa (40:44.920)
Now, most probable, it's the path that you actually see
Lex Fridman (40:49.280)
and deviation from that is very, very unlikely
Lex Fridman (40:51.840)
and probabilistically very minuscule.
Lex Fridman (40:53.800)
So in everyday experiments,
Cumrun Vafa (40:55.040)
we don't see anything deviated from what we expect,
Lex Fridman (40:58.000)
but quantum mechanics tells us that the things
Cumrun Vafa (41:00.760)
are more fuzzy.
Lex Fridman (41:01.600)
Things are not as precise as the line you draw.
Cumrun Vafa (41:05.760)
Things are a bit like cloud.
Lex Fridman (41:07.760)
So if you go to microscopic scales,
Cumrun Vafa (41:11.160)
like atomic scales and lower,
Lex Fridman (41:12.560)
these phenomena become more pronounced.
Cumrun Vafa (41:14.800)
You can see it much better.
Lex Fridman (41:16.280)
The electron is not at the point,
Lex Fridman (41:18.360)
but the cloud spread out around the nucleus.
Lex Fridman (41:21.320)
And so this fuzziness, this probabilistic aspect of reality
Cumrun Vafa (41:25.280)
is what quantum mechanics describes.
Lex Fridman (41:28.280)
Can I briefly pause on that idea?
Lex Fridman (41:31.600)
Do you think quantum mechanics
Lex Fridman (41:33.760)
is just a really damn good approximation,
Cumrun Vafa (41:37.040)
a tool for predicting reality,
Lex Fridman (41:40.200)
or does it actually describe reality?
Lex Fridman (41:43.120)
Do you think reality is fuzzy at that level?
Lex Fridman (41:45.800)
Well, I think that reality is fuzzy at that level,
Lex Fridman (41:48.440)
but I don't think quantum mechanics
Lex Fridman (41:49.840)
is necessarily the end of the story.
Lex Fridman (41:51.800)
So quantum mechanics is certainly an improvement
Lex Fridman (41:55.760)
over classical physics.
Cumrun Vafa (41:57.400)
That much we know by experiments and so forth.
Lex Fridman (42:00.200)
Whether I'm happy with quantum mechanics,
Cumrun Vafa (42:02.200)
whether I view quantum mechanics,
Lex Fridman (42:04.000)
for example, the thought,
Cumrun Vafa (42:05.400)
the measurement description of quantum mechanics,
Lex Fridman (42:08.960)
am I happy with it?
Lex Fridman (42:09.960)
Am I thinking that's the end stage or not?
Lex Fridman (42:11.760)
I don't.
Cumrun Vafa (42:12.720)
I don't think we're at the end of that story.
Lex Fridman (42:14.320)
And many physicists may or may not view this way.
Cumrun Vafa (42:17.440)
Some do, some don't.
Lex Fridman (42:18.960)
But I think that it's the best we have right now,
Cumrun Vafa (42:22.000)
that's for sure.
Lex Fridman (42:23.080)
It's the best approximation for reality we know today.
Lex Fridman (42:25.400)
And so far, we don't know what it is,
Lex Fridman (42:27.320)
the next thing that improves it or replaces it and so on.
Lex Fridman (42:30.520)
But as I mentioned before,
Lex Fridman (42:31.560)
I don't believe any of the laws of physics we know today
Cumrun Vafa (42:34.760)
are permanently exactly correct.
Lex Fridman (42:36.720)
That doesn't bother me.
Cumrun Vafa (42:38.080)
I'm not like dogmatic saying,
Lex Fridman (42:39.600)
I have figured out this is the law of nature.
Cumrun Vafa (42:41.960)
I know everything.
Lex Fridman (42:42.800)
No, no, that's the beauty about science
Cumrun Vafa (42:45.840)
is that we are not dogmatic.
Lex Fridman (42:47.600)
And we are willing to, in fact,
Cumrun Vafa (42:49.560)
we are encouraged to be skeptical of what we ourselves do.
Lex Fridman (42:53.960)
So you were talking about Dirac.
Cumrun Vafa (42:55.440)
Yes, I was talking about Dirac, right.
Lex Fridman (42:56.640)
So Dirac was trying to now combine
Cumrun Vafa (42:58.920)
this Schrodinger's equations,
Lex Fridman (43:01.440)
which was described in the context of trying to talk about
Lex Fridman (43:04.600)
how these probabilistic waves of electrons
Lex Fridman (43:06.680)
move for the atom,
Cumrun Vafa (43:07.840)
which was good for speeds
Lex Fridman (43:09.880)
which were not too close to the speed of light,
Cumrun Vafa (43:11.800)
to what happens when you get to the near the speed of light.
Lex Fridman (43:14.880)
So then you need relativity.
Lex Fridman (43:16.480)
So then Dirac tried to combine Einstein's relativity
Lex Fridman (43:19.440)
with quantum mechanics.
Lex Fridman (43:20.680)
So he tried to combine them
Lex Fridman (43:22.480)
and he wrote this beautiful equation, the Dirac equation,
Cumrun Vafa (43:26.880)
which roughly speaking,
Lex Fridman (43:28.560)
take the square root of the Einstein's equation
Cumrun Vafa (43:31.520)
in order to connect it to Schrodinger's
Lex Fridman (43:33.120)
time evolution operator,
Cumrun Vafa (43:34.280)
which is first order in time derivative
Lex Fridman (43:37.200)
to get rid of the naive thing
Cumrun Vafa (43:39.160)
that Einstein's equation would have given,
Lex Fridman (43:40.600)
which is second order.
Lex Fridman (43:41.440)
So you have to take a square root.
Lex Fridman (43:43.360)
Now square root usually has a plus or minus sign
Cumrun Vafa (43:45.600)
when you take it.
Lex Fridman (43:47.520)
And when he did this,
Cumrun Vafa (43:49.120)
he originally didn't notice this plus,
Lex Fridman (43:50.800)
didn't pay attention to this plus or minus sign,
Lex Fridman (43:52.600)
but later physicists pointed out to Dirac says,
Lex Fridman (43:55.120)
look, there's also this minus sign.
Lex Fridman (43:57.280)
And if you use this minus sign,
Lex Fridman (43:58.440)
you get negative energy.
Cumrun Vafa (44:01.120)
In fact, it was very, very annoying that, you know,
Lex Fridman (44:04.640)
somebody else tells you this obvious mistake you make.
Cumrun Vafa (44:06.680)
Pauli famous physicist told Dirac, this is nonsense.
Lex Fridman (44:09.840)
You're going to get negative energy with your equation,
Cumrun Vafa (44:11.640)
which negative energy without any bottom,
Lex Fridman (44:13.480)
you can go all the way down to negative.
Cumrun Vafa (44:15.520)
Infinite energy, so it doesn't make any sense.
Lex Fridman (44:18.080)
Dirac thought about it.
Lex Fridman (44:19.120)
And then he remembered Pauli's exclusion principle
Lex Fridman (44:22.200)
just before him.
Cumrun Vafa (44:23.040)
Pauli had said, you know,
Lex Fridman (44:24.240)
there's this principle called the exclusion principle
Cumrun Vafa (44:26.640)
that, you know, two electrons cannot be on the same orbit.
Lex Fridman (44:30.240)
And so Dirac said, okay, you know what?
Cumrun Vafa (44:32.680)
All these negative energy states are filled orbits,
Lex Fridman (44:37.680)
occupied.
Lex Fridman (44:38.920)
So according to you,
Lex Fridman (44:42.200)
Mr. Pauli, there's no place to go.
Lex Fridman (44:44.880)
So therefore they only have to go positive.
Lex Fridman (44:47.200)
Sounded like a big cheat.
Lex Fridman (44:49.160)
And then Pauli said, oh, you know what?
Lex Fridman (44:51.960)
We can change orbits from one orbit to another.
Lex Fridman (44:53.880)
What if I take one of these negative energy orbits
Lex Fridman (44:55.800)
and put it up there?
Cumrun Vafa (44:57.560)
Then it seems to be a new particle,
Lex Fridman (44:59.720)
which has opposite properties to the electron.
Cumrun Vafa (45:03.200)
It has positive energy, but it has positive charge.
Lex Fridman (45:06.000)
What is that?
Cumrun Vafa (45:09.160)
Dirac was a bit worried.
Lex Fridman (45:10.400)
He said, maybe that's proton
Cumrun Vafa (45:11.560)
because proton has plus charge.
Lex Fridman (45:13.480)
He wasn't sure.
Lex Fridman (45:14.720)
But then he said, oh, maybe it's proton.
Lex Fridman (45:16.480)
But then they said, no, no, no, no.
Cumrun Vafa (45:17.760)
It has the same mass as the electron.
Lex Fridman (45:19.360)
It cannot be proton because proton is heavier.
Cumrun Vafa (45:22.240)
Dirac was stuck.
Lex Fridman (45:23.080)
He says, well, then maybe another part we haven't seen.
Cumrun Vafa (45:27.600)
By that time, Dirac himself was getting a little bit worried
Lex Fridman (45:31.080)
about his own equation and his own crazy interpretation.
Cumrun Vafa (45:34.480)
Until a few years later, Anderson,
Lex Fridman (45:37.080)
in the photographic place that he had gotten
Cumrun Vafa (45:40.640)
from these cosmic rays,
Lex Fridman (45:42.000)
he discovered a particle which goes
Cumrun Vafa (45:45.920)
in the opposite direction that the electron goes
Lex Fridman (45:47.920)
when there's a magnetic field,
Lex Fridman (45:49.880)
and with the same mass,
Lex Fridman (45:52.320)
exactly like what Dirac had predicted.
Lex Fridman (45:55.160)
And this was what we call now positron.
Lex Fridman (45:57.640)
And in fact, beginning with the work of Dirac,
Cumrun Vafa (46:00.280)
we know that every particle has an antiparticle.
Lex Fridman (46:03.200)
And so this idea that there's an antiparticle
Cumrun Vafa (46:05.480)
came from this simple math.
Lex Fridman (46:06.840)
There's a plus and a minus
Cumrun Vafa (46:08.800)
from the Dirac's quote unquote mistake.
Lex Fridman (46:12.800)
So again, trying to combine ideas,
Cumrun Vafa (46:15.400)
sometimes the math is smarter than the person
Lex Fridman (46:18.080)
who uses it to apply it,
Lex Fridman (46:20.080)
and you try to resist it,
Lex Fridman (46:21.200)
and then you kind of confront it by criticism,
Cumrun Vafa (46:23.720)
which is the way it should be.
Lex Fridman (46:25.080)
So physicists comes and said, no, no, that's wrong,
Lex Fridman (46:26.920)
and you correct it, and so on.
Lex Fridman (46:27.840)
So that is a development of the idea
Cumrun Vafa (46:30.800)
there's particle, there's antiparticle, and so on.
Lex Fridman (46:32.800)
So this is the beginning of development
Cumrun Vafa (46:34.800)
of quantum mechanics and the connection with relativity,
Lex Fridman (46:37.440)
but the thing was more challenging
Cumrun Vafa (46:38.880)
because we had to also describe
Lex Fridman (46:40.760)
how electric and magnetic fields work with quantum mechanics.
Cumrun Vafa (46:44.760)
This was much more complicated
Lex Fridman (46:46.200)
because it's not just one point.
Cumrun Vafa (46:47.760)
Electric and magnetic fields were everywhere.
Lex Fridman (46:50.360)
So you had to talk about fluctuating
Lex Fridman (46:52.440)
and a fuzziness of electrical fields
Lex Fridman (46:54.200)
and magnetic fields everywhere.
Lex Fridman (46:56.320)
And the math for that was very difficult to deal with.
Lex Fridman (47:00.680)
And this led to a subject called quantum field theory.
Cumrun Vafa (47:03.560)
Fields like electric and magnetic fields had to be quantum,
Lex Fridman (47:06.680)
had to be described also in a wavy way.
Cumrun Vafa (47:09.080)
Feynman in particular was one of the pioneers
Lex Fridman (47:13.280)
along with Schrodingers and others
Cumrun Vafa (47:15.120)
to try to come up with a formalism
Lex Fridman (47:17.000)
to deal with fields like electric and magnetic fields,
Cumrun Vafa (47:20.800)
interacting with electrons in a consistent quantum fashion.
Lex Fridman (47:24.200)
And they developed this beautiful theory,
Cumrun Vafa (47:25.960)
quantum electrodynamics from that.
Lex Fridman (47:27.600)
And later on that same formalism,
Cumrun Vafa (47:30.040)
quantum field theory led to the discovery of other forces
Lex Fridman (47:33.640)
and other particles all consistent
Cumrun Vafa (47:35.520)
with the idea of quantum mechanics.
Lex Fridman (47:37.840)
So that was how physics progressed.
Lex Fridman (47:40.760)
And so basically we learned that all particles
Lex Fridman (47:43.600)
and all the forces are in some sense related
Cumrun Vafa (47:47.440)
to particle exchanges.
Lex Fridman (47:49.640)
And so for example, electromagnetic forces
Cumrun Vafa (47:52.320)
are mediated by a particle we call photon and so forth.
Lex Fridman (47:57.440)
And same for other forces that they discovered,
Cumrun Vafa (47:59.680)
strong forces and the weak forces.
Lex Fridman (48:01.040)
So we got the sense of what quantum field theory is.
Cumrun Vafa (48:03.880)
Is that a big leap of an idea that particles
Lex Fridman (48:09.280)
are fluctuations in the field?
Cumrun Vafa (48:12.200)
Like the idea that everything is a field.
Lex Fridman (48:15.040)
It's the old Einstein, light is a wave,
Cumrun Vafa (48:18.240)
both a particle and a wave kind of idea.
Lex Fridman (48:20.240)
Is that a huge leap in our understanding
Lex Fridman (48:23.920)
of conceiving the universe as fields?
Lex Fridman (48:26.360)
I would say so.
Cumrun Vafa (48:27.200)
I would say that viewing the particles,
Lex Fridman (48:29.720)
this duality that Bohr mentioned
Cumrun Vafa (48:31.800)
between particles and waves,
Lex Fridman (48:33.040)
that waves can behave sometimes like particles,
Cumrun Vafa (48:35.080)
sometimes like waves,
Lex Fridman (48:36.320)
is one of the biggest leaps of imagination
Cumrun Vafa (48:40.240)
that quantum mechanics made physics do.
Lex Fridman (48:42.800)
So I agree that that is quite remarkable.
Cumrun Vafa (48:45.480)
Is duality fundamental to the universe
Lex Fridman (48:50.240)
or is it just because we don't understand it fully?
Cumrun Vafa (48:52.200)
Like will it eventually collapse
Lex Fridman (48:54.320)
into a clean explanation that doesn't require duality?
Cumrun Vafa (48:57.800)
Like that a phenomena could be two things at once
Lex Fridman (49:02.440)
and both to be true.
Lex Fridman (49:04.400)
So that seems weird.
Lex Fridman (49:05.960)
So in fact I was going to get to that
Cumrun Vafa (49:08.320)
when we get to string theory
Lex Fridman (49:09.360)
but maybe I can comment on that now.
Cumrun Vafa (49:11.000)
Duality turns out to be running the show today
Lex Fridman (49:13.520)
and the whole thing that we are doing is string theory.
Cumrun Vafa (49:15.600)
Duality is the name of the game.
Lex Fridman (49:17.840)
So it's the most beautiful subject
Lex Fridman (49:19.440)
and I want to talk about it.
Lex Fridman (49:20.840)
Let's talk about it in the context of string theory then.
Lex Fridman (49:23.440)
So we do want to take a next step into,
Lex Fridman (49:27.200)
because we mentioned general relativity,
Cumrun Vafa (49:28.680)
we mentioned quantum mechanics,
Lex Fridman (49:30.480)
is there something to be said about quantum gravity?
Cumrun Vafa (49:32.640)
Yes, that's exactly the right point to talk about.
Lex Fridman (49:34.960)
So namely we have talked about quantum fields
Lex Fridman (49:37.480)
and I talked about electric forces,
Lex Fridman (49:39.760)
photon being the particle carrying those forces.
Lex Fridman (49:42.720)
So for gravity, quantizing gravitational field
Lex Fridman (49:46.600)
which is this curvature of space time according to Einstein,
Cumrun Vafa (49:49.600)
you get another particle called graviton.
Lex Fridman (49:52.520)
So what about gravitons?
Cumrun Vafa (49:55.000)
Should be there, no problem.
Lex Fridman (49:56.440)
So then you start computing it.
Lex Fridman (49:59.160)
What do I mean by computing it?
Lex Fridman (50:00.440)
Well, you compute scattering of one graviton
Cumrun Vafa (50:03.400)
off another graviton, maybe with graviton with an electron
Lex Fridman (50:06.080)
and so on, see what you get.
Cumrun Vafa (50:07.920)
Feynman had already mastered this quantum electrodynamics.
Lex Fridman (50:12.480)
He said, no problem, let me do it.
Cumrun Vafa (50:14.400)
Even though these are such weak forces,
Lex Fridman (50:17.200)
the gravity is very weak.
Lex Fridman (50:18.360)
So therefore to see them,
Lex Fridman (50:19.680)
these quantum effects of gravitational waves was impossible.
Cumrun Vafa (50:23.240)
It's even impossible today.
Lex Fridman (50:25.480)
So Feynman just did it for fun.
Cumrun Vafa (50:27.680)
He usually had this mindset that I want to do something
Lex Fridman (50:30.080)
which I will see in experiment,
Lex Fridman (50:31.160)
but this one, let's just see what it does.
Lex Fridman (50:34.360)
And he was surprised because the same techniques
Cumrun Vafa (50:36.680)
he was using for doing the same calculations,
Lex Fridman (50:39.960)
quantum electrodynamics, when applied to gravity failed.
Cumrun Vafa (50:44.720)
The formulas seem to make sense,
Lex Fridman (50:46.280)
but he had to do some integrals
Lex Fridman (50:47.560)
and he found that when he does those integrals,
Lex Fridman (50:49.040)
he got infinity and it didn't make any sense.
Cumrun Vafa (50:52.200)
Now there were similar infinities in the other pieces
Lex Fridman (50:54.520)
but he had managed to make sense out of those before.
Cumrun Vafa (50:56.920)
This was no way he could make sense out of it.
Lex Fridman (50:59.920)
He just didn't know what to do.
Cumrun Vafa (51:01.920)
He didn't feel it's an urgent issue
Lex Fridman (51:03.560)
because nobody could do the experiment.
Lex Fridman (51:05.800)
So he was kind of said, okay, there's this thing,
Lex Fridman (51:07.840)
but okay, we don't know how to exactly do it,
Lex Fridman (51:09.440)
but that's the way it is.
Lex Fridman (51:11.680)
So in some sense, a natural conclusion
Cumrun Vafa (51:14.080)
from what Feynman did could have been like,
Lex Fridman (51:16.640)
gravity cannot be consistent with quantum theory,
Lex Fridman (51:19.440)
but that cannot be the case
Lex Fridman (51:20.600)
because gravity is in our universe,
Cumrun Vafa (51:22.000)
quantum mechanics in our universe,
Lex Fridman (51:23.040)
they both together somehow should work.
Lex Fridman (51:25.560)
So it's not acceptable to say they don't work together.
Lex Fridman (51:28.960)
So that was a puzzle.
Lex Fridman (51:30.800)
How does it possibly work?
Lex Fridman (51:32.400)
It was left open.
Lex Fridman (51:34.660)
And then we get to the string theory.
Lex Fridman (51:37.120)
So this is the puzzle of quantum gravity.
Cumrun Vafa (51:38.880)
The particle description of quantum gravity failed.
Lex Fridman (51:41.420)
So the infinity shows up.
Lex Fridman (51:43.160)
What do we do with infinity?
Lex Fridman (51:45.840)
Let's get to the fun part.
Cumrun Vafa (51:47.120)
Let's talk about string theory.
Lex Fridman (51:48.760)
Yes.
Cumrun Vafa (51:50.840)
Let's discuss some technical basics of string theory.
Lex Fridman (51:56.300)
What is string theory?
Lex Fridman (51:57.740)
What is a string?
Lex Fridman (51:59.140)
How many dimensions are we talking about?
Lex Fridman (52:01.080)
What are the different states?
Lex Fridman (52:02.720)
How do we represent the elementary particles
Lex Fridman (52:04.960)
and the laws of physics using this new framework?
Lex Fridman (52:09.780)
So string theory is the idea
Cumrun Vafa (52:12.840)
that the fundamental entities are not particles,
Lex Fridman (52:16.280)
but extended higher dimensional objects
Cumrun Vafa (52:18.920)
like one dimensional strings, like loops.
Lex Fridman (52:22.360)
These loops could be open like with two ends,
Cumrun Vafa (52:25.000)
like an interval or a circle without any ends.
Lex Fridman (52:29.720)
And they're vibrating and moving around in space.
Lex Fridman (52:32.700)
So how big they are?
Lex Fridman (52:34.880)
Well, you can of course stretch it and make it big,
Cumrun Vafa (52:37.600)
or you can just let it be whatever it wants.
Lex Fridman (52:39.600)
It can be as small as a point
Cumrun Vafa (52:41.320)
because the circle can shrink to a point
Lex Fridman (52:44.280)
and be very light,
Cumrun Vafa (52:45.520)
or you can stretch it and becomes very massive,
Lex Fridman (52:48.080)
or it could oscillate and become massive that way.
Lex Fridman (52:50.240)
So it depends on which kind of state you have.
Lex Fridman (52:52.320)
In fact, the string can have infinitely many modes,
Cumrun Vafa (52:55.000)
depending on which kind of oscillation it's doing.
Lex Fridman (52:56.920)
Like a guitar has different harmonics,
Cumrun Vafa (52:59.040)
string has different harmonics,
Lex Fridman (53:00.140)
but for the string, each harmonic is a particle.
Lex Fridman (53:03.000)
So each particle will give you,
Lex Fridman (53:04.440)
ah, this is a more massive harmonic, this is a less massive.
Lex Fridman (53:07.480)
So the lightest harmonic, so to speak, is no harmonics,
Lex Fridman (53:10.000)
which means like the string shrunk to a point,
Lex Fridman (53:12.820)
and then it becomes like a massless particles
Lex Fridman (53:15.280)
or light particles like photon and graviton and so forth.
Lex Fridman (53:19.600)
So when you look at tiny strings,
Lex Fridman (53:22.640)
which are shrunk to a point, the lightest ones,
Cumrun Vafa (53:25.500)
they look like the particles that we think,
Lex Fridman (53:27.640)
they're like particles.
Cumrun Vafa (53:28.560)
In other words, from far away, they look like a point.
Lex Fridman (53:31.100)
But of course, if you zoom in,
Cumrun Vafa (53:32.140)
there's this tiny little circle that's there
Lex Fridman (53:35.080)
that's shrunk to almost a point.
Cumrun Vafa (53:37.080)
Should we be imagining, this is to the visual intuition,
Lex Fridman (53:40.560)
should we be imagining literally strings
Lex Fridman (53:42.720)
that are potentially connected as a loop or not?
Lex Fridman (53:47.120)
We knew, and when somebody outside of physics
Cumrun Vafa (53:50.640)
is imagining a basic element of string theory,
Lex Fridman (53:53.920)
which is a string,
Lex Fridman (53:56.520)
should we literally be thinking about a string?
Lex Fridman (53:58.720)
Yes, you should literally think about string,
Lex Fridman (54:00.560)
but string with zero thickness.
Lex Fridman (54:02.600)
With zero thickness.
Lex Fridman (54:03.920)
So notice, it's a loop of energy, so to speak,
Lex Fridman (54:07.720)
if you can think of it that way.
Lex Fridman (54:08.780)
And so there's a tension like a regular string,
Lex Fridman (54:11.080)
if you pull it, there's, you know, you have to stretch it.
Lex Fridman (54:14.120)
But it's not like a thickness, like you're made of something,
Lex Fridman (54:16.280)
it's just energy.
Cumrun Vafa (54:17.800)
It's not made of atoms or something like that.
Lex Fridman (54:19.680)
But it is very, very tiny.
Cumrun Vafa (54:21.800)
Very tiny.
Lex Fridman (54:22.640)
Much smaller than elementary particles of physics.
Cumrun Vafa (54:25.680)
Much smaller.
Lex Fridman (54:26.600)
So we think if you let the string to be by itself,
Cumrun Vafa (54:29.760)
the lowest state, there'll be like fuzziness
Lex Fridman (54:32.240)
or a size of that tiny little circle,
Cumrun Vafa (54:33.840)
which is like a point,
Lex Fridman (54:35.320)
about, could be anything between,
Cumrun Vafa (54:37.340)
we don't know the exact size,
Lex Fridman (54:38.380)
but in different models have different sizes,
Lex Fridman (54:40.320)
but something of the order of 10 to the minus,
Lex Fridman (54:42.160)
let's say 30 centimeters.
Lex Fridman (54:43.620)
So 10 to the minus 30 centimeters,
Lex Fridman (54:46.680)
just to compare it with the size of the atom,
Cumrun Vafa (54:48.360)
which is 10 to the minus eight centimeters,
Lex Fridman (54:50.400)
is 22 orders of magnitude smaller.
Cumrun Vafa (54:53.320)
So.
Lex Fridman (54:54.160)
Unimaginably small, I would say.
Cumrun Vafa (54:56.140)
Very small.
Lex Fridman (54:56.980)
So we basically think from far away,
Cumrun Vafa (54:58.800)
string is like a point particle.
Lex Fridman (55:00.920)
And that's why a lot of the things that we learned
Cumrun Vafa (55:03.720)
about point particle physics
Lex Fridman (55:04.920)
carries over directly to strings.
Lex Fridman (55:07.020)
So therefore there's not much of a mystery
Lex Fridman (55:09.000)
why particle physics was successful,
Cumrun Vafa (55:10.940)
because a string is like a particle
Lex Fridman (55:12.440)
when it's not stretched.
Lex Fridman (55:14.560)
But it turns out having this size,
Lex Fridman (55:17.000)
being able to oscillate, get bigger,
Cumrun Vafa (55:20.240)
turned out to be resolving this puzzles
Lex Fridman (55:22.460)
that Feynman was having in calculating his diagrams,
Lex Fridman (55:26.700)
and it gets rid of those infinities.
Lex Fridman (55:28.640)
So when you're trying to do those infinities,
Cumrun Vafa (55:31.360)
the regions that give infinities to Feynman,
Lex Fridman (55:34.400)
as soon as you get to those regions,
Cumrun Vafa (55:35.780)
then this string starts to oscillate,
Lex Fridman (55:38.080)
and these oscillation structure of the strings
Cumrun Vafa (55:40.560)
resolves those infinities to finite answer at the end.
Lex Fridman (55:43.040)
So the size of the string,
Cumrun Vafa (55:45.040)
the fact that it's one dimensional,
Lex Fridman (55:46.820)
gives a finite answer at the end.
Cumrun Vafa (55:48.720)
Resolves this paradox.
Lex Fridman (55:50.760)
Now, perhaps it's also useful to recount
Cumrun Vafa (55:54.160)
of how string theory came to be.
Lex Fridman (55:56.320)
Because it wasn't like somebody say,
Cumrun Vafa (55:58.360)
well, let me solve the problem of Einstein's,
Lex Fridman (56:01.920)
solve the problem that Feynman had with unifying
Cumrun Vafa (56:04.480)
Einstein's theory with quantum mechanics
Lex Fridman (56:06.560)
by replacing the point by a string.
Cumrun Vafa (56:08.120)
No, that's not the way the thought process,
Lex Fridman (56:10.200)
the thought process was much more random.
Cumrun Vafa (56:14.220)
Physicist, then it's John on this case,
Lex Fridman (56:16.020)
was trying to describe the interactions
Cumrun Vafa (56:17.960)
they were seeing in colliders, in accelerators.
Lex Fridman (56:22.160)
And they were seeing that some process,
Cumrun Vafa (56:23.600)
in some process, when two particles came together
Lex Fridman (56:26.520)
and joined together and when they were separately,
Cumrun Vafa (56:29.800)
in one way, and the opposite way, they behave the same way.
Lex Fridman (56:34.060)
In some way, there was a symmetry, a duality,
Cumrun Vafa (56:37.680)
which he didn't understand.
Lex Fridman (56:38.960)
The particles didn't seem to have that symmetry.
Cumrun Vafa (56:41.960)
He said, I don't know what it is,
Lex Fridman (56:43.060)
what's the reason that these colliders
Lex Fridman (56:44.760)
and experiments we're doing seems to have the symmetry,
Lex Fridman (56:46.780)
but let me write the mathematical formula,
Cumrun Vafa (56:49.840)
which exhibits that symmetry.
Lex Fridman (56:51.680)
He used gamma functions, beta functions and all that,
Cumrun Vafa (56:54.080)
you know, complete math, no physics,
Lex Fridman (56:56.580)
other than trying to get symmetry out of his equation.
Cumrun Vafa (56:59.320)
He just wrote down a formula as the answer for a process,
Lex Fridman (57:03.360)
not a method to compute it.
Lex Fridman (57:04.840)
Just say, wouldn't it be nice if this was the answer?
Lex Fridman (57:08.120)
Yes.
Cumrun Vafa (57:08.960)
Physics looked at this one, that's intriguing,
Lex Fridman (57:11.200)
it has the symmetry all right, but what is this?
Lex Fridman (57:13.480)
Where is this coming from?
Lex Fridman (57:14.800)
Which kind of physics gives you this?
Lex Fridman (57:17.400)
So I don't know.
Lex Fridman (57:19.660)
A few years later, people saw that,
Cumrun Vafa (57:21.760)
oh, the equation that you're writing,
Lex Fridman (57:23.520)
the process you're writing in the intermediate channels
Cumrun Vafa (57:26.400)
that particles come together,
Lex Fridman (57:27.800)
seems to have all the harmonics.
Cumrun Vafa (57:30.100)
Harmonics sounds like a string.
Lex Fridman (57:32.320)
Let me see if what you're describing
Cumrun Vafa (57:33.680)
has anything to do with the strings.
Lex Fridman (57:34.680)
And people try to see if what he's doing
Cumrun Vafa (57:36.400)
has anything to do with the strings.
Lex Fridman (57:37.400)
Oh, yeah, indeed.
Cumrun Vafa (57:38.960)
If I study scattering of two strings,
Lex Fridman (57:40.880)
I get exactly the formula you wrote down.
Cumrun Vafa (57:42.840)
That was the reinterpretation
Lex Fridman (57:45.280)
of what he had written in the formula as the strings,
Lex Fridman (57:48.500)
but still had nothing to do with gravity.
Lex Fridman (57:51.120)
It had nothing to do with resolving the problems
Cumrun Vafa (57:53.400)
of gravity with quantum mechanics.
Lex Fridman (57:55.200)
It was just trying to explain a process
Cumrun Vafa (57:57.600)
that people were seeing in hydronic physics collisions.
Lex Fridman (58:01.160)
So it took a few more years to get to that point.
Cumrun Vafa (58:04.420)
They did notice that,
Lex Fridman (58:07.460)
physicists noticed that whenever you try to find
Cumrun Vafa (58:10.340)
the spectrum of strings, you always get a massless particle
Lex Fridman (58:13.420)
which has exactly the properties
Cumrun Vafa (58:14.860)
that the graviton is supposed to have.
Lex Fridman (58:16.940)
And no particle in hydronic physics that had that property.
Cumrun Vafa (58:20.180)
You are getting a massless graviton
Lex Fridman (58:22.740)
as part of this scattering without looking for it.
Cumrun Vafa (58:25.720)
It was forced on you.
Lex Fridman (58:27.520)
People were not trying to solve quantum gravity.
Cumrun Vafa (58:29.760)
Quantum gravity was pushed on them.
Lex Fridman (58:31.860)
I don't want this graviton.
Cumrun Vafa (58:33.360)
Get rid of it.
Lex Fridman (58:34.200)
They couldn't get rid of it.
Cumrun Vafa (58:36.100)
They gave up trying to get rid of it.
Lex Fridman (58:38.380)
Physicists, Sherk and Schwartz said,
Cumrun Vafa (58:39.860)
you know what, string theory is theory of quantum gravity.
Lex Fridman (58:43.340)
They've changed their perspective altogether.
Cumrun Vafa (58:45.700)
We are not describing the hydronic physics.
Lex Fridman (58:47.620)
We are describing this theory of quantum gravity.
Lex Fridman (58:49.820)
And that's when string theory probably got like exciting
Lex Fridman (58:54.140)
that this could be the unifying theory.
Cumrun Vafa (58:56.380)
Exactly, it got exciting,
Lex Fridman (58:57.820)
but at the same time, not so fast.
Cumrun Vafa (58:59.520)
Namely, it should have been fast, but it wasn't
Lex Fridman (59:02.860)
because particle physics through quantum field theory
Cumrun Vafa (59:05.100)
were so successful at that time.
Lex Fridman (59:07.100)
This is mid seventies, standard model of physics,
Cumrun Vafa (59:10.060)
electromagnetism and unification of electromagnetic forces
Lex Fridman (59:12.860)
with all the other forces were beginning to take place
Cumrun Vafa (59:15.100)
without the gravity part.
Lex Fridman (59:17.380)
Everything was working beautifully for particle physics.
Lex Fridman (59:20.900)
And so that was the shining golden age
Lex Fridman (59:23.160)
of quantum field theory and all the experiments,
Cumrun Vafa (59:25.260)
standard model, this and that, unification,
Lex Fridman (59:27.980)
spontaneous symmetry breaking was taking place.
Cumrun Vafa (59:29.820)
All of them was nice.
Lex Fridman (59:31.020)
This was kind of like a side show
Lex Fridman (59:32.340)
and nobody was paying so much attention.
Lex Fridman (59:34.380)
This exotic string is needed for quantum gravity.
Cumrun Vafa (59:37.300)
Maybe there's other ways, maybe we should do something else.
Lex Fridman (59:39.520)
So, yeah, it wasn't paid much attention to.
Lex Fridman (59:41.940)
And this took a little bit more effort
Lex Fridman (59:44.060)
to try to actually connect it to reality.
Cumrun Vafa (59:48.060)
There are a few more steps.
Lex Fridman (59:49.100)
First of all, there was a puzzle
Cumrun Vafa (59:51.280)
that you were getting extra dimensions.
Lex Fridman (59:53.940)
String was not working well
Cumrun Vafa (59:55.600)
with three spatial dimension on one time.
Lex Fridman (59:57.820)
It needed extra dimension.
Cumrun Vafa (59:59.140)
Now, there are different versions of strings,
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