Transcript
00:00:00 The following is a conversation with Kamran Valfa,
00:00:02 a theoretical physicist at Harvard
00:00:04 specializing in string theory.
00:00:07 He is the winner of the 2017 Breakthrough Prize
00:00:10 in Fundamental Physics,
00:00:12 which is the most lucrative academic prize in the world.
00:00:15 Quick mention of our sponsors,
00:00:17 Headspace, Jordan Harmer’s show,
00:00:20 Squarespace, and Allform.
00:00:22 Check them out in the description to support this podcast.
00:00:25 As a side note, let me say that string theory
00:00:28 is a theory of quantum gravity
00:00:29 that unifies quantum mechanics and general relativity.
00:00:33 It says that quarks, electrons, and all other particles
00:00:36 are made up of much tinier strings of vibrating energy.
00:00:40 They vibrate in 10 or more dimensions,
00:00:42 depending on the flavor of the theory.
00:00:45 Different vibrating patterns result in different particles.
00:00:48 From its origins, for a long time,
00:00:50 string theory was seen as too good not to be true,
00:00:54 but has recently fallen out of favor
00:00:56 in the physics community,
00:00:57 partly because over the past 40 years,
00:01:00 it has not been able to make any novel predictions
00:01:03 that could then be validated through experiment.
00:01:06 Nevertheless, to this day,
00:01:08 it remains one of our best candidates
00:01:10 for a theory of everything,
00:01:12 or a theory that unifies the laws of physics.
00:01:15 Let me mention that a similar story happened
00:01:18 with neural networks
00:01:18 in the field of artificial intelligence,
00:01:21 where it fell out of favor
00:01:22 after decades of promise and research,
00:01:24 but found success again in the past decade
00:01:28 as part of the deep learning revolution.
00:01:30 So I think it pays to keep an open mind,
00:01:33 since we don’t know which of the ideas in physics
00:01:36 may be brought back decades later
00:01:38 and be found to solve the biggest mysteries
00:01:40 in theoretical physics.
00:01:42 String theory still has that promise.
00:01:45 This is the Lex Friedman podcast,
00:01:47 and here’s my conversation with Kamran Wafa.
00:01:51 What is the difference between mathematics
00:01:54 and physics?
00:01:55 Well, that’s a difficult question,
00:01:57 because in many ways,
00:01:58 math and physics are unified in many ways.
00:02:01 So to distinguish them is not an easy task.
00:02:04 I would say that perhaps the goals
00:02:06 of math and physics are different.
00:02:09 Math does not care to describe reality, physics does.
00:02:14 That’s the major difference.
00:02:16 But a lot of the thoughts, processes, and so on,
00:02:19 which goes to understanding the nature and reality,
00:02:22 are the same things that mathematicians do.
00:02:24 So in many ways, they are similar.
00:02:27 Mathematicians care about deductive reasoning,
00:02:32 and physicists or physics in general,
00:02:35 we care less about that.
00:02:37 We care more about interconnection of ideas,
00:02:40 about how ideas support each other,
00:02:42 or if there’s a puzzle, discord between ideas.
00:02:46 That’s more interesting for us.
00:02:48 And part of the reason is that we have learned in physics
00:02:50 that the ideas are not sequential.
00:02:53 And if we think that there’s one idea
00:02:54 which is more important,
00:02:56 and we start with there and go to the next idea,
00:02:58 and next one, and deduce things from that,
00:02:59 like mathematicians do,
00:03:01 we have learned that the third or fourth thing
00:03:03 we deduce from that principle
00:03:05 turns out later on to be the actual principle.
00:03:08 And from a different perspective,
00:03:10 starting from there leads to new ideas,
00:03:12 which the original one didn’t lead to,
00:03:14 and that’s the beginning of a new revolution in science.
00:03:18 So this kind of thing we have seen again and again
00:03:20 in the history of science,
00:03:21 we have learned to not like deductive reasoning
00:03:24 because that gives us a bad starting point,
00:03:27 to think that we actually have the original thought process
00:03:30 should be viewed as the primary thought,
00:03:32 and all these are deductions,
00:03:33 like the way mathematicians sometimes do.
00:03:35 So in physics, we have learned to be skeptical
00:03:37 of that way of thinking.
00:03:38 We have to be a bit open to the possibility
00:03:41 that what we thought is a deduction of a hypothesis
00:03:44 is actually the reason that’s true is the opposite.
00:03:47 And so we reverse the order.
00:03:48 And so this switching back and forth between ideas
00:03:52 makes us more fluid about deductive fashion.
00:03:56 Of course, it sometimes gives a wrong impression
00:03:59 like physicists don’t care about rigor.
00:04:00 They just say random things.
00:04:03 They are willing to say things that are not backed
00:04:05 by the logical reasoning.
00:04:07 That’s not true at all.
00:04:09 So despite this fluidity
00:04:12 in saying which one is a primary thought,
00:04:14 we are very careful about trying to understand
00:04:17 what we have really understood in terms of relationship
00:04:19 between ideas.
00:04:21 So that’s an important ingredient.
00:04:24 And in fact, solid math, being behind physics
00:04:27 is I think one of the attractive features
00:04:30 of a physical law.
00:04:32 So we look for beautiful math underpinning it.
00:04:35 Can we dig into that process of starting from one place
00:04:39 and then ending up at like the fourth step
00:04:43 and realizing all along that the place you started at
00:04:46 was wrong?
00:04:47 So is that happened when there’s a discrepancy
00:04:50 between what the math says
00:04:53 and what the physical world shows?
00:04:54 Is that how you then can go back
00:04:56 and do the revolutionary idea
00:04:59 for different starting place altogether?
00:05:02 Perhaps I give an example to see how it goes.
00:05:04 And in fact, the historical example is Newton’s work
00:05:08 on classical mechanics.
00:05:10 So Newton formulated the laws of mechanics,
00:05:14 the force F equals to MA and his other laws,
00:05:17 and they look very simple, elegant, and so forth.
00:05:20 Later, when we studied more examples of mechanics
00:05:25 and other similar things, physicists came up with the idea
00:05:28 that the notion of potential is interesting.
00:05:31 Potential was an abstract idea, which kind of came,
00:05:33 you could take its gradient and relate it to the force.
00:05:37 So you don’t really need it a priori,
00:05:38 but it solved, helped some thoughts.
00:05:41 And then later, Euler and Lagrange reformulated
00:05:45 Newtonian mechanics in a totally different way
00:05:49 in the following fashion.
00:05:50 They said, if you take,
00:05:51 if you wanna know where a particle at this point
00:05:53 and at this time, how does it get to this point
00:05:55 at the later time, is the following.
00:05:58 You take all possible paths connecting this particle
00:06:01 from going from the initial point to the final point,
00:06:04 and you compute the action.
00:06:07 And what is an action?
00:06:08 Action is the integral over time
00:06:10 of the kinetic term of the particle minus its potential.
00:06:15 So you take this integral,
00:06:16 and each path will give you some quantity.
00:06:19 And the path it actually takes, the physical path,
00:06:23 is the one which minimizes this integral or this action.
00:06:26 Now, this sounded like a backward step from Newton’s.
00:06:29 Newton’s formula seemed very simple.
00:06:32 F equals to ma, and you can write F is minus
00:06:35 the gradient of the potential.
00:06:36 So why would anybody start formulating such a simple thing
00:06:40 in terms of this complicated looking principle?
00:06:43 You have to study the space of all paths and all things
00:06:46 and find the minimum, and then you get the same equation.
00:06:48 So what’s the point?
00:06:50 So Euler and Lagrange’s formulation of Newton,
00:06:52 which was kind of recasting in this language,
00:06:56 is just a consequence of Newton’s law.
00:06:58 F equals to ma gives you the same fact
00:06:59 that this path is a minimum action.
00:07:02 Now, what we learned later, last century,
00:07:05 was that when we deal with quantum mechanics,
00:07:08 Newton’s law is only an average correct.
00:07:12 And the particle going from one to the other
00:07:15 doesn’t take exactly one path.
00:07:17 It takes all the paths with the amplitude,
00:07:21 which is proportional to the exponential
00:07:23 of the action times an imaginary number, i.
00:07:26 And so this fact turned out to be the reformulation
00:07:29 of quantum mechanics.
00:07:30 We should start there as the basis of the new law,
00:07:33 which is quantum mechanics, and Newton is only
00:07:35 an approximation on the average correct.
00:07:37 And when you say amplitude, you mean probability?
00:07:40 Yes, the amplitude means if you sum up all these paths
00:07:43 with exponential i times the action,
00:07:45 if you sum this up, you get the number, complex number.
00:07:48 You square the norm of this complex number,
00:07:50 gives you a probability to go from one to the other.
00:07:52 Is there ways in which mathematics can lead us astray
00:07:57 when we use it as a tool to understand the physical world?
00:08:01 Yes, I would say that mathematics can lead us astray
00:08:04 as much as old physical ideas can lead us astray.
00:08:08 So if you get stuck in something,
00:08:11 then you can easily fool yourself
00:08:13 that just like the thought process,
00:08:15 we have to free ourselves of that.
00:08:17 Sometimes math does that role, like say,
00:08:19 oh, this is such a beautiful math.
00:08:20 I definitely want to use it somewhere.
00:08:22 And so you just get carried away
00:08:23 and you just get maybe carried too far away.
00:08:25 So that is certainly true, but I wouldn’t say
00:08:28 it’s more dangerous than old physical ideas.
00:08:30 To me, new math ideas is as much potential
00:08:34 to lead us astray as old physical ideas,
00:08:36 which could be long held principles of physics.
00:08:38 So I’m just saying that we should keep an open mind
00:08:41 about the role the math plays,
00:08:43 not to be antagonistic towards it
00:08:46 and not to over, over welcoming it.
00:08:48 We should just be open to possibilities.
00:08:51 What about looking at a particular characteristics
00:08:53 of both physical ideas and mathematical ideas,
00:08:55 which is beauty?
00:08:56 You think beauty leads us astray, meaning,
00:09:00 and you offline showed me a really nice puzzle
00:09:03 that illustrates this idea a little bit.
00:09:06 Now, maybe you can speak to that or another example
00:09:09 where beauty makes it tempting for us to assume
00:09:13 that the law and the theory we found
00:09:16 is actually one that perfectly describes reality.
00:09:19 I think that beauty does not lead us astray
00:09:22 because I feel that beauty is a requirement
00:09:25 for principles of physics.
00:09:27 So beauty is a fundamental in the universe?
00:09:29 I think beauty is fundamental.
00:09:30 At least that’s the way many of us view it.
00:09:32 It’s not emergent.
00:09:33 It’s not emergent.
00:09:35 I think Hardy is the mathematician who said
00:09:37 that there’s no permanent place for ugly mathematics.
00:09:40 And so I think the same is true in physics
00:09:42 that if we find the principle which looks ugly,
00:09:47 we are not going to be, that’s not the end stage.
00:09:49 So therefore beauty is going to lead us somewhere.
00:09:52 Now, it doesn’t mean beauty is enough.
00:09:54 It doesn’t mean if you just have beauty,
00:09:56 if I just look at something is beautiful, then I’m fine.
00:09:58 No, that’s not the case.
00:10:00 Beauty is certainly a criteria that every good
00:10:03 physical theory should pass.
00:10:04 That’s at least the view we have.
00:10:06 Why do we have this view?
00:10:08 That’s a good question.
00:10:09 It is partly, you could say, based on experience
00:10:13 of science over centuries, partly is philosophical view
00:10:17 of what reality is or should be.
00:10:20 And in principle, it could have been ugly
00:10:23 and we might have had to deal with it,
00:10:25 but we have gotten maybe confident through examples
00:10:29 in the history of science to look for beauty.
00:10:32 And our sense of beauty seems to incorporate
00:10:34 a lot of things that are essential for us
00:10:36 to solve some difficult problems like symmetry.
00:10:37 We find symmetry beautiful
00:10:39 and the breaking of symmetry beautiful.
00:10:41 Somehow symmetry is a fundamental part
00:10:45 of how we conceive of beauty at all layers of reality,
00:10:50 which is interesting.
00:10:51 Like in both the visual space, like the way we look at art,
00:10:55 we look at each other as human beings,
00:10:57 the way we look at creatures in the biological space,
00:10:59 the way we look at chemistry,
00:11:01 and then into the physics world as the work you do.
00:11:04 It’s kind of interesting.
00:11:05 It makes you wonder like,
00:11:08 which one is the chicken or the egg?
00:11:10 Is symmetry the chicken and our conception of beauty
00:11:13 the egg or the other way around?
00:11:15 Or somehow the fact that the symmetry is part of reality,
00:11:20 it somehow creates a brain that then is able to perceive it.
00:11:24 Or maybe this is just because we,
00:11:27 maybe it’s so obvious, it’s almost trivial,
00:11:32 that symmetry, of course,
00:11:33 will be part of every kind of universe that’s possible.
00:11:37 And then any kind of organism that’s able to observe
00:11:40 that universe is going to appreciate symmetry.
00:11:44 Well, these are good questions.
00:11:46 We don’t have a deep understanding
00:11:47 of why we get attracted to symmetry.
00:11:49 Why do laws of nature seem to have symmetries underlying
00:11:54 them and the reasoning or the examples of whether,
00:11:57 if there wasn’t symmetry,
00:11:58 we would have understood it or not.
00:12:00 We could have said that, yeah, if there were, you know,
00:12:02 things which didn’t look that great,
00:12:04 we could understand them.
00:12:04 For example, we know that symmetries get broken
00:12:08 and we have appreciated nature
00:12:10 in the broken symmetry phase as well.
00:12:12 The world we live in has many things
00:12:14 which do not look symmetric,
00:12:16 but even those have underlying symmetry
00:12:18 when you look at it more deeply.
00:12:20 So we have gotten maybe spoiled perhaps
00:12:23 by the appearance of symmetry all over the place.
00:12:25 And we look for it.
00:12:26 And I think this is perhaps related to a sense of aesthetics
00:12:31 that scientists have.
00:12:33 And we don’t usually talk about it among scientists.
00:12:36 In fact, it’s kind of a philosophical view
00:12:39 of why do we look for simplicity or beauty or so forth.
00:12:43 And I think in a sense, scientists are a lot
00:12:47 like philosophers.
00:12:48 Sometimes I think, especially modern science
00:12:51 seems to shun philosophers and philosophical views.
00:12:54 And I think at their peril, I think in my view,
00:12:58 science owes a lot to philosophy.
00:13:01 And in my view, many scientists, in fact,
00:13:04 probably all good scientists
00:13:06 are perhaps amateur philosophers.
00:13:08 They may not state that they are philosophers
00:13:11 or they may not like to be labeled philosophers,
00:13:13 but in many ways what they do
00:13:14 is like what is philosophical takes of things.
00:13:18 Looking for simplicity or symmetry
00:13:20 is an example of that in my opinion, or seeing patterns.
00:13:23 You see, for example, another example of the symmetry
00:13:26 is like how you come up with new ideas in science.
00:13:29 You see, for example, an idea A
00:13:31 is connected with an idea B.
00:13:33 Okay, so you study this connection very deeply.
00:13:36 And then you find the cousin of an idea A,
00:13:39 let me call it A prime.
00:13:41 And then you immediately look for B prime.
00:13:44 If A is like B and if there’s an A prime,
00:13:46 then you look for B prime.
00:13:47 Why?
00:13:48 Well, it completes the picture.
00:13:50 Why?
00:13:51 Well, it’s philosophically appealing
00:13:53 to have more balance in terms of that.
00:13:55 And then you look for B prime and lo and behold,
00:13:57 you find this other phenomenon,
00:13:58 which is a physical phenomenon, which you call B prime.
00:14:01 So this kind of thinking motivates
00:14:03 asking questions and looking for things.
00:14:05 And it has guided scientists, I think, through many centuries
00:14:08 and I think it continues to do so today.
00:14:10 And I think if you look at the long arc of history,
00:14:12 I suspect that the things that will be remembered
00:14:16 is the philosophical flavor of the ideas of physics
00:14:21 and chemistry and computer science and mathematics.
00:14:24 Like, I think the actual details
00:14:29 will be shown to be incomplete or maybe wrong,
00:14:33 but the philosophical intuitions
00:14:34 will carry through much longer.
00:14:36 There’s a sense in which, if it’s true,
00:14:39 that we haven’t figured out most of how things work,
00:14:43 currently, that it’ll all be shown as wrong and silly.
00:14:47 It’d almost be a historical artifact.
00:14:49 But the human spirit, whatever,
00:14:52 like the longing to understand,
00:14:55 the way we perceive the world, the way we conceive of it,
00:14:59 of our place in the world, those ideas will carry on.
00:15:02 I completely agree.
00:15:03 In fact, I believe that almost,
00:15:05 well, I believe that none of the principles
00:15:08 or laws of physics we know today are exactly correct.
00:15:11 All of them are approximations to something.
00:15:13 They are better than the previous versions that we had,
00:15:15 but none of them are exactly correct,
00:15:17 and none of them are gonna stand forever.
00:15:19 So I agree that that’s the process we are heading,
00:15:22 we are improving.
00:15:24 And yes, indeed, the thought process
00:15:26 and that philosophical take is common.
00:15:28 So when we look at older scientists,
00:15:33 or maybe even all the way back to Greek philosophers
00:15:36 and the things that the way they thought and so on,
00:15:38 almost everything they said about nature was incorrect.
00:15:42 But the way they thought about it
00:15:43 and many things that they were thinking
00:15:45 is still valid today.
00:15:46 For example, they thought about symmetry breaking.
00:15:50 They were trying to explain the following.
00:15:51 This is a beautiful example, I think.
00:15:53 They had figured out that the Earth is round,
00:15:55 and they said, okay, Earth is round.
00:15:57 They have seen the length of the shadow of a meter stick,
00:16:01 and they have seen that if you go
00:16:02 from the equator upwards north,
00:16:04 they find that depending on how far away you are,
00:16:06 that the length of the shadow changes.
00:16:07 And from that, they had even measured
00:16:09 the radius of the Earth to good accuracy.
00:16:12 That’s brilliant, by the way, the fact that they did that.
00:16:14 Very brilliant, very brilliant.
00:16:15 So these Greek philosophers are very smart.
00:16:17 And so they had taken it to the next step.
00:16:20 They asked, okay, so the Earth is round,
00:16:23 why doesn’t it move?
00:16:25 They thought it doesn’t move.
00:16:26 They were looking around, nothing seemed to move.
00:16:28 So they said, okay, we have to have a good explanation.
00:16:31 It wasn’t enough for them to be there.
00:16:33 So they really wanna deeply understand that fact.
00:16:36 And they come up with a symmetry argument.
00:16:38 And the symmetry argument was,
00:16:40 oh, if the Earth is a spherical,
00:16:43 it must be at the center of the universe for sure.
00:16:45 So they said the Earth is at the center of the universe.
00:16:47 That makes sense.
00:16:48 And they said, if the Earth is going to move,
00:16:50 which direction does it pick?
00:16:52 Any direction it picks, it breaks that spherical symmetry
00:16:54 because you have to pick a direction.
00:16:57 And that’s not good because it’s not symmetrical anymore.
00:16:59 So therefore, the Earth decides to sit put
00:17:01 because it would break the symmetry.
00:17:03 So they had the incorrect science.
00:17:05 They thought Earth doesn’t move.
00:17:07 But they had this beautiful idea
00:17:08 that symmetry might explain it.
00:17:11 But they were even smarter than that.
00:17:12 Aristotle didn’t agree with this argument.
00:17:15 He said, why do you think symmetry prevents it from moving?
00:17:18 Because the preferred position?
00:17:19 Not so.
00:17:21 He gave an example.
00:17:22 He said, suppose you are a person
00:17:26 and we put you at the center of a circle
00:17:29 and we spread food around you on a circle around you,
00:17:32 loaves of bread, let’s say.
00:17:35 And we say, okay, stay at the center of the circle forever.
00:17:39 Are you going to do that
00:17:39 just because it’s a symmetric point?
00:17:43 No, you are going to get hungry.
00:17:44 You’re going to move towards one of those loaves of bread,
00:17:46 despite the fact that it breaks the symmetry.
00:17:49 So from this way, he tried to argue
00:17:51 being at the symmetric point
00:17:52 may not be the preferred thing to do.
00:17:55 And this idea of spontaneous symmetry breaking
00:17:57 is something we just use today
00:17:59 to describe many physical phenomena.
00:18:01 So spontaneous symmetry breaking
00:18:03 is the feature that we now use.
00:18:04 But this idea was there thousands of years ago,
00:18:08 but applied incorrectly to the physical world,
00:18:11 but now we are using it.
00:18:12 So these ideas are coming back in different forms.
00:18:14 So I agree very much that the thought process
00:18:17 is more important and these ideas are more interesting
00:18:20 than the actual applications that people may find today.
00:18:23 Did they use the language of symmetry
00:18:24 and the symmetry breaking and spontaneous symmetry breaking?
00:18:26 That’s really interesting.
00:18:28 Because I could see a conception of the universe
00:18:32 that kind of tends towards perfect symmetry
00:18:35 and is stuck there, not stuck there,
00:18:38 but achieves that optimal and stays there.
00:18:42 The idea that you would spontaneously
00:18:43 break out of symmetry, like have these perturbations,
00:18:47 like jump out of symmetry and back,
00:18:51 that’s a really difficult idea to load into your head.
00:18:55 Like where does that come from?
00:18:57 And then the idea that you may not be
00:18:59 at the center of the universe.
00:19:02 That is a really tough idea.
00:19:04 Right, so symmetry sometimes is an explanation
00:19:07 of being at the symmetric point.
00:19:08 It’s sometimes a simple explanation of many things.
00:19:10 Like if you have a bowl, a circular bowl,
00:19:15 then the bottom of it is the lowest point.
00:19:18 So if you put a pebble or something,
00:19:19 it will slide down and go there at the bottom
00:19:21 and stays there at the symmetric point
00:19:23 because it’s the preferred point, the lowest energy point.
00:19:26 But if that same symmetric circular bowl that you had
00:19:29 had a bump on the bottom, the bottom might not be
00:19:33 at the center, it might be on a circle on the table,
00:19:36 in which case the pebble would not end up at the center,
00:19:39 it would be the lower energy point.
00:19:40 Symmetrical, but it breaks the symmetry
00:19:43 once it takes a point on that circle.
00:19:45 So we can have symmetry reasoning for where things end up
00:19:48 or symmetry breakings, like this example would suggest.
00:19:52 We talked about beauty.
00:19:54 I find geometry to be beautiful.
00:19:56 You have a few examples that are geometric
00:20:01 in nature in your book.
00:20:04 How can geometry in ancient times or today
00:20:06 be used to understand reality?
00:20:09 And maybe how do you think about geometry
00:20:12 as a distinct tool in mathematics and physics?
00:20:17 Yes, geometry is my favorite part of math as well.
00:20:19 And Greeks were enamored by geometry.
00:20:22 They tried to describe physical reality using geometry
00:20:25 and principles of geometry and symmetry.
00:20:27 Platonic solids, the five solids they had discovered
00:20:31 had these beautiful solids.
00:20:33 They thought it must be good for some reality.
00:20:35 There must be explaining something.
00:20:37 They attached one to air, one to fire and so forth.
00:20:40 They tried to give physical reality to symmetric objects.
00:20:45 These symmetric objects are symmetries of rotation
00:20:48 and discrete symmetry groups we call today
00:20:50 of rotation group in three dimensions.
00:20:53 Now, we know now, we kind of laugh at the way
00:20:56 they were trying to connect that symmetry
00:20:57 to the laws of the realities of physics.
00:21:02 But actually it turns out in modern days,
00:21:05 we use symmetries in not too far away
00:21:09 exactly in these kinds of thoughts processes
00:21:12 in the following way.
00:21:14 In the context of string theory,
00:21:16 which is the field light study,
00:21:18 we have these extra dimensions.
00:21:20 And these extra dimensions are compact tiny spaces typically
00:21:24 but they have different shapes and sizes.
00:21:27 We have learned that if these extra shapes and sizes
00:21:30 have symmetries, which are related
00:21:32 to the same rotation symmetries
00:21:34 that the Greek we’re talking about,
00:21:36 if they enjoy those discrete symmetries
00:21:38 and if you take that symmetry and caution the space by it,
00:21:41 in other words, identify points under these symmetries,
00:21:44 you get properties of that space at the singular points
00:21:48 which force emanates from them.
00:21:51 What forces?
00:21:52 Forces like the ones we have seen in nature today,
00:21:54 like electric forces, like strong forces, like weak forces.
00:21:59 So these same principles that were driving them
00:22:02 to connect geometry and symmetries to nature
00:22:06 is driving today’s physics,
00:22:10 now much more modern ideas, but nevertheless,
00:22:13 the symmetries connecting geometry to physics.
00:22:17 In fact, often sometimes we ask the following question,
00:22:20 suppose I want to get this particular physical reality,
00:22:24 I wanna have this particles with these forces and so on,
00:22:27 what do I do?
00:22:28 It turns out that you can geometrically design
00:22:31 the space to give you that.
00:22:33 You say, oh, I put the sphere here, I will do this,
00:22:35 I will shrink them.
00:22:36 So if you have two spheres touching each other
00:22:39 and shrinking to zero size, that gives you strong forces.
00:22:43 If you have one of them, it gives you the weak forces.
00:22:45 If you have this, you get that.
00:22:46 And if you want to unify forces, do the other thing.
00:22:49 So these geometrical translation of physics
00:22:52 is one of my favorite things that we have discovered
00:22:54 in modern physics and the context of string theory.
00:22:57 The sad thing is when you go into multiple dimensions
00:22:59 and we’ll talk about it is we start to lose our capacity
00:23:05 to visually intuit the world we’re discussing.
00:23:09 And then we go into the realm of mathematics
00:23:11 and we’ll lose that.
00:23:12 Unfortunately, our brains are such that we’re limited.
00:23:15 But before we go into that mysterious, beautiful world,
00:23:19 let’s take a small step back.
00:23:21 And you also in your book have this kind of
00:23:24 through the space of puzzles, through the space of ideas,
00:23:27 have a brief history of physics, of physical ideas.
00:23:32 Now, we talked about Newtonian mechanics leading all
00:23:35 through different Lagrangian, Hamiltonian mechanics.
00:23:38 Can you describe some of the key ideas
00:23:41 in the history of physics?
00:23:42 Maybe lingering on each from electromagnetism to relativity
00:23:46 to quantum mechanics and to today,
00:23:49 as we’ll talk about with quantum gravity and string theory.
00:23:52 Sure, so I mentioned the classical mechanics
00:23:55 and the Euler Lagrangian formulation.
00:23:59 One of the next important milestones for physics
00:24:03 were the discoveries of laws of electricity and magnetism.
00:24:07 So Maxwell put the discoveries all together
00:24:10 in the context of what we call the Maxwell’s equations.
00:24:13 And he noticed that when he put these discoveries
00:24:16 that Faraday’s and others had made about electric
00:24:20 and magnetic phenomena in terms of mathematical equations,
00:24:23 it didn’t quite work.
00:24:25 There was a mathematical inconsistency.
00:24:27 Now, one could have had two attitudes.
00:24:31 One would say, okay, who cares about math?
00:24:32 I’m doing nature, electric force, magnetic force,
00:24:35 math I don’t care about.
00:24:36 But it bothered him.
00:24:37 It was inconsistent.
00:24:39 The equations he were writing, the two equations
00:24:40 he had written down did not agree with each other.
00:24:43 And this bothered him, but he figured out,
00:24:45 if you add this jiggle, this equation
00:24:47 by adding one little term there, it works.
00:24:50 At least it’s consistent.
00:24:51 What is the motivation for that term?
00:24:53 He said, I don’t know.
00:24:54 Have we seen it in experiments?
00:24:56 No.
00:24:57 Why did you add it?
00:24:58 Well, because of mathematical consistency.
00:24:59 So he said, okay, math forced him to do this term.
00:25:04 He added this term, which we now today call the Maxwell term.
00:25:08 And once he added that term, his equations were nice,
00:25:11 differential equations, mathematically consistent,
00:25:13 beautiful, but he also found the new physical phenomena.
00:25:17 He found that because of that term,
00:25:19 he could now get electric and magnetic waves
00:25:22 moving through space at a speed that he could calculate.
00:25:27 So he calculated the speed of the wave
00:25:29 and lo and behold, he found it’s the same
00:25:31 as the speed of light, which puzzled him
00:25:33 because he didn’t think light had anything
00:25:35 to do with electricity and magnetism.
00:25:37 But then he was courageous enough to say,
00:25:39 well, maybe light is nothing
00:25:40 but these electric and magnetic fields moving around.
00:25:44 And he wasn’t alive to see the verification
00:25:48 of that prediction and indeed it was true.
00:25:50 So this mathematical inconsistency,
00:25:53 which we could say this mathematical beauty drove him
00:25:58 to this physical, very important connection
00:26:02 between light and electric and magnetic phenomena,
00:26:05 which was later confirmed.
00:26:07 So then physics progresses and it comes to Einstein.
00:26:11 Einstein looks at Maxwell’s equation,
00:26:13 says, beautiful, these are nice equation,
00:26:15 except we get one speed light.
00:26:18 Who measures this light speed?
00:26:20 And he asked the question, are you moving?
00:26:23 Are you not moving?
00:26:24 If you move, the speed of light changes,
00:26:25 but Maxwell’s equation has no hint
00:26:27 of different speeds of light.
00:26:29 It doesn’t say, oh, only if you’re not moving,
00:26:31 you get the speed, it’s just you always get the speed.
00:26:33 So Einstein was very puzzled and he was daring enough
00:26:37 to say, well, you know, maybe everybody gets
00:26:39 the same speed for light.
00:26:40 And that motivated his theory of special relativity.
00:26:44 And this is an interesting example
00:26:45 because the idea was motivated from physics,
00:26:47 from Maxwell’s equations, from the fact
00:26:50 that people try to measure the properties of ether,
00:26:56 which was supposed to be the medium
00:26:58 in which the light travels through.
00:27:00 And the idea was that only in that medium,
00:27:03 the speed of, if you’re at risk with respect
00:27:06 to the ether, the speed, the speed of light,
00:27:08 then if you’re moving, the speed changes
00:27:10 and people did not discover it.
00:27:11 Michelson and Morley’s experiment showed there’s no ether.
00:27:15 So then Einstein was courageous enough to say,
00:27:17 you know, light is the same speed for everybody,
00:27:20 regardless of whether you’re moving or not.
00:27:22 And the interesting thing is about special theory
00:27:25 of relativity is that the math underpinning it
00:27:29 is very simple.
00:27:31 It’s a linear algebra, nothing terribly deep.
00:27:35 You can teach it at a high school level, if not earlier.
00:27:39 Okay, does that mean Einstein’s special relativity
00:27:42 is boring?
00:27:43 Not at all.
00:27:44 So this is an example where simple math, you know,
00:27:47 linear algebra leads to deep physics.
00:27:50 Einstein’s theory of special relativity.
00:27:53 Motivated by this inconsistency that Maxwell’s equation
00:27:56 would suggest for the speed of light,
00:27:58 depending on who observes it.
00:27:59 What’s the most daring idea there,
00:28:00 that the speed of light could be the same everywhere?
00:28:03 That’s the basic, that’s the guts of it.
00:28:05 That’s the core of Einstein’s theory.
00:28:07 That statement underlies the whole thing.
00:28:09 Speed of light is the same for everybody.
00:28:11 It’s hard to swallow and it doesn’t sound right.
00:28:13 It sounds completely wrong on the face of it.
00:28:16 And it took Einstein to make this daring statement.
00:28:19 It would be laughing in some sense.
00:28:22 How could anybody make this possibly ridiculous claim?
00:28:26 And it turned out to be true.
00:28:27 How does that make you feel?
00:28:28 Because it still sounds ridiculous.
00:28:31 It sounds ridiculous until you learn
00:28:33 that our intuition is at fault
00:28:34 about the way we conceive of space and time.
00:28:37 The way we think about space and time is wrong
00:28:40 because we think about the nature of time as absolute.
00:28:43 And part of it is because we live in a situation
00:28:46 where we don’t go with very high speeds.
00:28:49 There are speeds that are small
00:28:50 compared to the speed of light.
00:28:52 And therefore the phenomena we observe
00:28:54 does not distinguish the relativity of time.
00:28:57 The time also depends on who measures it.
00:28:59 There’s no absolute time.
00:29:00 When you say it’s noon today and now,
00:29:02 it depends on who’s measuring it.
00:29:04 And not everybody would agree with that statement.
00:29:07 And to see that you would have to have fast observer
00:29:10 moving speeds close to the speed of light.
00:29:12 So this shows that our intuition is at fault.
00:29:15 And a lot of the discoveries in physics
00:29:19 precisely is getting rid of the wrong old intuition.
00:29:23 And it is funny because we get rid of it,
00:29:25 but it’s always lingers in us in some form.
00:29:28 Like even when I’m describing it,
00:29:30 I feel like a little bit like, isn’t it funny?
00:29:32 As you’re just feeling the same way.
00:29:34 It is, it is.
00:29:35 But we kind of replace it by an intuition.
00:29:40 And actually there’s a very beautiful example of this,
00:29:43 how physicists do this, try to replace their intuition.
00:29:46 And I think this is one of my favorite examples
00:29:48 about how physicists develop intuition.
00:29:52 It goes to the work of Galileo.
00:29:54 So, again, let’s go back to Greek philosophers
00:29:58 or maybe Aristotle in this case.
00:30:00 Now, again, let’s make a criticism.
00:30:02 He thought that the heavier objects fall faster
00:30:05 than the lighter objects.
00:30:07 Makes sense.
00:30:07 It kind of makes sense.
00:30:08 And people say about the feather and so on,
00:30:10 but that’s because of the air resistance.
00:30:12 But you might think like,
00:30:13 if you have a heavy stone and a light pebble,
00:30:17 the heavy one will fall first.
00:30:18 If you don’t do any experiments,
00:30:20 that’s the first gut reaction.
00:30:21 I would say everybody would say that’s the natural thing.
00:30:24 Galileo did not believe this.
00:30:25 And he kind of did the experiment.
00:30:29 Famously it said he went on the top of Pisa Tower
00:30:32 and he dropped these heavy and light stones
00:30:34 and they fell at the same time
00:30:35 when he dropped it at the same time from the same height.
00:30:39 Okay, good.
00:30:39 So he said, I’m done.
00:30:41 I’ve showed that the heavy and lighter objects
00:30:43 fall at the same time.
00:30:44 I did the experiment.
00:30:45 Scientists at that time did not accept it.
00:30:49 Why was that?
00:30:50 Because at that time, science was not just experimental.
00:30:54 The experiment was not enough.
00:30:56 They didn’t think that they have to soil their hands
00:30:59 in doing experiments to get to the reality.
00:31:01 They said, why is it the case?
00:31:03 Why?
00:31:04 So Galileo had to come up with an explanation
00:31:06 of why heavier and lighter objects fall at the same rate.
00:31:09 This is the way he convinced them using symmetry.
00:31:13 He said, suppose you have three bricks,
00:31:16 the same shape, the same size, same mass, everything.
00:31:21 And we hold these three bricks at the same height
00:31:24 and drop them.
00:31:27 Which one will fall to the ground first?
00:31:29 Everybody said, of course, we know it’s symmetry
00:31:32 tells you they’re all the same shape,
00:31:33 same size, same height.
00:31:35 Of course, they fall at the same time.
00:31:36 Yeah, we know that.
00:31:37 Next, next.
00:31:38 It’s trivial.
00:31:39 He said, okay, what if we move these bricks around
00:31:42 with the same height?
00:31:42 Does it change the time they hit the ground?
00:31:45 They said, if it’s the same height,
00:31:46 again, by the symmetry principle,
00:31:47 because the height translation horizontal
00:31:49 translates to the symmetry, no, it doesn’t matter.
00:31:52 They all fall at the same rate.
00:31:53 Good.
00:31:54 Does it matter how close I bring them together?
00:31:55 No, it doesn’t.
00:31:56 Okay, suppose I make the two bricks touch
00:31:59 and then let them go.
00:31:59 Do they fall at the same rate?
00:32:01 Yes, they do.
00:32:02 But then he said, well, the two bricks that touch
00:32:04 are twice more mass than this other brick.
00:32:07 And you just agreed that they fall at the same rate.
00:32:09 They say, yeah, yeah, we just agreed.
00:32:10 That’s right, that’s great.
00:32:12 Yes.
00:32:13 So he deconfused them by the symmetry reasoning.
00:32:15 So this way of repackaging some intuition,
00:32:18 a different type of intuition.
00:32:19 When the intuitions clash,
00:32:21 then you side on the, you replace the intuition.
00:32:24 That’s brilliant.
00:32:26 In some of these more difficult physical ideas,
00:32:31 physics ideas in the 20th century and the 21st century,
00:32:34 it starts becoming more and more difficult
00:32:36 to then replace the intuition.
00:32:38 What does the world look like
00:32:39 for an object traveling close to the speed of light?
00:32:42 You start to think about the edges
00:32:44 of supermassive black holes,
00:32:47 and you start to think like, what’s that look like?
00:32:51 Or I’ve been into gravitational waves recently.
00:32:55 It’s like when the fabric of space time
00:32:58 is being morphed by gravity,
00:33:01 like what’s that actually feel like?
00:33:03 If I’m riding a gravitational wave, what’s that feel like?
00:33:09 I mean, I think some of those are more sort of hippy,
00:33:12 not useful intuitions to have,
00:33:15 but if you’re an actual physicist
00:33:18 or whatever the particular discipline is,
00:33:20 I wonder if it’s possible to meditate,
00:33:23 to sort of escape through thinking,
00:33:27 prolong thinking and meditation on a world,
00:33:31 like live in a visualized world that’s not like our own
00:33:35 in order to understand a phenomenon deeply.
00:33:38 So like replace the intuition,
00:33:41 like through rigorous meditation on the idea
00:33:44 in order to conceive of it.
00:33:46 I mean, if we talk about multiple dimensions,
00:33:48 I wonder if there’s a way to escape
00:33:51 with a three dimensional world in our mind
00:33:53 in order to then start to reason about it.
00:33:56 It’s, the more I talk to topologists,
00:34:01 the more they seem to not operate at all
00:34:04 in the visual space.
00:34:05 They really trust the mathematics,
00:34:07 like which is really annoying to me because topology
00:34:10 and differential geometry feels like it has a lot
00:34:15 of potential for beautiful pictures.
00:34:17 Yes, I think they do.
00:34:18 Actually, I would not be able to do my research
00:34:23 if I don’t have an intuitive feel about geometry.
00:34:26 And we’ll get to it as you mentioned before
00:34:29 that how, for example, in strength theory,
00:34:32 you deal with these extra dimensions.
00:34:33 And I’ll be very happy to describe how we do it
00:34:35 because without intuition, we will not get anywhere.
00:34:37 And I don’t think you can just rely on formalism.
00:34:40 I don’t.
00:34:41 I don’t think any physicist just relies on formalism.
00:34:44 That’s not physics.
00:34:45 That’s not understanding.
00:34:46 So we have to intuit it.
00:34:48 And that’s crucial.
00:34:49 And there are steps of doing it.
00:34:50 And we learned it might not be trivial,
00:34:52 but we learn how to do it.
00:34:53 Similar to what this Galileo picture I just told you,
00:34:56 you have to build these gradually.
00:34:59 But you have to connect the bricks.
00:35:02 Exactly, you have to connect the bricks, literally.
00:35:04 So yeah, so then, so going back to your question
00:35:07 about the path of the history of the science.
00:35:10 So I was saying about the electricity and magnetism
00:35:12 and the special relativity where simple idea
00:35:14 led to special relativity.
00:35:16 But then he went further thinking about acceleration
00:35:20 in the context of relativity.
00:35:21 And he came up with general relativity
00:35:23 where he talked about the fabric of space time
00:35:26 being curved and so forth and matter
00:35:28 affecting the curvature of the space and time.
00:35:32 So this gradually became a connection
00:35:36 between geometry and physics.
00:35:38 Namely, he replaced Newton’s gravitational force
00:35:43 with a very geometrical, beautiful picture.
00:35:46 It’s much more elegant than Newton’s,
00:35:47 but much more complicated mathematically.
00:35:49 So when we say it’s simpler,
00:35:52 we mean in some form it’s simpler,
00:35:55 but not in pragmatic terms of equation solving.
00:35:57 The equations are much harder to solve
00:35:59 in Einstein’s theory.
00:36:01 And in fact, so much harder that Einstein himself
00:36:03 couldn’t solve many of the cases.
00:36:06 He thought, for example, you couldn’t solve the equation
00:36:07 for a spherical symmetric matter,
00:36:10 like if you had a symmetric sun,
00:36:12 he didn’t think you can actually solve his equation for that.
00:36:15 And a year after he said that it was solved by Schwarzschild.
00:36:19 So it was that hard
00:36:21 that he didn’t think it’s gonna be that easy.
00:36:22 So yeah, deformism is hard.
00:36:24 But the contrast between the special relativity
00:36:27 and general relativity is very interesting
00:36:29 because one of them has almost trivial math
00:36:31 and the other one has super complicated math.
00:36:34 Both are physically amazingly important.
00:36:37 And so we have learned that, you know,
00:36:40 the physics may or may not require complicated math.
00:36:44 We should not shy from using complicated math
00:36:47 like Einstein did.
00:36:48 Nobody, Einstein wouldn’t say,
00:36:49 I’m not gonna touch this math because it’s too much,
00:36:52 you know, tensors or, you know, curvature
00:36:54 and I don’t like the four dimensional space time
00:36:56 because I can’t see four dimension.
00:36:57 He wasn’t doing that.
00:36:59 He was willing to abstract from that
00:37:01 because physics drove him in that direction.
00:37:03 But his motivation was physics.
00:37:05 Physics pushed him.
00:37:06 Just like Newton pushed to develop calculus
00:37:09 because physics pushed him that he didn’t have the tools.
00:37:12 So he had to develop the tools
00:37:14 to answer his physics questions.
00:37:16 So his motivation was physics again.
00:37:18 So to me, those are examples which show
00:37:20 that math and physics have this symbiotic relationship
00:37:24 which kind of reinforce each other.
00:37:26 Here I’m using, I’m giving you examples of both of them,
00:37:30 namely Newton’s work led to development
00:37:32 of mathematics, calculus.
00:37:34 And in the case of Einstein, he didn’t develop
00:37:36 Riemannian geometry, he just used them.
00:37:38 So it goes both ways and in the context of modern physics,
00:37:42 we see that again and again, it goes both ways.
00:37:44 Let me ask a ridiculous question.
00:37:46 You know, you talk about your favorite soccer player,
00:37:48 the bar, I’ll ask the same question about Einstein’s ideas
00:37:52 which is, which one do you think
00:37:54 is the biggest leap of genius?
00:37:56 Is it the E equals MC squared?
00:37:59 Is it Brownian motion?
00:38:01 Is it special relativity, is it general relativity?
00:38:05 Which of the famous set of papers he’s written in 1905
00:38:09 and in general, his work was the biggest leap of genius?
00:38:13 In my opinion, it’s special relativity.
00:38:16 The idea that speed of light is the same for everybody
00:38:19 is the beginning of everything he did.
00:38:20 The beginning is the seed.
00:38:21 The beginning.
00:38:22 Once you embrace that weirdness,
00:38:24 all the weirdness, all the rest.
00:38:25 I would say that’s, even though he says
00:38:27 the most beautiful moment for him,
00:38:29 he says that is when he realized that if you fall
00:38:31 in an elevator, you don’t know if you’re falling
00:38:33 or whether you’re in the falling elevator
00:38:36 or whether you’re next to the earth, gravitational.
00:38:39 That to him was his aha moment,
00:38:41 which inertial mass and gravitational mass
00:38:43 being identical geometrically and so forth
00:38:46 as part of the theory, not because of, you know,
00:38:49 some funny coincidence.
00:38:52 That’s for him, but I feel from outside at least,
00:38:54 it feels like the speed of light being the same
00:38:56 is the really aha moment.
00:38:59 The general relativity to you is not
00:39:02 like the conception of space time.
00:39:04 In a sense, the conception of space time
00:39:06 already was part of the special relativity
00:39:08 when you talk about length contraction.
00:39:10 So general relativity takes that to the next step,
00:39:13 but beginning of it was already space,
00:39:15 length contracts, time dilates.
00:39:17 So once you talk about those, then yeah,
00:39:19 you can dilate more or less different places
00:39:20 than its curvature.
00:39:21 So you don’t have a choice.
00:39:22 So it kind of started just with that same simple thought.
00:39:26 Speed of light is the same for all.
00:39:28 Where does quantum mechanics come into view?
00:39:32 Exactly, so this is the next step.
00:39:33 So Einstein’s, you know, developed general relativity
00:39:36 and he’s beginning to develop the foundation
00:39:38 of quantum mechanics at the same time,
00:39:39 the photoelectric effects and others.
00:39:42 And so quantum mechanics overtakes, in fact,
00:39:45 Einstein in many ways because he doesn’t like
00:39:47 the probabilistic interpretation of quantum mechanics
00:39:50 and the formulas that’s emerging,
00:39:52 but fits his march on and try to, for example,
00:39:56 combine Einstein’s theory of relativity
00:39:59 with quantum mechanics.
00:40:01 So Dirac takes special relativity,
00:40:04 tries to see how is it compatible with quantum mechanics.
00:40:07 Can we pause and briefly say what is quantum mechanics?
00:40:10 Oh yes, sure.
00:40:11 So quantum mechanics, so I discussed briefly
00:40:14 when I talked about the connection
00:40:16 between Newtonian mechanics
00:40:18 and the Euler Lagrange reformulation
00:40:20 of the Newtonian mechanics and interpretation
00:40:23 of this Euler Lagrange formulas in terms of the paths
00:40:27 that the particle take.
00:40:28 So when we say a particle goes from here to here,
00:40:31 we usually think it classically follows
00:40:34 a specific trajectory, but actually in quantum mechanics,
00:40:38 it follows every trajectory with different probabilities.
00:40:42 And so there’s this fuzziness.
00:40:44 Now, most probable, it’s the path that you actually see
00:40:49 and deviation from that is very, very unlikely
00:40:51 and probabilistically very minuscule.
00:40:53 So in everyday experiments,
00:40:55 we don’t see anything deviated from what we expect,
00:40:58 but quantum mechanics tells us that the things
00:41:00 are more fuzzy.
00:41:01 Things are not as precise as the line you draw.
00:41:05 Things are a bit like cloud.
00:41:07 So if you go to microscopic scales,
00:41:11 like atomic scales and lower,
00:41:12 these phenomena become more pronounced.
00:41:14 You can see it much better.
00:41:16 The electron is not at the point,
00:41:18 but the cloud spread out around the nucleus.
00:41:21 And so this fuzziness, this probabilistic aspect of reality
00:41:25 is what quantum mechanics describes.
00:41:28 Can I briefly pause on that idea?
00:41:31 Do you think quantum mechanics
00:41:33 is just a really damn good approximation,
00:41:37 a tool for predicting reality,
00:41:40 or does it actually describe reality?
00:41:43 Do you think reality is fuzzy at that level?
00:41:45 Well, I think that reality is fuzzy at that level,
00:41:48 but I don’t think quantum mechanics
00:41:49 is necessarily the end of the story.
00:41:51 So quantum mechanics is certainly an improvement
00:41:55 over classical physics.
00:41:57 That much we know by experiments and so forth.
00:42:00 Whether I’m happy with quantum mechanics,
00:42:02 whether I view quantum mechanics,
00:42:04 for example, the thought,
00:42:05 the measurement description of quantum mechanics,
00:42:08 am I happy with it?
00:42:09 Am I thinking that’s the end stage or not?
00:42:11 I don’t.
00:42:12 I don’t think we’re at the end of that story.
00:42:14 And many physicists may or may not view this way.
00:42:17 Some do, some don’t.
00:42:18 But I think that it’s the best we have right now,
00:42:22 that’s for sure.
00:42:23 It’s the best approximation for reality we know today.
00:42:25 And so far, we don’t know what it is,
00:42:27 the next thing that improves it or replaces it and so on.
00:42:30 But as I mentioned before,
00:42:31 I don’t believe any of the laws of physics we know today
00:42:34 are permanently exactly correct.
00:42:36 That doesn’t bother me.
00:42:38 I’m not like dogmatic saying,
00:42:39 I have figured out this is the law of nature.
00:42:41 I know everything.
00:42:42 No, no, that’s the beauty about science
00:42:45 is that we are not dogmatic.
00:42:47 And we are willing to, in fact,
00:42:49 we are encouraged to be skeptical of what we ourselves do.
00:42:53 So you were talking about Dirac.
00:42:55 Yes, I was talking about Dirac, right.
00:42:56 So Dirac was trying to now combine
00:42:58 this Schrodinger’s equations,
00:43:01 which was described in the context of trying to talk about
00:43:04 how these probabilistic waves of electrons
00:43:06 move for the atom,
00:43:07 which was good for speeds
00:43:09 which were not too close to the speed of light,
00:43:11 to what happens when you get to the near the speed of light.
00:43:14 So then you need relativity.
00:43:16 So then Dirac tried to combine Einstein’s relativity
00:43:19 with quantum mechanics.
00:43:20 So he tried to combine them
00:43:22 and he wrote this beautiful equation, the Dirac equation,
00:43:26 which roughly speaking,
00:43:28 take the square root of the Einstein’s equation
00:43:31 in order to connect it to Schrodinger’s
00:43:33 time evolution operator,
00:43:34 which is first order in time derivative
00:43:37 to get rid of the naive thing
00:43:39 that Einstein’s equation would have given,
00:43:40 which is second order.
00:43:41 So you have to take a square root.
00:43:43 Now square root usually has a plus or minus sign
00:43:45 when you take it.
00:43:47 And when he did this,
00:43:49 he originally didn’t notice this plus,
00:43:50 didn’t pay attention to this plus or minus sign,
00:43:52 but later physicists pointed out to Dirac says,
00:43:55 look, there’s also this minus sign.
00:43:57 And if you use this minus sign,
00:43:58 you get negative energy.
00:44:01 In fact, it was very, very annoying that, you know,
00:44:04 somebody else tells you this obvious mistake you make.
00:44:06 Pauli famous physicist told Dirac, this is nonsense.
00:44:09 You’re going to get negative energy with your equation,
00:44:11 which negative energy without any bottom,
00:44:13 you can go all the way down to negative.
00:44:15 Infinite energy, so it doesn’t make any sense.
00:44:18 Dirac thought about it.
00:44:19 And then he remembered Pauli’s exclusion principle
00:44:22 just before him.
00:44:23 Pauli had said, you know,
00:44:24 there’s this principle called the exclusion principle
00:44:26 that, you know, two electrons cannot be on the same orbit.
00:44:30 And so Dirac said, okay, you know what?
00:44:32 All these negative energy states are filled orbits,
00:44:37 occupied.
00:44:38 So according to you,
00:44:42 Mr. Pauli, there’s no place to go.
00:44:44 So therefore they only have to go positive.
00:44:47 Sounded like a big cheat.
00:44:49 And then Pauli said, oh, you know what?
00:44:51 We can change orbits from one orbit to another.
00:44:53 What if I take one of these negative energy orbits
00:44:55 and put it up there?
00:44:57 Then it seems to be a new particle,
00:44:59 which has opposite properties to the electron.
00:45:03 It has positive energy, but it has positive charge.
00:45:06 What is that?
00:45:09 Dirac was a bit worried.
00:45:10 He said, maybe that’s proton
00:45:11 because proton has plus charge.
00:45:13 He wasn’t sure.
00:45:14 But then he said, oh, maybe it’s proton.
00:45:16 But then they said, no, no, no, no.
00:45:17 It has the same mass as the electron.
00:45:19 It cannot be proton because proton is heavier.
00:45:22 Dirac was stuck.
00:45:23 He says, well, then maybe another part we haven’t seen.
00:45:27 By that time, Dirac himself was getting a little bit worried
00:45:31 about his own equation and his own crazy interpretation.
00:45:34 Until a few years later, Anderson,
00:45:37 in the photographic place that he had gotten
00:45:40 from these cosmic rays,
00:45:42 he discovered a particle which goes
00:45:45 in the opposite direction that the electron goes
00:45:47 when there’s a magnetic field,
00:45:49 and with the same mass,
00:45:52 exactly like what Dirac had predicted.
00:45:55 And this was what we call now positron.
00:45:57 And in fact, beginning with the work of Dirac,
00:46:00 we know that every particle has an antiparticle.
00:46:03 And so this idea that there’s an antiparticle
00:46:05 came from this simple math.
00:46:06 There’s a plus and a minus
00:46:08 from the Dirac’s quote unquote mistake.
00:46:12 So again, trying to combine ideas,
00:46:15 sometimes the math is smarter than the person
00:46:18 who uses it to apply it,
00:46:20 and you try to resist it,
00:46:21 and then you kind of confront it by criticism,
00:46:23 which is the way it should be.
00:46:25 So physicists comes and said, no, no, that’s wrong,
00:46:26 and you correct it, and so on.
00:46:27 So that is a development of the idea
00:46:30 there’s particle, there’s antiparticle, and so on.
00:46:32 So this is the beginning of development
00:46:34 of quantum mechanics and the connection with relativity,
00:46:37 but the thing was more challenging
00:46:38 because we had to also describe
00:46:40 how electric and magnetic fields work with quantum mechanics.
00:46:44 This was much more complicated
00:46:46 because it’s not just one point.
00:46:47 Electric and magnetic fields were everywhere.
00:46:50 So you had to talk about fluctuating
00:46:52 and a fuzziness of electrical fields
00:46:54 and magnetic fields everywhere.
00:46:56 And the math for that was very difficult to deal with.
00:47:00 And this led to a subject called quantum field theory.
00:47:03 Fields like electric and magnetic fields had to be quantum,
00:47:06 had to be described also in a wavy way.
00:47:09 Feynman in particular was one of the pioneers
00:47:13 along with Schrodingers and others
00:47:15 to try to come up with a formalism
00:47:17 to deal with fields like electric and magnetic fields,
00:47:20 interacting with electrons in a consistent quantum fashion.
00:47:24 And they developed this beautiful theory,
00:47:25 quantum electrodynamics from that.
00:47:27 And later on that same formalism,
00:47:30 quantum field theory led to the discovery of other forces
00:47:33 and other particles all consistent
00:47:35 with the idea of quantum mechanics.
00:47:37 So that was how physics progressed.
00:47:40 And so basically we learned that all particles
00:47:43 and all the forces are in some sense related
00:47:47 to particle exchanges.
00:47:49 And so for example, electromagnetic forces
00:47:52 are mediated by a particle we call photon and so forth.
00:47:57 And same for other forces that they discovered,
00:47:59 strong forces and the weak forces.
00:48:01 So we got the sense of what quantum field theory is.
00:48:03 Is that a big leap of an idea that particles
00:48:09 are fluctuations in the field?
00:48:12 Like the idea that everything is a field.
00:48:15 It’s the old Einstein, light is a wave,
00:48:18 both a particle and a wave kind of idea.
00:48:20 Is that a huge leap in our understanding
00:48:23 of conceiving the universe as fields?
00:48:26 I would say so.
00:48:27 I would say that viewing the particles,
00:48:29 this duality that Bohr mentioned
00:48:31 between particles and waves,
00:48:33 that waves can behave sometimes like particles,
00:48:35 sometimes like waves,
00:48:36 is one of the biggest leaps of imagination
00:48:40 that quantum mechanics made physics do.
00:48:42 So I agree that that is quite remarkable.
00:48:45 Is duality fundamental to the universe
00:48:50 or is it just because we don’t understand it fully?
00:48:52 Like will it eventually collapse
00:48:54 into a clean explanation that doesn’t require duality?
00:48:57 Like that a phenomena could be two things at once
00:49:02 and both to be true.
00:49:04 So that seems weird.
00:49:05 So in fact I was going to get to that
00:49:08 when we get to string theory
00:49:09 but maybe I can comment on that now.
00:49:11 Duality turns out to be running the show today
00:49:13 and the whole thing that we are doing is string theory.
00:49:15 Duality is the name of the game.
00:49:17 So it’s the most beautiful subject
00:49:19 and I want to talk about it.
00:49:20 Let’s talk about it in the context of string theory then.
00:49:23 So we do want to take a next step into,
00:49:27 because we mentioned general relativity,
00:49:28 we mentioned quantum mechanics,
00:49:30 is there something to be said about quantum gravity?
00:49:32 Yes, that’s exactly the right point to talk about.
00:49:34 So namely we have talked about quantum fields
00:49:37 and I talked about electric forces,
00:49:39 photon being the particle carrying those forces.
00:49:42 So for gravity, quantizing gravitational field
00:49:46 which is this curvature of space time according to Einstein,
00:49:49 you get another particle called graviton.
00:49:52 So what about gravitons?
00:49:55 Should be there, no problem.
00:49:56 So then you start computing it.
00:49:59 What do I mean by computing it?
00:50:00 Well, you compute scattering of one graviton
00:50:03 off another graviton, maybe with graviton with an electron
00:50:06 and so on, see what you get.
00:50:07 Feynman had already mastered this quantum electrodynamics.
00:50:12 He said, no problem, let me do it.
00:50:14 Even though these are such weak forces,
00:50:17 the gravity is very weak.
00:50:18 So therefore to see them,
00:50:19 these quantum effects of gravitational waves was impossible.
00:50:23 It’s even impossible today.
00:50:25 So Feynman just did it for fun.
00:50:27 He usually had this mindset that I want to do something
00:50:30 which I will see in experiment,
00:50:31 but this one, let’s just see what it does.
00:50:34 And he was surprised because the same techniques
00:50:36 he was using for doing the same calculations,
00:50:39 quantum electrodynamics, when applied to gravity failed.
00:50:44 The formulas seem to make sense,
00:50:46 but he had to do some integrals
00:50:47 and he found that when he does those integrals,
00:50:49 he got infinity and it didn’t make any sense.
00:50:52 Now there were similar infinities in the other pieces
00:50:54 but he had managed to make sense out of those before.
00:50:56 This was no way he could make sense out of it.
00:50:59 He just didn’t know what to do.
00:51:01 He didn’t feel it’s an urgent issue
00:51:03 because nobody could do the experiment.
00:51:05 So he was kind of said, okay, there’s this thing,
00:51:07 but okay, we don’t know how to exactly do it,
00:51:09 but that’s the way it is.
00:51:11 So in some sense, a natural conclusion
00:51:14 from what Feynman did could have been like,
00:51:16 gravity cannot be consistent with quantum theory,
00:51:19 but that cannot be the case
00:51:20 because gravity is in our universe,
00:51:22 quantum mechanics in our universe,
00:51:23 they both together somehow should work.
00:51:25 So it’s not acceptable to say they don’t work together.
00:51:28 So that was a puzzle.
00:51:30 How does it possibly work?
00:51:32 It was left open.
00:51:34 And then we get to the string theory.
00:51:37 So this is the puzzle of quantum gravity.
00:51:38 The particle description of quantum gravity failed.
00:51:41 So the infinity shows up.
00:51:43 What do we do with infinity?
00:51:45 Let’s get to the fun part.
00:51:47 Let’s talk about string theory.
00:51:48 Yes.
00:51:50 Let’s discuss some technical basics of string theory.
00:51:56 What is string theory?
00:51:57 What is a string?
00:51:59 How many dimensions are we talking about?
00:52:01 What are the different states?
00:52:02 How do we represent the elementary particles
00:52:04 and the laws of physics using this new framework?
00:52:09 So string theory is the idea
00:52:12 that the fundamental entities are not particles,
00:52:16 but extended higher dimensional objects
00:52:18 like one dimensional strings, like loops.
00:52:22 These loops could be open like with two ends,
00:52:25 like an interval or a circle without any ends.
00:52:29 And they’re vibrating and moving around in space.
00:52:32 So how big they are?
00:52:34 Well, you can of course stretch it and make it big,
00:52:37 or you can just let it be whatever it wants.
00:52:39 It can be as small as a point
00:52:41 because the circle can shrink to a point
00:52:44 and be very light,
00:52:45 or you can stretch it and becomes very massive,
00:52:48 or it could oscillate and become massive that way.
00:52:50 So it depends on which kind of state you have.
00:52:52 In fact, the string can have infinitely many modes,
00:52:55 depending on which kind of oscillation it’s doing.
00:52:56 Like a guitar has different harmonics,
00:52:59 string has different harmonics,
00:53:00 but for the string, each harmonic is a particle.
00:53:03 So each particle will give you,
00:53:04 ah, this is a more massive harmonic, this is a less massive.
00:53:07 So the lightest harmonic, so to speak, is no harmonics,
00:53:10 which means like the string shrunk to a point,
00:53:12 and then it becomes like a massless particles
00:53:15 or light particles like photon and graviton and so forth.
00:53:19 So when you look at tiny strings,
00:53:22 which are shrunk to a point, the lightest ones,
00:53:25 they look like the particles that we think,
00:53:27 they’re like particles.
00:53:28 In other words, from far away, they look like a point.
00:53:31 But of course, if you zoom in,
00:53:32 there’s this tiny little circle that’s there
00:53:35 that’s shrunk to almost a point.
00:53:37 Should we be imagining, this is to the visual intuition,
00:53:40 should we be imagining literally strings
00:53:42 that are potentially connected as a loop or not?
00:53:47 We knew, and when somebody outside of physics
00:53:50 is imagining a basic element of string theory,
00:53:53 which is a string,
00:53:56 should we literally be thinking about a string?
00:53:58 Yes, you should literally think about string,
00:54:00 but string with zero thickness.
00:54:02 With zero thickness.
00:54:03 So notice, it’s a loop of energy, so to speak,
00:54:07 if you can think of it that way.
00:54:08 And so there’s a tension like a regular string,
00:54:11 if you pull it, there’s, you know, you have to stretch it.
00:54:14 But it’s not like a thickness, like you’re made of something,
00:54:16 it’s just energy.
00:54:17 It’s not made of atoms or something like that.
00:54:19 But it is very, very tiny.
00:54:21 Very tiny.
00:54:22 Much smaller than elementary particles of physics.
00:54:25 Much smaller.
00:54:26 So we think if you let the string to be by itself,
00:54:29 the lowest state, there’ll be like fuzziness
00:54:32 or a size of that tiny little circle,
00:54:33 which is like a point,
00:54:35 about, could be anything between,
00:54:37 we don’t know the exact size,
00:54:38 but in different models have different sizes,
00:54:40 but something of the order of 10 to the minus,
00:54:42 let’s say 30 centimeters.
00:54:43 So 10 to the minus 30 centimeters,
00:54:46 just to compare it with the size of the atom,
00:54:48 which is 10 to the minus eight centimeters,
00:54:50 is 22 orders of magnitude smaller.
00:54:53 So.
00:54:54 Unimaginably small, I would say.
00:54:56 Very small.
00:54:56 So we basically think from far away,
00:54:58 string is like a point particle.
00:55:00 And that’s why a lot of the things that we learned
00:55:03 about point particle physics
00:55:04 carries over directly to strings.
00:55:07 So therefore there’s not much of a mystery
00:55:09 why particle physics was successful,
00:55:10 because a string is like a particle
00:55:12 when it’s not stretched.
00:55:14 But it turns out having this size,
00:55:17 being able to oscillate, get bigger,
00:55:20 turned out to be resolving this puzzles
00:55:22 that Feynman was having in calculating his diagrams,
00:55:26 and it gets rid of those infinities.
00:55:28 So when you’re trying to do those infinities,
00:55:31 the regions that give infinities to Feynman,
00:55:34 as soon as you get to those regions,
00:55:35 then this string starts to oscillate,
00:55:38 and these oscillation structure of the strings
00:55:40 resolves those infinities to finite answer at the end.
00:55:43 So the size of the string,
00:55:45 the fact that it’s one dimensional,
00:55:46 gives a finite answer at the end.
00:55:48 Resolves this paradox.
00:55:50 Now, perhaps it’s also useful to recount
00:55:54 of how string theory came to be.
00:55:56 Because it wasn’t like somebody say,
00:55:58 well, let me solve the problem of Einstein’s,
00:56:01 solve the problem that Feynman had with unifying
00:56:04 Einstein’s theory with quantum mechanics
00:56:06 by replacing the point by a string.
00:56:08 No, that’s not the way the thought process,
00:56:10 the thought process was much more random.
00:56:14 Physicist, then it’s John on this case,
00:56:16 was trying to describe the interactions
00:56:17 they were seeing in colliders, in accelerators.
00:56:22 And they were seeing that some process,
00:56:23 in some process, when two particles came together
00:56:26 and joined together and when they were separately,
00:56:29 in one way, and the opposite way, they behave the same way.
00:56:34 In some way, there was a symmetry, a duality,
00:56:37 which he didn’t understand.
00:56:38 The particles didn’t seem to have that symmetry.
00:56:41 He said, I don’t know what it is,
00:56:43 what’s the reason that these colliders
00:56:44 and experiments we’re doing seems to have the symmetry,
00:56:46 but let me write the mathematical formula,
00:56:49 which exhibits that symmetry.
00:56:51 He used gamma functions, beta functions and all that,
00:56:54 you know, complete math, no physics,
00:56:56 other than trying to get symmetry out of his equation.
00:56:59 He just wrote down a formula as the answer for a process,
00:57:03 not a method to compute it.
00:57:04 Just say, wouldn’t it be nice if this was the answer?
00:57:08 Yes.
00:57:08 Physics looked at this one, that’s intriguing,
00:57:11 it has the symmetry all right, but what is this?
00:57:13 Where is this coming from?
00:57:14 Which kind of physics gives you this?
00:57:17 So I don’t know.
00:57:19 A few years later, people saw that,
00:57:21 oh, the equation that you’re writing,
00:57:23 the process you’re writing in the intermediate channels
00:57:26 that particles come together,
00:57:27 seems to have all the harmonics.
00:57:30 Harmonics sounds like a string.
00:57:32 Let me see if what you’re describing
00:57:33 has anything to do with the strings.
00:57:34 And people try to see if what he’s doing
00:57:36 has anything to do with the strings.
00:57:37 Oh, yeah, indeed.
00:57:38 If I study scattering of two strings,
00:57:40 I get exactly the formula you wrote down.
00:57:42 That was the reinterpretation
00:57:45 of what he had written in the formula as the strings,
00:57:48 but still had nothing to do with gravity.
00:57:51 It had nothing to do with resolving the problems
00:57:53 of gravity with quantum mechanics.
00:57:55 It was just trying to explain a process
00:57:57 that people were seeing in hydronic physics collisions.
00:58:01 So it took a few more years to get to that point.
00:58:04 They did notice that,
00:58:07 physicists noticed that whenever you try to find
00:58:10 the spectrum of strings, you always get a massless particle
00:58:13 which has exactly the properties
00:58:14 that the graviton is supposed to have.
00:58:16 And no particle in hydronic physics that had that property.
00:58:20 You are getting a massless graviton
00:58:22 as part of this scattering without looking for it.
00:58:25 It was forced on you.
00:58:27 People were not trying to solve quantum gravity.
00:58:29 Quantum gravity was pushed on them.
00:58:31 I don’t want this graviton.
00:58:33 Get rid of it.
00:58:34 They couldn’t get rid of it.
00:58:36 They gave up trying to get rid of it.
00:58:38 Physicists, Sherk and Schwartz said,
00:58:39 you know what, string theory is theory of quantum gravity.
00:58:43 They’ve changed their perspective altogether.
00:58:45 We are not describing the hydronic physics.
00:58:47 We are describing this theory of quantum gravity.
00:58:49 And that’s when string theory probably got like exciting
00:58:54 that this could be the unifying theory.
00:58:56 Exactly, it got exciting,
00:58:57 but at the same time, not so fast.
00:58:59 Namely, it should have been fast, but it wasn’t
00:59:02 because particle physics through quantum field theory
00:59:05 were so successful at that time.
00:59:07 This is mid seventies, standard model of physics,
00:59:10 electromagnetism and unification of electromagnetic forces
00:59:12 with all the other forces were beginning to take place
00:59:15 without the gravity part.
00:59:17 Everything was working beautifully for particle physics.
00:59:20 And so that was the shining golden age
00:59:23 of quantum field theory and all the experiments,
00:59:25 standard model, this and that, unification,
00:59:27 spontaneous symmetry breaking was taking place.
00:59:29 All of them was nice.
00:59:31 This was kind of like a side show
00:59:32 and nobody was paying so much attention.
00:59:34 This exotic string is needed for quantum gravity.
00:59:37 Maybe there’s other ways, maybe we should do something else.
00:59:39 So, yeah, it wasn’t paid much attention to.
00:59:41 And this took a little bit more effort
00:59:44 to try to actually connect it to reality.
00:59:48 There are a few more steps.
00:59:49 First of all, there was a puzzle
00:59:51 that you were getting extra dimensions.
00:59:53 String was not working well
00:59:55 with three spatial dimension on one time.
00:59:57 It needed extra dimension.
00:59:59 Now, there are different versions of strings,
01:00:02 but the version that ended up being related
01:00:04 to having particles like electron,
01:00:06 what we call fermions, needed 10 dimensions,
01:00:09 what we call super string.
01:00:12 Now, why super?
01:00:13 Why the word super?
01:00:13 It turns out this version of the string,
01:00:17 which had fermions, had an extra symmetry,
01:00:21 which we call supersymmetry.
01:00:23 This is a symmetry between a particle and another particle
01:00:27 with exactly the same properties,
01:00:29 same mass, same charge, et cetera.
01:00:31 The only difference is that one of them
01:00:33 has a little different spin than the other one.
01:00:35 And one of them is a boson, one of them is a fermion
01:00:38 because of that shift of spin.
01:00:41 Otherwise, they’re identical.
01:00:42 So there was this symmetry.
01:00:43 String theory had this symmetry.
01:00:45 In fact, supersymmetry was discovered
01:00:48 through string theory, theoretically.
01:00:51 So theoretically, the first place that this was observed
01:00:53 when you were describing these fermionic strings.
01:00:57 So that was the beginning of the study of supersymmetry
01:01:00 was via string theory.
01:01:02 And then it had remarkable properties
01:01:05 that the symmetry meant and so forth
01:01:07 that people began studying supersymmetry after that.
01:01:10 And that was a kind of a tangent direction
01:01:13 at the beginning for string theory.
01:01:15 But people in particle physics started also thinking,
01:01:17 oh, supersymmetry is great.
01:01:19 Let’s see if we can have supersymmetry
01:01:21 in particle physics and so forth.
01:01:22 Forget about strings.
01:01:23 And they developed on a different track as well.
01:01:25 Supersymmetry in different models
01:01:27 became a subject on its own right,
01:01:29 understanding supersymmetry and what does this mean?
01:01:32 Because it unified bosons and fermion,
01:01:34 unified some ideas together.
01:01:36 So photon is a boson, electron is a fermion.
01:01:39 Could things like that be somehow related?
01:01:41 It was a kind of a natural kind of a question
01:01:43 to try to kind of unify
01:01:44 because in physics, we love unification.
01:01:48 Now, gradually, string theory was beginning
01:01:50 to show signs of unification.
01:01:51 It had graviton, but people found that you also have
01:01:54 things like photons in them,
01:01:56 different excitations of string behave like photons,
01:01:59 another one behaves like electron.
01:02:01 So a single string was unifying all these particles
01:02:04 into one object.
01:02:06 That’s remarkable.
01:02:08 It’s in 10 dimensions though.
01:02:10 It is not our universe
01:02:11 because we live in three plus one dimension.
01:02:13 How could that be possibly true?
01:02:15 So this was a conundrum.
01:02:18 It was elegant, it was beautiful,
01:02:19 but it was very specific
01:02:21 about which dimension you’re getting,
01:02:23 which structure you’re getting.
01:02:25 It wasn’t saying, oh, you just put D equals to four,
01:02:27 you’ll get your space time dimension that you want.
01:02:29 No, it didn’t like that.
01:02:30 It said, I want 10 dimensions and that’s the way it is.
01:02:34 So it was very specific.
01:02:35 Now, so people try to reconcile this
01:02:37 by the idea that, you know,
01:02:39 maybe these extra dimensions are tiny.
01:02:41 So if you take three macroscopic spatial dimensions
01:02:45 on one time and six extra tiny spatial dimensions,
01:02:49 like tiny spheres or tiny circles,
01:02:51 then it avoids contradiction with manifest fact
01:02:55 that we haven’t seen extra dimensions in experiments today.
01:02:59 So that was a way to avoid conflict.
01:03:03 Now, this was a way to avoid conflict,
01:03:05 but it was not observed in experiments.
01:03:09 A string observed in experiments?
01:03:10 No, because it’s so small.
01:03:12 So it’s beginning to sound a little bit funny.
01:03:16 Similar feeling to the way perhaps Dirac had felt
01:03:19 about this positron plus or minus, you know,
01:03:21 it was beginning to sound a little bit like,
01:03:24 oh yeah, not only I have to have 10 dimension,
01:03:25 but I have to have this, I have to also this.
01:03:28 And so conservative physicists would say,
01:03:31 hmm, you know, I haven’t seen these experiments.
01:03:34 I don’t know if they are really there.
01:03:35 Are you pulling my leg?
01:03:37 Do you want me to imagine things that are not there?
01:03:40 So this was an attitude of some physicists
01:03:42 towards string theory, despite the fact
01:03:45 that the puzzle of gravity and quantum mechanics
01:03:47 merging together work, but still was this skepticism.
01:03:50 You’re putting all these things that you want me
01:03:52 to imagine there, these extra dimensions
01:03:54 that I cannot see, aha, aha.
01:03:56 And you want me to believe that string
01:03:57 that you have not even seen the experiments are real,
01:03:59 aha, okay, what else do you want me to believe?
01:04:01 So this kind of beginning to sound a little funny.
01:04:03 Now, I will pass forward a little bit further.
01:04:08 A few decades later, when string theory became
01:04:11 the mainstream of efforts to unify the forces
01:04:13 and particles together, we learned
01:04:16 that these extra dimensions actually solved problems.
01:04:20 They weren’t a nuisance the way they originally appeared.
01:04:24 First of all, the properties of these extra dimensions
01:04:28 reflected the number of particles we got in four dimensions.
01:04:31 If you took these six dimensions to have like five holes
01:04:34 or four holes, change the number of particles
01:04:37 that you see in four dimensional space time,
01:04:39 you get one electron and one muon if you had this,
01:04:42 but if you did the other J shape, you get something else.
01:04:44 So geometrically, you could get different kinds of physics.
01:04:47 So it was kind of a mirroring of geometry by physics
01:04:51 down in the macroscopic space.
01:04:53 So these extra dimension were becoming useful.
01:04:56 Fine, but we didn’t need the extra dimension
01:04:58 to just write an electron in three dimensions,
01:05:00 we did rewrote it, so what?
01:05:02 Was there any other puzzle?
01:05:04 Yes, there were, Hawking.
01:05:07 Hawking had been studying black holes in mid 70s
01:05:10 following the work of Bekenstein,
01:05:12 who had predicted that black holes have entropy.
01:05:17 So Bekenstein had tried to attach the entropy
01:05:20 to the black hole.
01:05:21 If you throw something into the black hole,
01:05:23 the entropy seems to go down
01:05:25 because you had something entropy outside the black hole
01:05:28 and you throw it, black hole was unique,
01:05:30 so the entropy did not have any, black hole had no entropy.
01:05:33 So the entropy seemed to go down.
01:05:35 And so that’s against the laws of thermodynamics.
01:05:38 So Bekenstein was trying to say, no, no,
01:05:40 therefore black hole must have an entropy.
01:05:42 So he was trying to understand that he found that
01:05:43 if you assign entropy to be proportional
01:05:47 to the area of the black hole, it seems to work.
01:05:49 And then Hawking found not only that’s correct,
01:05:52 he found the correct proportionality factor
01:05:54 of a one quarter of the area and Planck units
01:05:57 is the correct amount of entropy.
01:05:59 And he gave an argument using
01:06:01 quantum semi classical arguments,
01:06:03 which means basically using a little bit
01:06:05 of a quantum mechanics,
01:06:06 because he didn’t have the full quantum mechanics
01:06:09 of string theory, he could do some aspects
01:06:11 of approximate quantum arguments.
01:06:12 So he heuristic quantum arguments led
01:06:14 to this entropy formula.
01:06:17 But then he didn’t answer the following question.
01:06:20 He was getting a big entropy for the black hole,
01:06:23 the black hole with the size of the horizon
01:06:25 of a black hole is huge, has a huge amount of entropy.
01:06:27 What are the microstates of this entropy?
01:06:29 When you say, for example, the gas is entropy,
01:06:32 you count where the atoms are,
01:06:33 you count this bucket or that bucket,
01:06:35 there’s an information about there and so on, you count them.
01:06:38 For the black hole, the way Hawking was thinking,
01:06:40 there was no degree of freedom, you throw them in,
01:06:43 and there was just one solution.
01:06:44 So where are these entropy?
01:06:46 What are these microscopic states?
01:06:50 They were hidden somewhere.
01:06:51 So later in string theory,
01:06:54 the work that we did with my colleague Strominger,
01:06:57 in particular showed that these ingredients
01:07:00 in string theory of black hole arise
01:07:04 from the extra dimensions.
01:07:06 So the degrees of freedom are hidden
01:07:08 in terms of things like strings,
01:07:10 wrapping these extra circles in these hidden dimensions.
01:07:13 And then we started counting how many ways
01:07:16 like the strings can wrap around this circle
01:07:18 and the extra dimension or that circle
01:07:19 and counted the microscopic degrees of freedom.
01:07:22 And lo and behold, we got the microscopic degrees
01:07:24 of freedom that Hawking was predicting four dimensions.
01:07:27 So the extra dimensions became useful
01:07:30 for resolving a puzzle in four dimensions.
01:07:32 The puzzle was where are the degrees of freedom
01:07:35 of the black hole hidden?
01:07:36 The answer, hidden in the extra dimensions.
01:07:39 The tiny extra dimensions.
01:07:41 So then by this time, it was beginning to,
01:07:43 we see aspects that extra dimensions
01:07:46 are useful for many things.
01:07:47 It’s not a nuisance.
01:07:48 It wasn’t to be kind of, you know, be ashamed of.
01:07:51 It was actually in the welcome features.
01:07:53 New feature, nevertheless.
01:07:54 How do you intuit the 10 dimensional world?
01:07:59 So yes, it’s a feature for describing certain phenomena
01:08:03 like the entropy in black holes,
01:08:06 but what you said that to you a theory becomes real
01:08:14 or becomes powerful when you can connect it
01:08:16 to some deep intuition.
01:08:18 So how do we intuit 10 dimensions?
01:08:20 Yes, so I will explain how some of the analogies work.
01:08:24 First of all, we do a lot of analogies.
01:08:28 And by analogies, we build intuition.
01:08:31 So I will start with this example.
01:08:33 I will try to explain that if we are in 10 dimensional space,
01:08:37 if we have a seven dimensional plane
01:08:40 and eight dimensional plane,
01:08:42 we ask typically in what space do they intersect each other
01:08:45 in what dimension?
01:08:46 That might sound like,
01:08:48 how do you possibly give an answer to this?
01:08:50 So we start with lower dimensions.
01:08:52 We start with two dimensions.
01:08:53 We say, if you have one dimension and a point,
01:08:56 do they intersect typically on a plane?
01:08:58 The answer is no.
01:08:59 So a line one dimensional, a point zero dimension
01:09:02 on a two dimensional plane, they don’t typically meet.
01:09:05 But if you have a one dimensional line and another line,
01:09:08 which is one plus one on a plane,
01:09:10 they typically intersect at a point.
01:09:13 Typically means if you’re not parallel,
01:09:15 typically they intersect at a point.
01:09:17 So one plus one is two and in two dimension,
01:09:20 they intersect at the zero dimensional point.
01:09:23 So you see two dimension, one and one, two,
01:09:25 two minus two is zero.
01:09:26 So you get point out of intersection.
01:09:29 Let’s go to three dimension.
01:09:31 You have a plane, two dimensional plane and a point.
01:09:33 Do they intersect?
01:09:34 No, two and zero.
01:09:37 How about the plane and a line?
01:09:39 A plane is two dimensional and a line is one.
01:09:41 Two plus one is three.
01:09:42 In three dimension, a plane and a line meet at points,
01:09:47 which is zero dimensional.
01:09:47 Three minus three is zero.
01:09:49 Okay, so plane and a line intersect
01:09:52 at a point in three dimension.
01:09:54 How about the plane and a plane in 3D?
01:09:56 Well, plane is two and this is two.
01:09:57 Two plus two is four.
01:09:59 In 3D, four minus three is one.
01:10:01 They intersect on a one dimensional line.
01:10:03 Okay, we’re beginning to see the pattern.
01:10:04 Okay, now come to the question.
01:10:06 We’re in 10 dimension.
01:10:06 Now we have the intuition.
01:10:08 We have a seven dimensional plane
01:10:09 and eight dimensional plane in 10 dimension.
01:10:11 They intersect on a plane.
01:10:13 What’s the dimension?
01:10:14 Well, seven plus eight is 15 minus 10 is five.
01:10:16 We draw the same picture as two planes
01:10:20 and we write seven dimension, eight dimension,
01:10:22 but we have gotten the intuition
01:10:23 from the lower dimensional one.
01:10:25 What to expect?
01:10:26 It doesn’t scare us anymore.
01:10:28 So we draw this picture.
01:10:30 We cannot see all the seven dimensions
01:10:32 by looking at this two dimensional visualization of it,
01:10:36 but it has all the features we want.
01:10:38 It has, so we draw this picture.
01:10:39 It says seven, seven,
01:10:40 and they meet at the five dimensional plane.
01:10:43 It says five.
01:10:44 So we have built this intuition.
01:10:46 Now, this is an example of how we come up with intuition.
01:10:51 Let me give you more examples of it
01:10:53 because I think this will show you
01:10:54 that people have to come up with intuitions to visualize it.
01:10:57 Otherwise, we will be a little bit lost.
01:11:00 So what you just described is kind of
01:11:02 in these high dimensional spaces,
01:11:04 focus on the meeting place of two planes
01:11:08 in high dimensional spaces.
01:11:10 Exactly, how the planes meet, for example,
01:11:12 what’s the dimension of their intersection and so on.
01:11:14 So how do we come up with intuition?
01:11:16 We borrow examples from lower dimensions,
01:11:19 build up intuition and draw the same pictures
01:11:21 as if we are talking about 10 dimensions,
01:11:24 but we are drawing the same as a two dimensional plane
01:11:26 because we cannot do any better.
01:11:28 But our words change, but not our pictures.
01:11:32 So your sense is we can have a deep understanding
01:11:35 of reality by looking at its slices,
01:11:39 at lower dimensional slices.
01:11:40 Exactly, exactly.
01:11:41 And this brings me to the next example I wanna mention,
01:11:45 which is sphere.
01:11:46 Let’s think about how do we think about the sphere?
01:11:48 Well, the sphere is a sphere, the round nice thing,
01:11:51 but sphere has a circular symmetry.
01:11:53 Now, I can describe the sphere in the following way.
01:11:57 I can describe it by an interval,
01:12:01 which is thinking about this going from the north
01:12:04 of the sphere to the south.
01:12:06 And at each point, I have a circle attached to it.
01:12:09 So you can think about the sphere as a line
01:12:11 with a circle attached with each point,
01:12:13 the circle shrinks to a point at end points
01:12:17 of the interval.
01:12:18 So I can say, oh, one way to think about the sphere
01:12:21 is an interval where at each point on that interval,
01:12:25 there’s another circle I’m not drawing.
01:12:27 But if you like, you can just draw it.
01:12:29 Say, okay, I won’t draw it.
01:12:30 So from now on, there’s this mnemonic.
01:12:32 I draw an interval when I wanna talk about the sphere
01:12:34 and you remember that the end points of the interval
01:12:37 mean a strong circle, that’s all.
01:12:39 And they say, yeah, I see, that’s a sphere, good.
01:12:41 Now, we wanna talk about the product of two spheres.
01:12:44 That’s four dimensional, how can I visualize it?
01:12:47 Easy, you just take an interval and another interval,
01:12:50 that’s just gonna be a square.
01:12:54 A square is a four dimensional space, yeah, why is that?
01:12:57 Well, at each point on the square, there’s two circles,
01:13:02 one for each of those directions you drew.
01:13:04 And when you get to the boundaries of each direction,
01:13:07 one of the circles shrink on each edge of that square.
01:13:09 And when you get to the corners of the square,
01:13:11 all both circles shrink.
01:13:13 This is a sphere times a sphere, I have defined interval.
01:13:17 I just described for you a four dimensional space.
01:13:20 Do you want a six dimensional space?
01:13:21 No problem, take a corner of a room.
01:13:25 In fact, if you want to have a sphere times a sphere
01:13:28 times a sphere times a sphere, take a cube.
01:13:32 A cube is a rendition of this six dimensional space,
01:13:36 two sphere times another sphere times another sphere,
01:13:39 where three of the circles I’m not drawing for you.
01:13:41 For each one of those directions, there’s another circle.
01:13:43 But each time you get to the boundary of the cube,
01:13:45 one circle shrinks.
01:13:47 When the boundaries meet, two circles shrinks.
01:13:48 When three boundaries meet, all the three circles shrink.
01:13:51 So I just give you a picture.
01:13:53 Now, mathematicians come up with amazing things.
01:13:55 Like, you know what, I want to take a point in space
01:13:58 and blow it up.
01:13:59 You know, these concepts like topology and geometry,
01:14:01 complicated, how do you do?
01:14:03 In this picture, it’s very easy.
01:14:05 Blow it up in this picture means the following.
01:14:07 You think about this cube, you go to the corner
01:14:10 and you chop off a corner.
01:14:12 Chopping off the corner replaces the point.
01:14:15 Yeah.
01:14:16 Replace the point by a triangle.
01:14:17 Yes.
01:14:18 So you’re blowing up a point and then this triangle
01:14:19 is what they call P2, projective two space.
01:14:22 But these pictures are very physical and you feel it.
01:14:25 There’s nothing amazing.
01:14:26 I’m not talking about six dimension.
01:14:28 Four plus six is 10, the dimension of string theory.
01:14:30 So we can visualize it, no problem.
01:14:32 Okay, so that’s building the intuition
01:14:34 to a complicated world of string theory.
01:14:36 Nevertheless, these objects are really small.
01:14:39 And just like you said, experimental validation
01:14:42 is very difficult because the objects are way smaller
01:14:45 than anything that we currently have the tools
01:14:48 and accelerators and so on to reveal through experiment.
01:14:53 So there’s a kind of skepticism
01:14:56 that’s not just about the nature of the theory
01:14:59 because of the 10 dimensions, as you’ve explained,
01:15:01 but in that we can’t experimentally validate it
01:15:05 and it doesn’t necessarily, to date,
01:15:07 maybe you can correct me,
01:15:09 predict something fundamentally new.
01:15:12 So it’s beautiful as an explaining theory,
01:15:16 which means that it’s very possible
01:15:18 that it is a fundamental theory
01:15:19 that describes reality and unifies the laws,
01:15:22 but there’s still a kind of skepticism.
01:15:25 And me, from sort of an outside observer perspective,
01:15:30 have been observing a little bit of a growing cynicism
01:15:34 about string theory in the recent few years.
01:15:37 Can you describe the cynicism about,
01:15:40 sort of by cynicism I mean a cynicism
01:15:42 about the hope for this theory
01:15:46 of pushing theoretical physics forward?
01:15:49 Yes.
01:15:50 Can you do describe why this is cynicism
01:15:53 and how do we reverse that trend?
01:15:56 Yes, first of all, the criticism for string theory
01:16:01 is healthy in a sense that in science
01:16:04 we have to have different viewpoints and that’s good.
01:16:07 So I welcome criticism and the reason for criticism
01:16:12 and I think that is a valid reason
01:16:13 is that there has been zero experimental evidence
01:16:15 for string theory.
01:16:17 That is no experiment has been done
01:16:20 to show that there’s this loop of energy moving around.
01:16:24 And so that’s a valid objection and valid worry.
01:16:28 And if I were to say, you know what,
01:16:30 string theory can never be verified
01:16:31 or experimentally checked, that’s the way it is,
01:16:34 they would have every right to say
01:16:36 what you’re talking about is not science.
01:16:37 Because in science we will have to have
01:16:39 experimental consequences and checks.
01:16:42 The difference between string theory
01:16:44 and something which is not scientific
01:16:45 is that string theory has predictions.
01:16:47 The problem is that the predictions we have today
01:16:49 of string theory is hard to access by experiments
01:16:52 available with the energies we can achieve
01:16:55 with the colliders today.
01:16:56 It doesn’t mean there’s a problem with string theory,
01:16:58 it just means technologically we’re not that far ahead.
01:17:01 Now, we can have two attitudes.
01:17:04 You say, well, if that’s the case, why are you studying
01:17:06 this subject?
01:17:07 Because you can’t do experiment today.
01:17:09 Now, this is becoming a little bit more like mathematics
01:17:12 in that sense.
01:17:13 You say, well, I want to learn,
01:17:15 I want to know how the nature works
01:17:16 even though I cannot prove it today
01:17:18 that this is it because of experiments.
01:17:21 That should not prevent my mind not to think about it.
01:17:24 So that’s the attitude many string theorists follow,
01:17:26 that it should be like this.
01:17:28 Now, so that’s an answer to the criticism,
01:17:30 but there’s actually a better answer to the criticism,
01:17:33 I would say.
01:17:34 We don’t have experimental evidence for string theory,
01:17:37 but we have theoretical evidence for string theory.
01:17:39 And what do I mean by theoretical evidence
01:17:41 for string theory?
01:17:42 String theory has connected different parts
01:17:45 of physics together.
01:17:47 It didn’t have to.
01:17:49 It has brought connections between part of physics,
01:17:52 although suppose you’re just interested
01:17:53 in particle physics.
01:17:54 Suppose you’re not even interested in gravity at all.
01:17:57 It turns out there are properties
01:17:59 of certain particle physics models
01:18:02 that string theory has been able to solve using gravity,
01:18:06 using ideas from string theory,
01:18:08 ideas known as holography,
01:18:10 which is relating something which has to do with particles
01:18:13 to something having to do with gravity.
01:18:15 Why did it have to be this rich?
01:18:17 The subject is very rich.
01:18:20 It’s not something we were smart enough to develop.
01:18:23 It came at us.
01:18:23 As I explained to you,
01:18:24 the development of string theory
01:18:25 came from accidental discovery.
01:18:28 It wasn’t because we were smart enough
01:18:29 to come up with the idea,
01:18:30 oh yeah, string of course has gravity in it.
01:18:32 No, it was accidental discovery.
01:18:34 So some people say it’s not fair to say
01:18:36 we have no evidence for string theory.
01:18:38 Graviton, gravity is an evidence for string theory.
01:18:41 It’s predicted by string theory.
01:18:43 We didn’t put it by hand, we got it.
01:18:45 So there’s a qualitative check.
01:18:47 Okay, gravity is a prediction of string theory.
01:18:51 It’s a postdiction because we know gravity existed.
01:18:53 But still, logically it is a prediction
01:18:56 because really we didn’t know it had the graviton
01:19:00 that we later learned that, oh, that’s the same as gravity.
01:19:02 So literally that’s the way it was discovered.
01:19:04 It wasn’t put in by hand.
01:19:06 So there are many things like that,
01:19:08 that there are different facets of physics,
01:19:11 like questions in condensed matter physics,
01:19:13 questions of particle physics,
01:19:15 questions about this and that have come together
01:19:18 to find beautiful answers by using ideas
01:19:21 from string theory at the same time
01:19:24 as a lot of new math has emerged.
01:19:27 That’s an aspect which I wouldn’t emphasize
01:19:29 as evidence to physicists necessarily,
01:19:31 because they would say, okay, great, you got some math,
01:19:33 but what’s it do with reality?
01:19:35 But as I explained, many of the physical principles
01:19:38 we know of have beautiful math underpinning them.
01:19:41 So it certainly leads further confidence
01:19:45 that we may not be going astray,
01:19:46 even though that’s not the full proof as we know.
01:19:49 So there are these aspects that give further evidence
01:19:52 for string theory, connections between each other,
01:19:55 connection with the real world,
01:19:56 but then there are other things that come about
01:19:58 and I can try to give examples of that.
01:20:01 So these are further evidences
01:20:03 and these are certain predictions of string theory.
01:20:05 They are not as detailed as we want,
01:20:08 but there are still predictions.
01:20:11 Why is the dimension of space and time three plus one?
01:20:16 Say, I don’t know, just deal with it, three plus one.
01:20:20 But in physics, we want to know why.
01:20:23 Well, take a random dimension from one to infinity.
01:20:26 What’s your random dimension?
01:20:28 A random dimension from one to infinity would not be four.
01:20:33 Eight would most likely be a humongous number,
01:20:35 if not infinity.
01:20:36 I mean, there’s no, if you choose any reasonable distribution
01:20:39 which goes from one to infinity,
01:20:41 three or four would not be your pick.
01:20:44 The fact that we are in three or four dimension
01:20:45 is already strange.
01:20:48 The fact that strings are sorry,
01:20:49 I cannot go beyond 10 or maybe 11 or something.
01:20:52 The fact that there’s this upper bound,
01:20:54 the range is not from one to infinity,
01:20:56 it’s from one to 10 or 11 or whatnot,
01:20:58 it already brings a natural prior.
01:21:00 Oh yeah, three or four is just on the average.
01:21:03 If you pick some of the compactification,
01:21:05 then it could easily be that.
01:21:06 So in other words, it makes it much more possible
01:21:08 that it could be three of our universe.
01:21:11 So the fact that the dimension already is so small,
01:21:14 it should be surprising.
01:21:16 We don’t ask that question.
01:21:17 We should be surprised because we could have conceived
01:21:20 of universes with our pre dimension.
01:21:22 Why is it that we have such a small dimension?
01:21:24 That’s number one.
01:21:25 So good theory of the universe should give you
01:21:28 an intuition of the why it’s four or three plus one.
01:21:33 And it’s not obvious that it should be.
01:21:36 That should be explained.
01:21:37 We take that as an assumption,
01:21:40 but that’s a thing that should be explained.
01:21:43 Yeah, so we haven’t explained that in string theory.
01:21:45 Actually, I did write a model within string theory
01:21:47 to try to describe why we end up
01:21:49 with three plus one space time dimensions,
01:21:52 which are big compared to the rest of them.
01:21:54 And even though this has not been,
01:21:57 the technical difficulties to prove it is still not there,
01:22:00 but I will explain the idea because the idea connects
01:22:02 to some other piece of elegant math,
01:22:05 which is the following.
01:22:06 Consider a universe made of a box, three dimensional box.
01:22:11 Or in fact, if we start in string theory,
01:22:13 nine dimensional box,
01:22:14 because we have nine spatial dimension on one time.
01:22:17 So imagine a nine dimensional box.
01:22:20 So we should imagine the box of a typical size of the string,
01:22:23 which is small.
01:22:25 So the universe would naturally start
01:22:27 with a very tiny nine dimensional box.
01:22:30 What do strings do?
01:22:31 Well, strings go around the box
01:22:34 and move around and vibrate and all that,
01:22:35 but also they can wrap around one side of the box
01:22:38 to the other because I’m imagining a box
01:22:41 with periodic boundary conditions.
01:22:42 So what we call the torus.
01:22:44 So the string can go from one side to the other.
01:22:46 This is what we call a winding string.
01:22:48 The string can wind around the box.
01:22:50 Now, suppose you have, you’ve now evolved the universe.
01:22:54 Because there’s energy, the universe starts to expand.
01:22:57 But it doesn’t expand too far.
01:23:00 Why is it?
01:23:01 Well, because there are these strings
01:23:03 which are wrapped around
01:23:04 from one side of the wall to the other.
01:23:07 When the universe, the walls of the universe are growing,
01:23:10 it is stretching the string
01:23:11 and the strings are becoming very, very massive.
01:23:15 So it becomes difficult to expand.
01:23:16 It kind of puts a halt on it.
01:23:18 In order to not put a halt,
01:23:19 a string which is going this way
01:23:21 and a string which is going that way
01:23:22 should intersect each other
01:23:25 and disconnect each other and unwind.
01:23:27 So a string which winds this way
01:23:29 and the string which finds the opposite way
01:23:31 should find each other to reconnect
01:23:35 and this way disappear.
01:23:37 So if they find each other and they disappear.
01:23:40 But how can strings find each other?
01:23:42 Well, the string moves and another string moves.
01:23:45 A string is one dimensional, one plus one is two
01:23:48 and one plus one is two and two plus two is four.
01:23:52 In four dimensional space time, they will find each other.
01:23:55 In a higher dimensional space time,
01:23:57 they typically miss each other.
01:23:59 Oh, interesting.
01:24:00 So if the dimension were too big,
01:24:01 they would miss each other,
01:24:02 they wouldn’t be able to expand.
01:24:04 So in order to expand, they have to find each other
01:24:06 and three of them can find each other
01:24:08 and those can expand and the other one will be stuck.
01:24:10 So that explains why within string theory,
01:24:13 these particular dimensions are really big
01:24:15 and full of exciting stuff.
01:24:16 That could be an explanation.
01:24:17 That’s a model we suggested with my colleague Brandenberger.
01:24:21 But it turns out to be related to a deep piece of math.
01:24:23 You see, for mathematicians,
01:24:26 manifolds of dimension bigger than four are simple.
01:24:31 Four dimension is the hardest dimension for math,
01:24:34 it turns out.
01:24:35 And it turns out the reason it’s difficult is the following.
01:24:38 It turns out that in higher dimension,
01:24:41 you use what’s called surgery in mathematical terminology,
01:24:45 where you use these two dimensional tubes
01:24:48 to maneuver them off of each other.
01:24:50 So you have two plus two becoming four.
01:24:53 In higher than four dimension,
01:24:54 you can pass them through each other
01:24:56 without them intersecting.
01:24:57 In four dimension, two plus two
01:25:01 doesn’t allow you to pass them through each other.
01:25:03 So the same techniques that work in higher dimension
01:25:05 don’t work in four dimension because two plus two is four.
01:25:08 The same reasoning I was just telling you
01:25:10 about strings finding each other in four
01:25:12 ends up to be the reason why four is much more complicated
01:25:16 to classify for mathematicians as well.
01:25:18 So there might be these things.
01:25:20 So I cannot say that this is the reason
01:25:22 that string theory is giving you three plus one,
01:25:25 but it could be a model for it.
01:25:26 And so there are these kinds of ideas
01:25:28 that could underlie why we have three extra dimensions
01:25:31 which are large and the rest of them are small.
01:25:32 But absolutely, we have to have a good reason.
01:25:35 We cannot leave it like that.
01:25:36 Can I ask a tricky human question?
01:25:38 So you are one of the seminal figures in string theory.
01:25:42 You got the Breakthrough Prize.
01:25:44 You’ve worked with Edward Witten.
01:25:45 There’s no Nobel Prize that has been given on string theory.
01:25:51 Credit assignment is tricky in science.
01:25:54 It makes you quite sad, especially big, like LIGO,
01:25:57 big experimental projects when so many incredible people
01:26:01 have been involved and yet the Nobel Prize is annoying
01:26:04 in that it’s only given to three people.
01:26:06 Who do you think gets the Nobel Prize
01:26:08 for string theory at first?
01:26:12 If it turns out that it, if not in full, then in part,
01:26:19 is a good model of the way the physics of the universe works.
01:26:24 Who are the key figures?
01:26:26 Maybe let’s put Nobel Prize aside.
01:26:28 Who are the key figures?
01:26:29 Okay, I like the second version of the question.
01:26:31 Because I think to try to give a prize to one person
01:26:34 in string theory doesn’t do justice to the diversity
01:26:37 of the subject.
01:26:37 That to me is.
01:26:38 So there was quite a lot of incredible people
01:26:41 in the history of string theory.
01:26:41 Quite a lot of people.
01:26:43 I mean, starting with Veneziano,
01:26:44 who wasn’t talking about strings.
01:26:46 I mean, he wrote down the beginning of the strings.
01:26:48 We cannot ignore that for sure.
01:26:50 And so you start with that and you go on
01:26:52 with various other figures and so on.
01:26:54 So there are different epochs in string theory
01:26:56 and different people have been pushing it.
01:26:57 And so for example, the early epoch,
01:26:59 we just told you people like Veneziano,
01:27:02 and Nambu, and the Susskind, and others were pushing it.
01:27:05 Green and Schwarz were pushing it and so forth.
01:27:07 So this was, or Scherck and so on.
01:27:09 So these were the initial periods of pioneers,
01:27:13 I would say, of string theory.
01:27:14 And then there were the mid 80s that Edward Witten
01:27:18 was the major proponent of string theory.
01:27:20 And he really changed the landscape of string theory
01:27:23 in terms of what people do and how we view it.
01:27:26 And I think his efforts brought a lot of attention
01:27:29 to the community of string theory.
01:27:31 To the community about high energy community
01:27:34 to focus on this effort as the correct theory
01:27:37 of unification of forces.
01:27:38 So he brought a lot of research as well as, of course,
01:27:41 the first rate work he himself did to this area.
01:27:44 So that’s in mid 80s and onwards,
01:27:45 and also in mid 90s where he was one of the proponents
01:27:49 of the duality revolution in string theory.
01:27:51 And with that came a lot of these other ideas
01:27:54 that led to breakthroughs involving, for example,
01:27:58 the example I told you about black holes and holography,
01:28:00 and the work that was later done by Maldacena
01:28:03 about the properties of duality between particle physics
01:28:06 and quantum gravity and the deeper connections
01:28:10 of holography, and it continues.
01:28:12 And there are many people within this range,
01:28:15 which I haven’t even mentioned.
01:28:16 They have done fantastic important things.
01:28:20 How it gets recognized, I think, is secondary,
01:28:22 in my opinion, than the appreciation
01:28:25 that the effort is collective.
01:28:27 That, in fact, that to me is the more important part
01:28:30 of science that gets forgotten.
01:28:32 For some reason, humanity likes heroes,
01:28:35 and science is no exception.
01:28:36 We like heroes, but I personally try to avoid that trap.
01:28:40 I feel, in my work, most of my work is with colleagues.
01:28:44 I have much more collaborations than sole author papers,
01:28:49 and I enjoy it, and I think that that’s, to me,
01:28:51 one of the most satisfying aspects of science
01:28:54 is to interact and learn and debate ideas with colleagues
01:28:59 because that influx of ideas enriches it,
01:29:02 and that’s why I find it interesting.
01:29:05 To me, science, if I was on an island,
01:29:08 and if I was developing string theory by myself
01:29:10 and had nothing to do with anybody,
01:29:11 it would be much less satisfying, in my opinion.
01:29:14 Even if I could take credit I did it,
01:29:17 it won’t be as satisfying.
01:29:18 Sitting alone with a big metal drinking champagne, no.
01:29:22 I think, to me, the collective work is more exciting,
01:29:25 and you mentioned my getting the breakthrough.
01:29:28 When I was getting it, I made sure to mention
01:29:30 that it is because of the joint work
01:29:32 that I’ve done with colleagues.
01:29:33 At that time, it was around 180 or so collaborators,
01:29:36 and I acknowledged them in the webpage for them.
01:29:39 I write all of their names
01:29:41 and the collaborations that led to this.
01:29:42 So, to me, science is fun when it’s collaboration,
01:29:46 and yes, there are more important
01:29:48 and less important figures, as in any field,
01:29:51 and that’s true, that’s true in string theory as well,
01:29:53 but I think that I would like to view this
01:29:55 as a collective effort.
01:29:56 So, setting the heroes aside,
01:30:00 the Nobel Prize is a celebration of,
01:30:04 what’s the right way to put it,
01:30:05 that this idea turned out to be right.
01:30:08 So, like, you look at Einstein
01:30:11 didn’t believe in black holes,
01:30:13 and then black holes got their Nobel Prize.
01:30:17 Do you think string theory will get its Nobel Prize,
01:30:22 Nobel Prizes, if you were to bet money?
01:30:25 If this was an investment meeting
01:30:27 and we had to bet all our money,
01:30:29 do you think he gets the Nobel Prizes?
01:30:31 I think it’s possible that none of the living physicists
01:30:34 will get the Nobel Prize in string theory,
01:30:35 but somebody will.
01:30:37 Because, unfortunately, the technology available today
01:30:41 is not very encouraging
01:30:43 in terms of seeing directly evidence for string theory.
01:30:46 Do you think it ultimately boils down to
01:30:48 the Nobel Prize will be given
01:30:49 when there is some direct or indirect evidence?
01:30:53 There would be, but I think that part of this
01:30:55 breakthrough prize was precisely the appreciation
01:30:58 that when we have sufficient evidence,
01:31:01 theoretical as it is, not experiment,
01:31:04 because of this technology lag,
01:31:06 you appreciate what you think is the correct path.
01:31:08 So, there are many people who have been recognized precisely
01:31:12 because they may not be around
01:31:14 when it actually gets experimented,
01:31:16 even though they discovered it.
01:31:17 So, there are many things like that
01:31:19 that’s going on in science.
01:31:21 So, I think that I would want to attach less significance
01:31:25 to the recognitions of people.
01:31:28 And I have a second review on this,
01:31:31 which is there are people who look at these works
01:31:35 that people have done and put them together
01:31:37 and make the next big breakthrough.
01:31:39 And they get identified with, perhaps rightly,
01:31:43 with many of these new visions.
01:31:47 But they are on the shoulders of these little scientists.
01:31:51 Which don’t get any recognition.
01:31:54 You know, yeah, you did this little work.
01:31:55 Oh yeah, you did this little work.
01:31:56 Oh yeah, yeah, five of you.
01:31:57 Oh yeah, these showed this pattern.
01:31:59 And then somebody else, it’s not fair.
01:32:01 To me, those little guys, which kind of like,
01:32:05 like seem to do the little calculation here,
01:32:07 a little thing there, which doesn’t rise to the occasion
01:32:10 of this grandiose kind of thing,
01:32:11 doesn’t make it to the New York Times headlines and so on,
01:32:15 deserve a lot of recognition.
01:32:17 And I think they don’t get enough.
01:32:18 I would say that there should be this Nobel prize
01:32:20 for, you know, they have these Doctors Without Borders,
01:32:23 they’re a huge group.
01:32:24 They should do a similar thing.
01:32:25 And these String Theors Without Borders kind of,
01:32:27 everybody is doing a lot of work.
01:32:29 And I think that I would like to see that effort recognized.
01:32:32 I think in the long arc of history,
01:32:35 we’re all little guys and girls
01:32:38 standing on the shoulders of each other.
01:32:40 I mean, it’s all going to look tiny in retrospect.
01:32:44 If we celebrate, the New York Times,
01:32:46 you know, as a newspaper,
01:32:51 or the idea of a newspaper in a few centuries from now
01:32:55 will be long forgotten.
01:32:56 Yes, I agree with that.
01:32:57 Especially in the context of String Theory,
01:32:59 we should have a very long term view.
01:33:00 Yes, exactly.
01:33:01 Just as a tiny tangent, we mentioned Edward Witten.
01:33:05 And he, in a bunch of walks of life for me as an outsider,
01:33:09 comes up as a person who is widely considered as like
01:33:14 one of the most brilliant people in the history of physics,
01:33:17 just as a powerhouse of a human,
01:33:21 like the exceptional places that a human mind can rise to.
01:33:27 Yes.
01:33:28 You’ve gotten the chance to work with him.
01:33:29 What’s he like?
01:33:30 Yes, more than that.
01:33:31 He was my advisor, PhD advisor.
01:33:34 So I got to know him very well
01:33:35 and I benefited from his insights.
01:33:37 In fact, what you said about him is accurate.
01:33:40 He is not only brilliant,
01:33:42 but he is also multifaceted in terms of the impact
01:33:46 he has had in not only physics, but also mathematics.
01:33:49 He has gotten the Fields Medal
01:33:50 because of his work in mathematics.
01:33:52 And rightly so, he has used his knowledge of physics
01:33:58 in a way which impacted deep ideas in modern mathematics.
01:34:01 And that’s an example of the power of these ideas
01:34:05 in modern high energy physics and string theory,
01:34:08 the applicability of it to modern mathematics.
01:34:11 So he’s quite an exceptional individual.
01:34:16 We don’t come across such people a lot in history.
01:34:19 So I think, yes, indeed,
01:34:20 he’s one of the rare figures in this history of subject.
01:34:24 He has had great impact on a lot of aspects
01:34:26 of not just string theory,
01:34:27 a lot of different areas in physics,
01:34:29 and also, yes, in mathematics as well.
01:34:32 So I think what you said about him is accurate.
01:34:34 I had the pleasure of interacting with him as a student
01:34:37 and later on as colleagues writing papers together
01:34:40 and so on.
01:34:41 What impact did he have on your life?
01:34:43 Like what have you learned from him?
01:34:46 If you were to look at the trajectory of your mind
01:34:48 of the way you approach science and physics and mathematics,
01:34:51 how did he perturb that trajectory in a way?
01:34:54 Yes, he did actually.
01:34:55 So I can explain because when I was a student,
01:34:57 I had the biggest impact by him,
01:35:01 clearly as a grad student at Princeton.
01:35:02 So I think that was a time where I was a little bit confused
01:35:06 about the relation between math and physics.
01:35:08 I got a double major in mathematics and physics
01:35:11 at MIT because I really enjoyed both.
01:35:14 And I liked the elegance and the rigor of mathematics.
01:35:18 And I liked the power of ideas in physics
01:35:21 and its applicability to reality
01:35:22 and what it teaches about the real world around us.
01:35:26 But I saw this tension between rigorous thinking
01:35:30 in mathematics and lack thereof in physics.
01:35:33 And this troubled me to no end.
01:35:36 I was troubled by that.
01:35:38 So I was at crossroads when I decided
01:35:40 to go to graduate school in physics
01:35:42 because I did not like some of the lack of rigors
01:35:44 I was seeing in physics.
01:35:47 On the other hand, to me, mathematics,
01:35:48 even though it was rigorous,
01:35:49 I didn’t see the point of it.
01:35:53 In other words, the math theorem by itself could be beautiful
01:35:57 but I really wanted more than that.
01:35:58 I wanted to say, okay, what does it teach us
01:36:00 about something else, something more than just math?
01:36:02 So I wasn’t that enamored with just math
01:36:05 but physics was a little bit bothersome.
01:36:07 Nevertheless, I decided to go to physics
01:36:08 and I decided to go to Princeton
01:36:10 and I started working with Edward Witten
01:36:13 as my thesis advisor.
01:36:15 And at that time I was trying to put physics
01:36:20 in rigorous mathematical terms.
01:36:22 I took quantum field theory.
01:36:23 I tried to make rigorous out of it and so on.
01:36:26 And no matter how hard I was trying,
01:36:29 I was not being able to do that.
01:36:31 And I was falling behind from my classes.
01:36:33 I was not learning much physics
01:36:35 and I was not making it rigorous.
01:36:37 And to me, it was this dichotomy between math and physics.
01:36:40 What am I doing?
01:36:41 I like math but this is not exactly this.
01:36:45 There comes Edward Witten as my advisor
01:36:47 and I see him in action thinking about math and physics.
01:36:52 He was amazing in math.
01:36:53 He knew all about the math.
01:36:54 It was no problem with him.
01:36:56 But he thought about physics in a way
01:36:58 which did not find this tension between the two.
01:37:02 It was much more harmonious.
01:37:04 For him, he would draw the Feynman diagrams
01:37:06 but he wouldn’t view it as a formalism.
01:37:08 He was viewed, oh yeah, the particle goes over there
01:37:10 and this is what’s going on.
01:37:11 And so wait, you’re thinking really,
01:37:13 is this particle, this is really electron going there?
01:37:15 Oh, yeah, yeah.
01:37:16 It’s not the form or the result perturbation.
01:37:18 No, no, no.
01:37:19 You just feel like the electron.
01:37:21 You’re moving with this guy and do that and so on.
01:37:23 And you’re thinking invariantly about physics
01:37:24 or the way he thought about relativity.
01:37:27 Like I was thinking about this momentum system.
01:37:29 He was thinking invariantly about physics,
01:37:31 just like the way you think about invariant concepts
01:37:34 and relativity, which don’t depend on the frame of reference.
01:37:36 He was thinking about the physics in invariant ways,
01:37:39 the way that doesn’t, that gives you a bigger perspective.
01:37:42 So this gradually helped me appreciate
01:37:46 that interconnections between ideas and physics
01:37:50 replaces mathematical rigor.
01:37:53 That the different facets reinforce each other.
01:37:56 They say, oh, I cannot rigorously define
01:37:58 what I mean by this,
01:37:59 but this thing connects with this other physics I’ve seen
01:38:01 and this other thing.
01:38:02 And they together form an elegant story.
01:38:06 And that replaced for me what I believed as a solidness,
01:38:09 which I found in math as a rigor, solid.
01:38:13 I found that replaced the rigor and solidness in physics.
01:38:16 So I found, okay, that’s the way you can hang onto.
01:38:19 It is not wishy washy.
01:38:20 It’s not like somebody is just not being able to prove it,
01:38:23 just making up a story.
01:38:24 It was more than that.
01:38:25 And it was no tension with mathematics.
01:38:28 In fact, mathematics was helping it, like friends.
01:38:31 And so much more harmonious and gives insights to physics.
01:38:34 So that’s, I think, one of the main things I learned
01:38:36 from interactions with Witten.
01:38:38 And I think that now perhaps I have taken that
01:38:42 to a far extreme.
01:38:43 Maybe he wouldn’t go this far as I have.
01:38:45 Namely, I use physics to define new mathematics
01:38:48 in a way which would be far less rigorous
01:38:50 than a physicist might necessarily believe,
01:38:53 because I take the physical intuition,
01:38:55 perhaps literally in many ways that could teach us about.
01:38:58 So now I’ve gained so much confidence
01:39:01 in physical intuition that I make bold statements
01:39:03 that sometimes takes math friends off guard.
01:39:08 So an example of it is mirror symmetry.
01:39:10 So we were studying these compactification
01:39:14 of string geometries.
01:39:15 This is after my PhD now.
01:39:17 I’ve, by the time I come to Harvard,
01:39:19 we’re studying these aspects of string compactification
01:39:21 on these complicated manifolds,
01:39:23 six dimensional spaces called Kalabial manifolds,
01:39:26 very complicated.
01:39:28 And I noticed with a couple other colleagues
01:39:31 that there was a symmetry in physics suggested
01:39:35 between different Kalabials.
01:39:36 It suggested that you couldn’t actually compute
01:39:40 the Euler characteristic of a Kalabia.
01:39:42 Euler characteristic is counting the number of points
01:39:45 minus the number of edges plus the number of faces minus.
01:39:48 So you can count the alternating sequence
01:39:50 of properties of a space,
01:39:51 which is a topological property of a space.
01:39:54 So Euler characteristics of the Kalabia
01:39:56 was a property of the space.
01:39:57 And so we noticed that from the physics formalism,
01:40:01 if string moves in a Kalabia,
01:40:03 you cannot distinguish,
01:40:05 we cannot compute the Euler characteristic.
01:40:07 You can only compute the absolute value of it.
01:40:10 Now this bothered us
01:40:11 because how could you not compute the actual sign
01:40:15 unless the both sides were the same?
01:40:18 So I conjectured maybe for every Kalabia
01:40:21 with Euler characteristics positive,
01:40:22 there’s one with negative.
01:40:23 I told this to my colleague Yao
01:40:25 who’s namesake is Kalabia,
01:40:30 that I’m making this conjecture.
01:40:31 Is it possible that for every Kalabia,
01:40:33 there’s one with the opposite Euler characteristic?
01:40:36 Sounds not reasonable.
01:40:37 I said, why?
01:40:38 He said, well, we know more Kalabias
01:40:40 with negative Euler characteristics than positive.
01:40:44 I said, but physics says we cannot distinguish them.
01:40:46 At least I don’t see how.
01:40:47 So we conjectured that for every Kalabia
01:40:50 with one sign, there’s the other one,
01:40:51 despite the mathematical evidence,
01:40:54 despite the mathematical evidence,
01:40:55 despite the expert telling us it’s not the right idea.
01:40:59 If a few years later, this symmetry, mirror symmetry
01:41:02 between the sign with the opposite sign
01:41:04 was later confirmed by mathematicians.
01:41:06 So this is actually the opposite view.
01:41:09 That is physics is so sure about it
01:41:11 that you’re going against the mathematical wisdom,
01:41:13 telling them they better look for it.
01:41:15 So taking the physical intuition literally
01:41:19 and then having that drive the mathematics.
01:41:22 Exactly.
01:41:22 And now we are so confident about many such examples
01:41:26 that has affected modern mathematics in ways like this,
01:41:30 that we are much more confident
01:41:31 about our understanding of what string theory is.
01:41:33 These are another aspects,
01:41:35 other aspects of why we feel string theory is correct.
01:41:37 It’s doing these kinds of things.
01:41:39 I’ve been hearing your talk quite a bit
01:41:41 about string theory, landscape and the swamp land.
01:41:46 What the heck are those two concepts?
01:41:47 Okay, very good question.
01:41:48 So let’s go back to what I was describing about Feynman.
01:41:51 Feynman was trying to do these diagrams for graviton
01:41:55 and electrons and all that.
01:41:57 He found that he’s getting infinities he cannot resolve.
01:42:01 Okay, the natural conclusion is that field theories
01:42:04 and gravity and quantum theory don’t go together
01:42:06 and you cannot have it.
01:42:08 So in other words, field theories and gravity
01:42:11 are inconsistent with quantum mechanics, period.
01:42:14 String theory came up with examples
01:42:18 but didn’t address the question more broadly
01:42:20 that is it true that every field theory
01:42:23 can be coupled to gravity in a quantum mechanical way?
01:42:27 It turns out that Feynman was essentially right.
01:42:30 Almost all particle physics theories,
01:42:33 no matter what you add to it,
01:42:35 when you put gravity in it, doesn’t work.
01:42:38 Only rare exceptions work.
01:42:41 So string theory are those rare exceptions.
01:42:44 So therefore the general principle
01:42:46 that Feynman found was correct.
01:42:47 Quantum field theory and gravity and quantum mechanics
01:42:50 don’t go together except for Joule’s exceptional cases.
01:42:54 There are exceptional cases.
01:42:56 Okay, the total vastness of quantum field theories
01:43:00 that are there we call the set of quantum field theories,
01:43:04 possible things.
01:43:05 Which ones can be consistently coupled to gravity?
01:43:09 We call that subspace the landscape.
01:43:13 The rest of them we call the swampland.
01:43:16 It doesn’t mean they are bad quantum field theories,
01:43:18 they are perfectly fine.
01:43:19 But when you couple them to gravity,
01:43:21 they don’t make sense, unfortunately.
01:43:24 And it turns out that the ratio of them,
01:43:27 the number of theories which are consistent with gravity
01:43:29 to the ones without,
01:43:31 the ratio of the area of the landscape
01:43:33 to the swampland, in other words, is measure zero.
01:43:37 So the swampland’s infinitely large?
01:43:40 The swampland’s infinitely large.
01:43:41 So let me give you one example.
01:43:43 Take a theory in four dimension with matter
01:43:46 with maximum amount of supersymmetry.
01:43:48 Can you get, it turns out a theory in four dimension
01:43:51 with maximum amount of supersymmetry
01:43:53 is characterized just with one thing, a group.
01:43:56 What we call the gauge group.
01:43:58 Once you pick a group, you have to find the theory.
01:44:01 Okay, so does every group make sense?
01:44:04 Yeah.
01:44:05 As far as quantum field theory, every group makes sense.
01:44:07 There are infinitely many groups,
01:44:08 there are infinitely many quantum field theories.
01:44:10 But it turns out there are only finite number of them
01:44:13 which are consistent with gravity out of that same list.
01:44:16 So you can take any group but only finite number of them,
01:44:19 the ones who’s, what we call the rank of the group,
01:44:22 the ones whose rank is less than 23.
01:44:26 Any one bigger than rank 23 belongs to the swampland.
01:44:29 There are infinitely many of them.
01:44:31 They’re beautiful field theories,
01:44:33 but not when you include gravity.
01:44:35 So then this becomes a hopeful thing.
01:44:37 So in other words, in our universe, we have gravity.
01:44:41 Therefore, we are part of that jewel subset.
01:44:44 Now, is this jewel subset small or large?
01:44:49 Yeah.
01:44:50 It turns out that subset is humongous,
01:44:54 but we believe still finite.
01:44:57 The set of possibilities is infinite,
01:44:59 but the set of consistent ones,
01:45:02 I mean, the set of quantum field theories are infinite,
01:45:04 but the consistent ones are finite, but humongous.
01:45:08 The fact that they’re humongous
01:45:10 is the problem we are facing in string theory,
01:45:12 because we do not know which one of these possibilities
01:45:16 the universe we live in.
01:45:18 If we knew, we could make more specific predictions
01:45:20 about our universe.
01:45:21 We don’t know.
01:45:22 And that is one of the challenges when string theory,
01:45:24 which point on the landscape,
01:45:26 which corner of this landscape do we live in?
01:45:28 We don’t know.
01:45:30 So what do we do?
01:45:31 Well, there are principles that are beginning to emerge.
01:45:35 So I will give you one example of it.
01:45:38 You look at the patterns of what you’re getting
01:45:40 in terms of these good ones,
01:45:41 the ones which are in the landscape
01:45:43 compared to the ones which are not.
01:45:45 You find certain patterns.
01:45:46 I’ll give you one pattern.
01:45:49 You find in all the ones that you get from string theory,
01:45:52 gravitational force is always there,
01:45:55 but it’s always, always the weakest force.
01:46:00 However, you could easily imagine field theories
01:46:03 for which gravity is not the weakest force.
01:46:05 For example, take our universe.
01:46:08 If you take mass of the electron,
01:46:10 if you increase the mass of electron by a huge factor,
01:46:14 the gravitational attraction of the electrons
01:46:16 will be bigger than the electric repulsion
01:46:17 between two electrons.
01:46:19 And the gravity will be stronger.
01:46:20 That’s all.
01:46:22 It happens that it’s not the case in our universe
01:46:25 because electron is very tiny in mass compared to that.
01:46:28 Just like our universe, gravity is the weakest force.
01:46:31 We find in all these other ones,
01:46:33 which are part of the good ones,
01:46:36 the gravity is the weakest force.
01:46:37 This is called the weak gravity conjecture.
01:46:40 We conjecture that all the points in the landscape
01:46:43 have this property.
01:46:45 Our universe being just an example of it.
01:46:47 So there are these qualitative features
01:46:49 that we are beginning to see.
01:46:50 But how do we argue for this?
01:46:52 Just by looking patterns?
01:46:53 Just by looking string theory as this?
01:46:55 No, that’s not enough.
01:46:58 We need more reason, more better reasoning.
01:47:00 And it turns out there is.
01:47:01 The reasoning for this turns out to be studying black holes.
01:47:05 Ideas of black holes turn out to put certain restrictions
01:47:09 of what a good quantum filter should be.
01:47:12 It turns out using black hole,
01:47:14 the fact that the black holes evaporate,
01:47:17 the fact that the black holes evaporate
01:47:20 gives you a way to check the relation
01:47:23 between the mass and the charge of elementary particle.
01:47:25 Because what you can do, you can take a charged particle
01:47:28 and throw it into a charged black hole
01:47:30 and wait it to evaporate.
01:47:32 And by looking at the properties of evaporation,
01:47:34 you find that if it cannot evaporate particles
01:47:37 whose mass is less than their charge,
01:47:39 then it will never evaporate.
01:47:40 You will be stuck.
01:47:42 And so the possibility of a black hole evaporation
01:47:44 forces you to have particles whose mass
01:47:47 is sufficiently small so that the gravity is weaker.
01:47:50 So you connect this fact to the other fact.
01:47:52 So we begin to find different facts
01:47:55 that reinforce each other.
01:47:56 So different parts of the physics reinforce each other.
01:47:59 And once they all kind of come together,
01:48:02 you believe that you’re getting the principle correct.
01:48:04 So weak gravity conjecture
01:48:05 is one of the principles we believe in
01:48:07 as a necessity of these conditions.
01:48:09 So these are the predictions string theory are making.
01:48:12 Is that enough?
01:48:13 Well, it’s qualitative.
01:48:14 It’s a semi quantity.
01:48:16 It’s just the mass of the electron
01:48:17 should be less than some number.
01:48:19 But that number is, if I call that number one,
01:48:23 the mass of the electron
01:48:23 turns out to be 10 to the minus 20 actually.
01:48:25 So it’s much less than one.
01:48:26 It’s not one.
01:48:28 But on the other hand,
01:48:30 there’s a similar reasoning for a big black hole
01:48:32 in our universe.
01:48:34 And if that evaporation should take place,
01:48:36 gives you another restriction,
01:48:37 tells you the mass of the electron
01:48:39 is bigger than 10 to the,
01:48:41 now in this case, bigger than something.
01:48:43 It shows bigger than 10 to the minus 30 in the Planck unit.
01:48:45 So you find, huh,
01:48:47 the mass of the electron should be less than one,
01:48:49 but bigger than 10 to the minus 30.
01:48:51 In our universe,
01:48:52 the mass of the electron is 10 to the minus 20.
01:48:54 Okay, now this kind of you could call postiction,
01:48:57 but I would say it follows from principles
01:48:59 that we now understand from string theory, first principle.
01:49:01 So we are making, beginning to make
01:49:04 these kinds of predictions,
01:49:05 which are very much connected to aspects of particle physics
01:49:09 that we didn’t think are related to gravity.
01:49:12 We thought, just take any electron mass you want.
01:49:14 What’s the problem?
01:49:15 It has a problem with gravity.
01:49:17 And so that conjecture
01:49:20 has also a happy consequence
01:49:22 that it explains that our universe,
01:49:24 like why the heck is gravity so weak as a force
01:49:28 and that’s not only an accident, but almost a necessity
01:49:32 if these forces are to coexist effectively?
01:49:35 Exactly, so that’s the reinforcement
01:49:38 of what we know in our universe,
01:49:40 but we are finding that as a general principle.
01:49:43 So we want to know what aspects of our universe
01:49:46 is forced on us,
01:49:47 like the weak gravity conjecture and other aspects.
01:49:50 How much of them do we understand?
01:49:52 Can we have particles lighter than neutrinos?
01:49:54 Or maybe that’s not possible.
01:49:56 You see the neutrino mass,
01:49:57 it turns out to be related to dark energy
01:49:59 in a mysterious way.
01:50:01 Naively, there’s no relation between dark energy
01:50:04 and the mass of a particle.
01:50:06 We have found arguments
01:50:07 from within the swampland kind of ideas,
01:50:10 why it has to be related.
01:50:12 And so there are beginning to be these connections
01:50:15 between graph consistency of quantum gravity
01:50:17 and aspects of our universe gradually being sharpened.
01:50:22 But we are still far from a precise quantitative prediction
01:50:25 like we have to have such and such, but that’s the hope,
01:50:27 that we are going in that direction.
01:50:29 Coming up with the theory of everything
01:50:31 that unifies general relativity and quantum field theory
01:50:34 is one of the big dreams of human civilization.
01:50:39 Us descendants of apes wondering about how this world works.
01:50:43 So a lot of people dream.
01:50:46 What are your thoughts about sort of other out there ideas,
01:50:50 theories of everything or unifying theories?
01:50:56 So there’s a quantum loop gravity.
01:50:59 There’s also more sort of like a friend of mine,
01:51:03 Eric Weinstein beginning to propose
01:51:05 something called geometric unity.
01:51:07 So these kinds of attempts,
01:51:09 whether it’s through mathematical physics
01:51:10 or through other avenues,
01:51:12 or with Stephen Wolfram,
01:51:13 a more computational view of the universe.
01:51:16 Again, in his case, it’s these hyper graphs
01:51:18 that are very tiny objects as well.
01:51:21 Similarly, a string theory
01:51:23 and trying to grapple with this world.
01:51:25 What do you think?
01:51:26 Is there any of these theories that are compelling to you,
01:51:30 that are interesting that may turn out to be true
01:51:33 or at least may turn out to contain ideas that are useful?
01:51:36 Yes, I think the latter.
01:51:37 I would say that the containing ideas that are true
01:51:40 is my opinion was what some of these ideas might be.
01:51:43 For example, loop quantum gravity
01:51:45 is to me not a complete theory of gravity in any sense,
01:51:47 but they have some nuggets of truth in them.
01:51:50 And typically what I expect to happen,
01:51:53 and I have seen examples of this within string theory,
01:51:55 aspects which we didn’t think are part of string theory
01:51:57 come to be part of it.
01:51:58 For example, I’ll give you one example.
01:52:00 String was believed to be 10 dimensional.
01:52:03 And then there was this 11 dimensional super gravity.
01:52:05 Nobody know what the heck is that?
01:52:08 Why are we getting 11 dimensional super gravity
01:52:10 whereas string is saying it should be 10 dimensional?
01:52:11 11 was the maximum dimension you can have a super gravity,
01:52:14 but string was saying, sorry, we’re 10 dimensional.
01:52:17 So for a while we thought that theory is wrong
01:52:20 because how could it be?
01:52:21 Because string theory is definitely a theory of everything.
01:52:23 We later learned that one of the circles
01:52:25 of string theory itself was tiny,
01:52:28 that we had not appreciated that fact.
01:52:30 And we discovered by doing thought experiments
01:52:32 of string theory that there’s gotta be an extra circle
01:52:35 and that circle is connected
01:52:36 to an 11 dimensional perspective.
01:52:38 And that’s what later on got called M theory.
01:52:40 So there are these kinds of things
01:52:43 that we do not know what exactly string theory is.
01:52:45 We’re still learning.
01:52:47 So we do not have a final formulation of string theory.
01:52:50 It’s very well could be the different facets
01:52:52 of different ideas come together
01:52:53 like loop quantum gravity or whatnot,
01:52:55 but I wouldn’t put them on par.
01:52:56 Namely, loop quantum gravity is a scatter of ideas
01:53:01 about what happens to space when they get very tiny.
01:53:03 For example, you replace things by discrete data
01:53:06 and try to quantize it and so on.
01:53:08 And it sounds like a natural idea to quantize space.
01:53:13 If you were naively trying to do quantum space,
01:53:15 you might think about trying to take points
01:53:17 and put them together in some discrete fashion
01:53:20 in some way that is reminiscent of loop quantum gravity.
01:53:24 String theory is more subtle than that.
01:53:27 For example, I will just give you an example.
01:53:29 And this is the kind of thing that we didn’t put in by hand,
01:53:31 we got it out.
01:53:32 And so it’s more subtle than,
01:53:33 so what happens if you squeeze the space
01:53:35 to be smaller and smaller?
01:53:37 Well, you think that after a certain distance,
01:53:41 the notion of distance should break down.
01:53:43 You know, when you go smaller than Planck scale,
01:53:47 should break down.
01:53:48 What happens in string theory?
01:53:50 We do not know the full answer to that,
01:53:52 but we know the following.
01:53:53 Namely, if you take a space
01:53:55 and bring it smaller and smaller,
01:53:56 if the box gets smaller than the Planck scale
01:53:58 by a factor of 10,
01:54:00 it is equivalent by the duality transformation
01:54:04 to a space which is 10 times bigger.
01:54:05 So there’s a symmetry called T duality,
01:54:10 which takes L to one over L.
01:54:12 Well, L is measured in Planck units,
01:54:14 or more precisely string units.
01:54:16 This inversion is a very subtle effect.
01:54:20 And I would not have been,
01:54:21 or any physicist would not have been able to design a theory
01:54:23 which has this property,
01:54:25 that when you make the space smaller,
01:54:27 it is as if you’re making it bigger.
01:54:29 That means there is no experiment you can do
01:54:32 to distinguish the size of the space.
01:54:34 This is remarkable.
01:54:35 For example, Einstein would have said,
01:54:37 of course I can’t measure the size of the space.
01:54:39 What do I do?
01:54:40 Well, I take a flashlight,
01:54:41 I send the light around,
01:54:43 measure how long it takes for the light
01:54:44 to go around the space,
01:54:45 and bring back and find the radius
01:54:47 or circumference of the universe.
01:54:48 What’s the problem?
01:54:50 I said, well, suppose you do that,
01:54:52 and you shrink it.
01:54:52 He said, well, it gets smaller and smaller.
01:54:54 So what?
01:54:54 I said, well, it turns out in string theory,
01:54:56 there are two different kinds of photons.
01:55:00 One photon measures one over L,
01:55:02 the other one measures L.
01:55:03 And so this duality reformulates.
01:55:07 And when the space gets smaller,
01:55:08 it says, oh no, you better use the bigger perspective
01:55:10 because the smaller one is harder to deal with.
01:55:13 So you do this one.
01:55:13 So these examples of loop quantum gravity
01:55:16 have none of these features.
01:55:17 These features that I’m telling you about,
01:55:18 we have learned from string theory.
01:55:20 But they nevertheless have some of these ideas
01:55:22 like topological gravity aspects
01:55:24 are emphasized in the context of loop quantum gravity
01:55:28 in some form.
01:55:28 And so these ideas might be there in some kernel,
01:55:31 in some corners of string theory.
01:55:32 In fact, I wrote a paper about topological string theory
01:55:35 and some connections with potentially loop quantum gravity,
01:55:38 which could be part of that.
01:55:39 So there are little facets of connections.
01:55:41 I wouldn’t say they’re complete,
01:55:43 but I would say most probably what will happen
01:55:46 to some of these ideas, the good ones at least,
01:55:48 they will be absorbed to string theory,
01:55:50 if they are correct.
01:55:51 Let me ask a crazy out there question.
01:55:54 Can physics help us understand life?
01:55:59 So we spoke so confidently about the laws of physics
01:56:06 being able to explain reality.
01:56:07 But, and we even said words like theory of everything,
01:56:11 implying that the word everything
01:56:13 is actually describing everything.
01:56:15 Is it possible that the four laws we’ve been talking about
01:56:20 are actually missing,
01:56:22 they are accurate in describing what they’re describing,
01:56:24 but they’re missing the description
01:56:26 of a lot of other things,
01:56:27 like emergence of life
01:56:31 and emergence of perhaps consciousness.
01:56:35 So is there, do you ever think about this kind of stuff
01:56:39 where we would need to understand extra physics
01:56:44 to try to explain the emergence of these complex pockets
01:56:51 of interesting weird stuff that we call life
01:56:54 and consciousness in this big homogeneous universe
01:56:58 that’s mostly boring and nothing is happening yet?
01:57:00 So first of all, we don’t claim that string theory
01:57:03 is the theory of everything in the sense that
01:57:05 we know enough what this theory is.
01:57:07 We don’t know enough about string theory itself,
01:57:09 we are learning it.
01:57:10 So I wouldn’t say, okay, give me whatever,
01:57:12 I will tell you how it works, no.
01:57:14 However, I would say by definition,
01:57:16 by definition to me physics is checking all reality.
01:57:20 Any form of reality, I call it physics,
01:57:22 that’s my definition.
01:57:23 I mean, I may not know a lot of it,
01:57:25 like maybe the origin of life and so on,
01:57:27 maybe a piece of that,
01:57:29 but I would call that as part of physics.
01:57:30 To me, reality is what we’re after.
01:57:33 I don’t claim I know everything about reality.
01:57:35 I don’t claim string theory necessarily has the tools
01:57:38 right now to describe all the reality either,
01:57:41 but we are learning what it is.
01:57:42 So I would say that I would not put a border to say,
01:57:44 no, from this point onwards, it’s not my territory,
01:57:47 it’s somebody else’s.
01:57:48 But whether we need new ideas in string theory
01:57:50 to describe other reality features, for sure I believe,
01:57:53 as I mentioned, I don’t believe any of the laws
01:57:57 we know today is final.
01:57:58 So therefore, yes, we will need new ideas.
01:58:00 This is a very tricky thing for us to understand
01:58:05 and be precise about.
01:58:08 But just because you understand the physics
01:58:12 doesn’t necessarily mean that you understand
01:58:17 the emergence of chemistry, biology, life,
01:58:21 intelligence, consciousness.
01:58:23 So those are built, it’s like you might understand
01:58:27 the way bricks work, but to understand what it means
01:58:32 to have a happy family, you don’t get from the bricks.
01:58:37 So directly, in theory you could,
01:58:42 if you ran the universe over again,
01:58:44 but just understanding the rules of the universe
01:58:47 doesn’t necessarily give you a sense
01:58:49 of the weird, beautiful things that emerge.
01:58:52 Right, no, so let me describe what you just said.
01:58:55 So there are two questions.
01:58:56 One is whether or not the techniques I use
01:58:58 in let’s say quantum field theory and so on
01:59:00 will describe how the society works.
01:59:02 Yes.
01:59:03 Okay, that’s far different scales of questions
01:59:06 that we’re asking here.
01:59:08 The question is, is there a change of,
01:59:10 is there a new law which takes over
01:59:12 that cannot be connected to the older laws
01:59:15 that we know, or more fundamental laws that we know?
01:59:18 Do you need new laws to describe it?
01:59:20 I don’t think that’s necessarily the case
01:59:21 in many of these phenomena like chemistry
01:59:23 or so on you mentioned.
01:59:25 So we do expect in principle chemistry
01:59:27 can be described by quantum mechanics.
01:59:29 We don’t think there’s gonna be a magical thing,
01:59:31 but chemistry is complicated.
01:59:32 Yeah, indeed, there are rules of chemistry
01:59:34 that chemists have put down which has not been explained yet
01:59:37 using quantum mechanics.
01:59:39 Do I believe that they will be something
01:59:41 described by quantum mechanics?
01:59:42 Yes, I do.
01:59:43 I don’t think they are going to be sitting there
01:59:44 in this just forever, but maybe it’s too complicated
01:59:47 and maybe we’ll wait for very powerful quantum computers
01:59:50 or whatnot to solve those problems.
01:59:51 I don’t know.
01:59:52 But I don’t think in that context
01:59:54 we have new principles to be added to fix those.
01:59:57 So I’m perfectly fine in the intermediate situation
02:00:01 to have rules of thumb or principles that chemists have found
02:00:04 which are working, which are not founded
02:00:06 on the basis of quantum mechanical laws, which does the job.
02:00:10 Similarly, as biologists do not found everything
02:00:13 in terms of chemistry, but they think,
02:00:15 there’s no reason why chemistry cannot.
02:00:16 They don’t think necessarily they’re doing something
02:00:18 amazingly not possible with chemistry.
02:00:20 Coming back to your question,
02:00:22 does consciousness, for example, bring this new ingredient?
02:00:26 If indeed it needs a new ingredient,
02:00:28 I will call that new ingredient part of physical law.
02:00:30 We have to understand it.
02:00:31 To me that, so I wouldn’t put a line to say,
02:00:34 okay, from this point onwards, it’s disconnected.
02:00:37 It’s fully disconnected from string theory or whatever.
02:00:39 We have to do something else.
02:00:41 It’s not a line.
02:00:42 What I’m referring to is can physics of a few centuries
02:00:45 from now that doesn’t understand consciousness
02:00:48 be much bigger than the physics of today,
02:00:51 where the textbook grows?
02:00:53 It definitely will.
02:00:54 I would say, it will grow.
02:00:55 I don’t know if it grows because of consciousness
02:00:58 being part of it or we have different view of consciousness.
02:01:01 I do not know where the consciousness will fit.
02:01:03 It’s gonna be hard for me to guess.
02:01:07 I mean, I can make random guesses now
02:01:09 which probably most likely is wrong,
02:01:11 but let me just do just for the sake of discussion.
02:01:14 I could say, brain could be their quantum computer,
02:01:18 classical computer.
02:01:19 Their arguments against this being a quantum thing,
02:01:20 so it’s probably classical, and if it’s classical,
02:01:22 it could be like what we are doing in machine learning,
02:01:24 slightly more fancy and so on.
02:01:26 Okay, people can go to this argument to no end
02:01:28 and to some whether consciousness exists or not,
02:01:30 or life, does it have any meaning?
02:01:32 Or is there a phase transition where you can say,
02:01:34 does electron have a life or not?
02:01:36 At what level does a particle become life?
02:01:39 Maybe there’s no definite definition of life
02:01:41 in that same way that, we cannot say electron,
02:01:43 if you, I like this example quite a bit.
02:01:48 We distinguish between liquid and a gas phase,
02:01:51 like water is liquid or vapor is gas,
02:01:53 and we say they’re different.
02:01:54 You can distinguish them.
02:01:55 Actually, that’s not true.
02:01:57 It’s not true because we know from physics
02:01:59 that you can change temperatures and pressure
02:02:01 to go from liquid to the gas
02:02:03 without making any phase transition.
02:02:05 So there is no point that you can say this was a liquid
02:02:08 and this was a gas.
02:02:10 You can continuously change the parameters
02:02:12 to go from one to the other.
02:02:13 So at the end, it’s very different looking.
02:02:15 Like, I know that water is different from vapor,
02:02:18 but there’s no precise point this happens.
02:02:21 I feel many of these things that we think,
02:02:24 like consciousness, clearly dead person
02:02:25 is not conscious and the other one is.
02:02:27 So there’s a difference like water and vapor,
02:02:30 but there’s no point you could say that this is conscious.
02:02:32 There’s no sharp transition.
02:02:34 So it could very well be that what we call heuristically
02:02:38 in daily life, consciousness is similar,
02:02:41 or life is similar to that.
02:02:43 I don’t know if it’s like that or not.
02:02:44 I’m just hypothesizing it’s possible.
02:02:46 Like there’s no.
02:02:48 There’s no discrete phases.
02:02:49 There’s no discrete phase transition like that.
02:02:51 Yeah, yeah, but there might be concepts of temperature
02:02:56 and pressure that we need to understand
02:02:59 to describe what the head consciousness in life is
02:03:02 that we’re totally missing.
02:03:04 I think that’s not a useless question.
02:03:07 Even those questions,
02:03:08 they is back to our original discussion of philosophy.
02:03:11 I would say consciousness and free will, for example,
02:03:15 are topics that are very much so
02:03:18 in the realm of philosophy currently.
02:03:20 Yes.
02:03:21 But I don’t think they will always be.
02:03:22 I agree with you.
02:03:23 I agree with you.
02:03:24 And I think I’m fine with some topics
02:03:27 being part of a different realm than physics today
02:03:29 because we don’t have the right tools,
02:03:32 just like biology was.
02:03:33 I mean, before we had DNA and all that genetics
02:03:35 and all that gradually began to take hold.
02:03:37 I mean, when people were beginning phase experiments
02:03:42 with biology and chemistry and so on,
02:03:44 gradually they came together.
02:03:46 So it wasn’t like together.
02:03:47 So yeah, I’d be perfectly understanding of a situation
02:03:49 where we don’t have the tools.
02:03:51 So do these experiments that you think
02:03:53 as defines a conscious in different form
02:03:55 and gradually we’ll build it and connect it.
02:03:57 And yes, we might discover new principles of nature
02:03:59 that we didn’t know.
02:04:01 I don’t know, but I would say that if they are,
02:04:03 they will be deeply connected with the else.
02:04:04 We have seen in physics,
02:04:06 we don’t have things in isolation.
02:04:08 You cannot compartmentalize,
02:04:10 this is gravity, this is electricity, this is that.
02:04:13 We have learned they all talk to each other.
02:04:15 There’s no way to make them in one corner and don’t talk.
02:04:19 So the same thing with anything, anything which is real.
02:04:21 So consciousness is real.
02:04:22 So therefore we have to connect it to everything else.
02:04:25 So to me, once you connect it,
02:04:26 you cannot say it’s not reality.
02:04:27 And once it’s reality, it’s physics.
02:04:29 I call it physics.
02:04:30 It may not be the physics I know today, for sure it’s not,
02:04:32 but I would be surprised if there’s disconnected realities
02:04:37 that you cannot imagine them as part of the same soup.
02:04:41 So I guess God doesn’t have a biology or chemistry textbook
02:04:45 and mostly, or maybe he or she reads it for fun,
02:04:49 biology and chemistry,
02:04:50 but when you’re trying to get some work done,
02:04:52 it’ll be going to the physics textbook.
02:04:54 Okay, what advice, let’s put on your wise visionary hat.
02:04:59 What advice do you have for young people today?
02:05:03 You’ve dedicated your book actually to your kids,
02:05:08 to your family.
02:05:09 What advice would you give to them?
02:05:11 What advice would you give to young people today
02:05:13 thinking about their career, thinking about life,
02:05:16 of how to live successful life, how to live a good life?
02:05:19 Yes, yes, I have three sons.
02:05:23 And in fact, to them, I have tried not to give
02:05:26 too much advice.
02:05:28 So even though I’ve tried to kind of not give advice,
02:05:31 maybe indirectly it has been some impact.
02:05:33 My oldest one is doing biophysics, for example,
02:05:36 and the second one is doing machine learning
02:05:38 and the third one is doing theoretical computer science.
02:05:40 So there are these facets of interest
02:05:42 which are not too far from my area,
02:05:44 but I have not tried to impact them in that way,
02:05:47 but they have followed their own interests.
02:05:49 And I think that’s the advice I would give
02:05:51 to any young person, follow your own interests
02:05:54 and let that take you wherever it takes you.
02:05:58 And this I did in my own case that I was planning
02:06:03 to study economics and electrical engineering
02:06:06 when I started at MIT.
02:06:08 And I discovered that I’m more passionate
02:06:10 about math and physics.
02:06:11 And at that time I didn’t feel math and physics
02:06:14 would make a good career.
02:06:15 And so I was kind of hesitant to go in that direction,
02:06:18 but I did because I kind of felt that
02:06:20 that’s what I’m driven to do.
02:06:22 So I don’t regret it, I’m lucky in the sense
02:06:26 that society supports people like me
02:06:28 who are doing these abstract stuff,
02:06:29 which may or may not be experimentally verified
02:06:32 even let alone applied to the technology in our lifetimes.
02:06:36 I’m lucky I’m doing that.
02:06:37 And I feel that if people follow their interests,
02:06:41 they will find the niche that they’re good at.
02:06:43 And this coincidence of hopefully their interests
02:06:48 and abilities are kind of aligned,
02:06:51 at least some extent to be able to drive them
02:06:54 to something which is successful.
02:06:56 And not to be driven by things like,
02:06:58 this doesn’t make a good career,
02:07:00 or this doesn’t do that, and my parents expect that,
02:07:02 or what about this?
02:07:03 And I think ultimately you have to live with yourself
02:07:06 and you only have one life and it’s short, very short.
02:07:08 I can tell you I’m getting there.
02:07:10 So I know it’s short.
02:07:11 So you really want not to do things
02:07:14 that you don’t want to do.
02:07:15 So I think following an interest
02:07:17 is my strongest advice to young people.
02:07:19 Yeah, it’s scary when your interest
02:07:22 doesn’t directly map to a career of the past or of today.
02:07:26 So you’re almost anticipating future careers
02:07:28 that could be created.
02:07:29 It’s scary.
02:07:32 But yeah, there’s something to that,
02:07:34 especially when the interest and the ability align,
02:07:36 that you will pave a path,
02:07:39 that will find a way to make money,
02:07:41 especially in this society,
02:07:42 in a capitalistic United States society.
02:07:46 It feels like ability and passion paves the way.
02:07:52 Yes.
02:07:54 At the very least, you can sell funny T shirts.
02:07:56 Yes.
02:07:57 You’ve mentioned life is short.
02:08:00 Do you think about your mortality?
02:08:04 Are you afraid of death?
02:08:05 I don’t think about my mortality.
02:08:09 I think that I don’t think about my death.
02:08:12 I don’t think about death in general too much.
02:08:14 First of all, it’s something that I can’t do much about,
02:08:16 and I think it’s something
02:08:18 that it doesn’t drive my everyday action.
02:08:21 It is natural to expect
02:08:23 that it’s somewhat like the time reversal situation.
02:08:25 So we believe that we have this approximate symmetry
02:08:27 in nature, time reversal.
02:08:29 Going forward, we die.
02:08:30 Going backwards, we get born.
02:08:32 So what was it to get born?
02:08:35 It wasn’t such a good or bad feeling.
02:08:37 I have no feeling of it.
02:08:38 So who knows what the death will feel like,
02:08:42 the moment of death or whatnot.
02:08:43 So I don’t know.
02:08:44 It is not known,
02:08:45 but in what form do we exist before or after?
02:08:50 Again, it’s something that it’s partly philosophical maybe.
02:08:53 I like how you draw comfort from symmetry.
02:08:55 It does seem that there is something asymmetric here,
02:08:58 a breaking of symmetry,
02:08:59 because there’s something to the creative force
02:09:05 of the human spirit that goes only one way.
02:09:09 Right.
02:09:10 That it seems the finiteness of life
02:09:13 is the thing that drives the creativity.
02:09:15 And so it does seem that at least the contemplation
02:09:21 of the finiteness of life, of mortality,
02:09:24 is the thing that helps you get your stuff together.
02:09:27 Yes, I think that’s true,
02:09:28 but actually I have a different perspective
02:09:29 on that a little bit.
02:09:30 Yes.
02:09:31 Namely, suppose I told you you’re immortal.
02:09:34 Yes.
02:09:37 I think your life will be totally boring after that,
02:09:39 because you will not,
02:09:41 I think part of the reason we have enjoyment in life
02:09:45 is the finiteness of it.
02:09:47 Yes.
02:09:48 And so I think mortality might be a blessing,
02:09:52 and immortality may not.
02:09:54 So I think that we value things
02:09:55 because we have that finite life.
02:09:58 We appreciate things.
02:09:59 We want to do this.
02:10:00 We want to do that.
02:10:00 We have motivation.
02:10:01 If I told you, you know, you have infinite life.
02:10:03 Oh, I don’t, I don’t need to do this today.
02:10:04 I have another billion or trillion or infinite life.
02:10:08 So why do I do now?
02:10:10 There is no motivation.
02:10:11 A lot of the things that we do
02:10:13 are driven by that finiteness of these resources.
02:10:16 So I think it is a blessing in disguise.
02:10:20 I don’t regret it that we have more finite life.
02:10:23 And I think that the process of being part of this thing,
02:10:31 that, you know, the reality,
02:10:33 to me, part of what attracts me to science
02:10:36 is to connect to that immortality kind of,
02:10:39 namely the laws, the reality beyond us.
02:10:43 To me, I’m resigned to the fact that not only me,
02:10:47 everybody’s going to die.
02:10:49 So this is a little bit of a consolation.
02:10:51 None of us are going to be around.
02:10:53 So therefore, okay,
02:10:55 and none of the people before me are around.
02:10:57 So therefore, yeah, okay,
02:10:58 this is something everybody goes through.
02:10:59 So taking that minuscule version of,
02:11:03 okay, how tiny we are and how short time it is and so on,
02:11:07 to connect to the deeper truth beyond us,
02:11:10 the reality beyond us,
02:11:11 is what sense of, quote unquote, immortality I would get.
02:11:16 Namely, at least I can hang on
02:11:18 to this little piece of truth,
02:11:20 even though I know, I know it’s not complete.
02:11:23 I know it’s going to be imperfect.
02:11:25 I know it’s going to change and it’s going to be improved.
02:11:28 But having a little bit deeper insight
02:11:30 than just the naive thing around us,
02:11:32 little earth here and little galaxy and so on,
02:11:35 makes me feel a little bit more pleasure to live this life.
02:11:40 So I think that’s the way I view my role as a scientist.
02:11:43 Yeah, the scarcity of this life helps us appreciate
02:11:48 the beauty of the immortal,
02:11:50 the universal truths of that physics present us.
02:11:53 And maybe one day physics will have something to say
02:11:58 about that beauty in itself,
02:12:03 explaining why the heck it’s so beautiful
02:12:06 to appreciate the laws of physics,
02:12:08 and yet why it’s so tragic that we would die so quickly.
02:12:14 Yes, we die so quickly.
02:12:16 So that can be a bit longer, that’s for sure.
02:12:18 It would be very nice.
02:12:19 Maybe physics will help out.
02:12:20 Well, Kamran, it was an incredible conversation.
02:12:23 Thank you so much once again
02:12:25 for painting a beautiful picture of the history of physics.
02:12:28 And it kind of presents a hopeful view
02:12:32 of the future of physics.
02:12:33 So I really, really appreciate that.
02:12:35 It’s a huge honor that you would talk to me
02:12:37 and waste all your valuable time with me.
02:12:39 I really appreciate it.
02:12:40 Thanks, Lex.
02:12:40 It was a pleasure, and I loved talking with you.
02:12:42 And this is wonderful set of discussions.
02:12:44 I really enjoyed my time with this discussion.
02:12:46 Thank you.
02:12:47 Thanks for listening to this conversation
02:12:49 with Kamran Vafa.
02:12:50 And thank you to Headspace, Jordan Harmerjee Show,
02:12:54 Squarespace, and Allform.
02:12:56 Check them out in the description to support this podcast.
02:13:00 And now, let me leave you with some words
02:13:02 from the great Richard Feynman.
02:13:04 “‘Physics isn’t the most important thing.
02:13:07 “‘Love is.’”
02:13:08 Thank you for listening, and hope to see you next time.