Cumrun Vafa: String Theory | Lex Fridman Podcast #204

00:00:00.000 | The following is a conversation with Kamran Vafa,

00:00:02.720 | a theoretical physicist at Harvard

00:00:04.760 | specializing in string theory.

00:00:07.200 | He is the winner of the 2017 Breakthrough Prize

00:00:10.960 | in Fundamental Physics,

00:00:12.320 | which is the most lucrative academic prize in the world.

00:00:15.760 | Quick mention of our sponsors, Headspace,

00:00:18.440 | Jordan Harberger Show, Squarespace, and Allform.

00:00:22.880 | Check them out in the description to support this podcast.

00:00:26.000 | As a side note, let me say that string theory

00:00:28.040 | is a theory of quantum gravity

00:00:29.960 | that unifies quantum mechanics and general relativity.

00:00:33.280 | It says that quarks, electrons, and all other particles

00:00:36.680 | are made up of much tinier strings of vibrating energy.

00:00:40.320 | They vibrate in 10 or more dimensions,

00:00:42.920 | depending on the flavor of the theory.

00:00:45.040 | Different vibrating patterns result in different particles.

00:00:48.600 | From its origins, for a long time,

00:00:50.680 | string theory was seen as too good not to be true,

00:00:54.680 | but has recently fallen out of favor

00:00:56.360 | in the physics community,

00:00:57.720 | partly because over the past 40 years,

00:01:00.200 | it has not been able to make any novel predictions

00:01:03.120 | that could then be validated through experiment.

00:01:06.320 | Nevertheless, to this day,

00:01:08.280 | it remains one of our best candidates

00:01:10.440 | for a theory of everything,

00:01:12.160 | or a theory that unifies the laws of physics.

00:01:15.560 | Let me mention that a similar story happened

00:01:18.040 | with neural networks

00:01:19.000 | in the field of artificial intelligence,

00:01:21.120 | where it fell out of favor

00:01:22.600 | after decades of promise and research,

00:01:24.840 | but found success again in the past decade

00:01:28.000 | as part of the deep learning revolution.

00:01:30.360 | So I think it pays to keep an open mind,

00:01:33.140 | since we don't know which of the ideas in physics

00:01:36.040 | may be brought back decades later

00:01:38.080 | and be found to solve the biggest mysteries

00:01:40.640 | in theoretical physics.

00:01:42.360 | String theory still has that promise.

00:01:45.980 | This is the Lex Friedman Podcast,

00:01:47.760 | and here's my conversation with Kamran Vafa.

00:01:51.880 | What is the difference between mathematics and physics?

00:01:55.700 | - Well, that's a difficult question,

00:01:57.460 | because in many ways,

00:01:58.400 | math and physics are unified in many ways,

00:02:01.640 | so to distinguish them is not an easy task.

00:02:04.480 | I would say that perhaps the goals

00:02:06.840 | of math and physics are different.

00:02:08.540 | Math does not care to describe reality, physics does.

00:02:14.440 | That's the major difference,

00:02:16.200 | but a lot of the thoughts, processes, and so on,

00:02:19.360 | which goes to understanding the nature and reality

00:02:22.760 | are the same things that mathematicians do.

00:02:24.680 | So in many ways, they are similar.

00:02:26.360 | Mathematicians care about deductive reasoning,

00:02:31.660 | and physicists or physics in general,

00:02:35.480 | we care less about that.

00:02:36.760 | We care more about interconnection of ideas,

00:02:41.000 | about how ideas support each other,

00:02:42.880 | or if there's a puzzle, discord between ideas.

00:02:46.240 | That's more interesting for us.

00:02:48.200 | And part of the reason is that we have learned in physics

00:02:50.560 | that the ideas are not sequential,

00:02:53.320 | and if we think that there's one idea

00:02:54.920 | which is more important,

00:02:56.040 | and we start with there and go to the next idea,

00:02:58.200 | next one, and deduce things from that

00:02:59.760 | like mathematicians do,

00:03:01.600 | we have learned that the third or fourth thing

00:03:03.720 | we deduce from that principle

00:03:05.580 | turns out later on to be the actual principle,

00:03:08.380 | and from a different perspective,

00:03:10.900 | starting from there leads to new ideas,

00:03:12.740 | which the original one didn't lead to,

00:03:14.920 | and that's the beginning of a new revolution in science.

00:03:18.140 | So this kind of thing we have seen again and again

00:03:20.240 | in the history of science,

00:03:21.080 | we have learned to not like deductive reasoning,

00:03:24.120 | because that gives us a bad starting point

00:03:27.200 | to think that we actually have the original thought process

00:03:30.260 | should be viewed as the primary thought,

00:03:32.220 | and all these are deductions,

00:03:33.920 | like the way mathematicians sometimes do.

00:03:35.600 | So in physics, we have learned to be skeptical

00:03:38.000 | of that way of thinking.

00:03:38.920 | We have to be a bit open to the possibility

00:03:41.240 | that what we thought is a deduction of a hypothesis

00:03:44.280 | actually the reason that's true is the opposite,

00:03:47.080 | and so we reverse the order.

00:03:48.940 | And so this switching back and forth between ideas

00:03:52.640 | makes us more fluid about a deductive fashion.

00:03:56.640 | Of course, it sometimes gives a wrong impression

00:03:59.460 | like physicists don't care about rigor,

00:04:00.960 | they just say random things,

00:04:03.400 | they are willing to say things that are not backed

00:04:05.900 | by the logical reasoning, that's not true at all.

00:04:09.220 | So despite this fluidity

00:04:12.620 | in seeing which one is a primary thought,

00:04:15.000 | we are very careful about trying to understand

00:04:17.080 | what we have really understood

00:04:18.680 | in terms of relationship between ideas.

00:04:21.120 | So that's an important ingredient,

00:04:24.000 | and in fact, solid math being behind physics

00:04:27.340 | is I think one of the attractive features of a physical law.

00:04:32.200 | So we look for beautiful math underpinning it.

00:04:35.280 | - Can we dig into that process of starting from one place

00:04:39.760 | and then ending up at like the fourth step

00:04:43.040 | and realizing all along that the place

00:04:45.440 | you started at was wrong?

00:04:47.000 | So is that happen when there's a discrepancy

00:04:50.560 | between what the math says

00:04:53.120 | and what the physical world shows?

00:04:54.960 | Is that how you then can go back

00:04:56.840 | and do the revolutionary idea

00:04:59.680 | for different starting place altogether?

00:05:02.040 | - Perhaps I give an example to see how it goes,

00:05:04.800 | and in fact, the historical example

00:05:06.800 | is Newton's work on classical mechanics.

00:05:10.160 | So Newton formulated the laws of mechanics,

00:05:14.000 | you know, the force F equals to ma and his other laws,

00:05:17.840 | and they look very simple, elegant, and so forth.

00:05:20.960 | Later, when we studied more examples of mechanics

00:05:25.840 | and other similar things,

00:05:27.640 | physicists came up with the idea

00:05:28.960 | that the notion of potential is interesting.

00:05:31.400 | Potential was an abstract idea which kind of came,

00:05:33.800 | you could take its gradient and relate it to the force,

00:05:37.000 | so you don't really need a a priori,

00:05:38.720 | but it solved, helped some thoughts.

00:05:41.200 | And then later, Euler and Lagrange reformulated

00:05:45.960 | Newtonian mechanics in a totally different way

00:05:49.240 | in the following fashion.

00:05:50.360 | They said, if you wanna know where a particle

00:05:52.880 | at this point and at this time,

00:05:54.520 | how does it get to this point at the later time,

00:05:58.000 | is the following.

00:05:58.920 | You take all possible paths connecting this particle

00:06:01.640 | from going from the initial point to the final point,

00:06:04.680 | and you compute the action.

00:06:07.160 | And what is an action?

00:06:08.400 | Action is the integral over time

00:06:10.760 | of the kinetic term of the particle minus its potential.

00:06:14.160 | So you take this integral,

00:06:16.800 | and each path will give you some quantity.

00:06:19.080 | And the path it actually takes, the physical path,

00:06:23.060 | is the one which minimizes this integral or this action.

00:06:26.580 | Now, this sounded like a backwards step from Newton's.

00:06:29.880 | Newton's formula seemed very simple.

00:06:32.720 | F equals to ma, and you can write F

00:06:34.680 | is minus the gradient of the potential.

00:06:36.840 | So why would anybody start formulating such a simple thing

00:06:40.040 | in terms of this complicated looking principle?

00:06:43.240 | You have to study the space of all paths and all things

00:06:46.760 | and find the minimum, and then you get the same equation.

00:06:48.760 | So what's the point?

00:06:50.040 | So Euler and Lagrange's formulation of Newton,

00:06:52.440 | which was kind of recasting in this language,

00:06:56.440 | is just a consequence of Newton's law.

00:06:58.040 | F equals to ma gives you the same fact

00:06:59.800 | that this path is a minimum action.

00:07:02.200 | Now, what we learned later, last century,

00:07:05.440 | was that when we deal with quantum mechanics,

00:07:08.520 | Newton's law is only an average correct.

00:07:11.760 | And the particle going from one to the other

00:07:15.840 | doesn't take exactly one path.

00:07:17.880 | It takes all the paths with the amplitude,

00:07:21.800 | which is proportional to the exponential of the action

00:07:24.240 | times an imaginary number, i.

00:07:26.940 | And so this fact turned out to be the reformulation

00:07:29.960 | of quantum mechanics.

00:07:30.780 | We should start there as the basis of the new law,

00:07:33.520 | which is quantum mechanics.

00:07:34.880 | And Newton is only an approximation

00:07:36.560 | on the average correct.

00:07:38.000 | - When we say amplitude, you mean probability?

00:07:40.360 | - Yes, the amplitude means if you sum up all these paths

00:07:43.200 | with exponential i times the action,

00:07:45.100 | if you sum this up, you get the number, complex number.

00:07:48.400 | You square the norm of this complex number,

00:07:50.480 | gives you a probability to go from one to the other.

00:07:52.760 | - Is there ways in which mathematics can lead us astray

00:07:57.760 | when we use it as a tool to understand the physical world?

00:08:01.480 | - Yes, I would say that mathematics can lead us astray

00:08:04.580 | as much as old physical ideas can lead us astray.

00:08:08.240 | So if you get stuck in something,

00:08:11.680 | then you can easily fool yourself

00:08:13.280 | that just like the thought process,

00:08:15.520 | we have to free ourselves of that.

00:08:17.300 | Sometimes math does that role.

00:08:18.720 | Like say, oh, this is such a beautiful math.

00:08:20.560 | I definitely wanna use it somewhere.

00:08:22.040 | And so you just get carried away

00:08:23.840 | and you just get maybe carried too far away.

00:08:25.760 | So that is certainly true,

00:08:26.960 | but I wouldn't say it's more dangerous

00:08:29.040 | than old physical ideas.

00:08:30.640 | To me, new math ideas is as much potential

00:08:34.400 | to lead us astray as old physical ideas,

00:08:36.380 | which could be long-held principles of physics.

00:08:38.780 | So I'm just saying that we should keep an open mind

00:08:41.860 | about the role that math plays,

00:08:43.900 | not to be antagonistic towards it

00:08:46.020 | and not to over-welcoming it.

00:08:48.980 | We should just be open to possibilities.

00:08:51.060 | - What about looking at a particular characteristics

00:08:53.660 | of both physical ideas and mathematical ideas,

00:08:55.700 | which is beauty?

00:08:56.820 | You think beauty leads us astray,

00:08:58.780 | meaning, and you offline showed me a really nice puzzle

00:09:03.180 | that illustrates this idea a little bit.

00:09:06.400 | Now, maybe you can speak to that or another example

00:09:09.020 | where beauty makes it tempting for us to assume

00:09:13.520 | that the law and the theory we found

00:09:16.360 | is actually one that perfectly describes reality.

00:09:19.740 | - I think that beauty does not lead us astray

00:09:22.800 | because I feel that beauty is a requirement

00:09:25.920 | for principles of physics.

00:09:27.760 | - So beauty is a fundamental in the universe?

00:09:29.600 | - I think beauty is fundamental.

00:09:30.920 | At least that's the way many of us view it.

00:09:32.800 | - It's not emergent?

00:09:33.920 | - It's not emergent.

00:09:35.120 | I think Hardy is the mathematician who said

00:09:37.720 | that there's no permanent place for ugly mathematics.

00:09:40.760 | And so I think the same is true in physics,

00:09:42.900 | that if we find a principle which looks ugly,

00:09:46.800 | we are not going to be, that's not the end stage.

00:09:49.640 | So therefore, beauty is going to lead us somewhere.

00:09:52.000 | Now, it doesn't mean beauty is enough.

00:09:54.800 | It doesn't mean if you just have beauty,

00:09:56.440 | if I just look at something is beautiful, then I'm fine.

00:09:58.920 | No, that's not the case.

00:10:00.240 | Beauty is certainly a criteria

00:10:02.240 | that every good physical theory should pass.

00:10:04.480 | That's at least the view we have.

00:10:06.520 | Why do we have this view?

00:10:08.280 | That's a good question.

00:10:09.760 | It is partly, you could say,

00:10:12.000 | based on experience of science over centuries,

00:10:15.080 | partly is philosophical view of what reality is or should be.

00:10:20.080 | And in principle, it could have been ugly

00:10:23.760 | and we might have had to deal with it,

00:10:25.520 | but we have gotten maybe confident

00:10:28.560 | through examples after examples in the history of science

00:10:31.000 | to look for beauty.

00:10:32.480 | - And our sense of beauty seems to incorporate

00:10:34.280 | a lot of things that are essential for us

00:10:36.040 | to solve some difficult problems like symmetry.

00:10:38.000 | We find symmetry beautiful

00:10:39.360 | and the breaking of symmetry beautiful.

00:10:41.260 | Somehow symmetry is a fundamental part

00:10:45.440 | of how we conceive of beauty at all layers of reality,

00:10:50.040 | which is interesting.

00:10:51.240 | Like in both the visual space, like where we look at art,

00:10:55.240 | we look at each other as human beings,

00:10:57.120 | the way we look at creatures in the biological space,

00:10:59.800 | the way we look at chemistry,

00:11:01.280 | and then to the physics world as the work you do.

00:11:04.400 | It's kind of interesting.

00:11:05.440 | It makes you wonder like,

00:11:07.040 | which one is the chicken or the egg?

00:11:10.000 | Is symmetry the chicken

00:11:11.800 | and our conception of beauty the egg

00:11:13.600 | or the other way around?

00:11:15.360 | Or somehow the fact that the symmetry is part of reality,

00:11:20.360 | it somehow creates a brain that then is able to perceive it?

00:11:24.200 | Or maybe this is just 'cause we,

00:11:27.040 | maybe it's so obvious it's almost trivial

00:11:32.040 | that symmetry, of course,

00:11:33.520 | will be part of every kind of universe that's possible.

00:11:36.400 | And then any kind of organism

00:11:40.000 | that's able to observe that universe

00:11:41.600 | is going to appreciate symmetry.

00:11:44.320 | - Well, these are good questions.

00:11:46.040 | We don't have a deep understanding

00:11:47.380 | of why we get attracted to symmetry.

00:11:49.720 | Why do laws of nature seem to have symmetries

00:11:53.920 | underlying them?

00:11:55.080 | And the reasoning, the examples of whether,

00:11:57.900 | if it wasn't symmetry, we would have understood it or not.

00:12:00.720 | We could have said that, yeah,

00:12:01.840 | if there were things which didn't look that great,

00:12:04.040 | we could understand them.

00:12:04.900 | For example, we know that symmetries get broken

00:12:08.040 | and we have appreciated nature

00:12:10.280 | in the broken symmetry phase as well.

00:12:12.400 | The world we live in has many things

00:12:14.280 | which do not look symmetric,

00:12:16.120 | but even those have underlying symmetry

00:12:18.960 | when you look at it more deeply.

00:12:20.640 | So we have gotten maybe spoiled perhaps

00:12:23.080 | by the appearance of symmetry all over the place

00:12:25.960 | and we look for it.

00:12:27.000 | And I think this is perhaps related

00:12:30.200 | to the sense of aesthetics that scientists have.

00:12:33.320 | And we don't usually talk about it among scientists.

00:12:36.760 | In fact, it's kind of a philosophical view

00:12:39.560 | of why do we look for simplicity or beauty or so forth.

00:12:43.240 | And I think in a sense,

00:12:45.440 | scientists are a lot like philosophers.

00:12:48.880 | Sometimes I think, especially modern science

00:12:51.200 | seems to shun philosophers and philosophical views.

00:12:54.880 | And I think at their peril.

00:12:56.400 | I think in my view, science owes a lot to philosophy.

00:13:01.360 | And in my view, many scientists,

00:13:04.160 | in fact, probably all good scientists

00:13:06.180 | are perhaps amateur philosophers.

00:13:08.840 | They may not state that they are philosophers

00:13:11.020 | or they may not like to be labeled philosophers,

00:13:13.400 | but in many ways, what they do

00:13:14.740 | is like what is philosophical takes of things.

00:13:18.640 | Looking for simplicity or symmetry

00:13:20.320 | is an example of that in my opinion, or seeing patterns.

00:13:23.640 | You see, for example, another example of the symmetry

00:13:26.680 | is like how you come up with new ideas in science.

00:13:29.440 | You see, for example, an idea A

00:13:31.880 | is connected with an idea B.

00:13:33.640 | Okay, so you study this connection very deeply

00:13:36.800 | and then you find the cousin of an idea A,

00:13:39.280 | let me call it A prime.

00:13:41.120 | And then you immediately look for B prime.

00:13:44.060 | If A is like B and if there's an A prime,

00:13:46.260 | then you look for B prime.

00:13:47.360 | Why?

00:13:48.180 | Because it completes the picture.

00:13:50.920 | Why?

00:13:51.760 | Well, it's philosophically appealing

00:13:53.180 | to have more balance in terms of that.

00:13:55.280 | And then you look for B prime

00:13:56.560 | and lo and behold, you find this other phenomenon,

00:13:58.600 | which is a physical phenomenon, which you call B prime.

00:14:01.160 | So this kind of thinking motivates

00:14:03.600 | asking questions and looking for things.

00:14:05.500 | And it has guided scientists, I think, through many centuries

00:14:08.560 | and I think it continues to do so today.

00:14:10.840 | - And I think if you look at the long arc of history,

00:14:12.960 | I suspect that the things that will be remembered

00:14:16.440 | is the philosophical flavor of the ideas of physics

00:14:21.440 | and chemistry and computer science and mathematics.

00:14:24.540 | Like I think the actual details

00:14:29.320 | will be shown to be incomplete or maybe wrong,

00:14:33.220 | but the philosophical intuitions

00:14:34.980 | will carry through much longer.

00:14:36.880 | There's a sense in which, if it's true,

00:14:39.380 | that we haven't figured out most of how things work currently

00:14:44.260 | that it'll all be shown as wrong and silly.

00:14:47.180 | It'd almost be a historical artifact.

00:14:49.940 | But the human spirit, whatever,

00:14:52.700 | like the longing to understand the way we perceive the world,

00:14:57.140 | the way we conceive of it, of our place in the world,

00:15:00.780 | those ideas will carry on.

00:15:02.540 | - I completely agree.

00:15:03.380 | In fact, I believe that almost,

00:15:05.620 | well, I believe that none of the principles

00:15:08.260 | or laws of physics we know today are exactly correct.

00:15:11.620 | All of them are approximations to something.

00:15:13.740 | They're better than the previous versions that we had,

00:15:15.700 | but none of them are exactly correct

00:15:17.780 | and none of them are gonna stand forever.

00:15:19.980 | So I agree that that's the process we are heading,

00:15:22.820 | we are improving.

00:15:24.140 | And yes, indeed, the thought process

00:15:26.420 | and that philosophical take is common.

00:15:28.820 | So when we look at older scientists

00:15:33.460 | or maybe even all the way back to Greek philosophers

00:15:36.060 | and the things that the way they thought and so on,

00:15:38.420 | almost everything they said about nature was incorrect.

00:15:42.140 | But the way they thought about it

00:15:43.900 | and many things that they were thinking

00:15:45.820 | is still valid today.

00:15:46.940 | For example, they thought about symmetry breaking.

00:15:50.060 | They were trying to explain the following.

00:15:51.980 | This is a beautiful example, I think.

00:15:53.660 | They had figured out that the earth is round

00:15:55.900 | and they said, okay, earth is round.

00:15:57.420 | They have seen the length of the shadow of a meter stick

00:16:01.300 | and they have seen that

00:16:02.140 | if you go from the equator upwards north,

00:16:04.540 | they find that depending on how far away you are,

00:16:06.460 | that the length of the shadow changes.

00:16:07.900 | And from that, they had even measured

00:16:09.900 | the radius of the earth to good accuracy.

00:16:12.260 | - That's brilliant, by the way,

00:16:13.460 | the fact that they did that.

00:16:14.540 | - Very brilliant, very brilliant.

00:16:15.660 | So these Greek philosophers are very smart.

00:16:17.740 | And so they had taken it to the next step.

00:16:20.640 | They asked, okay, so the earth is round.

00:16:23.100 | Why doesn't it move?

00:16:24.200 | They thought it doesn't move.

00:16:26.260 | They were looking around, nothing seemed to move.

00:16:28.520 | So they said, okay, we have to have a good explanation.

00:16:31.420 | It wasn't enough for them to be there.

00:16:33.300 | So they really wanna deeply understand that fact

00:16:36.060 | and they come up with a symmetry argument.

00:16:38.660 | And the symmetry argument was,

00:16:40.300 | oh, if the earth is a spherical,

00:16:43.460 | it must be at the center of the universe for sure.

00:16:45.500 | So they said the earth is at the center of the universe.

00:16:47.420 | - That makes sense.

00:16:48.260 | - And they said, if the earth is going to move,

00:16:50.700 | which direction does it pick?

00:16:52.180 | Any direction it picks, it breaks that spherical symmetry

00:16:54.820 | because you have to pick a direction.

00:16:57.060 | And that's not good because it's not symmetrical anymore.

00:16:59.340 | So therefore, the earth decides to sit put

00:17:01.940 | because it would break the symmetry.

00:17:03.940 | So they had the incorrect science.

00:17:05.780 | They thought earth doesn't move

00:17:06.900 | and they had this beautiful idea

00:17:08.660 | that symmetry might explain it.

00:17:10.220 | But they were even smarter than that.

00:17:12.620 | Aristotle didn't agree with this argument.

00:17:14.780 | He said, why do you think symmetry prevents it from moving?

00:17:18.420 | Because the preferred position?

00:17:19.920 | Not so.

00:17:21.340 | He gave an example.

00:17:22.300 | He said, suppose you are a person

00:17:26.100 | and we put you at the center of a circle

00:17:28.300 | and we spread food around you on a circle around you,

00:17:32.900 | loaves of bread, let's say.

00:17:35.020 | And we say, okay, stay at the center of the circle forever.

00:17:39.060 | Are you going to do that just because of the symmetric point?

00:17:42.060 | No, you are going to get hungry.

00:17:44.620 | You're going to move towards one of those loaves of bread

00:17:46.940 | despite the fact that it breaks the symmetry.

00:17:49.460 | So from this way, he tried to argue

00:17:51.340 | being at the symmetric point

00:17:52.620 | may not be the preferred thing to do.

00:17:55.360 | And this idea of spontaneous symmetry breaking

00:17:57.540 | is something we just used today

00:17:59.620 | to describe many physical phenomena.

00:18:01.580 | So spontaneous symmetry breaking

00:18:03.600 | is the feature that we now use.

00:18:04.940 | But this idea was there thousands of years ago,

00:18:08.040 | but applied incorrectly to the physical world,

00:18:11.020 | but now we are using it.

00:18:12.020 | So these ideas are coming back in different forms.

00:18:14.740 | So I agree very much that the thought process

00:18:17.360 | is more important and these ideas are more interesting

00:18:20.000 | than the actual applications that people may find today.

00:18:23.180 | - Did they use the language of symmetry

00:18:24.660 | and the symmetry breaking and spontaneous symmetry?

00:18:26.700 | But that's really interesting.

00:18:28.180 | 'Cause I could see a conception of the universe

00:18:32.500 | that kind of tends towards perfect symmetry

00:18:35.180 | and is stuck there.

00:18:36.980 | Like not stuck there, but achieves that optimal

00:18:40.740 | and stays there.

00:18:42.100 | The idea that you would spontaneously break out of symmetry,

00:18:45.900 | like have these perturbations,

00:18:47.460 | jump out of symmetry and back.

00:18:51.180 | That's a really difficult idea to load into your head.

00:18:55.300 | Like where does that come from?

00:18:57.300 | And then the idea that you may not be

00:18:59.940 | at the center of the universe.

00:19:02.420 | That is a really tough idea.

00:19:04.940 | - Right, so symmetry sometimes an explanation

00:19:07.320 | of being at the symmetric point

00:19:08.940 | is sometimes a simple explanation of many things.

00:19:10.940 | Like if you have a ball, a circular ball,

00:19:15.540 | then the bottom of it is the lowest point.

00:19:18.140 | So if you put a pebble or something,

00:19:19.740 | it will slide down and go there at the bottom

00:19:21.580 | and stays there at the symmetric point,

00:19:23.740 | because the preferred point, the lowest energy point.

00:19:26.420 | But if that same symmetric circular ball that you had

00:19:29.260 | had a bump on the bottom,

00:19:32.580 | the bottom might not be at the center,

00:19:34.380 | it might be on a circle on the table.

00:19:36.940 | In which case the pebble would not end up at the center,

00:19:39.300 | it would be the lower energy point.

00:19:40.900 | Symmetrical, but it breaks the symmetry

00:19:43.020 | once it picks a point on that circle.

00:19:45.180 | So we can have symmetry reasoning for where things end up

00:19:48.920 | or symmetry breakings, like this example would suggest.

00:19:52.740 | - We talked about beauty.

00:19:54.500 | I find geometry to be beautiful.

00:19:58.100 | You have a few examples that are geometric in nature

00:20:02.420 | in your book.

00:20:04.500 | How can geometry in ancient times or today

00:20:06.840 | be used to understand reality?

00:20:09.500 | And maybe how do you think about geometry

00:20:12.600 | as a distinct tool in mathematics and physics?

00:20:17.540 | - Yes, geometry is my favorite part of math as well.

00:20:19.940 | And Greeks were enamored by geometry.

00:20:22.480 | They tried to describe physical reality

00:20:24.500 | using geometry and principles of geometry and symmetry.

00:20:27.980 | Platonic solids, the five solids they had discovered,

00:20:31.220 | had these beautiful solids.

00:20:33.220 | They thought it must be good for some reality.

00:20:35.300 | They must be explaining something.

00:20:36.980 | They attached one to air, one to fire and so forth.

00:20:40.700 | They tried to give physical reality to symmetric objects.

00:20:44.400 | These symmetric objects are symmetries of rotation

00:20:48.580 | and discrete symmetry groups we call today

00:20:50.740 | of rotation group in three dimensions.

00:20:53.400 | Now, we know now, we kind of laugh at the way

00:20:56.220 | they were trying to connect that symmetry to, you know,

00:20:58.500 | the laws of the realities of physics.

00:21:02.020 | But actually it turns out in modern days,

00:21:05.840 | we use symmetries in not too far away

00:21:08.820 | exactly in these kinds of thoughts,

00:21:11.860 | processes in the following way.

00:21:13.580 | In the context of string theory,

00:21:16.420 | which is the field light study,

00:21:18.620 | we have these extra dimensions.

00:21:20.840 | And these extra dimensions are compact, tiny spaces

00:21:23.660 | typically, but they have different shapes and sizes.

00:21:26.260 | We have learned that if these extra shapes and sizes

00:21:30.220 | have symmetries, which are related to the same

00:21:32.900 | rotation symmetries that the Greek were talking about,

00:21:36.200 | if they enjoy those discrete symmetries,

00:21:38.540 | and if you take that symmetry and quotient the space by it,

00:21:41.780 | in other words, identify points under these symmetries,

00:21:44.540 | you get properties of that space at the singular points,

00:21:48.540 | which force emanates from them.

00:21:50.460 | What forces?

00:21:52.220 | Forces like the ones we have seen in nature today,

00:21:54.980 | like electric forces, like strong forces, like weak forces.

00:21:59.100 | So these same principles that were driving them

00:22:02.900 | to connect geometry and symmetries to nature

00:22:06.620 | is driving today's physics.

00:22:10.440 | Now much more, you know, modern ideas,

00:22:13.060 | but nevertheless the symmetries connecting geometry

00:22:16.540 | to physics.

00:22:17.360 | In fact, often we sometimes we have,

00:22:19.460 | we ask the following question,

00:22:20.580 | suppose I want to get this particular, you know,

00:22:23.840 | physical reality, I want to have this particles

00:22:26.140 | with these forces and so on, what do I do?

00:22:28.780 | It turns out that you can geometrically design

00:22:31.480 | the space to give you that.

00:22:33.180 | You say, oh, I put the sphere here, I will do this,

00:22:35.400 | I will shrink them.

00:22:36.660 | So if you have two spheres touching each other

00:22:39.540 | and shrinking to zero size, that gives you strong forces.

00:22:43.900 | If you have one of them, it gives you the weak forces.

00:22:45.740 | If you have this, you get that.

00:22:46.860 | And if you want to unify forces, do the other thing.

00:22:49.060 | So these geometrical translation of physics

00:22:52.500 | is one of my favorite things that we have discovered

00:22:54.860 | in modern physics in the context of string theory.

00:22:57.580 | - The sad thing is when you go into multiple dimensions

00:22:59.820 | and we'll talk about it is we start to lose our capacity

00:23:04.820 | to visually intuit the world we're discussing.

00:23:09.540 | And then we go into the realm of mathematics

00:23:11.660 | and we'll lose that.

00:23:12.740 | Unfortunately, our brains are such that we're limited.

00:23:15.680 | But before we go into that mysterious, beautiful world,

00:23:19.860 | let's take a small step back.

00:23:21.440 | And you also in your book have this kind of,

00:23:23.840 | through the space of puzzles, through the space of ideas,

00:23:27.260 | have a brief history of physics, of physical ideas.

00:23:32.020 | Now, we talked about Newtonian mechanics,

00:23:34.960 | a leading all through different Lagrangian,

00:23:37.280 | Hamiltonian mechanics.

00:23:38.960 | Can you describe some of the key ideas

00:23:41.160 | in the history of physics, maybe lingering on each

00:23:44.520 | from electromagnetism to relativity to quantum mechanics

00:23:48.600 | and to today as we'll talk about with quantum gravity

00:23:51.440 | and string theory?

00:23:52.720 | - Sure, so I mentioned the classical mechanics

00:23:55.880 | and the Euler-Lagrange formulation.

00:23:57.780 | One of the next important milestones for physics

00:24:03.200 | were the discoveries of laws of electricity and magnetism.

00:24:07.240 | So Maxwell put the discoveries all together

00:24:10.500 | in the context of what we call the Maxwell's equations.

00:24:13.600 | And he noticed that when he put these discoveries

00:24:16.880 | that Faraday's and others had made

00:24:19.640 | about electric and magnetic phenomena

00:24:22.000 | in terms of mathematical equations, it didn't quite work.

00:24:25.220 | There was a mathematical inconsistency.

00:24:27.780 | Now, one could have two attitudes.

00:24:31.240 | One would say, okay, who cares about math?

00:24:32.760 | I'm doing nature, electric force, magnetic force,

00:24:35.400 | math I don't care about.

00:24:36.840 | But it bothered him.

00:24:37.920 | It was inconsistent.

00:24:39.000 | The equations he were writing,

00:24:40.080 | the two equations he had written down

00:24:41.460 | did not agree with each other.

00:24:43.360 | And this bothered him.

00:24:44.320 | But he figured out, you know,

00:24:45.300 | if you add this jiggle this equation

00:24:47.400 | by adding one little term there, it works.

00:24:50.080 | At least it's consistent.

00:24:51.560 | What is the motivation for that term?

00:24:53.440 | He said, I don't know.

00:24:54.520 | Have we seen it in experiments?

00:24:56.080 | No.

00:24:57.240 | Why did you add it?

00:24:58.060 | Well, because of mathematical consistency.

00:24:59.840 | So he said, okay, math forced him to do this term.

00:25:04.560 | He added this term,

00:25:05.460 | which we now today call the Maxwell term.

00:25:08.240 | And once he added that term,

00:25:09.920 | his equations were nice, you know, differential equations,

00:25:12.440 | mathematically consistent, beautiful.

00:25:14.480 | But he also found a new physical phenomena.

00:25:17.160 | He found that because of that term,

00:25:19.040 | he could now get electric and magnetic waves

00:25:22.560 | moving through space at a speed that he could calculate.

00:25:27.340 | So he calculated the speed of the wave.

00:25:29.600 | And lo and behold,

00:25:30.440 | he found it's the same as the speed of light,

00:25:32.760 | which puzzled him because he didn't think

00:25:34.520 | light had anything to do with electricity and magnetism.

00:25:37.820 | But then he was courageous enough to say,

00:25:39.440 | well, maybe light is nothing

00:25:40.960 | but these electric and magnetic fields moving around.

00:25:43.660 | And he wasn't alive to see the verification

00:25:48.960 | of that prediction, and indeed it was true.

00:25:50.440 | So this mathematical inconsistency,

00:25:53.480 | which we could say, you know,

00:25:55.440 | this mathematical beauty drove him

00:25:58.280 | to this physical, very important connection

00:26:02.380 | between light and electric and magnetic phenomena,

00:26:05.120 | which was later confirmed.

00:26:07.560 | So then physics progresses and it comes to Einstein.

00:26:11.120 | Einstein looks at Maxwell's equation, says,

00:26:13.720 | beautiful, these are nice equation,

00:26:15.160 | except we get one speed light.

00:26:17.700 | Who measures this light speed?

00:26:20.480 | And he asked the question,

00:26:21.600 | are you moving, are you not moving?

00:26:24.320 | If you move, the speed of light changes,

00:26:25.800 | but Maxwell's equation has no hint

00:26:27.760 | of different speeds of light.

00:26:29.400 | It doesn't say, oh, only if you're not moving,

00:26:31.680 | you get the speed.

00:26:32.520 | It's just, you always get the speed.

00:26:33.640 | So Einstein was very puzzled

00:26:35.680 | and he was daring enough to say,

00:26:37.680 | well, you know, maybe everybody

00:26:38.960 | gets the same speed for light.

00:26:40.960 | And that motivated his theory of special relativity.

00:26:44.360 | And this is an interesting example

00:26:45.720 | because the idea was motivated from physics,

00:26:47.760 | from Maxwell's equations,

00:26:49.520 | from the fact that people tried to measure

00:26:54.160 | the properties of ether,

00:26:56.720 | which was supposed to be the medium

00:26:58.760 | in which the light travels through.

00:27:00.720 | And the idea was that only in that medium,

00:27:03.920 | the speed of, if you're at rest

00:27:06.120 | with respect to the ether,

00:27:07.480 | the speed of light,

00:27:08.600 | and if you're moving, the speed changes.

00:27:10.520 | And people did not discover it.

00:27:11.960 | Michelson and Morley's experiments

00:27:13.360 | showed there is no ether.

00:27:15.080 | So then Einstein was courageous enough to say,

00:27:17.600 | you know, light is the same speed for everybody,

00:27:20.120 | regardless of whether you're moving or not.

00:27:22.480 | And the interesting thing

00:27:24.480 | is about special theory of relativity

00:27:26.520 | is that the math underpinning it is very simple.

00:27:31.360 | It's linear algebra.

00:27:33.480 | Nothing terribly deep.

00:27:35.680 | You can teach it at high school level,

00:27:37.840 | if not earlier.

00:27:38.720 | Okay, does that mean Einstein's

00:27:41.880 | special relativity is boring?

00:27:43.360 | Not at all.

00:27:44.600 | So this is an example where simple math,

00:27:46.600 | you know, linear algebra,

00:27:49.000 | leads to deep physics.

00:27:50.920 | Einstein's theory of special relativity,

00:27:53.120 | motivated by this inconsistency

00:27:54.960 | that Maxwell's equation would suggest

00:27:57.280 | for the speed of light,

00:27:58.120 | depending on who observes it.

00:27:59.120 | - What's the most daring idea there,

00:28:01.200 | that the speed of light could be the same everywhere?

00:28:03.920 | - That's the basic, that's the guts of it.

00:28:05.720 | That's the core of Einstein's theory.

00:28:07.120 | That statement underlies the whole thing.

00:28:09.520 | Speed of light is the same for everybody,

00:28:11.040 | it's hard to swallow,

00:28:12.480 | and it doesn't sound right.

00:28:13.360 | It sounds completely wrong on the face of it.

00:28:16.320 | And it took Einstein to make this daring statement.

00:28:19.960 | It would be laughing in some sense.

00:28:22.560 | How could anybody make this possibly ridiculous claim?

00:28:26.520 | And it turned out to be true.

00:28:27.720 | - How does that make you feel?

00:28:28.880 | 'Cause it still sounds ridiculous.

00:28:31.360 | - It sounds ridiculous until you learn

00:28:33.080 | that our intuition is at fault

00:28:34.960 | about the way we conceive of space and time.

00:28:37.560 | The way we think about space and time is wrong,

00:28:40.120 | because we think about the nature of time as absolute.

00:28:43.360 | And part of it is because we live in a situation

00:28:46.720 | where we don't go with very high speeds,

00:28:49.400 | that our speeds are small compared to the speed of light,

00:28:51.960 | and therefore the phenomena we observe

00:28:54.720 | does not distinguish the relativity of time.

00:28:57.440 | The time also depends on who measures it.

00:28:59.200 | There's no absolute time.

00:29:00.800 | When you say it's noon today now,

00:29:02.920 | it depends on who's measuring it,

00:29:04.380 | and not everybody would agree with that statement.

00:29:07.040 | And to see that, you would have to have fast observer

00:29:10.560 | moving close to the speed of light.

00:29:12.760 | So this shows that our intuition is at fault.

00:29:15.800 | And a lot of the discoveries in physics

00:29:19.360 | precisely is getting rid of the wrong old intuition.

00:29:23.680 | And it is funny because we get rid of it,

00:29:26.000 | but it always lingers in us in some form.

00:29:28.240 | Like even when I'm describing it,

00:29:30.020 | I feel like a little bit like, isn't it funny?

00:29:32.960 | As you're just feeling the same way.

00:29:34.480 | It is, it is.

00:29:35.800 | But we kind of replace it by an intuition.

00:29:40.600 | And actually there's a very beautiful example of this,

00:29:43.440 | how physicists do this, try to replace their intuition.

00:29:46.040 | And I think this is one of my favorite examples

00:29:48.400 | about how physicists develop intuition.

00:29:51.240 | It goes to the work of Galileo.

00:29:55.000 | So again, let's go back to Greek philosophers

00:29:58.600 | or maybe Aristotle in this case.

00:30:00.520 | Now again, let's make a criticism.

00:30:02.560 | He thought that objects, the heavier objects

00:30:05.320 | fall faster than the lighter objects.

00:30:07.000 | - Makes sense.

00:30:07.880 | - It kind of makes sense.

00:30:08.840 | And people say about the feather and so on,

00:30:10.960 | but that's because of the air resistance.

00:30:12.680 | But you might think like if you have a heavy stone

00:30:15.600 | and a light pebble, the heavy one will fall first.

00:30:18.480 | If you don't do any experiments,

00:30:20.280 | that's the first gut reaction.

00:30:21.480 | I would say, everybody would say that's the natural thing.

00:30:24.080 | Galileo did not believe this

00:30:25.600 | and he kind of did the experiment.

00:30:29.920 | Famously it said he went on the top of Pisa Tower

00:30:32.680 | and he dropped these heavy and light stones

00:30:34.680 | and they fell at the same time

00:30:35.840 | when he dropped it at the same time, from the same height.

00:30:39.040 | Okay, good.

00:30:39.960 | So he said, I'm done.

00:30:41.400 | I've showed that the heavy and lighter objects

00:30:43.680 | fall at the same time, I did the experiment.

00:30:45.880 | Scientists at that time did not accept it.

00:30:49.480 | Why was that?

00:30:50.800 | Because at that time science was not just experimental.

00:30:54.280 | The experiment was not enough.

00:30:55.880 | They didn't think that they have to soil their hands

00:30:59.600 | in doing experiments to get to the reality.

00:31:01.880 | They said, why is it the case?

00:31:03.360 | - Why?

00:31:04.200 | - So Galileo had to come up with an explanation

00:31:06.400 | of why heavier and lighter objects fall at the same rate.

00:31:09.560 | This is the way he convinced them, using symmetry.

00:31:13.280 | He said, suppose you have three bricks,

00:31:16.160 | the same shape, the same size, same mass, everything.

00:31:21.360 | And we hold these three bricks at the same height

00:31:24.200 | and drop them.

00:31:25.920 | Which one will fall to the ground first?

00:31:30.160 | Everybody said, of course, we know that symmetry

00:31:32.320 | tells you they're all the same shape,

00:31:34.040 | same size, same height.

00:31:35.440 | Of course they fall at the same time.

00:31:36.960 | Yeah, we know that, next, next.

00:31:38.880 | It's trivial.

00:31:40.080 | He said, okay, what if we move these bricks around

00:31:42.160 | with the same height?

00:31:43.000 | Does it change the time they hit the ground?

00:31:45.360 | They said, if it's the same height,

00:31:46.520 | again, by the symmetry principle,

00:31:48.000 | because the height translation,

00:31:49.160 | horizontal translation is the symmetry.

00:31:51.000 | No, it doesn't matter.

00:31:52.280 | They all fall at the same rate.

00:31:53.600 | Good, doesn't matter how close I bring them together?

00:31:55.880 | No, it doesn't.

00:31:56.920 | Okay, suppose I make the two bricks touch

00:31:59.040 | and then let them go.

00:31:59.880 | Do they fall at the same rate?

00:32:01.160 | Yes, they do.

00:32:02.640 | But then he said, well, the two bricks that touch

00:32:04.960 | are twice more mass than this other brick.

00:32:07.200 | And you just agreed that they fall at the same rate.

00:32:09.680 | They say, yeah, yeah, we just agreed.

00:32:10.800 | That's right, that's great.

00:32:12.440 | Yes, so he de-confused them by the symmetry reasoning.

00:32:15.480 | So this way of repackaging some intuition,

00:32:18.360 | a different intuition.

00:32:19.880 | When the intuitions clash,

00:32:21.960 | then you side on the, you replace the intuition.

00:32:24.960 | - That's brilliant.

00:32:25.960 | In some of these more difficult physical ideas,

00:32:31.320 | physics ideas in the 20th century and the 21st century,

00:32:34.240 | it starts becoming more and more difficult

00:32:36.160 | to then replace the intuition.

00:32:38.000 | You know, what does the world look like

00:32:39.880 | for an object traveling close to the speed of light?

00:32:42.880 | You start to think about like the edges

00:32:44.960 | of supermassive black holes.

00:32:47.480 | And you start to think like, what's that look like?

00:32:51.000 | Or I've been into gravitational waves recently.

00:32:54.640 | It's like when the fabric of space-time

00:32:58.040 | is being morphed by gravity.

00:33:01.400 | Like what's that actually feel like?

00:33:03.400 | If I'm riding a gravitational wave, what's that feel like?

00:33:06.860 | I mean, I think some of those are more sort of hippie,

00:33:12.120 | not useful intuitions to have.

00:33:15.800 | But if you're an actual physicist

00:33:18.760 | or whatever the particular discipline is,

00:33:20.600 | I wonder if it's possible to meditate,

00:33:23.520 | to sort of escape through thinking,

00:33:26.680 | prolonged thinking and meditation on a world,

00:33:31.760 | like live in a visualized world that's not like our own

00:33:35.320 | in order to understand a phenomenon deeply.

00:33:38.120 | So like replace the intuition

00:33:39.880 | like through rigorous meditation on the idea

00:33:44.600 | in order to conceive of it.

00:33:46.360 | I mean, if we're talking about multiple dimensions,

00:33:48.720 | I wonder if there's a way to escape

00:33:51.600 | with a three-dimensional world in our mind

00:33:53.960 | in order to then start to reason about it.

00:33:56.040 | The more I talk to topologists,

00:33:59.080 | the more they seem to not operate at all

00:34:04.280 | in the visual space.

00:34:05.720 | They really trust the mathematics,

00:34:07.880 | which is really annoying to me

00:34:09.400 | because topology and differential geometry

00:34:14.400 | feels like it has a lot of potential

00:34:16.040 | for beautiful pictures.

00:34:17.520 | - Yes, I think they do.

00:34:18.640 | Actually, I would not be able to do my research

00:34:23.320 | if I don't have an intuitive feel about geometry.

00:34:26.200 | And we'll get to it, as you mentioned before,

00:34:29.680 | that how, for example, in string theory,

00:34:32.120 | you deal with these extra dimensions.

00:34:33.440 | And I'll be very happy to describe how we do it

00:34:35.480 | because without intuition, we will not get anywhere.

00:34:37.640 | And I don't think you can just rely on formalism.

00:34:40.480 | I don't.

00:34:41.400 | I don't think any physicist just relies on formalism.

00:34:44.120 | That's not physics.

00:34:45.040 | That's not understanding.

00:34:46.720 | So we have to intuit it.

00:34:48.120 | And that's crucial.

00:34:49.280 | And there are steps of doing it, and we learned.

00:34:51.320 | It might not be trivial, but we learned how to do it.

00:34:53.880 | Similar to this Galileo picture I just told you,

00:34:56.560 | you have to build these gradually.

00:34:59.440 | - You have to connect the bricks.

00:35:00.920 | - You have to connect the bricks.

00:35:02.000 | Exactly, you have to connect the bricks, literally.

00:35:04.800 | So yeah, so then, going back to your question

00:35:07.760 | about the path of the history of the science,

00:35:10.080 | so I was saying about the refusal of magnetism

00:35:12.400 | and the special relativity were simple idea

00:35:14.600 | led to special relativity,

00:35:16.400 | but then he went further thinking about acceleration

00:35:20.020 | in the context of relativity,

00:35:21.760 | and he came up with general relativity

00:35:23.840 | where he talked about the fabric of space-time being curved

00:35:26.720 | and so forth and matter affecting the curvature

00:35:30.640 | of the space and time.

00:35:31.520 | So this gradually became a connection

00:35:34.520 | between geometry and physics.

00:35:38.580 | Namely, he replaced Newton's gravitational force

00:35:43.180 | with a very geometrical beautiful picture.

00:35:46.000 | It's much more elegant than Newton's,

00:35:47.540 | but much more complicated mathematically.

00:35:49.980 | So when we say it's simpler,

00:35:52.780 | we mean in some form it's simpler,

00:35:55.020 | but not in pragmatic terms of equation solving.

00:35:57.780 | The equations are much harder to solve in Einstein's theory,

00:36:01.340 | and in fact, so much harder that Einstein himself

00:36:03.920 | couldn't solve many of the cases.

00:36:06.020 | He thought, for example, he couldn't solve the equation

00:36:07.940 | for a spherical symmetric matter,

00:36:10.800 | like if you had a symmetric sun,

00:36:12.980 | he didn't think you can actually solve his equation for that

00:36:15.740 | and a year after he said that it was solved by Schwarzschild.

00:36:19.380 | So it was that hard that he didn't think

00:36:22.020 | it's gonna be that easy.

00:36:22.940 | So yeah, the formalism is hard,

00:36:25.000 | but the contrast between the special relativity

00:36:27.460 | and general relativity is very interesting

00:36:29.060 | because one of them has almost trivial math

00:36:31.540 | and the other one has super complicated math.

00:36:34.580 | Both are physically amazingly important.

00:36:37.620 | And so we have learned that the physics

00:36:41.740 | may or may not require complicated math.

00:36:44.860 | We should not shy from using complicated math

00:36:47.620 | like Einstein did.

00:36:48.820 | Nobody, Einstein wouldn't say,

00:36:49.940 | I'm not gonna touch this math

00:36:51.300 | because it's too much tensors or curvature

00:36:54.700 | and I don't like the four dimensional space time

00:36:56.380 | because I can't see four dimension.

00:36:58.020 | He wasn't doing that.

00:36:59.140 | He was willing to abstract from that

00:37:01.520 | because physics drove him in that direction,

00:37:03.660 | but his motivation was physics.

00:37:05.460 | Physics pushed him.

00:37:06.560 | Just like Newton pushed to develop calculus

00:37:09.980 | because physics pushed him that he didn't have the tools.

00:37:12.540 | So he had to develop the tools

00:37:14.140 | to answer his physics questions.

00:37:16.020 | So his motivation was physics again.

00:37:18.780 | So to me, those are examples which show

00:37:20.820 | that math and physics have this symbiotic relationship

00:37:24.580 | which kind of reinforce each other.

00:37:26.900 | Here I'm giving you examples of both of them,

00:37:30.120 | namely Newton's work led to development

00:37:32.540 | of mathematics calculus.

00:37:34.540 | And in the case of Einstein,

00:37:35.680 | he didn't develop Riemannian geometry, just used them.

00:37:38.800 | So it goes both ways.

00:37:40.360 | And in the context of modern physics,

00:37:42.200 | we see that again and again, it goes both ways.

00:37:44.560 | - Let me ask a ridiculous question.

00:37:46.960 | You know, you talk about your favorite soccer player,

00:37:48.880 | the bar, I'll ask the same question about Einstein's ideas,

00:37:52.440 | which is, which one do you think

00:37:54.640 | is the biggest leap of genius?

00:37:56.520 | Is it the E equals MC squared?

00:37:59.960 | Is it Brownian motion?

00:38:01.560 | Is it special relativity?

00:38:03.160 | Is it general relativity?

00:38:05.180 | Which of the famous set of papers he's written in 1905

00:38:09.780 | and in general, his work was the biggest leap of genius?

00:38:13.740 | - In my opinion, it's special relativity.

00:38:16.260 | The idea that speed of light is the same for everybody

00:38:19.140 | is the beginning of everything he did.

00:38:20.500 | - The beginning is the-- - The beginning.

00:38:22.220 | - Once you embrace that weirdness, all the weirdness--

00:38:25.580 | - I would say that's, even though he says

00:38:27.780 | the most beautiful moment for him,

00:38:29.740 | he says that is when he realized

00:38:31.060 | that if you fall in an elevator,

00:38:32.620 | you don't know if you're falling

00:38:33.700 | or whether you're in the falling elevator

00:38:36.420 | or whether you're next to the Earth gravitational field,

00:38:39.100 | that to him was his aha moment,

00:38:41.800 | which inertial mass and gravitational mass

00:38:43.620 | being identical geometrically and so forth

00:38:46.460 | as part of the theory, not because of,

00:38:48.500 | you know, some funny coincidence.

00:38:52.280 | That's for him, but I feel from outside at least,

00:38:54.380 | it feels like the speed of light being the same

00:38:56.860 | is the really aha moment.

00:38:59.180 | - The general relativity to you is not,

00:39:02.160 | like the conception of space-time.

00:39:04.940 | - In a sense, the conception of space-time

00:39:06.580 | already was part of spatial relativity

00:39:08.620 | when you talk about length contraction.

00:39:10.900 | So general relativity takes that to the next step,

00:39:13.060 | but beginning of it was already space-length contracts,

00:39:16.460 | time dilates, so once you talk about those,

00:39:18.780 | then yeah, you can dilate more or less different places

00:39:20.740 | than its curvature, so you don't have a choice.

00:39:22.900 | So it's kind of started just with that same simple thought,

00:39:26.500 | speed of light is the same for all.

00:39:28.660 | - Where does quantum mechanics come into view?

00:39:32.060 | - Exactly, so this is the next step.

00:39:33.580 | So Einstein's, you know, developed general relativity

00:39:36.820 | and he's beginning to develop the foundation

00:39:38.500 | of quantum mechanics at the same time,

00:39:39.980 | the photoelectric effects and others.

00:39:42.260 | And so quantum mechanics overtakes, in fact,

00:39:45.740 | Einstein in many ways because he doesn't like

00:39:47.600 | the probabilistic interpretation of quantum mechanics

00:39:50.380 | and the formulas that's emerging,

00:39:52.380 | but physicists march on and try to, for example,

00:39:56.320 | combine Einstein's theory of relativity

00:39:59.860 | with quantum mechanics.

00:40:01.060 | So Dirac takes special relativity,

00:40:04.080 | tries to see how is it compatible with quantum mechanics.

00:40:07.820 | - Can we pause and briefly say what is quantum mechanics?

00:40:10.540 | - Oh yes, sure, so quantum mechanics,

00:40:12.860 | so I discussed briefly when I talked about the connection

00:40:16.500 | between Newtonian mechanics and the Euler-Lagrange

00:40:20.100 | reformulation of the Newtonian mechanics

00:40:22.340 | and interpretation of this Euler-Lagrange formalism

00:40:26.140 | in terms of the paths that the particle take.

00:40:28.620 | So when we say a particle goes from here to here,

00:40:31.500 | we usually think it classically,

00:40:34.100 | it follows a specific trajectory,

00:40:36.340 | but actually in quantum mechanics,

00:40:38.560 | it follows every trajectory with different probabilities.

00:40:43.020 | And so there's this fuzziness.

00:40:44.940 | Now, most probable, it's the path that you actually see

00:40:49.300 | and the deviation from that is very, very unlikely

00:40:51.860 | and probabilistically very minuscule.

00:40:53.820 | So in everyday experiment, we don't see anything deviated

00:40:56.380 | from what we expect.

00:40:58.040 | But quantum mechanics tells us that things are more fuzzy,

00:41:01.540 | things are not as precise as the line you draw.

00:41:05.780 | Things are a bit like cloud.

00:41:07.780 | So if you go to microscopic scales,

00:41:11.180 | like atomic scales and lower,

00:41:12.580 | these phenomena become more pronounced.

00:41:14.800 | You can see it much better.

00:41:16.300 | The electron is not at the point,

00:41:18.380 | but the clouds spread out around the nucleus.

00:41:21.340 | And so this fuzziness, this probabilistic aspect of reality

00:41:25.300 | is what quantum mechanics describes.

00:41:28.300 | - Can I briefly pause on that idea?

00:41:31.640 | Do you think this is, quantum mechanics

00:41:33.780 | is just a really damn good approximation,

00:41:37.080 | a tool for predicting reality?

00:41:40.240 | Or does it actually describe reality?

00:41:42.900 | Do you think reality is fuzzy at that level?

00:41:45.840 | - Well, I think that reality is fuzzy at that level,

00:41:48.460 | but I don't think quantum mechanics

00:41:49.880 | is necessarily the end of the story.

00:41:51.800 | So quantum mechanics is certainly an improvement

00:41:55.780 | over classical physics.

00:41:57.420 | That much we know by experiments and so forth.

00:42:00.220 | Whether I'm happy with quantum mechanics,

00:42:02.240 | whether I view quantum mechanics, for example,

00:42:04.660 | the thought, the measurement description

00:42:08.060 | of quantum mechanics, am I happy with it?

00:42:09.980 | Am I thinking that's the end stage or not?

00:42:11.780 | I don't.

00:42:12.740 | I don't think we're at the end of that story.

00:42:14.340 | And many physicists may or may not view this way.

00:42:17.460 | Some do, some don't.

00:42:18.980 | But I think that it's the best we have right now.

00:42:22.000 | That's for sure.

00:42:23.080 | It's the best approximation for reality we know today.

00:42:25.400 | And so far, we don't know what it is the next thing

00:42:27.840 | that improves it or replaces it and so on.

00:42:30.520 | But as I mentioned before, I don't believe any

00:42:33.080 | of the laws of physics we know today

00:42:34.760 | are apparently exactly correct.

00:42:35.600 | - It's the end of the story.

00:42:36.440 | - And it doesn't bother me.

00:42:38.040 | I'm not like dogmatic, saying, I have figured out,

00:42:40.440 | this is the law of nature, I know everything.

00:42:42.560 | No, no.

00:42:43.840 | That's the beauty about science, that we are not dogmatic.

00:42:47.600 | And we are willing to, in fact, we are encouraged

00:42:50.380 | to be skeptical of what we ourselves do.

00:42:53.940 | - So you were talking about Dirac.

00:42:55.420 | - Yes, I was talking about Dirac, right.

00:42:56.640 | So Dirac was trying to now combine

00:42:58.900 | this Schrodinger's equations, which was described

00:43:02.760 | in the context of trying to talk about how

00:43:04.780 | these probabilistic waves of electrons move for the atom,

00:43:07.860 | which was good for speeds which were not too close

00:43:10.740 | to the speed of light, to what happens when you get

00:43:12.980 | to the near the speed of light.

00:43:14.860 | So then you need relativity.

00:43:16.460 | So then Dirac tried to combine Einstein's relativity

00:43:19.460 | with quantum mechanics.

00:43:20.700 | So he tried to combine them and he wrote

00:43:23.740 | this beautiful equation, the Dirac equation,

00:43:26.900 | which roughly speaking, take the square root

00:43:29.660 | of the Einstein's equation in order to connect it

00:43:32.460 | to Schrodinger's time evolution operator,

00:43:34.300 | which is first order in time derivative,

00:43:37.220 | to get rid of the naive thing that Einstein's equation

00:43:40.100 | would have given, which is second order.

00:43:41.380 | So you have to take a square root.

00:43:43.380 | Now square root usually has a plus or minus sign

00:43:45.580 | when you take it.

00:43:46.500 | And when he did this, he originally didn't notice this,

00:43:50.780 | didn't pay attention to this plus or minus sign,

00:43:52.600 | but later physicists pointed out to Dirac,

00:43:54.940 | says, look, there's also this minus sign.

00:43:57.260 | And if you use this minus sign, you get negative energy.

00:44:00.060 | In fact, it was very, very annoying that, you know,

00:44:04.620 | somebody else tells you this obvious mistake you make.

00:44:06.660 | Pauli, famous physicist, told Dirac, this is nonsense.

00:44:09.840 | You're gonna get negative energy with your equation,

00:44:11.620 | which negative energy without any bottom.

00:44:13.460 | You can go all the way down to negative infinite energy,

00:44:16.500 | so it doesn't make any sense.

00:44:18.140 | Dirac thought about it, and then he remembered

00:44:20.220 | Pauli's exclusion principle.

00:44:22.300 | Just before him, Pauli had said, you know,

00:44:24.320 | there's this principle called the exclusion principle

00:44:26.720 | that two electrons cannot be on the same orbit.

00:44:29.420 | And so Dirac said, okay, you know what?

00:44:32.740 | All these negative energy states are filled orbits,

00:44:37.740 | occupied.

00:44:38.980 | So according to you, Mr. Pauli, there's no place to go,

00:44:43.980 | so therefore they only have to go positive.

00:44:47.220 | Sounded like a big cheat.

00:44:49.220 | And then Pauli said, oh, you know what?

00:44:52.000 | We can change orbits from one orbit to another.

00:44:53.900 | What if I take one of these negative energy orbits

00:44:55.860 | and put it up there?

00:44:57.620 | Then it seems to be a new particle,

00:44:59.740 | which has opposite properties to the electron.

00:45:03.220 | It has positive energy, but it has positive charge.

00:45:06.060 | What is that?

00:45:07.980 | Dirac was a bit worried.

00:45:10.460 | He said, maybe that's proton,

00:45:11.620 | because proton has plus charge.

00:45:13.540 | He wasn't sure.

00:45:14.820 | But then he said, oh, maybe it's proton.

00:45:16.580 | But then they said, no, no, no, no.

00:45:17.860 | It has the same mass as the electron.

00:45:19.420 | It cannot be proton, because proton is heavier.

00:45:22.340 | Dirac was stuck.

00:45:23.180 | He says, well, then maybe another particle we haven't seen.

00:45:26.080 | By that time, Dirac himself was getting a little bit

00:45:30.780 | worried about his own equation

00:45:32.100 | and his own crazy interpretation.

00:45:34.600 | Until a few years later, Anderson,

00:45:37.160 | in the photographic place that he had gotten

00:45:40.740 | from these cosmic rays, he discovered a particle

00:45:45.640 | which goes in the opposite direction that the electron goes

00:45:47.960 | when there's a magnetic field,

00:45:49.920 | and with the same mass,

00:45:52.360 | exactly like what Dirac had predicted.

00:45:55.200 | And this was what we call now positron.

00:45:57.680 | And in fact, beginning with the work of Dirac,

00:46:00.320 | we know that every particle has an antiparticle.

00:46:03.240 | And so this idea that there's an antiparticle

00:46:05.520 | came from this simple math.

00:46:06.880 | There's a plus and a minus

00:46:08.840 | from the Dirac's quote-unquote mistake.

00:46:12.840 | So again, trying to combine ideas,

00:46:15.440 | sometimes the math is smarter than the person

00:46:18.120 | who uses it to apply it.

00:46:20.120 | And we try to resist it, and then you're kind of confronted

00:46:22.920 | by criticism, which is the way it should be.

00:46:25.100 | So physicists come and say, no, no, no, that's wrong,

00:46:26.940 | and you correct it, and so on.

00:46:27.860 | So that is a development of the idea

00:46:30.840 | there's particle, there's antiparticle, and so on.

00:46:32.840 | So this is the beginning of development of quantum mechanics

00:46:35.720 | and the connection with relativity.

00:46:37.440 | But the thing was more challenging,

00:46:38.900 | because we had to also describe

00:46:40.760 | how electric and magnetic fields work with quantum mechanics.

00:46:44.780 | This was much more complicated,

00:46:46.200 | because it's not just one point.

00:46:47.760 | Electric and magnetic fields were everywhere,

00:46:50.380 | so you had to talk about fluctuating and a fuzziness

00:46:53.300 | of electrical field and magnetic fields everywhere,

00:46:56.320 | and the math for that was very difficult to deal with.

00:47:00.680 | And this led to a subject called quantum field theory.

00:47:03.560 | Fields, like electric and magnetic fields,

00:47:05.320 | had to be quantum, had to be described also in a wavy way.

00:47:09.080 | Feynman, in particular, was one of the pioneers,

00:47:13.280 | along with Schrodinger and others,

00:47:15.100 | to try to come up with a formalism to deal with fields,

00:47:18.880 | like electric and magnetic fields,

00:47:20.800 | interacting with electrons in a consistent quantum fashion,

00:47:24.200 | and they developed this beautiful theory,

00:47:25.960 | quantum electrodynamics, from that.

00:47:27.600 | And later on, that same formalism,

00:47:30.000 | quantum field theory, led to the discovery

00:47:32.760 | of other forces and other particles,

00:47:34.560 | all consistent with the idea of quantum mechanics.

00:47:37.840 | So that was how physics progressed,

00:47:40.720 | and so basically we learned that all particles

00:47:43.560 | and all the forces are in some sense

00:47:46.880 | related to particle exchanges.

00:47:49.640 | And so, for example, electromagnetic forces

00:47:52.320 | are mediated by a particle we call photon, and so forth.

00:47:57.320 | And same for other forces that they discovered,

00:47:59.640 | strong forces and the weak forces.

00:48:01.080 | So we got the sense of what quantum field theory is.

00:48:03.920 | - Is that a big leap of an idea

00:48:07.880 | that particles are fluctuations in the field?

00:48:12.240 | Like the idea that everything is a field?

00:48:15.080 | Is the old Einstein, light is a wave,

00:48:18.320 | both a particle and a wave, kind of idea?

00:48:20.480 | Is that a huge leap in our understanding

00:48:23.960 | of conceiving the universe as fields?

00:48:26.400 | - I would say so.

00:48:27.240 | I would say that viewing the particles,

00:48:29.720 | this duality that Bohr mentioned

00:48:31.800 | between particles and waves,

00:48:33.040 | that waves can behave sometimes like particles,

00:48:35.080 | sometimes like waves, is one of the biggest leaps

00:48:38.400 | of imagination that quantum mechanics made physics do.

00:48:42.820 | So I agree that that is quite remarkable.

00:48:45.480 | - Is duality fundamental to the universe,

00:48:50.240 | or is it just because we don't understand it fully?

00:48:52.200 | Like we'll eventually collapse into a clean explanation

00:48:56.460 | that doesn't require duality?

00:48:57.960 | That a phenomena could be two things at once

00:49:02.440 | and both to be true?

00:49:04.400 | That seems weird.

00:49:05.960 | - So in fact, I was going to get to that

00:49:08.320 | when we get to string theory,

00:49:09.360 | but maybe I can comment on that now.

00:49:11.000 | Duality turns out to be running the show today,

00:49:13.520 | is the whole thing that we are doing is string theory.

00:49:15.600 | Duality is the name of the game.

00:49:17.820 | So it's the most beautiful subject,

00:49:19.440 | and I want to talk about it.

00:49:20.840 | - Let's talk about it in the context of string theory.

00:49:23.440 | So do you want to take a next step into,

00:49:27.160 | 'cause we mentioned general relativity,

00:49:28.680 | we mentioned quantum mechanics,

00:49:30.480 | is there something to be said about quantum gravity?

00:49:32.640 | - Yes, that's exactly the right point to talk about.

00:49:34.960 | So namely, we have talked about quantum fields,

00:49:37.480 | and I talked about electric forces,

00:49:39.760 | photon being the particle carrying those forces.

00:49:42.720 | So for gravity, quantizing gravitational field,

00:49:46.600 | which is this curvature of space-time

00:49:48.200 | according to Einstein,

00:49:49.580 | you get another particle called graviton.

00:49:51.680 | So what about gravitons?

00:49:54.980 | Should be there, no problem.

00:49:56.440 | So then you start computing it.

00:49:59.160 | What do I mean by computing it?

00:50:00.440 | Well, you compute scattering of one graviton

00:50:03.400 | off another graviton,

00:50:04.500 | maybe with graviton with an electron and so on,

00:50:06.480 | see what you get.

00:50:07.920 | Feynman had already mastered this quantum electrodynamics,

00:50:12.480 | he said, "No problem, let me do it."

00:50:14.380 | Even though these are such weak forces,

00:50:17.200 | the gravity is very weak,

00:50:18.360 | so therefore to see them,

00:50:19.680 | these quantum effects of gravitational waves

00:50:22.080 | was impossible, it's even impossible today.

00:50:25.480 | So Feynman just did it for fun.

00:50:27.680 | He usually had this mindset that I wanna do something

00:50:30.080 | which I will see in experiment,

00:50:31.160 | but this one, let's just see what it does.

00:50:33.240 | And he was surprised because the same techniques

00:50:36.680 | he was using for doing the same calculations,

00:50:39.960 | quantum electrodynamics, when applied to gravity failed.

00:50:43.160 | The formulas seemed to make sense,

00:50:46.300 | but he had to do some integrals,

00:50:47.560 | and he found that when he does those integrals,

00:50:49.040 | he got infinity, and it didn't make any sense.

00:50:52.200 | Now, there were similar infinities in the other pieces,

00:50:54.520 | but he had managed to make sense out of those before.

00:50:56.920 | This was no way he could make sense out of it,

00:50:59.920 | he just didn't know what to do.

00:51:01.960 | He didn't feel it's an urgent issue

00:51:03.560 | because nobody could do the experiment,

00:51:05.800 | so he was kind of said, "Okay, there's this thing,

00:51:07.760 | "but okay, we don't know how to exactly do it,

00:51:09.360 | "but that's the way it is."

00:51:11.700 | So in some sense, a natural conclusion

00:51:14.120 | from what Feynman did could have been like,

00:51:16.640 | gravity cannot be consistent with quantum theory,

00:51:19.440 | but that cannot be the case

00:51:20.600 | because gravity is in our universe,

00:51:22.000 | quantum mechanics is in our universe,

00:51:23.020 | they're both together, somehow it should work.

00:51:25.600 | So it's not acceptable to say they don't work together.

00:51:29.000 | So that was a puzzle, how does it possibly work?

00:51:32.400 | It was left open.

00:51:34.660 | And then we get to the string theory.

00:51:37.120 | So this is the puzzle of quantum gravity,

00:51:38.880 | the particle description of quantum gravity failed.

00:51:41.440 | - So the infinity shows up, what do we do with infinity?

00:51:45.820 | Let's get to the fun part, let's talk about string theory.

00:51:48.760 | - Yes.

00:51:49.600 | - Let's discuss some technical basics of string theory.

00:51:55.840 | What is string theory?

00:51:57.740 | What is a string?

00:51:59.140 | How many dimensions are we talking about?

00:52:01.080 | What are the different states?

00:52:02.720 | How do we represent the elementary particles

00:52:04.960 | and the laws of physics using this new framework?

00:52:09.780 | - So string theory is the idea

00:52:12.840 | that the fundamental entities are not particles,

00:52:16.280 | but extended higher dimensional objects,

00:52:18.920 | like one dimensional strings, like loops.

00:52:22.380 | These loops could be open, like the two ends,

00:52:25.000 | like an interval or a circle without any ends.

00:52:29.100 | So, and they're vibrating and moving around in space.

00:52:32.700 | So how big they are?

00:52:34.880 | Well, you can of course stretch it and make it big,

00:52:37.620 | or you can just let it be whatever it wants.

00:52:39.620 | It can be as small as a point

00:52:41.300 | because the circle can shrink to a point

00:52:43.380 | and be very light, or you can stretch it

00:52:46.940 | and becomes very massive,

00:52:48.060 | or it could oscillate and become massive that way.

00:52:50.220 | So it depends on which kind of state you have.

00:52:52.340 | In fact, the string can have infinitely many modes

00:52:54.980 | depending on which kind of oscillation it's doing.

00:52:56.900 | Like a guitar has different harmonics,

00:52:59.020 | string has different harmonics,

00:53:00.140 | but for the string, each harmonic is a particle.

00:53:02.980 | So each particle will give you,

00:53:04.420 | ah, this is a more massive harmonic,

00:53:06.180 | this is a less massive.

00:53:07.480 | So the lightest harmonic, so to speak, is no harmonics,

00:53:10.000 | which means like the string shrunk to a point,

00:53:12.820 | and then it becomes like a massless particles

00:53:15.260 | or light particles like photon and graviton and so forth.

00:53:19.600 | So when you look at tiny strings,

00:53:22.620 | which are shrunk to a point, the lightest ones,

00:53:25.500 | they look like the particles that we think of,

00:53:27.660 | they're like particles.

00:53:28.540 | In other words, from far away, they look like a point.

00:53:31.100 | But of course, if you zoom in,

00:53:32.140 | there's this tiny little circle that's there

00:53:35.060 | that's shrunk to almost a point.

00:53:37.060 | - Should we be imagining,

00:53:38.540 | this is to the visual intuition,

00:53:40.540 | should we be imagining literally strings

00:53:42.720 | that are potentially connected as a loop or not?

00:53:47.120 | When you and when somebody outside of physics

00:53:50.640 | is imagining a basic element of string theory,

00:53:53.920 | which is a string,

00:53:55.080 | should we literally be thinking about a string?

00:53:58.740 | - Yes, you should literally think about string,

00:54:00.560 | but string with zero thickness.

00:54:02.580 | - With zero thickness.

00:54:03.920 | - So notice it's a loop of energy, so to speak.

00:54:07.720 | If you can think of it that way.

00:54:08.780 | And so there's a tension like a regular string.

00:54:11.080 | If you pull it, you have to stretch it.

00:54:14.100 | But it's not like a thickness,

00:54:15.180 | like you're made of something, it's just energy.

00:54:17.800 | It's not made of atoms or something like that.

00:54:19.680 | - But it is very, very tiny.

00:54:21.800 | - Very tiny.

00:54:22.640 | - Much smaller than elementary particles of physics.

00:54:25.680 | - Much smaller.

00:54:26.600 | So we think if you let the string to be by itself,

00:54:29.720 | with the lowest state, there'll be like a fuzziness

00:54:32.260 | or a size of that tiny little circle,

00:54:33.840 | which is like a point, about, could be anything between,

00:54:37.340 | we don't know the exact size,

00:54:38.400 | but in different models have different sizes,

00:54:40.340 | but something of the order of 10 to the minus,

00:54:42.160 | let's say 30 centimeters.

00:54:43.600 | So 10 to the minus 30 centimeters,

00:54:46.660 | just to compare with the size of the atom,

00:54:48.320 | which is 10 to the minus eight centimeters,

00:54:50.380 | is 22 orders of magnitude smaller.

00:54:52.460 | So it's--

00:54:54.160 | - Unimaginably small, I would say.

00:54:56.120 | - Very small.

00:54:56.960 | So we basically think from far away,

00:54:58.800 | string is like a point particle.

00:55:00.920 | And that's why a lot of the things that we learned

00:55:03.700 | about point particle physics

00:55:04.920 | carries over directly to strings.

00:55:07.020 | So therefore there's not much of a mystery

00:55:09.000 | why particle physics was successful,

00:55:10.940 | because string is like a particle when it's not stretched.

00:55:14.520 | But it turns out having this size,

00:55:16.960 | being able to oscillate, get bigger,

00:55:20.200 | turned out to be resolving this puzzles

00:55:22.400 | that Feynman was having in calculating his diagrams,

00:55:26.660 | and it gets rid of those infinities.

00:55:28.600 | So when you're trying to do those infinities,

00:55:31.440 | the regions that give infinities to Feynman,

00:55:34.360 | as soon as you get to those regions,

00:55:35.740 | then this string starts to oscillate,

00:55:38.040 | and these oscillation structure of the strings

00:55:40.520 | resolves those infinities to find the answer at the end.

00:55:43.000 | So the size of the string,

00:55:45.000 | the fact that it's one dimensional,

00:55:46.800 | gives a finite answer at the end,

00:55:48.680 | resolves this paradox.

00:55:50.720 | Now, perhaps it's also useful to recount

00:55:54.080 | of how string theory came to be.

00:55:56.280 | Because it wasn't like somebody say,

00:55:58.240 | "Well, let me solve the problem of Einstein's,

00:56:01.800 | "solve the problem that Feynman had

00:56:03.040 | "with unifying Einstein's theory with quantum mechanics

00:56:06.440 | "by replacing the point by a string."

00:56:08.080 | No, that's not the way the thought process,

00:56:10.160 | the thought process was much more random.

00:56:12.800 | Physicist, Veneziano in this case,

00:56:16.000 | was trying to describe the interactions

00:56:17.920 | they were seeing in colliders, in accelerators.

00:56:22.120 | And they were seeing that some process,

00:56:23.560 | in some process when two particles came together

00:56:26.480 | and joined together, and when they were separately,

00:56:29.760 | in one way, and the opposite way,

00:56:32.500 | they behave the same way.

00:56:34.020 | In some way, there was a symmetry, a duality,

00:56:37.640 | which he didn't understand.

00:56:38.920 | The particles didn't seem to have that symmetry.

00:56:41.960 | He said, "I don't know what it is,

00:56:42.980 | "what's the reason that these colliders

00:56:44.680 | "and experiments we're doing seems to have the symmetry,

00:56:46.700 | "but let me write the mathematical formula

00:56:48.880 | "which exhibits that symmetry."

00:56:51.680 | He used gamma functions, beta functions, and all that,

00:56:54.080 | you know, complete math, no physics,

00:56:56.580 | other than trying to get symmetry out of his equation.

00:56:59.320 | He just wrote down a formula

00:57:01.100 | as the answer for a process, not a method to compute it.

00:57:04.840 | Just say, "Wouldn't it be nice if this was the answer?"

00:57:08.120 | - Yes.

00:57:08.960 | - "Physicist looked at this formula,

00:57:10.380 | "hmm, that's intriguing, it has this symmetry, all right,

00:57:12.400 | "but what is this, where is this coming from?

00:57:14.720 | "Which kind of physics gives you this?"

00:57:17.440 | Said, "I don't know."

00:57:18.280 | (laughing)

00:57:19.440 | - Yeah. - A few years later,

00:57:21.020 | people saw that, oh, the equation that you're writing,

00:57:23.520 | the process that you're writing

00:57:24.640 | in the intermediate channels that particles come together

00:57:27.800 | seems to have all the harmonics.

00:57:30.120 | Harmonics sounds like a string.

00:57:32.320 | Let me see if what you're describing

00:57:33.680 | has anything to do with the strings,

00:57:34.680 | and people try to see if what he's doing

00:57:36.400 | has anything to do with the strings.

00:57:37.400 | Oh, yeah, indeed, if I study scattering of two strings,

00:57:40.880 | I get exactly the formula you wrote down.

00:57:42.840 | That was the reinterpretation of what he had written

00:57:46.360 | in the formula as a string,

00:57:48.500 | but still had nothing to do with gravity.

00:57:51.120 | It had nothing to do with resolving the problems

00:57:53.400 | of gravity with quantum mechanics.

00:57:55.200 | It was just trying to explain a process

00:57:57.600 | that people were seeing in hadronic physics collisions.

00:58:01.140 | So it took a few more years to get to that point.

00:58:04.420 | They noticed that, physicists noticed that

00:58:09.120 | whenever you try to find the spectrum of strings,

00:58:11.240 | you always get a massless particle,

00:58:13.420 | which has exactly the properties

00:58:14.840 | that a graviton is supposed to have,

00:58:16.940 | and no particle in hadronic physics that had that property.

00:58:20.160 | You are getting a massless graviton

00:58:22.740 | as part of the scattering without looking for it.

00:58:25.720 | It was forced on you.

00:58:27.520 | People were not trying to solve quantum gravity.

00:58:29.760 | Quantum gravity was pushed on them.

00:58:31.880 | I don't want this graviton, get rid of it.

00:58:33.960 | They couldn't get rid of it.

00:58:36.080 | They gave up trying to get rid of it.

00:58:38.360 | Physicists said, Sherkin-Schwartz said, you know what?

00:58:40.640 | String theory is theory of quantum gravity.

00:58:43.300 | They changed the perspective altogether.

00:58:45.640 | We are not describing the hadronic physics.

00:58:47.560 | We're describing this theory of quantum gravity.

00:58:49.800 | - And that's when string theory probably got exciting,

00:58:54.080 | that this could be the unifying theory.

00:58:56.340 | - Exactly, it got exciting,

00:58:57.780 | but at the same time, not so fast.

00:58:59.480 | Namely, it should have been fast, but it wasn't,

00:59:02.840 | because particle physics through quantum field theory

00:59:05.080 | were so successful at that time.

00:59:07.120 | This is mid '70s.

00:59:08.560 | Standard model of physics, electromagnetism

00:59:11.000 | and unification of electromagnetic forces

00:59:12.840 | with all the other forces were beginning to take place

00:59:15.100 | without the gravity part.

00:59:16.400 | Everything was working beautifully for particle physics.

00:59:20.900 | And so that was the shining golden age

00:59:23.160 | of quantum field theory and all the experiments,

00:59:25.240 | standard model, this and that, unification,

00:59:27.800 | and spontaneous symmetry breaking was taking place.

00:59:29.800 | All of them was nice.

00:59:31.040 | This was kind of like a side show

00:59:32.320 | and nobody was paying so much attention.

00:59:34.360 | This exotic string is needed for quantum gravity.

00:59:37.000 | Ah, maybe there's other ways.

00:59:38.180 | Maybe we should do something else.

00:59:39.520 | So, and yet, it wasn't paid much attention to.

00:59:41.920 | And this took a little bit more effort

00:59:44.040 | to try to actually connect it to reality.

00:59:48.060 | There were a few more steps.

00:59:49.080 | First of all, there was a puzzle

00:59:51.280 | that you were getting extra dimensions.

00:59:53.960 | String was not working well

00:59:55.600 | with three spatial dimensions at one time.

00:59:57.800 | It needed extra dimension.

00:59:59.140 | Now, there are different versions of strings,

01:00:02.520 | but the version that ended up being related

01:00:04.800 | to having particles like electron, what we call fermions,

01:00:08.300 | needed 10 dimensions, what we call super string.

01:00:11.200 | Now, why super?

01:00:13.060 | Why the word super?

01:00:13.900 | It turns out this version of the string,

01:00:17.360 | which had fermions, had an extra symmetry,

01:00:20.100 | which we call supersymmetry.

01:00:23.720 | This is a symmetry between a particle and another particle

01:00:27.960 | with exactly the same properties,

01:00:29.640 | same mass, same charge, et cetera.

01:00:31.640 | The only difference is that one of them

01:00:33.080 | has a little different spin than the other one.

01:00:35.800 | And one of them is a boson.

01:00:37.720 | One of them is a fermion because of that shift of spin.

01:00:41.080 | Otherwise, they're identical.

01:00:42.120 | So there was this symmetry.

01:00:43.560 | String theory had this symmetry.

01:00:45.520 | In fact, supersymmetry was discovered

01:00:48.360 | through string theory, theoretically.

01:00:51.560 | So theoretically, the first place that this was observed

01:00:53.880 | when you were describing these fermionic strings.

01:00:57.680 | So that was the beginning of the study of supersymmetry

01:01:00.200 | was via string theory.

01:01:02.160 | And then it had remarkable properties

01:01:05.000 | that the symmetry meant and so forth

01:01:07.200 | that people began studying supersymmetry after that.

01:01:10.680 | And that was a kind of a tangent direction

01:01:13.580 | at the beginning for string theory.

01:01:15.720 | But people in particle physics started also thinking,

01:01:17.920 | oh, supersymmetry is great.

01:01:19.040 | Let's see if we can have supersymmetry in particle physics

01:01:21.880 | and so forth.

01:01:22.720 | Forget about strings.

01:01:23.540 | And they developed on a different track as well.

01:01:25.720 | Supersymmetry in different models

01:01:27.640 | became a subject on its own right,

01:01:29.120 | understanding supersymmetry and what does this mean.

01:01:32.280 | Because it unified bosons and fermions,

01:01:34.120 | unifies some ideas together.

01:01:36.040 | So photon is a boson, electron is a fermion.

01:01:39.120 | Could things like that be somehow related?

01:01:41.680 | It was a kind of a natural kind of a question

01:01:43.660 | to try to kind of unify

01:01:44.920 | because in physics, we love unification.

01:01:48.080 | Now, gradually string theory was beginning

01:01:50.220 | to show signs of unification.

01:01:51.680 | It had graviton, but people found

01:01:53.720 | that you also have things like photons in them.

01:01:56.500 | Different excitations of string behave like photons.

01:01:59.080 | Another one behave like electron.

01:02:01.080 | So a single string was unifying all these particles

01:02:04.760 | into one object.

01:02:06.340 | That's remarkable.

01:02:07.380 | It's in 10 dimensions though.

01:02:10.560 | It is not our universe

01:02:11.720 | because we live in three plus one dimension.

01:02:13.520 | How could that be possibly true?

01:02:15.600 | So this was a conundrum.

01:02:18.240 | It was elegant, it was beautiful,

01:02:19.920 | but it was very specific

01:02:21.920 | about which dimension you're getting,

01:02:23.600 | which structure you're getting.

01:02:25.040 | It wasn't saying, oh, you just put D equals to four,

01:02:27.640 | you'll get your space time dimension that you want.

01:02:29.540 | No, it didn't like that.

01:02:30.880 | It said, I want 10 dimensions and that's the way it is.

01:02:34.300 | So it was very specific.

01:02:35.680 | Now, so people try to reconcile this by the idea

01:02:38.300 | that maybe these extra dimensions are tiny.

01:02:42.000 | So if you take three macroscopic spatial dimensions

01:02:45.240 | at one time and six extra tiny spatial dimensions,

01:02:49.440 | like tiny spheres or tiny circles,

01:02:51.760 | then it avoids contradiction with manifest fact

01:02:55.960 | that we haven't seen extra dimensions in experiments today.

01:02:59.720 | So that was a way to avoid conflict.

01:03:03.040 | Now, this was a way to avoid conflict,

01:03:05.880 | but it was not observed in experiments.

01:03:09.280 | A string observed in experiments?

01:03:10.580 | No, because it's so small.

01:03:12.960 | So it's beginning to sound a little bit funny.

01:03:16.040 | Similar feeling to the way perhaps Dirac had felt

01:03:19.640 | about this positron, plus or minus.

01:03:21.720 | You know, it was beginning to sound a little bit like,

01:03:24.200 | oh yeah, not only I have to have 10 dimension,

01:03:25.960 | but I have to also this and,

01:03:28.720 | and so conservative physicists would say,

01:03:31.200 | hmm, you know, I haven't seen these in experiments.

01:03:34.380 | I don't know if they are really there.

01:03:35.900 | Are you pulling my leg?

01:03:37.800 | Do you want me to imagine things that are not there?

01:03:40.520 | So this was an attitude of some physicists

01:03:42.720 | towards string theory, despite the fact

01:03:45.440 | that the puzzle of gravity and quantum mechanics

01:03:47.660 | merging together work, but still was this skepticism.

01:03:50.920 | You're putting all these things,

01:03:52.040 | like you want me to imagine there are these extra dimensions

01:03:54.240 | that I cannot see, uh-huh, uh-huh,

01:03:56.040 | and you want me to believe that string theory

01:03:57.400 | that you have not even seen in experiments are real,

01:03:59.080 | uh-huh, okay, what else do you want me to believe?

01:04:01.160 | So this kind of beginning to sound a little funny.

01:04:03.400 | Now, I will pass forward a little bit further.

01:04:07.160 | A few decades later, when string theory

01:04:10.960 | became the mainstream of efforts to unify the forces

01:04:14.000 | and particles together, we learned

01:04:16.560 | that these extra dimensions actually solved problems.

01:04:20.640 | They weren't a nuisance the way they originally appeared.

01:04:24.400 | First of all, the properties of these extra dimensions

01:04:28.000 | reflected the number of particles we got in four dimensions.

01:04:31.480 | If you took these six dimensions to have like six,

01:04:33.520 | five holes or four holes, it changed the number of particles

01:04:37.000 | that you see in four dimensional space-time.

01:04:39.440 | You get one electron and one muon if you had this,

01:04:42.000 | but if you did the other J shape, you get something else.

01:04:44.520 | So geometrically, you could get different kinds of physics.

01:04:47.640 | So it was kind of a mirroring of geometry by physics

01:04:51.800 | down in the macroscopic space.

01:04:53.360 | So these extra dimension were becoming useful.

01:04:56.760 | Fine, but we didn't need extra dimensions

01:04:58.800 | to just write an electron in three dimensions.

01:05:00.560 | We did, we wrote it, so what?

01:05:02.760 | Was there any other puzzle?

01:05:04.240 | Yes, there were.

01:05:05.960 | Hawking.

01:05:07.120 | Hawking had been studying black holes in mid '70s

01:05:09.680 | following the work of Bekenstein,

01:05:12.800 | who had predicted that black holes have entropy.

01:05:17.800 | So Bekenstein had tried to attach the entropy

01:05:20.240 | to the black hole.

01:05:21.520 | If you throw something into the black hole,

01:05:23.820 | the entropy seems to go down

01:05:25.280 | because you had something entropy outside the black hole

01:05:27.960 | and you throw it.

01:05:28.800 | Black hole was unique, so the entropy did not have any,

01:05:32.120 | black hole had no entropy,

01:05:33.460 | so the entropy seemed to go down.

01:05:35.920 | And so that's against the laws of thermodynamics.

01:05:38.000 | So Bekenstein was trying to say, no, no,

01:05:40.000 | therefore black hole must have an entropy.

01:05:42.120 | So he was trying to understand that.

01:05:43.320 | He found that if you assign entropy to be proportional

01:05:47.160 | to the area of the black hole, it seems to work.

01:05:49.200 | And then Hawking found not only that's correct,

01:05:52.600 | he found the correct proportionality factor

01:05:54.360 | of a factor of a one quarter of the area in Planck units

01:05:57.000 | is the correct amount of entropy.

01:05:59.360 | And he gave an argument using

01:06:01.080 | quantum semi-classical arguments,

01:06:03.280 | which means basically using a little bit

01:06:05.640 | of quantum mechanics,

01:06:06.880 | because he didn't have the full quantum mechanics

01:06:09.040 | of strength there, he could do some aspects

01:06:11.040 | of approximate quantum arguments.

01:06:12.920 | So heuristic quantum arguments led

01:06:14.880 | to this entropy formula.

01:06:16.520 | But then he didn't answer the following question.

01:06:20.760 | He was getting a big entropy for the black hole.

01:06:23.400 | The black hole with the size of the horizon

01:06:25.040 | of a black hole is huge, has a huge amount of entropy.

01:06:27.680 | What are the microstates of this entropy?

01:06:29.680 | When you say, for example, the gas has entropy,

01:06:32.240 | you count where the atoms are,

01:06:33.800 | you count this bucket or that bucket,

01:06:35.560 | there's an information about there and so on, you count them.

01:06:38.680 | For the black hole, the way Hawking was thinking,

01:06:40.840 | there was no degree of freedom,

01:06:41.840 | you throw them in, and there was just one solution.

01:06:44.480 | So where are these entropy?

01:06:46.680 | What are these microscopic states?

01:06:49.040 | They were hidden somewhere.

01:06:51.960 | So later in string theory,

01:06:54.360 | the work that we did with my colleague Strominger,

01:06:57.600 | in particular, showed that these ingredients

01:07:00.520 | in string theory of black hole arise

01:07:04.360 | from the extra dimensions.

01:07:06.080 | So the degrees of freedom are hidden

01:07:08.280 | in terms of things like strings,

01:07:10.000 | wrapping these extra circles in these hidden dimensions.

01:07:13.640 | And then we started counting how many ways,

01:07:16.040 | like the strings can wrap around this circle

01:07:18.160 | and the extra dimension or that circle,

01:07:19.960 | and counted the microscopic degrees of freedom.

01:07:22.480 | And lo and behold, we got the microscopic degrees

01:07:24.760 | of freedom that Hawking was predicting four dimensions.

01:07:27.760 | So the extra dimensions became useful

01:07:30.200 | for resolving a puzzle in four dimensions.

01:07:32.820 | The puzzle was, where are the degrees of freedom

01:07:35.360 | of the black hole hidden?

01:07:36.680 | The answer, hidden in the extra dimensions,

01:07:39.320 | the tiny extra dimensions.

01:07:41.040 | So then by this time, it was beginning to,

01:07:43.840 | we see aspects that extra dimensions

01:07:46.160 | are useful for many things.

01:07:47.360 | It's not a nuisance, it wasn't to be kind of,

01:07:49.920 | you know, be shamed of,

01:07:51.200 | it was actually in the welcome features.

01:07:53.520 | New feature, nevertheless.

01:07:54.920 | - How do you intuit the 10 dimensional world?

01:07:59.560 | So yes, it's a feature for describing certain phenomena,

01:08:03.140 | like the entropy in black holes,

01:08:06.380 | but what, you said that to you,

01:08:10.660 | a theory becomes real or becomes powerful

01:08:15.260 | when you can connect it to some deep intuition.

01:08:18.220 | So how do we intuit 10 dimensions?

01:08:20.580 | - Yes, so I will explain how some of the analogies work.

01:08:24.820 | First of all, we do a lot of analogies.

01:08:27.840 | And by analogies, we build intuition.

01:08:31.020 | So I will start with this example.

01:08:33.240 | I will try to explain

01:08:34.820 | that if we are in 10 dimensional space,

01:08:37.500 | if we have a seven dimensional plane,

01:08:40.260 | an eight dimensional plane,

01:08:41.900 | we ask typically in what space do they intersect each other

01:08:45.580 | in what dimension?

01:08:46.700 | That might sound like,

01:08:48.020 | how do you possibly give an answer to this?

01:08:50.540 | So we start with lower dimensions.

01:08:52.460 | We start with two dimensions.

01:08:53.620 | We say, if you have one dimension and a point,

01:08:56.520 | do they intersect typically on a plane?

01:08:58.600 | The answer is no.

01:08:59.440 | So a line, one dimensional, a point, zero dimension,

01:09:02.980 | on a two dimensional plane, they don't typically meet.

01:09:05.760 | But if you have a one dimensional line,

01:09:07.400 | another line, which is one plus one on a plane,

01:09:10.540 | they typically intersect at a point.

01:09:13.620 | Typically means if you're not parallel,

01:09:15.140 | typically they intersect at a point.

01:09:17.080 | So one plus one is two.

01:09:19.480 | And in two dimension,

01:09:20.620 | they intersect at a zero dimensional point.

01:09:23.020 | So you see two dimension, one and one, two,

01:09:25.740 | two minus two is zero.

01:09:26.900 | So you get point out of intersection.

01:09:29.560 | Okay, let's go to three dimension.

01:09:31.880 | You have a plane, two dimensional plane and a point.

01:09:33.880 | Do they intersect?

01:09:34.720 | No, two and zero.

01:09:35.860 | How about the plane and a line?

01:09:39.400 | A plane is two dimensional and a line is one,

01:09:41.400 | two plus one is three.

01:09:42.980 | In three dimension, a plane and a line meet at points,

01:09:47.000 | which is zero dimensional.

01:09:47.880 | Three minus three is zero.

01:09:49.720 | Okay, so plane and a line intersect

01:09:52.920 | at a point in three dimension.

01:09:54.080 | How about the plane and a plane in 3D?

01:09:56.000 | A plane is two and this is two, two plus two is four.

01:09:59.180 | In 3D, four minus three is one,

01:10:01.060 | they intersect on a one dimensional line.

01:10:03.100 | Okay, we're beginning to see the pattern.

01:10:04.540 | Okay, now come to the question.

01:10:06.060 | We're in 10 dimension, now we have the intuition.

01:10:08.140 | We have a seven dimensional plane

01:10:09.380 | and an eight dimensional plane in 10 dimension.

01:10:11.540 | They intersect on a plane.

01:10:13.140 | What's the dimension?

01:10:14.020 | Well, seven plus eight is 15 minus 10 is five.

01:10:16.860 | We draw the same picture as two planes

01:10:20.300 | and we write seven dimension, eight dimension,

01:10:22.520 | but we have gotten the intuition

01:10:23.940 | from the lower dimensional one, what to expect.

01:10:26.980 | It doesn't scare us anymore.

01:10:28.660 | So we draw this picture.

01:10:30.220 | We cannot see all the seven dimensions

01:10:32.700 | by looking at this two dimensional visualization of it,

01:10:36.240 | but it has all the features we want.

01:10:38.380 | It has, so we draw this picture,

01:10:39.900 | which is seven, seven,

01:10:40.840 | and they meet at the five dimensional plane,

01:10:43.660 | which is five.

01:10:44.620 | So we have built this intuition.

01:10:46.540 | Now, this is an example

01:10:50.020 | of how we've come up with intuition.

01:10:51.860 | Let me give you more examples of it

01:10:53.260 | because I think this will show you

01:10:54.860 | that people have to come up with intuitions to visualize it.

01:10:57.900 | Otherwise, we will be a little bit lost.

01:11:00.820 | - So what you just described is kind of

01:11:02.940 | in these high dimensional spaces,

01:11:04.260 | focus on the meeting place of two planes

01:11:08.780 | in high dimensional spaces.

01:11:10.200 | - Exactly.

01:11:11.040 | How the planes meet, for example,

01:11:12.220 | what's the dimension of their intersection and so on.

01:11:14.740 | So how do we come up with intuition?

01:11:16.600 | We borrow examples from lower dimensions,

01:11:19.560 | build up intuition and draw the same pictures

01:11:21.940 | as if we are talking about 10 dimensions,

01:11:24.780 | but we are drawing the same as a two dimensional plane

01:11:26.860 | because we cannot do any better.

01:11:28.260 | But our words change, but not our pictures.

01:11:32.580 | - So your sense is we can have a deep understanding

01:11:35.820 | of reality by looking at its slices,

01:11:39.220 | at lower dimensional slices.

01:11:40.500 | - Exactly, exactly.

01:11:41.780 | And this brings me to the next example I wanna mention,

01:11:45.180 | which is sphere.

01:11:46.440 | Let's think about how do we think about the sphere?

01:11:48.580 | Well, the sphere is a sphere, you know,

01:11:50.140 | the round, nice thing.

01:11:51.460 | But sphere has a circular symmetry.

01:11:53.520 | Now, I can describe the sphere in the following way.

01:11:57.900 | I can describe it by an interval,

01:12:01.540 | which is think about this going from the north of the sphere

01:12:04.980 | to the south.

01:12:06.380 | And at each point, I have a circle attached to it.

01:12:09.500 | So you can think about the sphere as a line

01:12:11.460 | with a circle attached with each point,

01:12:13.900 | the circle shrinks to a point at end points of the interval.

01:12:18.100 | So I can say, oh, one way to think about the sphere

01:12:21.900 | is an interval, where at each point on that interval,

01:12:25.300 | there's another circle I'm not drawing.

01:12:27.300 | But if you like, you can just draw it.

01:12:29.300 | Say, okay, I won't draw it.

01:12:30.220 | So from now on, there's this mnemonic.

01:12:32.500 | I draw an interval when I wanna talk about the sphere,

01:12:34.640 | and you remember that the end points of the interval

01:12:37.260 | mean a strong circle, that's all.

01:12:39.300 | And they say, yeah, I see, that's a sphere, good.

01:12:41.260 | Now, we wanna talk about the product of two spheres.

01:12:44.300 | That's four dimensional, how can I visualize it?

01:12:47.020 | Easy, you just take an interval and another interval,

01:12:50.900 | that's just gonna be a square.

01:12:52.340 | A square is a four dimensional space?

01:12:56.460 | Yeah, why is that?

01:12:57.620 | Well, at each point on the square, there's two circles,

01:13:02.140 | one for each of those directions you drew.

01:13:04.860 | And when you get to the boundaries of each direction,

01:13:07.100 | one of the circles shrink on each edge of that square.

01:13:09.940 | And when you get to the corners of the square,

01:13:11.700 | all both circles shrink.

01:13:13.660 | This is a sphere times a sphere.

01:13:15.740 | I have defined an interval.

01:13:17.460 | I just described for you a four dimensional space.

01:13:20.260 | Do you want a six dimensional space?

01:13:21.640 | No problem.

01:13:22.980 | Take a corner of a room.

01:13:25.780 | In fact, if you want to have a sphere times a sphere,

01:13:28.140 | times a sphere, times a sphere.

01:13:30.340 | Take a cube.

01:13:31.180 | A cube is a rendition of this six dimensional space.

01:13:36.780 | Two sphere times another sphere, times another sphere,

01:13:39.340 | where three of the circles I'm not drawing for you.

01:13:41.860 | For each one of those directions, there's another circle.

01:13:43.740 | But each time you get to the boundary of the cube,

01:13:45.860 | one circle shrinks.

01:13:47.100 | When the boundaries meet, two circles shrink.

01:13:48.780 | When three boundaries meet, all the three circles shrink.

01:13:51.940 | So I just give you a picture.

01:13:53.300 | Now, mathematicians come up with amazing things.

01:13:55.900 | Like, you know what?

01:13:56.740 | I want to take a point in space and blow it up.

01:13:59.420 | You know, these concepts like topology and geometry,

01:14:01.940 | complicated, how do you do?

01:14:03.540 | In this picture, it's very easy.

01:14:05.380 | Blow it up in this picture means the following.

01:14:07.860 | You think about this cube, you go to the corner

01:14:10.300 | and you chop off a corner.

01:14:12.620 | Chopping off the corner replaces a point.

01:14:15.140 | - Yeah.

01:14:15.980 | - It replaces a point by a triangle.

01:14:17.500 | That's called blowing up a point.

01:14:18.900 | And then this triangle is what they call P2,

01:14:20.820 | projective two space.

01:14:22.300 | But these pictures are very physical and you feel it.

01:14:25.040 | There's nothing amazing.

01:14:26.260 | I'm not talking about six dimension.

01:14:28.020 | Four plus six is 10, the dimension of string theory.

01:14:30.500 | So we can visualize it, no problem.

01:14:32.300 | - Okay, so that's building the intuition

01:14:34.260 | to a complicated world of string theory.

01:14:36.860 | Nevertheless, these objects are really small.

01:14:39.940 | And just like you said,

01:14:41.340 | experimental validation is very difficult

01:14:43.220 | because the objects are way smaller

01:14:45.340 | than anything that we currently have the tools

01:14:48.300 | and accelerators and so on to reveal through experiment.

01:14:53.300 | So there's a kind of skepticism

01:14:56.260 | that's not just about the nature of the theory

01:14:59.340 | because of the 10 dimensions as you've explained,

01:15:01.780 | but in that we can't experimentally validate it.

01:15:04.980 | And it doesn't necessarily, to date,

01:15:07.460 | maybe you can correct me,

01:15:08.980 | predict something fundamentally new.

01:15:12.020 | So it's beautiful as an explaining theory,

01:15:16.100 | which means that it's very possible

01:15:18.200 | that it is a fundamental theory

01:15:19.900 | that describes reality and unifies the laws,

01:15:22.580 | but there's still a kind of skepticism.

01:15:25.500 | And me from sort of an outside observer perspective

01:15:30.500 | have been observing a little bit of a growing cynicism

01:15:34.300 | about string theory in the recent few years.

01:15:37.300 | Can you describe the cynicism about sort of,

01:15:40.580 | by cynicism I mean a cynicism about the hope for this theory

01:15:45.580 | of pushing theoretical physics forward?

01:15:49.820 | - Yes.

01:15:50.820 | - Can you do describe why this is cynicism

01:15:53.980 | and how do we reverse that trend?

01:15:56.020 | - Yes, first of all,

01:15:57.100 | the criticism for string theory is healthy

01:16:02.100 | in a sense that in science,

01:16:04.620 | we have to have different viewpoints and that's good.

01:16:07.260 | So I welcome criticism.

01:16:09.180 | And the reason for criticism,

01:16:12.420 | and I think that is a valid reason,

01:16:13.860 | is that there has been zero experimental evidence

01:16:15.860 | for string theory.

01:16:17.100 | That is, no experiment has been done

01:16:20.300 | to show that there's this little loop

01:16:22.300 | of energy moving around.

01:16:24.100 | And so that's a valid objection and valid worry.

01:16:28.540 | And if I were to say, you know what,

01:16:30.220 | string theory can never be verified or experimentally checked

01:16:32.740 | that's the way it is,

01:16:34.460 | they would have every right to say

01:16:36.280 | what you're talking about is not science.

01:16:37.860 | Because in science,

01:16:38.740 | we will have to have experimental consequences and checks.

01:16:42.380 | The difference between string theory

01:16:44.580 | and something which is not scientific

01:16:45.780 | is that string theory has predictions.

01:16:47.660 | The problem is that the predictions we have today

01:16:49.660 | of string theory is hard to access

01:16:51.420 | by experiments available with the energies

01:16:54.060 | we can achieve with a colliders today.

01:16:56.660 | It doesn't mean there's a problem with string theory,

01:16:58.340 | it just means technologically we're not that far ahead.

01:17:01.620 | Now, we can have two attitudes.

01:17:04.300 | You say, well, if that's the case,

01:17:06.140 | why are you studying this subject?

01:17:07.300 | Because you can't do experiment today.

01:17:09.440 | Now, this is becoming a little bit more like mathematics

01:17:12.480 | in that sense.

01:17:13.320 | You say, well, I want to learn,

01:17:15.400 | I want to know how the nature works

01:17:16.680 | even though I cannot prove it today

01:17:18.480 | that this is it because of experiments.

01:17:21.060 | That should not prevent my mind not to think about it.

01:17:23.480 | - That's right.

01:17:24.320 | - So that's the attitude many string theorists follow

01:17:26.260 | that it should be like this.

01:17:28.200 | Now, so that's an answer to the criticism,

01:17:30.960 | but there's actually a better answer

01:17:32.320 | to the criticism I would say.

01:17:34.120 | We don't have experimental evidence for string theory,

01:17:37.420 | but we have theoretical evidence for string theory.

01:17:39.300 | And what do I mean by theoretical evidence

01:17:41.640 | for string theory?

01:17:42.900 | String theory has connected

01:17:44.480 | different parts of physics together.

01:17:46.320 | It didn't have to.

01:17:48.660 | It has brought connections between part of physics,

01:17:52.180 | although, suppose you're just interested

01:17:53.600 | in particle physics.

01:17:54.760 | Suppose you're not even interested in gravity at all.

01:17:57.880 | It turns out there are properties

01:17:59.700 | of certain particle physics models

01:18:02.340 | that string theory has been able to solve using gravity,

01:18:06.060 | using ideas from string theory,

01:18:08.200 | ideas known as holography,

01:18:10.680 | which is relating something which has to do with particles

01:18:13.140 | to something having to do with gravity.

01:18:15.600 | Why did it have to be this rich?

01:18:17.760 | The subject is very rich.

01:18:20.220 | It's not something we were smart enough to develop.

01:18:23.000 | It came at us.

01:18:23.840 | As I explained to you,

01:18:24.660 | the development of string theory

01:18:25.720 | came from accidental discovery.

01:18:28.080 | It wasn't because we were smart enough

01:18:29.760 | to come up with the idea,

01:18:30.840 | oh yeah, string of course has gravity in it.

01:18:32.360 | No, it was accidental discovery.

01:18:34.440 | So some people say it's not fair to say

01:18:36.160 | we have no evidence for string theory.

01:18:38.100 | Graviton, gravity is an evidence for string theory.

01:18:41.280 | It's predicted by string theory.

01:18:43.260 | We didn't put it by hand, we got it.

01:18:45.840 | So there's a qualitative check.

01:18:47.920 | Okay, gravity is a prediction of string theory.

01:18:51.080 | It's a postdiction because we know gravity existed.

01:18:53.480 | But still, logically, it is a prediction

01:18:56.740 | because really we didn't know it had,

01:18:59.320 | that graviton that we later learned that,

01:19:00.960 | oh, that's the same as gravity.

01:19:02.720 | So literally that's the way it was discovered.

01:19:04.440 | It wasn't put in by hand.

01:19:06.240 | So there are many things like that,

01:19:08.440 | that there are different facets of physics,

01:19:11.440 | like questions in condensed matter physics,

01:19:13.360 | questions of particle physics,

01:19:15.240 | questions about this and that have come together

01:19:18.040 | to find beautiful answers

01:19:20.160 | by using ideas from string theory

01:19:23.020 | at the same time as a lot of new math has emerged.

01:19:27.040 | That's an aspect which I wouldn't emphasize

01:19:29.640 | as evidence to physicists necessarily,

01:19:31.920 | because they would say, "Okay, great, you got some math,

01:19:33.800 | "but what does it do with reality?"

01:19:35.600 | But as I explained, many of the physical principles

01:19:38.680 | we know of have beautiful math underpinning them.

01:19:41.700 | So certainly leads further confidence

01:19:45.200 | that we may not be going astray,

01:19:46.800 | even though that's not the full proof as we know.

01:19:49.200 | So there are these aspects that give further evidence

01:19:52.280 | for string theory, connections between each other,

01:19:55.300 | connection with the real world,

01:19:56.240 | but then there are other things that come about,

01:19:58.480 | and I can try to give examples of that.

01:20:01.120 | So these are further evidences,

01:20:03.000 | and these are certain predictions of string theory.

01:20:05.840 | They are not as detailed as we want,

01:20:08.880 | but there are still predictions.

01:20:10.480 | Why is the dimension of space and time three plus one?

01:20:14.920 | Say, I don't know, just deal with it, three plus one.

01:20:20.160 | But in physics, we want to know why.

01:20:23.320 | Well, take a random dimension from one to infinity.

01:20:26.480 | What's your random dimension?

01:20:28.200 | A random dimension from one to infinity would not be four.

01:20:32.320 | Eight would most likely be a humongous number,

01:20:35.600 | if not infinity.

01:20:36.440 | I mean, if you choose any reasonable distribution,

01:20:39.600 | which goes from one to infinity,

01:20:41.320 | three or four would not be your pick.

01:20:43.160 | The fact that we are in three or four dimension

01:20:45.860 | is already strange.

01:20:47.380 | The fact that strings of story, I cannot go beyond 10,

01:20:50.880 | or maybe 11 or something.

01:20:52.700 | The fact that there's this upper bound,

01:20:54.600 | the range is not from one to infinity,

01:20:56.200 | it's from one to 10 or 11 or whatnot.

01:20:58.700 | It already brings a natural prior,

01:21:00.720 | oh yeah, three or four is, you know,

01:21:02.080 | it's just on the average.

01:21:03.160 | If you pick some of the compactifications,

01:21:05.120 | then it could easily be that.

01:21:06.000 | So in other words, it makes it much more possible

01:21:08.840 | that it could be theory of our universe.

01:21:11.040 | So the fact that the dimension already is so small,

01:21:14.200 | it should be surprising.

01:21:15.960 | We don't ask that question.

01:21:17.400 | We should be surprised,

01:21:18.960 | because we could have conceived of universes

01:21:21.000 | with our predimension.

01:21:22.400 | Why is it that we have such a small dimension?

01:21:24.260 | That's number one.

01:21:25.460 | - So, oh, so good theory of the universe

01:21:28.220 | should give you an intuition of the why it's four,

01:21:31.580 | or three plus one.

01:21:33.580 | And it's not obvious that it should be,

01:21:35.620 | that that should be explained.

01:21:37.200 | We take that as an assumption,

01:21:40.540 | but that's a thing that should be explained.

01:21:43.340 | - Yeah, so we haven't explained that in string theory.

01:21:45.020 | Actually, I did write a model within string theory

01:21:47.420 | to try to describe why we end up with three

01:21:50.920 | plus one space-time dimensions,

01:21:52.520 | which are big compared to the rest of them.

01:21:54.720 | And even though this has not been,

01:21:57.000 | the technical difficulties to prove it is still not there,

01:22:00.240 | but I will explain the idea,

01:22:01.640 | because the idea connects to some other piece

01:22:03.280 | of elegant math, which is the following.

01:22:06.280 | Consider a universe made of a box, a three-dimensional box.

01:22:11.280 | Or in fact, if we start in string theory,

01:22:13.280 | nine-dimensional box,

01:22:14.280 | because we have nine spatial dimensions at one time.

01:22:17.200 | So imagine a nine-dimensional box.

01:22:20.240 | So we should imagine the box of a typical size

01:22:23.380 | of the string, which is small.

01:22:25.620 | So the universe would naturally start

01:22:27.660 | with a very tiny nine-dimensional box.

01:22:29.640 | What do strings do?

01:22:31.400 | Well, strings go around the box

01:22:33.980 | and move around and vibrate and all that,

01:22:35.360 | but also they can wrap around one side of the box

01:22:38.580 | to the other, because I'm imagining a box

01:22:41.380 | with periodic boundary conditions,

01:22:42.740 | so what we call the torus.

01:22:44.580 | So the string can go from one side to the other.

01:22:46.940 | This is what we call a winding string.

01:22:48.540 | The string can wind around the box.

01:22:50.400 | Now, suppose you now evolve the universe.

01:22:55.180 | Because there's energy, the universe starts to expand,

01:22:58.540 | but it doesn't expand too far.

01:23:01.220 | Why is it?

01:23:02.320 | Well, because there are these strings

01:23:04.700 | which are wrapped around from one side of the wall

01:23:06.880 | to the other.

01:23:08.060 | When the universe, the walls of the universe are growing,

01:23:11.380 | it is stretching the string,

01:23:12.860 | and the strings are becoming very, very massive.

01:23:15.940 | So it becomes difficult to expand.

01:23:17.660 | It kind of puts a halt on it.

01:23:19.100 | In order to not put a halt,

01:23:21.420 | a string which is going this way

01:23:22.800 | and a string which is going that way

01:23:24.220 | should intersect each other

01:23:26.980 | and disconnect each other and unwind.

01:23:29.420 | So a string which winds this way

01:23:31.220 | and a string which winds the opposite way

01:23:33.260 | should find each other to reconnect

01:23:36.980 | and this way disappear.

01:23:38.700 | So if they find each other and they disappear.

01:23:42.180 | But how can strings find each other?

01:23:43.620 | Well, the string moves, and another string moves.

01:23:46.980 | A string is one dimensional.

01:23:48.460 | One plus one is two, and one plus one is two,

01:23:51.880 | and two plus two is four.

01:23:53.660 | In four dimensional space time, they will find each other.

01:23:57.000 | In a higher dimensional space time,

01:23:58.780 | they typically miss each other.

01:24:00.740 | - Oh, interesting.

01:24:01.580 | - So if the dimension were too big,

01:24:03.180 | they would miss each other.

01:24:04.140 | They wouldn't be able to expand.

01:24:05.860 | So in order to expand, they have to find each other,

01:24:08.140 | and three of them can find each other,

01:24:10.260 | and those can expand, and the other one would be stuck.

01:24:12.380 | - So that explains why within string theory,

01:24:14.580 | these particular dimensions are really big

01:24:16.540 | and full of exciting stuff.

01:24:17.820 | - That could be an explanation.

01:24:18.900 | That's a model we suggested with my colleague Brandenberger,

01:24:22.940 | but it turns out to be related to a deep piece of math.

01:24:24.980 | You see, for mathematicians,

01:24:28.000 | manifolds of dimension bigger than four are simple.

01:24:31.260 | Four dimension is the hardest dimension for math,

01:24:35.040 | it turns out, and it turns out the reason it's difficult

01:24:39.020 | is the following.

01:24:39.840 | It turns out that in higher dimension,

01:24:42.300 | you use what's called surgery in mathematical terminology

01:24:46.140 | where you use these two-dimensional tubes

01:24:49.160 | to maneuver them off of each other.

01:24:51.380 | So you have two plus two becoming four.

01:24:54.100 | In higher than four dimension,

01:24:55.340 | you can pass them through each other

01:24:57.140 | without them intersecting.

01:24:58.900 | In four dimension, two plus two

01:25:01.260 | doesn't allow you to pass them through each other,

01:25:04.100 | so the same techniques that work in higher dimension

01:25:06.220 | don't work in four dimension because two plus two is four.

01:25:09.180 | The same reasoning I was just telling you

01:25:11.060 | about strings finding each other in four

01:25:13.740 | ends up to be the reason why four is much more complicated

01:25:17.100 | to classify for mathematicians as well.

01:25:19.300 | So there might be these things.

01:25:21.140 | So I cannot say that this is the reason

01:25:23.740 | that string theory is giving you three plus one,

01:25:25.900 | but it could be a model for it.

01:25:27.340 | And so there are these kind of ideas

01:25:29.260 | that could underlie why we have three extra dimensions

01:25:32.420 | which are large and the rest of them are small.

01:25:33.760 | But absolutely, we have to have a good reason.

01:25:35.940 | We cannot leave it like that.

01:25:37.100 | - Can I ask a tricky human question?

01:25:40.540 | So you are one of the seminal figures in string theory.

01:25:44.660 | You got the Breakthrough Prize.

01:25:46.260 | You've worked with Edward Witten.

01:25:48.060 | There's no Nobel Prize that has been given

01:25:51.100 | on string theory.

01:25:52.280 | You know, credit assignment is tricky in science.

01:25:56.380 | It makes you quite sad, especially big, like LIGO,

01:26:00.020 | big experimental projects

01:26:01.460 | when so many incredible people have been involved

01:26:04.820 | and yet the Nobel Prize is annoying

01:26:06.880 | in that it's only given to three people.

01:26:08.900 | Who do you think gets the Nobel Prize

01:26:11.020 | for string theory at first?

01:26:14.620 | If it turns out that it,

01:26:16.620 | if not in full, then in part,

01:26:20.040 | is a good model of the way the physics

01:26:25.420 | of the universe works.

01:26:27.260 | Who are the key figures?

01:26:28.700 | Maybe let's put Nobel Prize aside.

01:26:31.100 | Who are the key figures?

01:26:31.940 | - Okay, I like the second version of the question.

01:26:34.060 | I think to try to give a prize to one person

01:26:36.820 | in string theory doesn't do justice

01:26:38.340 | to the diversity of the subject.

01:26:39.860 | That to me is--

01:26:40.820 | - So there was quite a lot of incredible people

01:26:43.100 | in the history of string theory.

01:26:43.940 | - Quite a lot of people.

01:26:45.220 | I mean, starting with Venetiano,

01:26:46.580 | who wasn't talking about strings.

01:26:48.180 | I mean, he wrote down the beginning of a string.

01:26:50.420 | So we cannot ignore that for sure.

01:26:51.980 | And so you start with that

01:26:53.860 | and you go on with various other figures and so on.

01:26:56.140 | So there are different epochs in string theory

01:26:58.460 | and different people have been pushing it.

01:27:00.380 | So for example, the early epoch,

01:27:01.800 | we just told you people like Venetiano and Nambu

01:27:05.740 | and the Suskind and others were pushing it,

01:27:07.560 | Green and Schwartz were pushing it and so forth.

01:27:09.860 | So this was, or Sherrick and so on.

01:27:11.620 | So these were the initial periods of pioneers,

01:27:15.180 | I would say, of string theory.

01:27:16.780 | And then there were the mid '80s

01:27:19.620 | that Edward Witten was the major proponent of string theory

01:27:22.660 | and he really changed the landscape of string theory

01:27:25.820 | in terms of what people do and how we view it.

01:27:28.100 | And I think his efforts brought a lot of attention

01:27:31.300 | to the community about high energy community

01:27:34.620 | to focus on this effort as the correct theory

01:27:37.060 | of unification of forces.

01:27:38.480 | So he brought a lot of research as well as, of course,

01:27:41.120 | the first rate work he himself did to this area.

01:27:44.420 | So that's in mid '80s and onwards and also in mid '90s

01:27:47.040 | where he was one of the proponents

01:27:49.040 | of the duality revolution in string theory.

01:27:51.640 | And with that came a lot of these other ideas

01:27:54.240 | that led to breakthroughs involving, for example,

01:27:58.400 | the example I told you about black holes and holography

01:28:00.920 | and the work that was later done by Maldacena

01:28:03.920 | about the properties of duality between particle physics

01:28:06.640 | and quantum gravity and the connections,

01:28:09.400 | deeper connections of holography and it continues.

01:28:12.960 | And there are many people within this range

01:28:15.360 | which I haven't even mentioned.

01:28:16.640 | They have done fantastic important things.

01:28:19.000 | How it gets recognized I think is secondary in my opinion

01:28:23.520 | than the appreciation that the effort is collective.

01:28:27.720 | That in fact, that to me is the more important part

01:28:30.440 | of science that gets forgotten.

01:28:32.260 | For some reason, humanity likes heroes

01:28:35.200 | and science is no exception.

01:28:36.440 | We like heroes, but I personally try to avoid that trap.

01:28:40.520 | I feel in my work, most of my work is with colleagues.

01:28:44.840 | I have much more collaborations than sole author papers

01:28:49.280 | and I enjoy it and I think that that's to me

01:28:51.880 | one of the most satisfying aspects of science

01:28:54.360 | is to interact and learn and debate ideas with colleagues

01:28:59.280 | because that influx of ideas enriches it

01:29:02.080 | and that's why I find it interesting.

01:29:05.720 | To me, science, if I was in an island

01:29:08.260 | and if I was developing string theory by myself

01:29:10.400 | and had nothing to do with anybody,

01:29:11.800 | it would be much less satisfying in my opinion.

01:29:14.160 | Even if I could take credit, I did it,

01:29:17.180 | it won't be as satisfying.

01:29:18.360 | - Sitting alone with a big metal drinking champagne.

01:29:22.200 | - No, I think to me the collective work is more exciting

01:29:25.760 | and you mentioned my getting the breakthrough.

01:29:28.320 | When I was getting it, I made sure to mention

01:29:30.520 | that it is because of the joint work

01:29:32.320 | that I've done with colleagues.

01:29:33.800 | At that time, it was around 180 or so collaborators

01:29:36.840 | and I acknowledged them in the webpage for them.

01:29:39.760 | I write all of their names

01:29:41.160 | and the collaborations that led to this.

01:29:42.720 | So to me, science is fun when it's collaboration

01:29:46.560 | and yes, there are more important

01:29:48.760 | and less important figures as in any field

01:29:51.240 | and that's true, that's true in string theory as well

01:29:53.320 | but I think that I would like to view this

01:29:55.940 | as a collective effort.

01:29:56.960 | - So setting the heroes aside,

01:30:00.520 | the Nobel Prize is a celebration of,

01:30:04.280 | what's the right way to put it?

01:30:05.760 | That this idea turned out to be right.

01:30:08.240 | (laughing)

01:30:09.640 | So like you look at Einstein didn't believe in black holes

01:30:13.760 | and then black holes got their Nobel Prize.

01:30:17.840 | Do you think string theory will get its Nobel Prize,

01:30:22.080 | Nobel Prizes?

01:30:23.800 | If you were to bet money,

01:30:25.440 | if this was an investment meeting

01:30:27.840 | and we had to bet all our money,

01:30:30.000 | do you think he gets the Nobel Prizes?

01:30:32.000 | - I think it's possible that none of the living physicists

01:30:34.560 | will get the Nobel Prize in string theory

01:30:36.000 | but somebody will.

01:30:37.040 | (laughing)

01:30:38.000 | Because unfortunately, the technology available today

01:30:41.600 | is not very encouraging

01:30:43.080 | in terms of seeing directly evidence for string theory.

01:30:46.280 | - Do you think it's ultimately boils down

01:30:48.120 | to the Nobel Prize will be given

01:30:49.640 | when there is some direct or indirect evidence?

01:30:53.280 | - There would be but I think that part

01:30:55.760 | of this breakthrough prize was precisely the appreciation

01:30:58.800 | that when we have sufficient evidence,

01:31:01.520 | theoretical as it is, not experiment,

01:31:04.200 | because of this technology lag,

01:31:06.240 | you appreciate what you think is the correct path.

01:31:08.960 | So there are many people who have been recognized precisely

01:31:12.560 | because they may not be around

01:31:14.080 | when it actually gets experimented,

01:31:16.160 | even though they discovered it.

01:31:17.880 | So there are many things like that

01:31:19.880 | that's going on in science.

01:31:21.520 | So I think that I would want to attach less significance

01:31:25.640 | to the recognitions of people.

01:31:28.280 | And I have a second review on this,

01:31:31.120 | which is there are people who look at these works

01:31:35.600 | that people have done and put them together

01:31:37.520 | and make the next big breakthrough.

01:31:40.000 | And they get identified with, perhaps rightly,

01:31:43.820 | with many of these new visions.

01:31:48.000 | But they are on the shoulders of these little scientists

01:31:51.400 | which don't get any recognition.

01:31:54.040 | You know, yeah, you did this little work.

01:31:55.480 | Oh yeah, you did this little work.

01:31:56.920 | Oh yeah, yeah, five of you.

01:31:57.760 | Oh yeah, these showed this pattern.

01:31:59.280 | And then somebody else, it's not fair.

01:32:01.800 | To me, those little guys,

01:32:04.120 | which kind of like seem to do a little calculation here,

01:32:07.400 | a little thing there,

01:32:08.240 | which doesn't rise to the occasion

01:32:10.120 | of this grandiose kind of thing,

01:32:11.800 | doesn't make it to the New York Times headlines and so on,

01:32:15.100 | deserve a lot of recognition.

01:32:17.080 | And I think they don't get enough.

01:32:18.400 | I would say that there should be this Nobel Prize for,

01:32:21.760 | you know, they have these Doctors Without Borders,

01:32:23.920 | they're a huge group, they should do similar thing.

01:32:26.080 | These String Theorists Without Borders,

01:32:27.400 | kind of everybody's doing a lot of work.

01:32:29.320 | And I think that I would like to see

01:32:31.040 | that effort recognized.

01:32:32.840 | - I think in the long arc of history,

01:32:35.460 | we're all little guys and girls

01:32:38.080 | standing on the shoulders of each other.

01:32:40.640 | I mean, it's all going to look tiny in retrospect.

01:32:44.560 | We celebrate the New York Times,

01:32:46.940 | you know, as a newspaper,

01:32:51.040 | or the idea of a newspaper in a few centuries from now

01:32:55.000 | will be long forgotten.

01:32:56.200 | - Yes, I agree with that.

01:32:57.680 | Especially in the context of string theory,

01:32:59.000 | we should have very long term view.

01:33:00.760 | - Yes, exactly.

01:33:01.960 | Just as a tiny tangent, we mentioned Edward Witten,

01:33:05.400 | and he, in a bunch of walks of life for me as an outsider,

01:33:09.800 | comes up as a person who is widely considered

01:33:13.560 | as like one of the most brilliant people

01:33:16.480 | in the history of physics,

01:33:17.920 | just as a powerhouse of a human.

01:33:21.520 | Like the exceptional places that a human mind can rise to.

01:33:26.520 | - Yes.

01:33:28.120 | - You've gotten a chance to work with him.

01:33:29.720 | What's he like?

01:33:30.560 | - Yes, more than that, he was my advisor, PhD advisor.

01:33:34.280 | So I got to know him very well,

01:33:35.960 | and I benefited from his insights.

01:33:37.520 | In fact, what you said about him is accurate.

01:33:40.120 | He's not only brilliant,

01:33:42.320 | but he's also multifaceted in terms of the impact

01:33:46.600 | he has had in not only physics, but also mathematics.

01:33:49.560 | He's gotten the Fields Medal

01:33:50.920 | because of his work in mathematics,

01:33:52.480 | and rightly so, he has used his knowledge of physics

01:33:57.480 | in a way which impacted deep ideas in modern mathematics.

01:34:01.680 | And that's an example of the power of these ideas

01:34:06.000 | in modern high energy physics and string theory,

01:34:08.000 | that the applicability of it to modern mathematics.

01:34:11.640 | So he's quite an exceptional individual.

01:34:16.360 | We don't come across such people a lot in history.

01:34:19.560 | So I think, yes, indeed,

01:34:20.720 | he's one of the rare figures in this history of subject.

01:34:24.160 | He has had great impact on a lot of aspects

01:34:26.520 | of not just string theory,

01:34:27.360 | a lot of different areas in physics,

01:34:29.400 | and also, yes, in mathematics as well.

01:34:32.760 | So I think what you said about him is accurate.

01:34:35.000 | I had the pleasure of interacting with him as a student,

01:34:37.600 | and later on as colleagues,

01:34:39.440 | writing papers together and so on.

01:34:41.520 | - What impact did he have on your life?

01:34:43.200 | Like, what have you learned from him?

01:34:46.080 | If you were to look at the trajectory of your mind,

01:34:48.240 | of the way you approach science and physics and mathematics,

01:34:51.560 | how did he perturb that trajectory in a way?

01:34:54.400 | - Yes, he did, actually.

01:34:55.620 | So I can explain, because when I was a student,

01:34:57.680 | I had the biggest impact by him,

01:35:00.160 | clearly as a grad student at Princeton.

01:35:02.420 | So I think that was a time

01:35:04.080 | where I was a little bit confused

01:35:06.680 | about the relation between math and physics.

01:35:08.800 | I got a double major in mathematics and physics at MIT,

01:35:12.140 | because I really enjoyed both,

01:35:14.600 | and I liked the elegance and the rigor of mathematics,

01:35:18.280 | and I liked the power of ideas in physics

01:35:21.180 | and its applicability to reality

01:35:22.760 | and what it teaches about the real world around us.

01:35:26.380 | But I saw this tension

01:35:27.980 | between rigorous thinking in mathematics

01:35:31.540 | and lack thereof in physics,

01:35:33.440 | and this troubled me to no end.

01:35:36.260 | I was troubled by that.

01:35:38.020 | So I was at crossroads

01:35:40.140 | when I decided to go to graduate school in physics,

01:35:42.200 | because I did not like some of the lack of rigors

01:35:44.800 | I was seeing in physics.

01:35:46.000 | On the other hand, to me,

01:35:48.200 | mathematics, even though it was rigorous,

01:35:49.920 | I'm thinking it sometimes were,

01:35:51.840 | I didn't see the point of it.

01:35:53.040 | In other words, when I see,

01:35:55.280 | when I, you know, the math theorem by itself

01:35:56.780 | could be beautiful, but I really wanted more than that.

01:35:58.760 | I want to say, okay, what does it teach us

01:36:00.320 | about something else, something more than just math?

01:36:02.880 | So I wasn't that enamored with just math,

01:36:05.240 | but physics was a little bit bothersome.

01:36:07.320 | Nevertheless, I decided to go to physics,

01:36:08.960 | and I decided to go to Princeton,

01:36:10.840 | and I started working with Edward Witten

01:36:13.280 | as my thesis advisor.

01:36:14.920 | And at that time, I was trying to put physics

01:36:20.480 | in rigorous mathematical terms.

01:36:22.400 | I took quantum field theory,

01:36:23.840 | I tried to make rigorous out of it, and so on.

01:36:26.800 | And no matter how hard I was trying,

01:36:29.080 | I was not being able to do that,

01:36:31.760 | and I was falling behind from my classes.

01:36:33.800 | I was not learning much physics,

01:36:35.680 | and I was not making it rigorous.

01:36:37.000 | And to me, it was this dichotomy between math and physics.

01:36:40.600 | What am I doing?

01:36:41.440 | I like math, but this is not exactly rigorous.

01:36:43.980 | There comes Edward Witten as my advisor,

01:36:47.520 | and I see him in action, thinking about math and physics.

01:36:52.080 | He was amazing in math.

01:36:53.680 | He knew all about the math.

01:36:54.920 | It was no problem with him.

01:36:56.280 | But he thought about physics in a way

01:36:58.600 | which did not find this tension between the two.

01:37:02.640 | It was much more harmonious.

01:37:04.080 | For him, he would draw the Feynman diagrams,

01:37:06.520 | but he wouldn't view it as a formalism.

01:37:08.920 | He was viewed, oh yeah, the particle goes over there,

01:37:10.580 | and this is what's going on.

01:37:11.520 | And I said, wait, you're thinking,

01:37:13.000 | really, is this particle, this is really electron

01:37:15.320 | going there, oh yeah, yeah.

01:37:16.160 | It's not the formal rules of perturbation.

01:37:18.840 | No, no, no.

01:37:19.920 | You just feel like the electron,

01:37:21.160 | you're moving with this guy and do that, and so on,

01:37:23.040 | and you're thinking invariantly about physics,

01:37:24.840 | or the way he thought about relativity.

01:37:27.760 | Like, I was thinking about this momentum system.

01:37:29.840 | He was thinking invariantly about physics,

01:37:31.500 | just like the way you think about invariant concepts

01:37:34.000 | in relativity, which don't depend on the frame of reference.

01:37:36.480 | He was thinking about the physics in invariant ways,

01:37:39.840 | the way that gives you a bigger perspective.

01:37:42.940 | So this gradually helped me appreciate

01:37:46.480 | that interconnections between ideas and physics

01:37:50.200 | replaces mathematical rigor.

01:37:52.760 | That the different facets reinforce each other.

01:37:56.360 | You say, oh, I cannot rigorously define

01:37:58.560 | what I mean by this, but this thing connects

01:38:00.400 | with this other physics I've seen, and this other thing,

01:38:02.920 | and they together form an elegant story.

01:38:06.360 | And that replaced for me what I believed as a solidness,

01:38:09.760 | which I found in math as a rigor, solid.

01:38:13.000 | I found that replaced the rigor and solidness in physics.

01:38:16.160 | So I found, okay, that's the way you can hang on to.

01:38:19.200 | It is not wishy-washy.

01:38:20.320 | It's not like somebody is just not being able to prove it,

01:38:23.200 | just making up a story.

01:38:24.720 | It was more than that,

01:38:25.960 | and it was no tension with mathematics.

01:38:28.480 | In fact, mathematics was helping it, like friends.

01:38:31.720 | And so much more harmonious and gives insights to physics.

01:38:34.820 | So that's, I think, one of the main things I learned

01:38:36.640 | from interactions with Witten.

01:38:38.680 | And I think that now perhaps I have taken

01:38:41.880 | that to a far extreme.

01:38:43.480 | Maybe he wouldn't go this far as I have.

01:38:45.080 | Namely, I use physics to define new mathematics

01:38:48.600 | in a way which would be far less rigorous

01:38:50.960 | than a physicist might necessarily believe,

01:38:53.400 | because I take the physical intuition,

01:38:55.720 | perhaps literally in many ways, that could teach us math.

01:38:58.880 | So now I've gained so much confidence in physical intuition

01:39:02.360 | that I make bold statements that sometimes

01:39:05.240 | takes math friends off guard.

01:39:08.440 | So an example of it is mirror symmetry.

01:39:10.880 | So we were studying these compactification

01:39:14.440 | of string geometries.

01:39:15.800 | This is after my PhD now.

01:39:17.760 | By the time I'd come to Harvard,

01:39:19.560 | we were studying these aspects of string compactification

01:39:21.680 | on these complicated manifolds, six-dimensional spaces,

01:39:24.520 | called Calabi-Yau manifolds, very complicated.

01:39:28.080 | And I noticed with a couple other colleagues

01:39:31.140 | that there was a symmetry in physics suggested

01:39:35.220 | between different Calabi-Yau's.

01:39:36.600 | It suggested that you couldn't actually compute

01:39:40.400 | the Euler characteristic of a Calabi-Yau.

01:39:42.560 | Euler characteristic is counting the number of points

01:39:45.560 | minus the number of edges plus the number of faces minus.

01:39:48.440 | So you can count the alternating sequence

01:39:50.520 | of properties of the space,

01:39:51.800 | which is the topological property of a space.

01:39:54.720 | So Euler characteristic of the Calabi-Yau

01:39:56.680 | was a property of the space.

01:39:57.920 | And so we noticed that from the physics formalism,

01:40:01.600 | if string moves in a Calabi-Yau, you cannot distinguish,

01:40:05.520 | we cannot compute the Euler characteristic.

01:40:07.360 | You can only compute the absolute value of it.

01:40:10.280 | Now this bothered us because how could you not compute

01:40:13.360 | the actual sign unless the both sides were the same?

01:40:17.280 | So I conjectured maybe for every Calabi-Yau

01:40:21.120 | with the Euler characteristic is positive,

01:40:22.360 | there's one with negative.

01:40:23.600 | I told this to my colleague Yao,

01:40:25.360 | whose namesake is Calabi-Yau,

01:40:28.480 | that I'm making this conjecture.

01:40:31.880 | Is it possible that for every Calabi-Yau,

01:40:33.640 | there's one with the opposite Euler characteristic?

01:40:36.560 | Sounds not reasonable.

01:40:37.840 | I said, why?

01:40:38.680 | He said, well, we know more Calabi-Yau's

01:40:40.320 | with negative Euler characteristics than positive.

01:40:42.820 | I said, but physics says we cannot distinguish them,

01:40:46.520 | at least I don't see how.

01:40:47.840 | So we conjectured that for every Calabi-Yau

01:40:50.480 | with one sign, there's the other one,

01:40:51.720 | despite the mathematical evidence,

01:40:54.080 | despite the mathematical evidence,

01:40:55.720 | despite the expert telling us it's not the right idea.

01:40:58.520 | A few years later, this symmetry, mirror symmetry

01:41:02.080 | between the sign with the opposite sign

01:41:04.520 | was later confirmed by mathematicians.

01:41:06.960 | So this is actually the opposite view.

01:41:09.200 | That is, physics is so sure about it

01:41:11.560 | that you're going against the mathematical wisdom,

01:41:13.720 | telling them they better look for it.

01:41:15.400 | - So taking the physical intuition literally

01:41:19.360 | and then having that drive the mathematics.

01:41:22.080 | - Exactly, and by now we are so confident

01:41:24.000 | about many such examples

01:41:26.160 | that has affected modern mathematics in ways like this,

01:41:30.320 | that we are much more confident

01:41:31.600 | about our understanding of what string theory is.

01:41:33.920 | These are other aspects of why

01:41:36.520 | we feel string theory is correct.

01:41:37.560 | It's doing these kinds of things.

01:41:39.880 | - I've been hearing your talk quite a bit

01:41:41.640 | about string theory, landscape and the swampland.

01:41:46.000 | What the heck are those two concepts?

01:41:47.840 | - Okay, very good question.

01:41:48.840 | So let's go back to what I was describing about Feynman.

01:41:51.920 | Feynman was trying to do these diagrams for graviton

01:41:55.800 | and electrons and all that.

01:41:57.280 | He found that he's getting infinities he cannot resolve.

01:42:00.880 | Okay, the natural conclusion is that field theories

01:42:04.040 | and gravity and quantum theory don't go together

01:42:06.560 | and he cannot have it.

01:42:07.600 | So in other words, field theories and gravity

01:42:11.240 | are inconsistent with quantum mechanics, period.

01:42:14.160 | String theory came up with examples,

01:42:18.320 | but didn't address the question more broadly

01:42:20.720 | that is it true that every field theory

01:42:23.080 | can be coupled to gravity in a quantum mechanical way?

01:42:27.360 | It turns out that Feynman was essentially right.

01:42:30.640 | Almost all particle physics theories,

01:42:33.200 | no matter what you add to it,

01:42:35.520 | when you put gravity in it, doesn't work.

01:42:38.320 | Only rare exceptions work.

01:42:40.440 | So string theory are those rare exceptions.

01:42:44.160 | So therefore the general principle

01:42:46.040 | that Feynman found was correct.

01:42:47.800 | Quantum field theory and gravity

01:42:49.620 | and quantum mechanics don't go together,

01:42:51.880 | except for Joule's exceptional cases.

01:42:54.780 | There are exceptional cases.

01:42:56.400 | Okay, the total vastness of quantum field theories

01:43:00.200 | that are there, we call the set of quantum field theories,

01:43:04.320 | possible things.

01:43:05.320 | Which ones can be consistently coupled to gravity?

01:43:08.300 | We call that subspace the landscape.

01:43:13.160 | The rest of them, we call the swampland.

01:43:16.200 | It doesn't mean they are bad quantum field theories,

01:43:18.000 | they are perfectly fine.

01:43:19.960 | But when you couple them to gravity,

01:43:21.880 | they don't make sense, unfortunately.

01:43:24.200 | And it turns out that the ratio of them,

01:43:26.940 | the number of theories which are consistent with gravity

01:43:29.720 | to the ones which are without,

01:43:31.320 | the ratio of the area of the landscape

01:43:33.960 | to the swampland, in other words, is measure zero.

01:43:36.780 | - So the swampland is infinitely large?

01:43:40.240 | - The swampland is infinitely large.

01:43:41.640 | So let me give you one example.

01:43:43.200 | Take a theory in four dimension with matter,

01:43:46.480 | with maximum amount of supersymmetry.

01:43:48.880 | Can you get, it turns out a theory in four dimension

01:43:51.680 | with maximum amount of supersymmetry

01:43:53.800 | is characterized just with one thing, a group.

01:43:56.660 | What we call the gauge group.

01:43:58.360 | Once you pick a group, you have to find the theory.

01:44:01.500 | Okay, so does every group make sense?

01:44:04.120 | Yeah.

01:44:05.280 | As far as quantum field theory, every group makes sense.

01:44:07.480 | There are infinitely many groups,

01:44:08.580 | there are infinitely many quantum field theories.

01:44:10.680 | But it turns out there are only finite number of them

01:44:13.760 | which are consistent with gravity out of that same list.

01:44:16.840 | So you can take any group but only finite number of them,

01:44:19.420 | the ones who's what we call the rank of the group,

01:44:22.680 | the ones whose rank is less than 23.

01:44:26.200 | Any one bigger than rank 23 belongs to the swampland.

01:44:29.720 | There are infinitely many of them.

01:44:31.220 | They're beautiful field theories

01:44:33.100 | but not when you include gravity.

01:44:34.760 | So then this becomes a hopeful thing.

01:44:37.760 | So in other words, in our universe, we have gravity.

01:44:41.920 | Therefore, we are part of that jewel subset.

01:44:44.660 | Now, is this jewel subset small or large?

01:44:50.240 | It turns out that subset is humongous

01:44:54.520 | but we believe still finite.

01:44:56.280 | The set of possibilities is infinite

01:44:59.440 | but the set of consistent ones,

01:45:02.320 | I mean, the set of quantum field theories are infinite

01:45:04.320 | but the consistent ones are finite but humongous.

01:45:07.180 | The fact that they're humongous

01:45:10.120 | is the problem we are facing in string theory

01:45:12.320 | because we do not know which one of these possibilities

01:45:16.600 | the universe we live in.

01:45:18.200 | If we knew, we could make more specific predictions

01:45:20.420 | about our universe.

01:45:21.440 | We don't know.

01:45:22.400 | And that is one of the challenges when string theory,

01:45:24.560 | which point on the landscape,

01:45:26.080 | which corner of this landscape do we live in?

01:45:28.700 | We don't know.

01:45:30.160 | So what do we do?

01:45:31.820 | Well, there are principles that are beginning to emerge.

01:45:35.720 | So I will give you one example of it.

01:45:38.080 | You look at the patterns of what you're getting

01:45:40.760 | in terms of these good ones,

01:45:42.000 | the ones which are in the landscape

01:45:43.420 | compared to the ones which are not.

01:45:45.560 | You find certain patterns.

01:45:46.620 | I'll give you one pattern.

01:45:48.020 | You find in all the ones that you get from string theory,

01:45:52.700 | gravitational force is always there,

01:45:55.560 | but it's always, always the weakest force.

01:45:58.600 | However, you could easily imagine field theories

01:46:03.800 | for which gravity is not the weakest force.

01:46:05.720 | For example, take our universe.

01:46:08.960 | If you take mass of the electron,

01:46:10.680 | if you increase the mass of electron by a huge factor,

01:46:14.080 | the gravitational attraction of the electrons

01:46:16.040 | will be bigger than the electric repulsion

01:46:17.840 | between two electrons,

01:46:19.440 | and the gravity will be stronger, that's all.

01:46:21.700 | It happens that it's not the case in our universe

01:46:25.080 | because electron is very tiny in mass compared to that.

01:46:28.620 | Just like our universe, gravity is the weakest force.

01:46:31.920 | We find in all these other ones

01:46:33.840 | which are part of the good ones,

01:46:36.180 | the gravity is the weakest force.

01:46:37.940 | This is called the weak gravity conjecture.

01:46:40.720 | We conjecture that all the points in the landscape

01:46:43.680 | have this property.

01:46:45.920 | Our universe being just an example of it.

01:46:47.680 | So there are these qualitative features

01:46:49.520 | that we are beginning to see.

01:46:50.920 | But how do we argue for this?

01:46:52.320 | Just by looking patterns?

01:46:53.960 | Just by looking string theory has this?

01:46:55.760 | No, that's not enough.

01:46:56.960 | We need more better reasoning, and it turns out there is.

01:47:01.840 | The reasoning for this turns out to be studying black holes.

01:47:05.040 | Ideas of black holes turn out to put certain restrictions

01:47:09.520 | of what a good quantum field theory should be.

01:47:12.040 | It turns out using black hole,

01:47:14.480 | the fact that the black holes evaporate,

01:47:16.600 | the fact that the black holes evaporate

01:47:20.120 | gives you a way to check the relation

01:47:23.560 | between the mass and the charge of elementary particle.

01:47:25.880 | Because what you can do, you can take a charged particle

01:47:28.520 | and throw it into a charged black hole

01:47:30.480 | and wait it to evaporate.

01:47:32.040 | And by looking at the properties of evaporation,

01:47:34.440 | you find that if it cannot evaporate,

01:47:37.240 | particles whose mass is less than their charge,

01:47:39.500 | then it will never evaporate.

01:47:40.800 | You will be stuck.

01:47:42.120 | And so the possibility of a black hole evaporation

01:47:44.520 | forces you to have particles

01:47:46.600 | whose mass is sufficiently small

01:47:48.720 | so that the gravity is weaker.

01:47:50.480 | So you connect this fact to the other fact.

01:47:52.920 | So we begin to find different facts

01:47:55.040 | that reinforce each other.

01:47:56.320 | So different parts of the physics reinforce each other.

01:47:59.360 | And once they all kind of come together,

01:48:02.360 | you believe that you're getting the principle correct.

01:48:04.260 | So weak gravity conjecture

01:48:05.560 | is one of the principles we believe in

01:48:07.480 | as a necessity of these conditions.

01:48:09.760 | So these are the predictions strictly you're making.

01:48:12.280 | Is that enough?

01:48:13.120 | Well, it's qualitative.

01:48:14.680 | It's a semi-quantity.

01:48:16.160 | It's just that mass of the electron

01:48:17.600 | should be less than some number.

01:48:19.560 | But that number is, if I call that number one,

01:48:23.040 | the mass of the electron turns out

01:48:24.200 | to be 10 to the minus 20 actually.

01:48:25.640 | So it's much less than one.

01:48:26.800 | It's not one.

01:48:28.000 | But on the other hand,

01:48:30.240 | there's a similar reasoning

01:48:31.640 | for a big black hole in our universe.

01:48:34.020 | And if that evaporation should take place,

01:48:36.460 | gives you another restriction,

01:48:37.520 | tells you the mass of the electron

01:48:39.280 | is bigger than 10 to the,

01:48:41.120 | is now in this case, bigger than something.

01:48:43.200 | It shows bigger than 10 to the minus 30 in the Planck unit.

01:48:45.760 | So you find, aha,

01:48:47.420 | the mass of the electron should be less than one,

01:48:49.400 | but bigger than 10 to the minus 30.

01:48:51.320 | In our universe,

01:48:52.160 | the mass of the electron is 10 to the minus 20.

01:48:54.440 | Okay, now this kind of, you could call postdiction,

01:48:57.040 | but I would say it follows from principles

01:48:59.160 | that we now understand from string theory, first principle.

01:49:01.920 | So we are beginning to make these kinds of predictions,

01:49:05.840 | which are very much connected

01:49:08.400 | to aspects of particle physics that we didn't think

01:49:11.120 | are related to gravity.

01:49:12.260 | We thought, just take any electron mass you want.

01:49:14.960 | What's the problem?

01:49:15.800 | It has a problem with gravity.

01:49:17.360 | - And so that conjecture has also a happy consequence

01:49:22.360 | that it explains that our universe,

01:49:24.700 | like why the heck is gravity so weak?

01:49:27.040 | There's a force and that's not only an accident,

01:49:30.580 | but almost a necessity

01:49:32.320 | if these forces are to coexist effectively.

01:49:35.320 | - Exactly, so that's the reinforcement

01:49:38.240 | of what we know in our universe,

01:49:40.720 | but we are finding that as a general principle.

01:49:43.260 | So we want to know what aspects of our universe

01:49:46.360 | is forced on us,

01:49:47.880 | like the weak gravity conjecture and other aspects.

01:49:50.680 | How much of them do we understand?

01:49:52.720 | Can we have particles lighter than neutrinos?

01:49:54.760 | Or maybe that's not possible.

01:49:56.320 | You see, the neutrino mass,

01:49:57.400 | it turns out to be related to dark energy

01:49:59.600 | in a mysterious way.

01:50:01.640 | Naively, there's no relation between dark energy

01:50:04.400 | and the mass of a particle.

01:50:06.520 | We have found arguments

01:50:07.560 | from within the swampland kind of ideas

01:50:10.020 | why it has to be related.

01:50:12.640 | And so there are beginning to be these connections

01:50:15.280 | between consistency of quantum gravity

01:50:17.920 | and aspects of our universe gradually being sharpened.

01:50:22.320 | But we are still far from a precise quantitative prediction

01:50:25.180 | like we have to have such and such,

01:50:26.840 | but that's the hope,

01:50:27.880 | that we are going in that direction.

01:50:29.520 | - Coming up with a theory of everything

01:50:31.000 | that unifies general relativity and quantum field theories

01:50:35.200 | is one of the big dreams of human civilization,

01:50:39.880 | us descendants of apes wondering about how this world works.

01:50:43.400 | So a lot of people dream.

01:50:44.840 | What are your thoughts about sort of other out there ideas,

01:50:50.920 | theories of everything,

01:50:52.760 | or unifying theories?

01:50:56.120 | So there's a quantum loop gravity.

01:50:59.880 | There's also more sort of,

01:51:02.360 | like a friend of mine, Eric Weinstein,

01:51:04.280 | beginning to propose something called geometric unity.

01:51:07.560 | So these kinds of attempts,

01:51:09.060 | whether it's through mathematical physics

01:51:10.760 | or through other avenues,

01:51:12.480 | or with Stephen Wolfram,

01:51:13.760 | a more computational view of the universe.

01:51:16.120 | Again, in his case,

01:51:17.680 | it's these hypergraphs that are very tiny objects as well,

01:51:21.480 | similarly a string theory,

01:51:23.640 | in trying to grapple with this world.

01:51:25.760 | What do you think,

01:51:26.840 | is there any of these theories

01:51:29.200 | that are compelling to you,

01:51:30.160 | that are interesting,

01:51:31.480 | that may turn out to be true,

01:51:33.480 | or at least may turn out to contain ideas that are useful?

01:51:36.200 | - Yes, I think the latter.

01:51:37.280 | I would say that the containing ideas that are true,

01:51:40.720 | is my opinion,

01:51:41.560 | was what some of these ideas might be.

01:51:43.560 | For example, loop quantum gravity,

01:51:45.680 | is to me not a complete theory of gravity in any sense,

01:51:47.880 | but they have some nuggets of truth in them.

01:51:50.320 | And typically what I expect happen,

01:51:52.960 | and I have seen examples of this within string theory,

01:51:55.720 | aspects which we didn't think are part of string theory

01:51:57.960 | come to be part of it.

01:51:58.840 | For example, I'll give you one example.

01:52:00.840 | String was believed to be 10 dimensional.

01:52:03.320 | And then there was this 11 dimensional super gravity.

01:52:05.960 | Nobody know what the heck is that.

01:52:08.120 | Why are we getting 11 dimensional super gravity,

01:52:10.040 | where a string is saying it should be 10 dimensional?

01:52:11.720 | 11 was the maximum dimension you can have a super gravity,

01:52:14.880 | but string was saying, sorry, we're 10 dimensional.

01:52:18.000 | So for a while we thought that theory is wrong,

01:52:20.520 | because how could it be?

01:52:21.400 | Because string theory is definitely a theory of everything.

01:52:23.380 | We later learned that one of the circles

01:52:25.440 | of string theory itself was tiny.

01:52:28.520 | That we had not appreciated that fact.

01:52:30.240 | And we discovered by doing thought experiments

01:52:32.200 | in string theory, that there's gotta be an extra circle,

01:52:35.000 | and that circle is connected

01:52:36.400 | to an 11 dimensional perspective.

01:52:38.360 | And that's what later on got called M-theory.

01:52:40.680 | So there are these kinds of things that,

01:52:44.000 | we do not know what exactly string theory is,

01:52:45.880 | we're still learning.

01:52:47.400 | So we do not have a final formulation of string theory.

01:52:50.520 | It's very well could be that different facets

01:52:52.380 | of different ideas come together,

01:52:53.940 | like loop quantum gravity or whatnot.

01:52:55.320 | But I wouldn't put them on par.

01:52:56.840 | Namely, loop quantum gravity is a scatter of ideas

01:53:01.080 | about what happens to space when they get very tiny.

01:53:03.800 | For example, you replace things by discrete data

01:53:06.480 | and try to quantize it and so on.

01:53:08.720 | And it sounds like a natural idea to quantize space.

01:53:13.520 | If you were naively trying to do quantum space,

01:53:15.220 | you might think about trying to take points

01:53:17.320 | and put them together in some discrete fashion,

01:53:20.200 | in some way that is reminiscent of loop quantum gravity.

01:53:24.800 | String theory is more subtle than that.

01:53:27.020 | For example, I would just give you an example.

01:53:29.160 | And this is the kind of thing that we didn't put in by hand,

01:53:31.240 | we got it out.

01:53:32.460 | And so it's more subtle than,

01:53:33.880 | so what happens if you squeeze the space

01:53:35.840 | to be smaller and smaller?

01:53:37.800 | Well, you think that after a certain distance,

01:53:41.080 | the notion of distance should break down.

01:53:43.440 | When it goes smaller than Planck scale, should break down.

01:53:48.740 | What happens in string theory?

01:53:50.680 | We do not know the full answer to that,

01:53:52.200 | but we know the following.

01:53:53.160 | Namely, if you take a space

01:53:55.160 | and bring it smaller and smaller,

01:53:56.600 | if the box gets smaller than the Planck scale

01:53:58.600 | by a factor of 10,

01:54:00.520 | it is equivalent by the duality transformation

01:54:04.040 | to a space which is 10 times bigger.

01:54:05.840 | So there's a symmetry called T-duality

01:54:10.080 | which takes L to one over L.

01:54:12.480 | Well, L is measured in Planck units

01:54:14.420 | or more precisely, string units.

01:54:16.280 | This inversion is a very subtle effect.

01:54:20.520 | And I would not have been,

01:54:21.660 | or any physicist would not have been able to design

01:54:23.400 | a theory which has this property,

01:54:25.060 | that when you make the space smaller,

01:54:27.160 | it is as if you're making it bigger.

01:54:29.440 | That means there is no experiment you can do

01:54:32.480 | to distinguish the size of the space.

01:54:34.860 | This is remarkable.

01:54:35.720 | For example, Einstein would have said,

01:54:37.760 | of course I can measure the size of the space.

01:54:39.400 | What do I do?

01:54:40.240 | Well, I take a flashlight, I send the light around,

01:54:43.000 | measure how long it takes for the light

01:54:44.340 | to go around the space and bring back

01:54:46.020 | and find the radius or circumference of the universe.

01:54:48.800 | What's the problem?

01:54:50.860 | I said, well, suppose you do that and you shrink it.

01:54:52.760 | He said, well, it gets smaller and smaller.

01:54:54.000 | So what?

01:54:54.840 | I said, well, it turns out in string theory,

01:54:56.840 | there are two different kinds of photons.

01:54:59.300 | One photon measures one over L, the other one measures L.

01:55:03.480 | And so this duality reformulates.

01:55:07.520 | And when the space gets smaller, it says,

01:55:09.080 | oh no, you better use the bigger perspective

01:55:10.720 | because the smaller one is harder to deal with.

01:55:13.020 | So you do this one.

01:55:13.880 | So these examples of loop quantum gravity

01:55:16.120 | have none of these features.

01:55:17.240 | These features that I'm telling you about,

01:55:18.640 | we have learned from string theory,

01:55:20.200 | but they nevertheless have some of these ideas

01:55:22.060 | like topological gravity aspects

01:55:24.500 | are emphasized in the context of loop quantum gravity

01:55:28.020 | in some form.

01:55:28.860 | And so these ideas might be there in some kernel,

01:55:31.260 | in some corners of string theory.

01:55:32.420 | In fact, I wrote a paper about topological string theory

01:55:35.340 | and some connections potentially loop quantum gravity,

01:55:38.220 | which could be part of that.

01:55:39.240 | So there are little facets of connections.

01:55:41.620 | I wouldn't say they're complete,

01:55:43.820 | but I would say most probably what will happen

01:55:46.100 | to some of these ideas, the good ones at least,

01:55:48.460 | they will be absorbed to string theory if they are correct.

01:55:51.840 | - Let me ask you a crazy out there question.

01:55:54.360 | Can physics help us understand life?

01:55:59.360 | So we spoke so confidently about the laws of physics

01:56:04.880 | being able to explain reality,

01:56:08.160 | and we even said words like theory of everything,

01:56:11.880 | implying that the word everything

01:56:13.560 | is actually describing everything.

01:56:15.800 | Is it possible that the four laws we've been talking about

01:56:20.500 | are actually missing,

01:56:22.040 | they are accurate in describing what they're describing,

01:56:24.800 | but they're missing the description

01:56:26.240 | of a lot of other things like emergence of life

01:56:30.500 | and emergence of perhaps consciousness.

01:56:35.060 | So is there, do you ever think about this kind of stuff

01:56:39.260 | where we would need to understand extra physics

01:56:44.440 | to try to explain the emergence of these complex pockets

01:56:49.440 | of interesting, weird stuff that we call life

01:56:54.240 | and consciousness in this big homogeneous universe

01:56:58.140 | that's mostly boring and nothing is happening in?

01:57:00.400 | - So first of all, we don't claim that string theory

01:57:03.600 | is the theory of everything in the sense

01:57:05.480 | that we know enough what this theory is.

01:57:07.680 | We don't know enough about string theory itself.

01:57:09.280 | We are learning it.

01:57:10.100 | So I wouldn't say, okay, give me whatever,

01:57:12.040 | I will tell you how it works.

01:57:13.240 | No, however, I would say by definition,

01:57:16.480 | by definition to me physics is checking all reality.

01:57:19.420 | Any form of reality, I call it physics.

01:57:22.560 | That's my definition.

01:57:23.400 | I mean, I may not know a lot of it,

01:57:25.600 | like maybe the origin of life and so on,

01:57:27.840 | maybe a piece of that,

01:57:29.280 | but I would call that as part of physics.

01:57:30.920 | To me, reality is what we're after.

01:57:33.560 | I don't claim I know everything about reality.

01:57:35.720 | I don't claim string theory necessarily

01:57:37.620 | has the tools right now to describe all the reality either,

01:57:41.320 | but we are learning what it is.

01:57:42.360 | So I would say that I would not put a border to say,

01:57:44.840 | no, you know, from this point onwards,

01:57:46.280 | it's not my territory, somebody else's.

01:57:48.420 | But whether we need new ideas in string theory

01:57:50.840 | to describe other reality features,

01:57:53.120 | for sure I believe, as I mentioned,

01:57:54.800 | I don't believe any of the laws we know today is final.

01:57:58.080 | So therefore, yes, we will need new ideas.

01:58:00.840 | - This is a very tricky thing for us to understand

01:58:03.520 | and be precise about.

01:58:08.080 | But just because you understand the physics

01:58:12.760 | doesn't necessarily mean that you understand

01:58:17.000 | the emergence of chemistry, biology,

01:58:19.800 | life, intelligence, consciousness.

01:58:23.800 | So those are built, it's like you might understand

01:58:27.760 | the way bricks work,

01:58:30.560 | but to understand what it means to have a happy family,

01:58:34.560 | you don't get from the bricks.

01:58:37.600 | So directly, in theory you could,

01:58:42.280 | if you ran the universe over again,

01:58:44.760 | but just understanding the rules of the universe

01:58:47.440 | doesn't necessarily give you a sense

01:58:49.880 | of the weird, beautiful things that emerge.

01:58:52.280 | - Right, no, so let me describe what you just said.

01:58:55.400 | So there are two questions.

01:58:56.240 | One is whether or not the techniques I use

01:58:58.520 | in let's say quantum field theory and so on

01:59:00.620 | will describe how the society works.

01:59:02.400 | - Yes.

01:59:03.240 | - Okay, that's far distance,

01:59:04.800 | far different scales of questions that we're asking here.

01:59:08.280 | The question is, is there a change of,

01:59:10.520 | is there a new law which takes over

01:59:12.960 | that cannot be connected to the older laws that we know

01:59:16.280 | or more fundamental laws that we know?

01:59:18.080 | Do you need new laws to describe it?

01:59:20.360 | I don't think that's necessarily the case

01:59:22.000 | in many of these phenomena like chemistry

01:59:23.680 | or so on you mentioned.

01:59:25.240 | So we do expect, in principle chemistry

01:59:27.680 | can be described by quantum mechanics.

01:59:29.600 | We don't think there's gonna be a magical thing,

01:59:31.640 | but chemistry is complicated.

01:59:32.960 | Yeah, indeed there are rules of chemistry

01:59:34.840 | that chemists have put down

01:59:36.440 | which has not been explained yet using quantum mechanics.

01:59:39.460 | Do I believe that they will be something

01:59:41.080 | described by quantum mechanics?

01:59:42.160 | Yes, I do.

01:59:43.000 | I don't think they are going to be sitting there

01:59:44.940 | in the sheds forever, but maybe it's too complicated

01:59:47.080 | and maybe we'll wait for very powerful quantum computers

01:59:50.320 | or whatnot to solve those problems.

01:59:51.640 | I don't know.

01:59:52.620 | But I don't think in that context

01:59:54.760 | we have new principles to be added to fix those.

01:59:57.960 | So I'm perfectly fine in the intermediate situation

02:00:01.780 | to have rules of thumb or principles

02:00:04.040 | that chemists have found which are working,

02:00:05.840 | which are not founded on the basis

02:00:07.920 | of quantum mechanical laws, which does the job.

02:00:10.420 | Similarly, as biologists do not found everything

02:00:13.320 | in terms of chemistry, but they think,

02:00:15.480 | there's no reason why chemistry cannot.

02:00:16.760 | They don't think necessarily they're doing something

02:00:18.800 | amazingly not possible with chemistry.

02:00:21.000 | Coming back to your question,

02:00:22.200 | does consciousness, for example, bring this new ingredient?

02:00:26.200 | If indeed it needs a new ingredient,

02:00:28.280 | I will call that new ingredient part of physical law.

02:00:30.560 | We have to understand it.

02:00:31.700 | To me, that, so I wouldn't put a line to say,

02:00:34.500 | okay, from this point onwards, you cannot,

02:00:36.380 | it's disconnected, it's totally disconnected

02:00:38.460 | from strength or whatever, we have to do something else.

02:00:41.100 | - It's not a line.

02:00:42.340 | What I'm referring to is, can physics of a few centuries

02:00:45.780 | from now that doesn't understand consciousness

02:00:48.220 | be much bigger than the physics of today,

02:00:51.620 | where the textbook grows?

02:00:53.980 | - It definitely will.

02:00:54.940 | I would say, it will grow.

02:00:55.980 | I don't know if it grows because of consciousness

02:00:58.820 | being part of it or we have different view

02:01:00.300 | of consciousness.

02:01:01.240 | I do not know where the consciousness will fit.

02:01:03.780 | It's gonna be hard for me to guess.

02:01:07.540 | I mean, I can make random guesses now,

02:01:09.420 | which probably most likely is wrong,

02:01:11.400 | but let me just do just for the sake of discussion.

02:01:14.700 | I could say, you know, brain could be

02:01:17.300 | their quantum computer, classical computer,

02:01:18.940 | their arguments against this being a quantum thing,

02:01:20.780 | so it's probably classical, and if it's classical,

02:01:22.940 | it could be like what we are doing in machine learning,

02:01:24.620 | slightly more fancy, and so on.

02:01:26.180 | Okay, people can go to this argument to no end

02:01:28.500 | and to say whether consciousness exists or not,

02:01:30.080 | or life, does it have any meaning,

02:01:32.280 | or is there a phase transition where you can say,

02:01:34.740 | does electron have a life, or not?

02:01:36.660 | At what level does a particle become life?

02:01:39.100 | Maybe there's no definite definition of life

02:01:41.260 | in that same way that, you know, we cannot say electron.

02:01:44.060 | I like this example quite a bit.

02:01:48.660 | You know, we distinguish between liquid and a gas phase,

02:01:51.120 | like water is liquid or vapor is gas,

02:01:53.820 | and we say they're different.

02:01:54.660 | You can distinguish them.

02:01:55.680 | Actually, that's not true.

02:01:57.300 | It's not true because we know from physics

02:01:59.820 | that you can change temperatures and pressure

02:02:01.900 | to go from liquid to the gas

02:02:03.620 | without making any phase transition.

02:02:05.660 | So there is no point that you can say this was a liquid

02:02:08.420 | and this was a gas.

02:02:10.180 | You can continuously change the parameters

02:02:12.020 | to go from one to the other.

02:02:13.800 | So at the end, it's very different looking.

02:02:15.820 | Like, you know, I know that water is different from vapor,

02:02:18.040 | but, you know, there's no precise point this happens.

02:02:21.220 | I feel many of these things that we think,

02:02:24.000 | like consciousness, clearly,

02:02:25.220 | dead person is not conscious and the other one is,

02:02:27.180 | so there's a difference, like water and vapor.

02:02:29.620 | But there's no point you could say that this is conscious.

02:02:32.820 | There's no sharp transition.

02:02:34.140 | So it could very well be that what we call,

02:02:37.580 | heuristically, in daily life,

02:02:39.780 | consciousness is similar, or life is similar to that.

02:02:43.120 | I don't know if it's like that or not.

02:02:44.460 | I'm just hypothesizing it's possible.

02:02:46.740 | Like, there's no--

02:02:48.260 | - There's no discrete phases of consciousness.

02:02:49.100 | - There's no discrete phase transition like that.

02:02:51.020 | - Yeah, yeah, but there might be, you know,

02:02:55.260 | concepts of temperature and pressure

02:02:57.300 | that we need to understand to describe

02:03:00.120 | what the heck consciousness in life is

02:03:02.700 | that we're totally missing.

02:03:04.700 | - Yes.

02:03:05.520 | - I think that's not a useless question.

02:03:07.380 | Even those questions that,

02:03:09.260 | back to our original discussion of philosophy,

02:03:11.700 | I would say consciousness and free will, for example,

02:03:15.180 | are topics that are very much so

02:03:18.860 | in the realm of philosophy currently.

02:03:20.820 | - Yes.

02:03:21.660 | - But I don't think they will always be.

02:03:22.940 | - I agree with you.

02:03:23.860 | I agree with you, and I think I'm fine

02:03:26.180 | with some topics being part of a different realm

02:03:28.300 | than physics today, because we don't have the right tools.

02:03:32.140 | Just like biology was.

02:03:33.320 | I mean, before we had DNA and all that,

02:03:35.060 | genetics and all that gradually began to take hold.

02:03:37.620 | I mean, when people were beginning

02:03:40.820 | with various experiments with biology and chemistry

02:03:43.700 | and so on, gradually they came together.

02:03:46.020 | So it wasn't like together.

02:03:47.260 | So yeah, I have a perfectly understanding

02:03:49.300 | of a situation where we don't have the tools.

02:03:51.520 | So do these experiments that you think

02:03:53.300 | as defines a conscious in different form

02:03:55.140 | and gradually we'll build it and connect it?

02:03:57.180 | And yes, we might discover new principles of nature

02:03:59.620 | that we didn't know.

02:04:01.060 | I don't know, but I would say that if they are,

02:04:03.380 | they will be deeply connected with the else.

02:04:04.660 | We have seen in physics, we don't have things in isolation.

02:04:08.260 | You cannot compartmentalize, you know,

02:04:10.820 | this is gravity, this is electricity, this is that.

02:04:13.300 | We have learned they all talk to each other.

02:04:15.220 | There's no way to make them in one corner and don't talk.

02:04:19.060 | So the same thing with anything, anything which is real.

02:04:21.300 | So consciousness is real.

02:04:22.380 | So therefore, we have to connect it to everything else.

02:04:25.200 | So to me, once you connect it, you cannot say

02:04:27.140 | it's not reality and once it's reality, it's physics.

02:04:29.900 | I call it physics.

02:04:30.740 | It may not be the physics I know today, for sure it's not.

02:04:32.980 | But I would be surprised if there's disconnected realities

02:04:37.300 | that you cannot imagine them as part of the same soup.

02:04:41.260 | - So I guess God doesn't have a biology

02:04:44.100 | or chemistry textbook and mostly,

02:04:46.000 | or maybe he or she reads it for fun, biology and chemistry,

02:04:50.500 | but when you're trying to get some work done,

02:04:52.100 | it'll be going to the physics textbook.

02:04:54.500 | Okay, what advice, let's put on your wise visionary hat.

02:04:59.500 | What advice do you have for young people today?

02:05:03.860 | You've dedicated your book actually to your kids,

02:05:08.060 | to your family.

02:05:09.620 | What advice would you give to them?

02:05:11.500 | What advice would you give to young people today

02:05:13.880 | thinking about their career, thinking about life,

02:05:16.780 | of how to live a successful life, how to live a good life?

02:05:19.820 | - Yes, I have three sons and in fact, to them,

02:05:24.340 | I have tried not to give too much advice.

02:05:28.260 | So even though I've tried to kind of not give advice,

02:05:31.380 | maybe indirectly there has been some impact.

02:05:33.900 | My oldest one is doing biophysics, for example,

02:05:35.980 | and the second one is doing machine learning

02:05:38.260 | and the third one is doing theoretical computer science.

02:05:40.580 | So there are these facets of interest

02:05:42.460 | which are not too far from my area,

02:05:44.620 | but I have not tried to impact them in that way

02:05:47.940 | but they have followed their own interests.

02:05:49.940 | And I think that's the advice I would give

02:05:51.820 | to any young person, follow your own interests

02:05:54.940 | and let that take you wherever it takes you.

02:05:57.260 | And this I did in my own case

02:06:01.900 | that I was planning to study economics

02:06:04.660 | and electrical engineering when I started at MIT.

02:06:08.100 | And I discovered that I'm more passionate

02:06:10.620 | about math and physics.

02:06:11.700 | And at that time, I didn't feel math and physics

02:06:14.020 | would make a good career.

02:06:15.860 | And so I was kind of hesitant to go in that direction,

02:06:18.540 | but I did because I kind of felt

02:06:20.420 | that that's what I'm driven to do.

02:06:22.580 | So I don't regret it.

02:06:24.700 | I'm lucky in the sense that society supports people like me

02:06:28.340 | who are doing these abstract stuff,

02:06:29.900 | which may or may not be experimentally verified

02:06:32.540 | even let alone applied to the daily technology

02:06:34.740 | in our lifetime.

02:06:36.380 | I'm lucky I'm doing that.

02:06:37.660 | And I feel that if people follow their interests,

02:06:41.300 | they will find the niche that they're good at.

02:06:43.940 | And this coincidence of hopefully their interests

02:06:48.020 | and abilities are kind of aligned,

02:06:51.860 | at least some extent to be able to drive them

02:06:54.420 | to something which is successful.

02:06:56.260 | And not to be driven by things like,

02:06:58.540 | this doesn't make a good career,

02:07:00.020 | or this doesn't do that,

02:07:01.060 | and my parents expect that, or what about this.

02:07:03.740 | I think ultimately you have to live with yourself

02:07:06.140 | and you only have one life and it's short, very short.

02:07:08.660 | I can tell you I'm getting there.

02:07:10.580 | So I know it's short.

02:07:11.500 | So you really want not to do things

02:07:14.980 | that you don't want to do.

02:07:15.900 | So I think following your interests

02:07:17.340 | is my strongest advice to young people.

02:07:19.300 | - Yeah, it's scary when your interest

02:07:22.180 | doesn't directly map to a career of the past or of today.

02:07:26.620 | So you're almost anticipating future careers

02:07:28.700 | that could be created, it's scary.

02:07:30.460 | But yeah, there's something to that,

02:07:34.180 | especially when the interest and the ability align,

02:07:36.820 | that you will pave a path,

02:07:39.620 | that we'll find a way to make money,

02:07:41.220 | especially in this society,

02:07:42.660 | in the capitalistic United States society.

02:07:46.260 | It feels like ability and passion paves the way.

02:07:51.260 | - Yes.

02:07:53.100 | - At the very least you can sell funny T-shirts.

02:07:56.940 | - Yes.

02:07:57.780 | - You've mentioned life is short.

02:07:59.920 | Do you think about your mortality?

02:08:04.140 | Are you afraid of death?

02:08:05.900 | - I don't think about my mortality.

02:08:09.820 | I think that I don't think about my death,

02:08:12.460 | I don't think about death in general too much.

02:08:14.700 | First of all, it's something that I can't do much about.

02:08:16.800 | And I think it's something that it doesn't,

02:08:19.540 | it doesn't drive my everyday action.

02:08:21.640 | It is natural to expect that it's somewhat

02:08:24.100 | like the time reversal situation.

02:08:25.940 | So we believe that we have this approximate symmetry

02:08:27.900 | in nature, time reversal.

02:08:29.420 | Going forward we die, going backwards we get born.

02:08:32.320 | So what was it to get born?

02:08:35.060 | It wasn't such a good or bad thing, I have no feeling of it.

02:08:38.780 | So who knows what the death will feel like,

02:08:42.140 | the moment of death or whatnot.

02:08:43.460 | So I don't know, it is not known.

02:08:45.460 | But in what form do we exist before or after?

02:08:50.140 | Again, it's something that it's partly philosophical maybe.

02:08:53.740 | - I like how you draw comfort from symmetry.

02:08:55.900 | It does seem that there is something asymmetric here,

02:08:58.700 | breaking of symmetry because there's something

02:09:02.540 | to the creative force of the human spirit

02:09:07.860 | that goes only one way.

02:09:09.540 | - Right.

02:09:10.380 | - That it seems the finiteness of life

02:09:13.140 | is the thing that drives the creativity.

02:09:15.940 | And so it does seem that at least contemplation

02:09:20.940 | of the finiteness of life, of mortality,

02:09:24.580 | is the thing that helps you get your stuff together.

02:09:27.060 | - Yes, I think that's true.

02:09:28.120 | But actually I have a different perspective

02:09:29.580 | on that a little bit.

02:09:30.420 | - Yes.

02:09:31.240 | - Namely, suppose I told you you're immortal.

02:09:34.820 | - Yes.

02:09:37.120 | - I think your life would be totally boring after that

02:09:39.780 | because you will not, there's,

02:09:42.640 | I think part of the reason we have enjoyment in life

02:09:45.940 | is the finiteness of it.

02:09:47.280 | - Yes.

02:09:48.120 | - And so I think mortality might be a blessing

02:09:52.180 | and immortality may not.

02:09:54.000 | So I think that we value things

02:09:55.960 | because we have that finite life.

02:09:58.080 | We appreciate things, we wanna do this,

02:10:00.000 | we wanna do that, we have motivation.

02:10:01.600 | If I told you, you know, you have infinite life,

02:10:03.080 | oh, I don't need to do this today,

02:10:04.680 | I have another billion or trillion or infinite life,

02:10:08.520 | so why do I do now?

02:10:10.160 | There is no motivation.

02:10:11.840 | A lot of the things that we do are driven

02:10:14.220 | by that finiteness, the finiteness of these resources.

02:10:16.960 | So I think it's a blessing in disguise.

02:10:20.100 | I don't regret it that we have more finite life.

02:10:23.400 | And I think that the process of being part of this thing,

02:10:28.400 | that, you know, the reality,

02:10:33.080 | to me, part of what attracts me to science

02:10:36.400 | is to connect to that immortality kind of,

02:10:39.700 | namely the loss, the reality beyond us.

02:10:43.920 | To me, I'm resigned to the fact that not only me,

02:10:47.880 | everybody's going to die.

02:10:49.800 | So this is a little bit of a consolation.

02:10:51.960 | None of us are going to be around.

02:10:53.860 | So therefore, okay,

02:10:55.240 | and none of the people before me are around.

02:10:57.360 | So therefore, yeah, okay,

02:10:58.620 | this is something everybody goes through.

02:11:00.780 | So taking that minuscule version of, okay,

02:11:03.880 | how tiny we are and how short time it is and so on,

02:11:07.520 | to connect to the deeper truth beyond us,

02:11:10.140 | the reality beyond us,

02:11:11.920 | is what sense of, quote unquote, immortality I would get.

02:11:16.820 | Namely, at least I can hang on

02:11:18.760 | to this little piece of truth,

02:11:20.720 | even though I know, I know it's not complete.

02:11:23.200 | I know it's going to be imperfect.

02:11:25.800 | I know it's going to change and it's going to be improved.

02:11:28.440 | But having a little bit deeper insight

02:11:30.580 | than just the naive thing around us,

02:11:32.700 | little Earth here and little galaxy and so on,

02:11:35.240 | makes me feel a little bit more pleasure to live this life.

02:11:40.140 | So I think that's the way I view my role as a scientist.

02:11:43.200 | - Yeah, the scarcity of this life

02:11:46.320 | helps us appreciate the beauty of the immortal,

02:11:50.200 | the universal truths that physics present us.

02:11:53.680 | So maybe one day physics will have something to say

02:11:58.680 | about that beauty in itself,

02:12:03.400 | explaining why the heck it's so beautiful

02:12:06.520 | to appreciate the laws of physics

02:12:08.280 | and yet why it's so tragic that we die so quickly.

02:12:13.280 | - Yes, we die so quickly.

02:12:16.160 | So that can be a bit longer, that's for sure.

02:12:18.080 | - It would be very nice.

02:12:19.200 | Maybe physics will help out.

02:12:20.820 | - Well, Kamran, it was an incredible conversation.

02:12:23.880 | Thank you so much once again for painting

02:12:25.600 | a beautiful picture of the history of physics.

02:12:28.360 | And it kind of presents a hopeful view

02:12:32.440 | of the future of physics.

02:12:33.520 | So I really, really appreciate that.

02:12:35.800 | It's a huge honor that you would talk to me

02:12:37.400 | and waste all your valuable time with me.

02:12:39.320 | I really appreciate it.

02:12:40.160 | - Thanks, Lex.

02:12:40.980 | It was a pleasure and I loved talking with you

02:12:42.760 | and this is wonderful set of discussions.

02:12:44.520 | I really enjoyed my time with this discussion.

02:12:46.480 | Thank you.

02:12:47.960 | - Thanks for listening to this conversation

02:12:49.440 | with Kamran Vafa and thank you to Headspace,

02:12:52.640 | Jordan Harberger Show, Squarespace and Allform.

02:12:56.660 | Check them out in the description to support this podcast.

02:13:00.200 | And now let me leave you with some words

02:13:02.600 | from the great Richard Feynman.

02:13:05.040 | Physics isn't the most important thing, love is.

02:13:08.540 | Thank you for listening and hope to see you next time.

02:13:12.320 | (upbeat music)

02:13:14.900 | (upbeat music)

02:13:17.480 | [BLANK_AUDIO]

Cumrun Vafa: String Theory | Lex Fridman Podcast #204

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