Consider a Spherical Cow with Lara Anderson

What is string theory, really? Why does it need extra dimensions? Neil deGrasse Tyson and comedian Chuck Nice welcome theoretical physicist and mathematician Lara Anderson to guide us through string theory, higher dimensions, and finding a unifying theory of everything.

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:00Neil deGrasse Tyson?

  So Brian Greene is not the only string theorist in town.

• 0:02Chuck Nice?

  I was not aware of this. Lead to Brian.

• 0:06Neil deGrasse Tyson?

  Is there room?

• 0:07Chuck Nice?

  Is there room for another string theorist?

• 0:09Neil deGrasse Tyson?

  At the OK Corral.

• 0:10Chuck Nice?

  Yeah.

• 0:11Neil deGrasse Tyson?

  All updates on string theory and how it connects to mathematics. Multiple branches of it, coming right up.

• 0:20Neil deGrasse Tyson?

  Welcome to StarTalk. Your place in the universe where science and pop culture collide. StarTalk begins right now.

• 0:34Neil deGrasse Tyson?

  This is StarTalk. Neil deGrasse Tyson, your personal astrophysicist. Got Chuck Nice with me. How you doing Chuck?

• 0:40Chuck Nice?

  Hey, I'm doing great, man.

• 0:41Neil deGrasse Tyson?

  All right, comedian, actor.

• 0:43Chuck Nice?

  Yeah.

• 0:44Neil deGrasse Tyson?

  Seen you a few TV commercials here and there.

• 0:46Chuck Nice?

  Yeah, you know, I'm, uh, listen, sometimes I have nothing to do.

• 0:53Neil deGrasse Tyson?

  Well, today we've got a really cool topic that's always on everybody's mind. Well, everybody who cares about the universe.

• 1:00Chuck Nice?

  You know, even people who don't, but I'll get, I'll get into that later. Yeah, you know, what we're talking about, I've heard some really weird stuff from people that, you know.

• 1:07Neil deGrasse Tyson?

  They want to get into it.

• 1:08Chuck Nice?

  That, yeah.

• 1:09Neil deGrasse Tyson?

  Okay, yeah, all right. All right, we're talking today about string theory.

• 1:12Chuck Nice?

  Yeah.

• 1:13Neil deGrasse Tyson?

  And now we have our man about town, string theorist Brian Greene, right up the street.

• 1:17Chuck Nice?

  Right.

• 1:18Neil deGrasse Tyson?

  But I said, you know, all the universe is not Brian Greene.

• 1:22Chuck Nice?

  You know, he would beg to differ.

• 1:25Neil deGrasse Tyson?

  So we reached out to the cosmos.

• 1:28Chuck Nice?

  Yes.

• 1:29Neil deGrasse Tyson?

  All right. And we found Laura Anderson. Laura, welcome to StarTalk.

• 1:33Speaker 02

  Thank you very much. It's a pleasure to be here.

• 1:35Neil deGrasse Tyson?

  Yeah, so you're associate professor of physics at Virginia Tech. One of our producers is from Virginia Tech. So you're, you're right at home here among us.

• 1:45Chuck Nice?

  Yeah, we, you're family now.

• 1:46Neil deGrasse Tyson?

  Yeah, you're family.

• 1:48Speaker 02

  Good to be among friends.

• 1:49Neil deGrasse Tyson?

  Also, like Brian Greene, you have a double affiliation also with the mathematics department.

• 1:56Speaker 02

  I do. I'm an affiliate professor of mathematics, which means I can supervise math grad students as well.

• 2:01Chuck Nice?

  So you just, you just taking jobs from everybody?

• 2:05Chuck Nice?

  His professors out on the street because of Laura Anderson.

• 2:16Neil deGrasse Tyson?

  I can try to picture that, how they, they're like, I'll do your math homework for money.

• 2:22Chuck Nice?

  Exactly. Well, math for food.

• 2:25Speaker 02

  I think unfortunately, that is a job.

• 2:27Neil deGrasse Tyson?

  It's called tutors, really, really.

• 2:29Chuck Nice?

  Exactly.

• 2:29Neil deGrasse Tyson?

  And your research includes, I have here, the geometry and particle phenomenology in string theory.

• 2:37Chuck Nice?

  Ooh.

• 2:38Neil deGrasse Tyson?

  So, we just want, let's just get to the bottom of this. What is string theory?

• 2:42Chuck Nice?

  Yes.

• 2:43Speaker 02

  String theory is an attempt to reconcile Einstein's theory of general relativity, a theory of gravity, with the formalism of quantum mechanics and quantum field theory. So it is a consistent quantum theory of gravity. Um, may not be the way that quantum gravity works in our universe, but at the very least, it's a theoretical playground where we get to ask questions about quantum gravity.

• 3:06Neil deGrasse Tyson?

  Now, you've made an assumption in there, and I agree with this assumption, but I want to hear you defend it. You, you are trying to absorb Einstein's general theory of relativity, our modern understanding of gravity.

• 3:18Chuck Nice?

  Right.

• 3:19Neil deGrasse Tyson?

  Into a quantum description. Why aren't you trying to take the quantum and absorb it into a general relativity description?

• 3:28Chuck Nice?

  Because inherent in what both of you said, there must be an incongruency that would cause you to have to do that.

• 3:36Neil deGrasse Tyson?

  Exactly. So, why don't you start with that, Laura? What is the incongruency here that you're trying to resolve?

• 3:41Speaker 02

  Two of the greatest intellectual accomplishments of the 20th century, in my opinion, are Einstein's theory of gravity and the description of fundamental interactions in nature, um, as described by particle physics. So, things like a description of, you know, quarks, fundamental particles, how they interact with each other, that gives us a description of things like electromagnetism and the strong and weak nuclear forces. These sort of basic building blocks of matter, these fundamental Legos that we can hook together, they're described very well by, by quantum field theory. The issue, the, the sort of discrepancy between these two is that each separately are able to make predictions that are incredibly accurate in our modern world. So we can make predictions to like 13 significant figures using either of these theoretical frameworks.

• 4:25Neil deGrasse Tyson?

  That'd be 13 decimal places.

• 4:27Chuck Nice?

  Right.

• 4:27Neil deGrasse Tyson?

  13 significant figures, yeah.

• 4:29Chuck Nice?

  I'm gonna say that's pretty accurate.

• 4:30Speaker 02

  It's pretty great, yeah, and like things like modern GPS wouldn't work without general relativities. So we have a lot of ways of testing these theories. They seem really robust and that they're telling us really important things about how the world behaves. Unfortunately, if you try and combine the two, so you try and describe phenomena that might need both tools. So for example, things where the interaction of particles and very short distance scales are in play, but also where there's really strong gravitation. So for example, inside a black hole, that would be a regime where you need both of these these theories, these frameworks to agree and give you concrete predictions. And unfortunately, the theories break down when you try and combine them and you don't end up getting useful answers. You get very manifestly wrong answers. They're called disastrous

• 5:14Speaker 02

  infinities, things that just don't predict anything.

• 5:18Chuck Nice?

  Disastrous.

• 5:19Neil deGrasse Tyson?

  Disastrous infinity. Man, you can't get more diss than that. Sounds like the worst marriage ever.

• 5:24Speaker 02

  Yeah, it's not great.

• 5:26Neil deGrasse Tyson?

  Disastrous infinities. So, so what gives you the confidence that it is the quantum physics understanding that will absorb gravity and not gravity absorbing quantum physics? Hmm.

• 5:40Speaker 02

  It's a great question. I think that it should be a two-way street. So in order to describe either phenomena, you need something that can be described in both frameworks. Einstein's theory is sort of intrinsically classical, meaning that this picture of the curvature of space and time, it's not designed for sort of the quantum mechanical uncertainties that we know and observe in particle physics. So in that sense, we know that Einstein's theory, probably at a granular level, if you sort of zoom in, should evolve into something quantum mechanical. But exact form of that is up for exploration.

• 6:12Neil deGrasse Tyson?

  Okay. Okay. So that works. Now, you mentioned fundamental particles, and you go back to ancient Greece, the atom was a fundamental particle. That was the smallest thing that you could be. That you could be, right? And then we break the atom, oh, there's other particles. And so you listed like electron that's fundamental, and you mentioned quarks. What gives you the confidence that we can't keep dividing matter?

• 6:36Speaker 02

  Cutting further.

• 6:37Neil deGrasse Tyson?

  Further. Yeah.

• 6:39Speaker 02

  That's a great question. I don't think, I think most theoretical physicists would not say that we are 100% confident that we stop there. This is the zoo of particles that we've observed so far that seem to match the phenomena, the forces and interactions and effects that we see in nature very well. But absolutely, there could be smaller things in play, and indeed, string theory posits that there are.

• 7:00Chuck Nice?

  Oh, really? Right, exactly, that's the little vibrating string.

• 7:04Neil deGrasse Tyson?

  So the string is the fundamental string.

• 7:06Chuck Nice?

  The string is the fundamental.

• 7:07Neil deGrasse Tyson?

  So take us there.

• 7:07Chuck Nice?

  Okay, yeah.

• 7:08Neil deGrasse Tyson?

  Now, how do strings come into this?

• 7:10Speaker 02

  So the idea behind string theory, the two-minute version of what string theory tries to do is it says, "Imagine that instead of describing particles as little point particles that move through space, imagine that instead if you were able to zoom in far enough that you could have an object that has an extended length associated to it." And the very rough idea is that just like a violin string can vibrate in different ways and produce different notes, these little fundamental strings can vibrate into different configurations, and it turns out they can change their properties. The mathematics of how you describe these things moving can change their properties based on how they vibrate. So they can vibrate one way and be an electron, it can vibrate another way and be a quark.

• 7:52Speaker 02

  And that seems like a very sort of cute idea for how to describe a lot of physics in a very simple framework.

• 7:59Neil deGrasse Tyson?

  But if that's the case, in principle, you ought to be able to pluck the string that is otherwise an electron and get a quark out of it. Have you done this?

• 8:11Speaker 02

  No, and the problem with this is, I'm going to tell you one thing that's really cool about this framework, and then also two things that are not cool. And one of them is very much to do with what you said. So why can't you just test or observe, you know, are these strings there? The theory predicts that these fundamental length scales of strings are so small that we would need a particle accelerator about the size of the solar system in order to smash atoms together and directly see those strings, which unfortunately, we do not have access to those type of energy scales yet.

• 8:42Neil deGrasse Tyson?

  Just to affirm what is implicit in your statement, the larger the particle accelerator, the faster you can speed the particles so that when they collide, there's much more energy in that collision, you'll probe regimes that previous accelerators could not.

• 9:00Speaker 02

  That's right.

• 9:00Neil deGrasse Tyson?

  Okay, and so you just scale up what we got going now, and you need something the size of the solar system to get to these energies.

• 9:07Speaker 02

  Right, which doesn't seem very viable.

• 9:09Neil deGrasse Tyson?

  No, it's not. So what's your, what's plan B?

• 9:13Speaker 02

  Before I answer that, let me, let me just throw one thing out there about string theory that I think is important to say. So if you ask about quantum mechanical point particles, and you say, what kind of spaces could they move through? It turns out that quantum field theory or quantum mechanics can be formulated in any type of space. So they can move through basically any background that you choose, any, any, you know, configuration of space and time. But if you ask the same question for these little one-dimensional strings, if you say, where can a quantum mechanical string move? It turns out that the only spaces that they're allowed to move in and do their, their thing of vibrating in different ways and being different particles, the only space they can do that in are spaces that obey Einstein's equations of general relativity.

• 9:53Neil deGrasse Tyson?

  That sounds good.

• 9:54Chuck Nice?

  Wow.

• 9:55Speaker 02

  Yeah, so you actually get gravity for free in this formulation of quantum mechanical strings. So we, we sometimes say that quantum gravity is consistent and also compulsory in string theory, because it's being, you know, you're forced on you by the equations that the strings must satisfy.

• 10:09Neil deGrasse Tyson?

  So that's a good feeling then, because it means

• 10:12Chuck Nice?

  Yeah.

• 10:12Neil deGrasse Tyson?

  something is talking to something else in the form, in the formulation.

• 10:16Chuck Nice?

  Right.

• 10:16Neil deGrasse Tyson?

  That, that, that was not crowbarred in to begin with.

• 10:21Speaker 02

  That's right. So it's sort of being handed to you. And that fact is, I think, something that early in the development of string theory got a lot of people excited. Unfortunately, like many good things, things come with a catch. And the catch in string theory is that this beautiful formulation I just described of, you know, you can describe all of particle physics by one little extended object, you get gravity for free, only seems to work if the universe that this happens in has more than three spatial dimensions and one time-like dimension that we seem to see in our universe.

• 10:48Chuck Nice?

  So you need extra dimensions, because right now we live in four dimensions, which is three spatial plus time. You need an X, you need more dimensions in order to make this thing work, but we don't have access to more dimensions, so we can't really say for sure.

• 11:09Speaker 02

  Well, it's, I mean, it seems like a really big intellectual leap, right? We're, we're pretty happy with our three dimensions of space and one dimension of time. So the first pass is like, could this at all, is this just a deal breaker, right? Is there any way that this could be consistent with what we've already observed about the universe? And as you were just alluding to, right? The question is, you know, could such extra dimensions exist? And if so, how would we try and probe whether that's the case? The, the requirement that we can see right off the bat about these extra dimensions is if they were to exist, they can't be the same size as the rest of the dimensions that we see in our universe. So if we look around, we can see that, you know, we have very large spatial extent for, you know, front, back, side to side, up and down, and of course, time.

• 11:52Speaker 02

  But if there were these other directions, they would have to be really, really small compared to the rest of our universe. And the analog for that is if you imagine looking at an extended object like a wire from really far away, it just looks one dimensional. It just looks like it has a length. But if you were able to get really close to that wire, you'd see that it also has something like a thickness, a radial direction. And so that extra direction is what's called compact, meaning that it's very small compared to, say, the length of the wire. So one thing that we do know is that if this had any chance of working, these extra dimensions would have to be compact and very small compared to the rest of our universe.

• 12:25Chuck Nice?

  Okay, you're freaking me out right now because, and this, I mean, I'm just gonna say it. So I was down in Costa Rica doing ayahuasca for a week. And in that time, I had an experience where I met these beings who told me about dimensions that existed inside of our dimension. So they were alongside of, yet inside of the dimension that we live in. And I can only think that maybe that was a presupposed, pre-planted, post-hypnotic suggestion

• 13:07Chuck Nice?

  because I have actually read about string theory because if it's not.

• 13:15Neil deGrasse Tyson?

  Or Laura, what drugs have you been taking?

• 13:18Chuck Nice?

  My, my follow-up question is, what did you do?

• 13:21Neil deGrasse Tyson?

  That's right.

• 13:23Speaker 02

  I, I hear the question, yeah.

• 13:24Neil deGrasse Tyson?

  Is it possible to have a compactified time dimension as well? Or is we only, is all of the models only stuck with one time dimension? Oh, wow. I've never even considered that. Yeah, imagine two dimensions of time. Wow. Holy crap. Go ahead. Yes.

• 13:39Speaker 02

  So the problem with two time dimensional theories and compact time dimensions in general is that it's very hard to maintain causality in such theories. So if you have a time direction that can loop back on itself, it's possible to have the whole go back in time and shoot your grandpa situation hitting you pretty hard. So to maintain consistent theories with multiple time directions or compact time directions, it's not, I'm not gonna say it's impossible, but most people don't consider that a very viable way forward to try and build through physics.

• 14:06Chuck Nice?

  You would have to discard causality altogether in order to do that? Or would it just be it violates?

• 14:12Speaker 02

  I think in general, the claim would be it would violate causality in such theories. Um, so there may be some creative ways to get around that, but generically, I think that's true.

• 14:20Neil deGrasse Tyson?

  By the way, we have a Stephen Hawking on one of our earlier episodes. You can find it in our archives.

• 14:25Chuck Nice?

  That's right.

• 14:25Neil deGrasse Tyson?

  We went to University of Cambridge.

• 14:27Chuck Nice?

  Right.

• 14:27Neil deGrasse Tyson?

  And chilled with him for a bit. Tell me, he proposed a time travel conjecture? Something like that? Tell us what that was? And did that save us from this? Is that?

• 14:38Speaker 02

  I mean, there are a number of conjectures, say, in the theory of gravity that say that causality is an important structure. So that, um, in general, one would not expect that consistent theories of gravity or indeed quantum mechanics should allow such.

• 14:52Neil deGrasse Tyson?

  You just can't get it to work. Okay.

• 14:53Chuck Nice?

  Okay.

• 14:54Neil deGrasse Tyson?

  That's fine.

• 14:55Chuck Nice?

  Yeah, I mean, listen.

• 14:56Neil deGrasse Tyson?

  That removes many movies in the repertoire where you gotta go back and change the past.

• 15:02Chuck Nice?

  Exactly.

• 15:03Neil deGrasse Tyson?

  Like Terminator among them.

• 15:04Chuck Nice?

  All the Terminator is done. Forget about it. We look back on the past, but we look forward to many futures. So the idea of being able to look back and say, at this particular point, all those many futures still exist. If I could get back to that point, then I could change it to one of these other tracks. You know, which, I mean, it's a great fantasy and it makes sense to have that fantasy. But what you're saying is it's a stupid fantasy because it ain't never gonna happen.

• 15:32Speaker 02

  Well, I would say, you know, never say never in science. You gotta be careful. But, um, certainly it's not something that I think most people have a good idea how to make work in a consistent way.

• 15:40Chuck Nice?

  And that's why I'm not a scientist.

• 15:42Neil deGrasse Tyson?

  And just consider to say never say never. You said never.

• 15:46Chuck Nice?

  You just said never.

• 15:47Neil deGrasse Tyson?

  You can't say never say never.

• 15:49Chuck Nice?

  Without saying never.

• 15:50Neil deGrasse Tyson?

  Without saying never.

• 16:04Speaker 03

  Hello, I'm Vinky Barok Allen and I support StarTalk on Patreon. This is StarTalk with Neill Grass Tyson.

• 16:22Neil deGrasse Tyson?

  Before we sort of pivot to this whole thing of phenomenology, because I want to know, it's a big word with a lot of syllables, I've heard it invoked before, especially in particle physics, but before we step there, I just want to understand, how will you ever test string theory? And we know in advance that you have naysayers out there, physicists among them, who are saying you're diverting time, resources, graduate students, faculty positions, to something that doesn't even classify as a legitimate scientific theory or hypothesis, because you need to be able to test it. You need to test it. Without the test, go home. So, let me hear your response to that.

• 17:01Speaker 02

  Absolutely. I think that's a really fair question. For any theoretical framework where you're trying to describe things, if there's a big intellectual leap, and for example, extra dimensions is a pretty big leap, you have to justify that with a payoff. You have to say, what is the benefit that I'm going to give you in terms of structure and predictions and what you're learning from this theory? So, to push back on the, how are we going to test it? Let me observe first that in particle theory in general, the time scales between predicting structure in particle theory and then being able to see it in experiment, over the last half-century have gone, increased in size considerably. So, one example of this is the prediction of the Higgs boson made by Peter Higgs, which

• 17:42Speaker 02

  took about 50 years from the prediction that this particle should exist to its observation at the LHC.

• 17:48Neil deGrasse Tyson?

  That means the theorists are just way ahead of the experimentalists. You've got some deadbeat experimentalists.

• 17:54Speaker 02

  I would not say that at all. I would say that that dialogue of theory and experiment is really important, but I'm just pointing out that direct experiment of, you know, direct verification by experiment of lots of things is hard. That doesn't mean that one shouldn't do it. It just means that, you know, you have to be deciding what time scales are relevant for that question. Um, for string theory, I would say the problem is a lot worse than it was for something like the prediction of the Higgs boson, because the energy scales are so massive to directly observe strings. So for me personally, I'm interested in trying to decide whether string theory is useful a lot faster than that in the point of view of my career. And I remain very agnostic as to whether that's the case. So

• 18:30Neil deGrasse Tyson?

  You want to be alive when

• 18:32Speaker 02

  I want to be alive when we decide this. And also, you know, if somebody could demonstrate to me right now that string theory was for sure not useful for our universe, I'd choose to work on something else. So what do I think is the, the sort of most direct way to those types of answers? Is that in string theory, many types of structure and, and results in physics are really inter-correlated. So it turns out that in something like, um, the, the particle physics description of all the particles we know about so far, the standard model of particle physics.

• 19:02Neil deGrasse Tyson?

  So that's a, the organization of all the particles that we know, just on one, in one chart. And you say this, this interacts with that and this connects here. And it's a, it's a beautiful thing, actually, when you step back and it's a try. I mean, that happened in, I'm older than both y'all. That happened in my lifetime. I mean, then basically 1970s. Wow. We started assembling a little earlier too, but the full picture was coming together as we found these other particles to flesh it out. So it's a periodic table of particles. Okay. All right. An organization, organization principle. Very good. Okay. So pick it up there. Sorry.

• 19:36Speaker 02

  Quick anecdote from when I was a kid. I got interested in physics, reading books like by people like Brian Greene when I was, you know, in my early teens. And I had a little, um, I'd written down the standard model, like on a piece of paper. I made my little zoo of particles. And I like carried around on a piece of paper for a bit as a, a super, you know, nerdy, young, young person.

• 19:52Neil deGrasse Tyson?

  Girl geek in the house. Love it. Girl geek alert.

• 19:55Speaker 02

  Yeah, totally, totally. But, um, but I totally agree. It's, it's the zoo of what, what we know is there. And the point I was going to make in relation to string theory is that in something like the standard model, there's a lot of free numbers. So for example, nothing in that theory tells you what the mass of the electron is or how the quarks couple to each other. Those numbers are just observed in our universe.

• 20:15Neil deGrasse Tyson?

  Well, you said three numbers, you mean not predicted.

• 20:18Speaker 02

  Correct, not predicted by the structure of the theory. Now, in contrast to that, in string theory, if you found a solution in string theory that produced particle physics like we see in our universe, none of those numbers are free. They're all determined by the configuration of these extra dimensions and the structure of the theory. So it's a huge array of physics that you have to get right all at once. And here I've been talking about particle physics, but you also have to answer questions about cosmology and the large-scale structure and history of the universe. So how are you going to decide if string theory is wrong? I think that it's most likely that we would be able to say that the structures that we see in nature, we can argue that we either can or can't get the right sort of regimes of numbers and effects that we already know are there

• 21:02Speaker 02

  much more rapidly than we're going to build an accelerator to see a string.

• 21:05Neil deGrasse Tyson?

  I love that.

• 21:06Chuck Nice?

  Oh, that's...

• 21:06Neil deGrasse Tyson?

  Yeah, so what you're saying is, whether or not you can test the dimensionality or the other sort of physicality of string theory, if your theorists can go in the back room and come out and say, "I pluck this string this way, it's going to give me an electron and it's going to give me this mass."

• 21:25Chuck Nice?

  Right.

• 21:26Neil deGrasse Tyson?

  And it's the only mass that's going to come out of this.

• 21:28Chuck Nice?

  Right, because that's the right number.

• 21:29Neil deGrasse Tyson?

  It's the right number and the right vibration and it's going to look like an electron. That's a Nobel Prize right there.

• 21:34Chuck Nice?

  Yeah. It's like the way they do modeling or weather modeling. We have a prediction and then we run the model on what's already been done. And if we get those numbers, then we can have confidence in the predictive model itself.

• 21:51Speaker 02

  That's right. Again, so the idea is, I just articulated it. This has been around for like 40 years. This was when people were first formulating string theory. Everybody was excited and they thought we were going to do exactly what you just said. You know, you're going to go around, you're going to look at the solutions of the string theory. You're going to say, "Boom, here's our universe, here's all these numbers. We just predict everything, it's great." That hasn't happened. And people are still trying to think about this. So what are some of the big obstacles? One of the issues is that it turns out that when you ask how many different configurations for these extra dimensions can there be? Initially, the hope was that maybe that was very restrictive. Maybe there was just a couple. The question of how many could you have? So if you said, what if we just had, say, two extra dimensions that obeyed the consistency

• 22:36Speaker 02

  equations from string theory? Turns out, there's a unique answer from the differential equations that tell you what that shape could be. If you ask, is there, you know, what happens if there are four extra dimensions? There's a unique answer. And then if you say, what happens if there were six extra dimensions, which happens to be the extra dimensions that we think needs to arise to give what we see in nature? Turns out, there are half a billion configurations and counting that have been found so far.

• 23:00Chuck Nice?

  So it sounds to me like what you guys are saying is, we have this instrument, and on the instrument, there's a certain amount of notes that are just resident in the instrument. And now we have to figure out one thong. Because all those notes can make however many billions of songs. And we gotta find the one song that all those notes can play.

• 23:27Neil deGrasse Tyson?

  But it also sounds like nature's just messing with us.

• 23:33Speaker 02

  It sounds, so this is one of the things that people push back on string theory. And as I say, okay, if all these possible solutions of string theory exist, how is it ever going to be predicted? And, you know, you could just have this big soup of things. And some people have even made that argument. They call it the string landscape, where they say, you know, you could just land anywhere. So, so what if there's some place in the string landscape that looks like our universe? There's all this other junk. What is the theory actually telling us then? And the argument I would, I would say against that is that in something like quantum field theory, which we already talked about for the standard model, this zoo of all the, you know, quarks and leptons that we know in nature, there are an infinite number of quantum field theories that I could write down that aren't our universe. But it doesn't matter, because we do know how to write down one that does look like our universe.

• 24:15Speaker 02

  So string theory is sort of a natural extension of quantum field theory in some ways, and it has a lot of flexibility that may have nothing to do with the physics that we observe in our universe. But the question is, once you zoom in on the, the parts of that theory that do, do you learn anything? So for example, do you find that if you See the particles that we already know that additional particles must be there or additional forces. Or can we correlate features in cosmology and the large scale structure of the universe like dark energy or dark matter? With particle physics that we know to be true.

• 24:45Neil deGrasse Tyson?

  I don't want to lessen the significance of how you describe that. But if I understand it, you're saying on this, like you said, this landscape of half a billion possible songs that it could be, and you want the one song that's yours. It's not useful if you find it unless upon finding it, you get other insights about the universe we're in.

• 25:05Chuck Nice?

  Uh huh.

• 25:06Neil deGrasse Tyson?

  Because otherwise it's just a just so story.

• 25:09Chuck Nice?

  Right.

• 25:09Neil deGrasse Tyson?

  Right. It's like the universe is just that and we and we explain it with just that.

• 25:13Chuck Nice?

  Right.

• 25:13Neil deGrasse Tyson?

  And it doesn't take you any further down the street. Is that a fair way to characterize it?

• 25:18Speaker 02

  Yeah. And another thing that people are asking in within string theory is how many possible quantum theories of gravity could there be? So imagine that string theory isn't how our universe works. We know it's a quantum theory of gravity, but it might be too idealized to describe our universe. So in physics, we talk about, you know, pretending that cows are spheres in order to make the math easier.

• 25:37Neil deGrasse Tyson?

  Consider a spherical cow.

• 25:39Chuck Nice?

  Oh, gotcha.

• 25:40Neil deGrasse Tyson?

  We do that all the time.

• 25:41Chuck Nice?

  Okay.

• 25:42Neil deGrasse Tyson?

  Yeah, it's just easier that.

• 25:46Speaker 02

  Our, our society of physics students has a spherical towel t-shirt here for, uh, at Virginia Tech.

• 25:51Neil deGrasse Tyson?

  Yeah, yeah, it's a thing.

• 25:52Chuck Nice?

  That's, that's pretty wild.

• 25:53Neil deGrasse Tyson?

  It's a thing. If you want to maximize the milk production of a cow.

• 25:56Chuck Nice?

  Right.

• 25:56Neil deGrasse Tyson?

  Start with a spherical cow.

• 25:58Chuck Nice?

  Start with a spherical cow.

• 25:59Neil deGrasse Tyson?

  Yeah.

• 25:59Chuck Nice?

  Right on.

• 26:00Neil deGrasse Tyson?

  Yeah.

• 26:00Chuck Nice?

  I don't want that milk. I don't want, I don't want milk from a spherical cow. I don't, I'm sorry. Yeah. Yeah, I'm going goat's milk from now on. Yeah.

• 26:09Speaker 02

  But yeah, the, the point I'm making is that maybe string theory isn't, you know, it's just too idealized to describe how quantum gravity works in our universe. But a lot of theorists are questioning, okay, if that's the case, we know this is a quantum theory of gravity. So if you had another one, right, like the right one that isn't a string theory, how could those two theories be related? And there's groups of string theorists who are trying to argue and, and provide mathematical theorems. Um, for example, someone named Kerman Bafa at Harvard has created something called the cobordism conjecture. And what he's positing is that perhaps if you had more than one quantum theory of gravity, they must be connected in some way. Otherwise you would develop inconsistencies in how you could describe quantum gravitational, quantum gravitational effects.

• 26:53Speaker 02

  So the the argument I'm making here is that even if string theory isn't the right one, whatever that might mean, maybe it's connected to the right one. So maybe we still learn structure about how string theory can inform what our universe should look like.

• 27:05Chuck Nice?

  Is this what, not exactly what you just said, but the comprehensive look at all of this, is this what gives rise to the multiverse and infinite number of universes?

• 27:18Speaker 02

  That's a different question, but an interesting one.

• 27:20Neil deGrasse Tyson?

  The answer is no.

• 27:21Chuck Nice?

  The answer is no. Okay.

• 27:23Neil deGrasse Tyson?

  We'll say that for the end.

• 27:25Chuck Nice?

  We'll say that for later.

• 27:26Neil deGrasse Tyson?

  Your specific specialty within string theory is particle phenomenology. And could you just introduce us to that?

• 27:32Speaker 02

  Absolutely. So that's the question of whether string theory can produce solutions that look like the particle physics that we see, the the standard model, for example, and the interactions of the other fundamental forces that are in gravity. And so one of the things that I've worked on at various times over my career is trying to ask, for the types of solutions that we we see in string theory, what characterizes those that would give us things like we see in nature? So again, coming back to this concept of the string landscape, there's a famous number of like possible solutions that you can get for this this string landscape, a number of like 10 to the 500, which is unimaginably large, is thrown around.

• 28:13Neil deGrasse Tyson?

  10 to the 500.

• 28:15Speaker 02

  10 to the 500.

• 28:16Chuck Nice?

  That's not a number.

• 28:18Speaker 02

  That's not a number. It's crazy.

• 28:20Chuck Nice?

  That's not even a number.

• 28:21Speaker 02

  But in that counting of solutions in string theory, so this is again, you know, something that people say, oh, you know, 10 to the 500, how are you going to learn anything? But all of those 10 to the 500 that people have counted historically, none of them included an electron. Okay? So if you know something about the universe you want to model, you don't care if there's 10 to the 500.

• 28:38Neil deGrasse Tyson?

  Oh, you can constrain it.

• 28:39Speaker 02

  Yeah, none of those can possibly give rise to the type of physics we see. So the type of of research that I do is trying to correlate what shapes for these extra dimensions, what properties of string theory will narrow the field down to things that are close to our universe.

• 28:53Neil deGrasse Tyson?

  Okay, so so have you gotten there yet? What's what, you know?

• 28:55Chuck Nice?

  What's the holdup here, Lara?

• 28:57Speaker 02

  We're getting better. I mean, in all honesty, you know, we have not delivered on that yet, but we are, I feel like, still making legitimate progress.

• 29:04Chuck Nice?

  Okay, so how about this? Because one of the great redeeming qualities of all scientific discovery, or the search thereof, is Even if I don't get to the thing I am trying to find out, along the way, I find out all this other great stuff that now gives us computers and digital cameras and GPS, but I didn't get to what I wanted. So, wha- wha- what have you guys contributed that has been your happy accident?

• 29:39Neil deGrasse Tyson?

  Well, I mean, I want to say that more tightly.

• 29:42Chuck Nice?

  Say it tighter.

• 29:43Neil deGrasse Tyson?

  You ready? No, no, I love that.

• 29:44Chuck Nice?

  Go ahead.

• 29:45Neil deGrasse Tyson?

  I loved it, but I want to say it another way.

• 29:46Chuck Nice?

  All right, go ahead.

• 29:47Neil deGrasse Tyson?

  Okay. Um, in your failures, how have you succeeded?

• 29:52Chuck Nice?

  Ooh!

• 29:55Speaker 02

  So, I think the answer to that is really big, and string theory as a field has really expanded to, you know, huge different numbers of subfields and researchers who do really different things. So, there are many different answers to that question. I could, like, you know, percolate in the back of my mind. One is, um, the discovery of the holographic principle, which says that, uh, phenomena like gravity are very deeply related to things called gauge theories, which, again, describe the interactions of particles and charges, um, that these things can be related, uh, in different dimensions of spaces. So, the statement is that, uh, gravitational theories can be related to gauge theories that live on the quote "boundary" of that space. Um, things like the holographic principle are an extremely deep, uh, bit of structure

• 30:40Speaker 02

  that says that, you know, gauge theories and particle physics and gravity are not as different as we thought they were. That's, that's a really, um, profound, I would say, uh, observation that has arisen in string theory.

• 30:50Neil deGrasse Tyson?

  So, the simplest example that I've heard of the holographic principle is the surface of the event horizon of a black hole.

• 30:59Chuck Nice?

  Okay.

• 30:59Neil deGrasse Tyson?

  And, and correct me if I'm wrong here. So, you fall through, the surface has a memory of everything that passed through.

• 31:06Chuck Nice?

  Interesting.

• 31:07Neil deGrasse Tyson?

  And so, you can think of the information content of the surface as the full... Understanding of anything.

• 31:14Chuck Nice?

  Everything that's inside.

• 31:15Neil deGrasse Tyson?

  That's inside.

• 31:16Chuck Nice?

  Cause there's no loss of information because it's all retained.

• 31:19Neil deGrasse Tyson?

  Retained, retained there. And so that inside the black hole, and if we are inside a black hole of our universe.

• 31:25Chuck Nice?

  Right.

• 31:25Neil deGrasse Tyson?

  Cause we have a horizon, which you can analogize to an event horizon.

• 31:30Chuck Nice?

  Right.

• 31:31Neil deGrasse Tyson?

  Then we would be the holographic projections of that. So is this a fair, did I, did I capture that correctly?

• 31:37Chuck Nice?

  Wow.

• 31:38Neil deGrasse Tyson?

  Okay. Given that, I've yet to heard a physicist rebut that. So is there general agreement that that's probably real?

• 31:45Speaker 02

  I mean, in an idealized sense, yes. Um, how much that, you know, pertains, uh, you know, what you learn from that in our universe, I think, is still up for grabs. And this is again, something people are, are thinking about very actively. Um, other analogies that I would give for, you know, useful stuff that's come out of string theory is, um, relationships again between things like particle physics and cosmology. So, you know, the study of, uh, you know, dark energy, dark matter, descriptions of inflation, those things being related to, you know, how particle physics, uh, realizes those. And also structure in mathematics. Um, so there are a lot of new fields of mathematics that were sparked due to that dialogue between, uh, mathematicians and physicists that arose through string theory and these shapes of extra dimensions.

• 32:28Neil deGrasse Tyson?

  So that's good. So you're exciting mathematicians.

• 32:30Chuck Nice?

  Right.

• 32:30Speaker 02

  Yes.

• 32:31Neil deGrasse Tyson?

  And then they reached out and wanted some of you in their department, right?

• 32:35Speaker 02

  Sometimes, yeah. And, and I think, you know, it's a very much a mutual relationship.

• 32:38Chuck Nice?

  Which is why you're taking a job away from another professor.

• 32:43Speaker 02

  So as one example of this, um, there's something called the minimal model program in, uh, geometry, which tries to classify basically all these, um, higher dimensional complex shapes, like all of them, you know, can you write down compact geometries in any number of dimensions and characterize all their properties and, and come up with sort of a zoo of every possible geometry?

• 33:02Neil deGrasse Tyson?

  Well, we get, this is, this is your other specialty.

• 33:04Chuck Nice?

  Right.

• 33:05Neil deGrasse Tyson?

  Algebraic geometry.

• 33:07Speaker 02

  That's right.

• 33:07Neil deGrasse Tyson?

  And was that a spinoff of the rest of these interests? Is that?

• 33:10Speaker 02

  No, it's very much tied to it. So the question of trying to produce particle physics from, uh, string theory, that's, that's a particle physics question, but the actual computation you have to do really rests on the properties of these compact extra dimensions. So you have to do a ton of geometry to extract the numbers that you want, like the mass of the electron, I'm a coupling of the quarks. So it's sort of intrinsically interdisciplinary in that sense.

• 33:32Chuck Nice?

  Wow.

• 33:32Neil deGrasse Tyson?

  So I want to hear more about this geometry.

• 33:34Chuck Nice?

  That's cra- I mean, that's crazy.

• 33:35Neil deGrasse Tyson?

  So here's something that is so simple and low dimensional. So don't laugh at me, but I was, but I want you to take this to your level. Uh, I was talking with a topologist, algebraic, or some one of these math folk. And we were talking about knots.

• 33:52Chuck Nice?

  Okay.

• 33:52Neil deGrasse Tyson?

  Just knots.

• 33:53Chuck Nice?

  All right.

• 33:53Neil deGrasse Tyson?

  And you take a string and tie a knot in it, and it's a knot.

• 33:56Chuck Nice?

  It's a square knot.

• 33:57Neil deGrasse Tyson?

  Or any, whatever your knot is.

• 33:58Chuck Nice?

  Whatever knot.

• 33:59Neil deGrasse Tyson?

  Okay, whatever your knot. And then we, that's, we're in three dimensions here. So a one dimensional string can make a knot. I said, what's that in four spatial dimensions? He said, you can't tie a knot in four dimensions.

• 34:13Chuck Nice?

  Hmm.

• 34:14Neil deGrasse Tyson?

  You would just lift it up and it would just unravel. And I, and I, so that just messed with me. And then I, then I thought, let's take away a dimension. Let's go to two dimensions. Okay?

• 34:25Chuck Nice?

  Okay.

• 34:25Neil deGrasse Tyson?

  Two dimensions, and we've got two dimensional people in a flat surface. If you take a rope and just loop it on itself, they cannot undo that. That is an unsolvable knot to them. Because they can't pass it back over itself to come around.

• 34:42Chuck Nice?

  Right.

• 34:42Neil deGrasse Tyson?

  All right. But I, in three dimensions, just pick it up and it's gone.

• 34:45Chuck Nice?

  Right.

• 34:46Neil deGrasse Tyson?

  So that was, so I was just, I couldn't sleep that night. And then I wondered to myself, what is going on in the mind of someone who's imagining all of this in even higher dimensions than that?

• 34:57Chuck Nice?

  What kind of drugs are you hiding? Come on, Larry, don't hold out on me. I shared my ayahuasca, give us the real dope.

• 35:06Neil deGrasse Tyson?

  You never told us that ayahuasca, high dimensionality person talked to you.

• 35:10Chuck Nice?

  Well, you know what, I, I have, you guys are the first to know. To be honest, because it was so freaky and freaked me out and I never talked about it. But then when she said-

• 35:19Neil deGrasse Tyson?

  It was a string theorist of the future.

• 35:21Chuck Nice?

  Exactly.

• 35:21Neil deGrasse Tyson?

  Said, you are our savior.

• 35:25Chuck Nice?

  I was the Neo of string theory.

• 35:29Neil deGrasse Tyson?

  Yeah, what is the Matrix Neo? So please tell us.

• 35:32Speaker 02

  So I really like the the knot analogy. And let me give another one that's sort of more directly related to the kind of stuff I do. So we talked about, you know, Einstein's theory of gravity. You could ask, imagine that the entire universe was two dimensional, right? Could you have curvature that could lead to like gravitational like theories in two dimensions? And it turns out there's only one number that you get to to specify. And it's basically, you know, if you imagine like the surface of a sphere, it's whether it's, you know, positively curved or negatively curved, like a saddle. That's it. And so you can't have dynamical gravity in two dimensions. Likewise, you know, the form gravitation takes will change as you go up in dimension. So absolutely, the this question of, you know, what can you not and not? What can you use to

• 36:15Speaker 02

  describe how space and time might curve? All of that changes with different dimensions.

• 36:21Neil deGrasse Tyson?

  Yeah. And so you have to get your brain up in that.

• 36:24Chuck Nice?

  I'm telling you right now, I'm, you know, I need a nap. Just from this conversation.

• 36:32Neil deGrasse Tyson?

  So so but the cop out thing here is all the higher dimensions are all compacted. So I don't have to think about or worry about it. I'll never see them. Right? How how impactful are the compacted dimensions if you if you're trying to manifest gravity in higher dimensions? Does gravity care if it's compacted or not? Does it just care about the dimensionality?

• 36:55Speaker 02

  It does. It does care if it's compact. It cares about the compactness. It cares about the shape of those extra dimensions. And indeed, we believe in string theory that those extra dimensions have to still obey Einstein's equations. So they still have to to be consistent gravity in in those extra dimensions.

• 37:09Neil deGrasse Tyson?

  Suppose we lived in eight dimensions, and he came up with general relativity in eight dimensions. Who are you to say his three plus one dimensional thing that higher dimensions have to obey that? He did that in this measly three plus one dimensional world. You can't put you can't put commandments on higher dimensions. They are superior to us in every way, and you know it.

• 37:30Chuck Nice?

  Yes, I hate to say it, but higher dimensions look down on us.

• 37:35Neil deGrasse Tyson?

  That's good.

• 37:39Chuck Nice?

  I'm sorry, I had to do it.

• 37:40Neil deGrasse Tyson?

  That's a T-shirt right there.

• 37:41Chuck Nice?

  It is. It is a good T-shirt.

• 37:43Speaker 02

  So I actually torture my undergraduate students in my class on Einstein's theory. Because Einstein's theory can be actually formulated in any number of dimensions very easily. So there's actually nothing special about four. So frequently for my students, I'll say, you know, imagine that this was in six dimensions or 10 dimensions or, you know.

• 38:01Neil deGrasse Tyson?

  She said it can be formulated easily, but she didn't say what she meant by easily.

• 38:05Speaker 02

  Very relative statement there.

• 38:09Speaker 02

  One thing that isn't easy, and this is actually related to why it's hard, again, to, you know, really bring string theory to its full fruition, is that when you do do Einstein's theory in higher dimensions, the equations you have to solve are nastier. So humans are not really good at nonlinear differential equations, and they are especially not good when they go into high numbers of variables and high numbers of dimensions.

• 38:31Neil deGrasse Tyson?

  So isn't that some... Okay, so why don't you just get a math-fluent AI bot to do this? I mean, that's the whole point of...

• 38:41Chuck Nice?

  Yeah, exactly.

• 38:42Neil deGrasse Tyson?

  It's hard for you, but give it to an AI.

• 38:45Chuck Nice?

  Yeah.

• 38:46Speaker 02

  So one of the things that we've had to try and do in string theory to extract some of these predictions is actually solve Einstein's equations for these extra-compact dimensions. And we don't know any analytic solutions for how... That means, you know, exact solutions you could write down on a piece of paper for Einstein's theory for the six dimensions that we would need for these string compactifications, they're called.

• 39:07Neil deGrasse Tyson?

  In science, you can solve a problem analytically with an equation, say, there's the answer. And some you can't, and you have to actually run the experiment or increment a model and see the results each time just to see where it goes.

• 39:22Chuck Nice?

  Right.

• 39:22Neil deGrasse Tyson?

  And so that's ugly, we hate those, but we kind of recognize that that's... Like in chaos theory, you have to sort of calculate it out.

• 39:30Chuck Nice?

  Right.

• 39:30Neil deGrasse Tyson?

  You can't just write down the solution. So if you're saying you can't in principle, or it's just... Too laborious.

• 39:38Speaker 02

  Just not known how to do it yet. So in general, solving Einstein's equations in any number of dimensions, you know, for any system is is hard because they're nonlinear. So what that means is that the the gravitational theory is actually backreacting or talking to itself. So the fact that you have gravitons, you know, the the quantum mechanical description of gravity in the space, that can create more gravity. So this is really wild in terms of the differential equations because normally you could say, I find one solution to the theory and I find another and I can just add them together and still get a solution. But in general relativity, that doesn't work. You can't add two solutions and get another solution. You have to start over every time.

• 40:18Neil deGrasse Tyson?

  Whoa. So when we when we model, we do this in astrophysics all the time. There's stuff that's just too complicated, but I know at any instant what's supposed to happen and then I just load load that up. But you what you're doing is you're calculating with these differential equations. These equations that you can calculate at every time step and it's following you on the time step. Right. All right, but you can't just solve out the whole the whole shebang. So. Gotcha. So, all right, so you were but so tell me again why you can't use AI.

• 40:49Speaker 02

  So we can actually, and that's that's a fun topic. So I was involved for a number of years with numeric simulations like you're describing, where you use a computer to try and solve the equations that you you can't otherwise. And historically, in order to do those computations, we had to put them on supercomputer clusters and like wait for months to get results. But now, actually, with the advent of AI, this is something that my collaborators and I have worked on.

• 41:12Neil deGrasse Tyson?

  And now you just do it on your iPhone, right?

• 41:15Speaker 02

  Now we can actually do it on a laptop. So we've started using machine learning algorithms to numerically solve some of these differential equations. So this is different than using like, you know, looking at photos on the Internet and then having AI generate a new photo. We don't have these solutions. So there isn't a database that you can train an AI model on, but you can still use the framework of these neural networks to try and solve really complicated equations. And indeed, I worked on that and lots of other people in the field have, and we found that using these techniques, we can speed up a lot of computations in a really substantive way. And this actually made it possible just recently for groups to compute quark masses in string theory for the first time. So to be clear, these are not the quark mass values that we actually observe in nature. That would be awesome, but we don't see that

• 42:00Speaker 02

  yet. But we can say, if you just hand me some extra dimensions, whatever they may be, and then say, what would the, you know, the quarks look like in that universe? Now we can actually come up with those numbers using machine learning algorithms.

• 42:12Neil deGrasse Tyson?

  Chuck will go back on his ayahuasca trip and get the person from that dimension to verify the quark mass. Yeah, exactly.

• 42:21Neil deGrasse Tyson?

  And Chuck will be the oracle of physics. And I'm, I'm up for it. I'm telling you right now. I'm ready to handle it. I'm ready to go do more ayahuasca. So I'm, I'm ready.

• 42:54Neil deGrasse Tyson?

  So let me ask another thing. You talk about what discoveries can come out of this. Is it possible, because this excites us all. We don't say it every day, but we feel it. Could any of this, could any of this bring forth new physics? Because up till now, everything you've said has been within the framework of the quantum, quantum field theory, general relativity. And imagine before Einstein was born, you would not even know that relativity was a thing you could use to solve your problems. So is there some new physics waiting to emerge either, either out of your work, or some yet to be born genius that you'll look at and

• 43:37Neil deGrasse Tyson?

  say, oh, my gosh. I'm gonna give a fast astro case here. Okay. A turn of the century, the orbit of Mercury around the sun wasn't quite following Newton's laws. You know, it was like, well, there's probably another planet there tugging on it. Yeah, we even named the planet. I mean, that's how. Oh, really? Yeah, we called it Vulcan. Oh, nice. Yeah, you go 1910, just look up planet Vulcan. Like, there it is. It's pretty wild. Okay. And so, and we were happy. Well, how come no one saw it? Oh, because it's too close to the sun. Right. It's in the glare. So we, we had, we had, it was all there. It was all worked out. All worked out. And then Einstein. Comes up with general relativity. He wasn't trying to explain it, but he showed that at very strong gravity, Newton's laws fail.

• 44:19Neil deGrasse Tyson?

  You plug into his equations and there's no disagreement with the thing. So Vulcan died overnight, but the new physics transformed the physics we were already working with. Right. And gave us better answers than to move on with. So what I learned from that is Einstein killed Vulcan, Tyson killed Pluto. Stop.

• 44:43Neil deGrasse Tyson?

  There's not the lesson.

• 44:46Speaker 02

  It's a good lesson.

• 44:47Neil deGrasse Tyson?

  No, it's not. No.

• 44:48Speaker 02

  So new physics could take a lot of different forms. Um, so, you know, one example might be perhaps there are more than the four fundamental forces that we've already observed in nature. So could there be, you know, so-called fifth force, uh, another version of something like electromagnetism or, um, you know, the strong weak nuclear force. Um, that would be an example of new physics. Uh, other things that we know we don't understand very well include things like dark energy and dark matter. Um, questions like, you know, general relativity tells us that there are these disastrous infinities, you know, in the center of black holes, there's singularities. So what actually repairs those singularities in a quantum, uh, you know, gravitational theory? What, what tells us how physics really behaves inside there?

• 45:29Speaker 02

  Um, that's definitely new physics and that would be the hope of the kind of thing you'd, you'd like to see.

• 45:34Neil deGrasse Tyson?

  Oh, wow. So, okay, so you think there's new physics out there?

• 45:39Speaker 02

  Not again, the claims on, you know, what constraint theory deliver? I'm, I'm still somewhat agnostic on that. Um, but I think it's really interesting to try and push the theory to find out. Say, you know, can you show that this just can't be used to model our universe, which is a real possibility and it's going to break somewhere you can't get there? Or can you, you push it to try and make some of this structure visible?

• 45:58Neil deGrasse Tyson?

  So I just, I'd love a hundred years from now, look back on this conversation and say, look at those idiots back in 2025. Yeah, this will be a kindergarten video a hundred years from now.

• 46:09Speaker 02

  I hope so.

• 46:12Neil deGrasse Tyson?

  So I got something else here about a duality in string theory. What's going on there?

• 46:17Speaker 02

  Yeah, this is something that I and my collaborators are working on at the moment.

• 46:19Neil deGrasse Tyson?

  That's a cool word, by the way, duality. I love duality. Duality.

• 46:22Speaker 02

  So the idea behind duality is that you could have two different theories or two different geometries as they arise for these compact extra dimensions in string theory that secretly are different sides of the same coin. So an analogy that I give sometimes in talks is if you ever looked at some of these optical illusions photos on the internet where you know you have a picture that's either a vase if you look at it one way or two faces if you look at it the other way, you can say, you know, is it a vase or is it faces? And the answer is it's both, right? It's both packaged. The question in string theory is you have all these different, you know, half a billion configurations for extra dimensions. Do they all lead to different physics? And the answer that we think is no. There are known equivalences of different so-called topological spaces. These are things that have, you know, a different geometric properties like their number of

• 47:07Speaker 02

  holes and their structure. Those different topological spaces can actually lead to the same physics that we would see. So that if there's redundancy in that, that is really powerful because it means you don't have to search through half a billion possibilities. You can, you know, maybe sort of fold those possibilities in half and only look at some portion of them. Some of these dualities have been around for, you know, 20 years in string theory and my collaborators and I think we have new examples which require less supersymmetry. So a less spherical cow than people had assumed in the past.

• 47:40Chuck Nice?

  Okay.

• 47:41Speaker 02

  Yeah, we're growing some legs in the cow, for example. And we think that this may improve our ability to calculate lots of things and also teach us some new properties mathematically about how these spaces can behave.

• 47:52Neil deGrasse Tyson?

  Well, catch us up on supersymmetry.

• 47:54Speaker 02

  So supersymmetry is something that comes along for the ride for some formulations of string theory, which says that all the quarks and leptons that we see in nature may have additional partners. So for example, instead of, you know, a quark, you have a squark, another partner that would be much heavier than the existing particles that we've seen.

• 48:15Neil deGrasse Tyson?

  So when you're describing supersymmetry, it's a symmetry beyond the symmetries that are already known and loved in the standard model?

• 48:23Speaker 02

  That's right. So all these, these sort of three generations of quarks and leptons that we've seen already, there would be another whole set of those particles that would share many of their properties but be heavier in mass and sort of the opposites in that each theory, you know, each particle that was, say, a boson would have a fermionic partner and so on.

• 48:41Neil deGrasse Tyson?

  All right. So what you're saying is? You're not content with just these three regimes we have in the standard model. Just hand us somewhere in the universe other regimes above that and see what properties they might have. That could explain stuff that we don't now understand.

• 48:57Speaker 02

  That's an idea. And people initially thought this idea might explain some really important questions in particle physics, for example, to do with the mass of the Higgs boson. That would be what's called low-scale supersymmetry. And particle experiments like the LHC searched very hard for this and didn't see it. So some people consider supersymmetry not a very useful idea because they thought, you know, it might appear in these regimes and it would not be useful. People are re-investigating this question in string theory. You know, some of the solutions in string theory are supersymmetric, some are not. What we generally would agree on is that if you did have supersymmetry, it would have to be at a very high scale. So it would be much, these particles would be much heavier than you could see at an

• 49:38Speaker 02

  experiment like the LHC, and that the symmetry would be spontaneously broken in universes like ours, so that by the time you got down to, you know, where we live now, you would only see the standard model particles of the energy.

• 49:50Neil deGrasse Tyson?

  And not the other ones that would have helped birth it. So, so...

• 49:57Chuck Nice?

  I'm gonna say that's rather convenient, though.

• 50:02Chuck Nice?

  I'm just gonna say.

• 50:04Speaker 02

  So the question is like, why do you need that symmetry, right? If you're kind of getting rid of it back when, you know, you really want to be talking about the physics? Yeah.

• 50:11Neil deGrasse Tyson?

  Conveniently discarding it.

• 50:13Chuck Nice?

  That's good, yeah.

• 50:14Speaker 02

  That's a great, great question. In some string theories, it still plays a role in terms of regulating the quantum mechanical behavior of the theory and making it well-behaved. So in that sense, you know, you're still using it theoretically for something, even though you don't need it to describe the particle physics that we are observing in nature. But I think all string theorists would ask, it's a really interesting question to say, how much of this supersymmetry can you get rid of and then still preserve the features that are of interest to us? So the types of dualities I was describing, these are perhaps new because they involve less reliance on supersymmetry than we had in the past. And so we're still observing this sort of redundancy or these different descriptions of the same physics packaged different ways. But we don't need as much supersymmetry.

• 50:58Neil deGrasse Tyson?

  If memory serves, the graviton is not in the standard model. Is that correct?

• 51:03Speaker 02

  That's right.

• 51:04Neil deGrasse Tyson?

  So that means no one is thinking about a supersymmetric particle to the graviton. Because that would be kind of interesting.

• 51:12Speaker 02

  Yeah, you certainly could for any particle, any gauge boson, you could have what's called a gaugeino. So a supersymmetric particle.

• 51:20Neil deGrasse Tyson?

  Gaugeino. They're just making sense.

• 51:22Chuck Nice?

  I love it. Yes, it sounds like you're naming like pharmaceutical products.

• 51:27Speaker 02

  This is before my era in physics, but I do feel that some of these names are very much a product of the 1970s.

• 51:32Neil deGrasse Tyson?

  I was in high school, that's how old I am, in the 1970s and I'm just, you know, there was like the particle of the month club. What new particle was being discovered in the new accelerators in California and elsewhere. And we were just building this fabric of the universe out of that.

• 51:47Chuck Nice?

  That's kind of cool, actually.

• 51:48Neil deGrasse Tyson?

  To be witness to it, it was fun.

• 51:49Chuck Nice?

  That's fun.

• 51:50Neil deGrasse Tyson?

  Yeah, not participant, but witness. Could you spend a moment just celebrating the idea of symmetry in physics?

• 51:59Speaker 02

  Yeah, this is a really, really great question. So symmetry in physics. This is something that is extremely deep and has been very, very predictive and powerful over the last hundred years of physics. So this is a question that in my classes to undergraduates, I try and convey that a lot of physics is based on looking at a phenomenon that you see. Like an apple falling from a tree and saying, you know, how do I model the path that that's going to fall, right? Like, how do I write an equation that describes that? But once you start talking about symmetries, and these are basically rules for how you might change a space or an equation in ways that can leave it alone, once you start talking about symmetries, you actually have the power to ask, could the theory that I'm describing be any different?

• 52:43Speaker 02

  It allows you to ask questions about not just what you observe, but whether any theory that you write down could have arisen differently in nature. So there's some kind of highfalutin quote by Einstein that, you know, saying he wanted to probe God's thoughts, you know, in terms of his theories, which sounds extremely grandiose. But the tangible, non-theological underpinning of that is that you can ask for a theory. If I write down this theory, like Einstein's theory of relativity, could it have been different? What is the freedom to change that theory at all? Could there have been any other theory of gravity that could have worked?

• 53:20Neil deGrasse Tyson?

  What does it have to do with our understanding of what the word symmetry means?

• 53:24Speaker 02

  Great. So the idea behind symmetry is that if I tell you about the rules for what I can do for a theory and leave it alone, that is equivalent to specifying the theory. So for example, if you want to ask, how have the laws of physics impacted by the fact that if I do an experiment here, and then I move that experiment five feet to the left and do the same experiment, I should get the same answer, right? What is that, the implication of that for the laws of physics? It turns out that that phenomena, that you know, the laws of physics shouldn't care whether your laboratory is here versus five feet to the left, that's very much linked to something like the conservation of momentum in classical physics. Or the fact that you should do an experiment today and get the same answer tomorrow is related

• 54:08Speaker 02

  to the idea that energy is conserved in classical physics. So all of these, sort of, can I shift something, you know, in some concrete way? There I talked about, you know, moving an experiment or doing the same experiment at different points in time. But in more general, you can say, if I could characterize all the different ways that I can, you know, pick something up, turn it over, look at it, you know, shift its description, and if it stays the same, that actually tells me what equations are compatible with that in a really predictive way.

• 54:35Neil deGrasse Tyson?

  I've got another symmetry here, mirror symmetry. So what do you have for us there?

• 54:39Speaker 02

  So mirror symmetry was originally discovered in the context of these string compactifications. So considering solutions of string theory that could lead to, you know, physics like we see in our universe, the compact extra dimensions that people were trying to write down, they discovered that all the solutions they could find seemed to come in pairs. And these pairs involved interchanging topological numbers. So a topological number is all the ways that you can change a geometry, or sorry, is characterized by all the ways you can change a geometry without intrinsically changing what it is. So the classic example is that you can change a donut into a coffee cup. So you picture, you know, taking yourself a donut, and if you imagine the material was

• 55:24Speaker 02

  all rubbery and you could stretch it and squish it any way you want, but not cut it, how can you deform that shape or change it?

• 55:31Neil deGrasse Tyson?

  Or put another hole in it. And this is a coffee cup with a little finger hole.

• 55:36Speaker 02

  With a handle.

• 55:37Neil deGrasse Tyson?

  Yeah, yeah, it wouldn't work if it didn't have a handle, right? Right.

• 55:39Speaker 02

  So it was just like a drinking glass, doesn't work?

• 55:41Neil deGrasse Tyson?

  Yeah, not the coffee cup from Starbucks, because that doesn't have a handle. Right, okay.

• 55:45Speaker 02

  The idea there is that a geometry with a single hole, you can't change the number of holes, whether that's the center of the donut, or if you could smush it around and turn it into the handle of a coffee cup. That's what's called a topological invariant. But the number of holes is one of these examples of topological numbers. So a donut has one hole. You could imagine, you know, a donut that was built to have two holes, and so on. You know, this kind of thing describes or characterizes geometry. So in mirror symmetry, all these geometries come with topological numbers, but any combinations you could have, it turns out, you could have in more than one configuration. And this, again, sort of divides the space of possible geometries in half, in this case. It tells you that all of them are interchangeable in concrete ways.

• 56:30Chuck Nice?

  Wow. Man, that is insane. Personal. Why? Why?

• 56:40Speaker 02

  It's a really great question.

• 56:42Chuck Nice?

  That's so crazy. I love it, though.

• 56:45Neil deGrasse Tyson?

  But I got more here. We're just cracking this egg. You know, what do I have? Calabi-Yau manifolds?

• 56:53Speaker 02

  So these are examples of configurations for the shapes of extra dimensions in string theory that satisfy Einstein's equations. So these are the half a billion possibilities that I was talking about.

• 57:04Neil deGrasse Tyson?

  Oh, manifolds. Now I need to get the word manifold.

• 57:07Chuck Nice?

  Y'all got that from Star Trek. Get out of here.

• 57:11Speaker 02

  Named after two very clever mathematicians. Yeah, these manifolds were conjectured to exist by a mathematician named Eugene Calabi, and proved by Yao, who won a Fields Medal for his proof that these things solved Einstein's equations.

• 57:26Neil deGrasse Tyson?

  Do you know about the Fields Medal?

• 57:27Chuck Nice?

  No, I don't.

• 57:28Speaker 02

  So it's kind of like the Nobel Prize in Mathematics. It's given to major substantive discoveries in mathematics. But the catch is you got to be under 40 to get it. So you have to be young and clever.

• 57:39Chuck Nice?

  Oh, man, of course, the mathematicians would use chronology as a determining factor. Yeah, discriminating.

• 57:46Neil deGrasse Tyson?

  It's ageist.

• 57:47Chuck Nice?

  Yes, yeah, exactly.

• 57:49Neil deGrasse Tyson?

  Homological.

• 57:51Neil deGrasse Tyson?

  Field mirror symmetry.

• 57:53Speaker 02

  So this is an example of something where there was a dialogue between string theory and mathematics that was really fruitful. So these observations about Calabi-Yau manifolds and their topology were first observed in string theory, and mathematicians went away and tried to sort of explain why that was happening and found a correspondence between a lot of deeper mathematical structures that actually led to another Fields medal for a gentleman named Konsevich.

• 58:14Neil deGrasse Tyson?

  It's Maxim Konsevich. Maxim Konsevich. That sounds Russian. Yeah, exactly.

• 58:19Speaker 02

  So this type of structure, you know, this dialogue between maths and physics, I personally think is really fruitful. We've learned a lot from mathematicians from building these kinds of things. And then the fun question that I'm asking recently is, you know, could there be new variants of this that could lead to new physics, new predictions for particle physics, but also new mathematical structures? So some of the dualities we're looking at right now involve not just changing two manifolds or two configurations of geometry, but actually mixing things like electromagnetic fields in those backgrounds with geometry. So there's all sorts of weird and wonderful, you know, mixing of possible degrees of freedom in the theory that could still magically leave the physics alone.

• 58:57Neil deGrasse Tyson?

  There's surely plenty of physicists out there who are perfectly trained in all their physics but don't have your math background. So in a way, they're kind of researching with blinders on, given how much more you see in the mathematical regimes. Is that a fair characterization here?

• 59:14Speaker 02

  The pushback, somebody might say, you know, you're sort of torturing yourself by trying to solve really hard problems in math and physics at the same time. So there are lots of questions where you don't need this degree of math. But unfortunately, the path to the physical questions we want to answer leads through this crazy hairy geometry in high dimensions to be able to answer the physics we want in string theory. So we kind of don't have a choice.

• 59:36Neil deGrasse Tyson?

  This evokes Einstein, where I don't think he was totally up on non-Euclidean geometry.

• 59:40Speaker 02

  Differential geometry, all that.

• 59:42Neil deGrasse Tyson?

  Different. He wasn't totally up on that and needed some help, right? Even though he had the physics. Oh, man. I would love to meet Einstein's tutor. What do you do? I tutor Einstein in geometry. That's all. Yeah.

• 1:00:01Neil deGrasse Tyson?

  This marriage of frontier physics and emergent math, I mean, this is, it's been going since the very beginning. And I might offer a cosmic perspective on that as we close it out. Alrighty. One of the features of research scientists in academia is to get really smart people working on problems where there's no obligation or expectation that there's a sellable product at the end of that exercise. What it means is that the mind can roam freely on the boundaries of what is known and unknown in the universe. And in physics, that has always occurred. It's always occurred in tandem with advances in mathematics.

• 1:00:45Neil deGrasse Tyson?

  You go back to ancient Greece. They're trying to measure the shape of the earth. Is it round? Is it not? The word geometry gets introduced. And if you look at what that means, it means earth measurement. Geometry. And so you look at this juxtaposition of our advances in science and our advances in mathematics, and these are two fields that so often people in school say, "I'm not good at math and I'm good at physics." Meanwhile, is the foundation of our understanding of our place and existence in this universe. So I look forward to further advances on the frontier of physics and how they marry with further advances in mathematics, no matter how obscure it might look to the passerby.

• 1:01:30Neil deGrasse Tyson?

  One day, in the end, you'll be living with it as we currently are with all the trappings of modern engineering, technology, and society at large. And that's a cosmic perspective. So Laura, I mean, this has been a delightful conversation. We've learned a lot, or you've told us a lot. Yeah. Maybe I learned maybe two-thirds of it. What's your fraction on that? I am just as dumb as I ever have been. But I feel smart. There you go. That's all that matters. And so, again, you're at Virginia Tech and you teach a course on general relativity, Einstein's relativity. I'd love it. So you're a pure theorist, right?

• 1:02:14Neil deGrasse Tyson?

  So you don't, they don't invite you into the particle accelerators, correct?

• 1:02:17Speaker 02

  I have been invited, but I'm kept on a very short leash and then let out again.

• 1:02:22Neil deGrasse Tyson?

  See, I was on a mountain once, observing in the mountains of Chile. I invited a theorist to come, and when he came, there was an earthquake. Wow, so that he's never invited back? Yeah, this is, it was clear. The theorists are just, "Get out of my, get out of my lab." Do you know what you end on the telescope to look through? Oh, no. No, it's a fun riff over time, but we know we need each other. Yes, yes. So, thanks again, Laura, for being on StarTalk. And Chuck, always good to have you, man. Always a pleasure. All right. This has been StarTalk, String Theory Edition. Neil deGrasse Tyson, keep looking up.