Part 5! How did String Theory get started? What has made the idea so popular over the decades? Can we ever truly have a theory of quantum gravity? What is supersymmetry, the landscape, and the AdS/CFT Correspondence? What do holograms have to do with this? How many dimensions do we live in? Why does String Theory have such a hard time making predictions? How are we supposed to judge a theory that isn’t done yet? It’s a non-stop String Theory bonanza as I discuss these questions and more in today’s Ask a Spaceman!
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EPISODE TRANSCRIPTION (AUTO-GENERATED)
We've come a pretty long way, haven't we? Our journey started in the nineteen tens. That's over a hundred years ago with the first hints of some techniques, some ideas that would wouldn't lead to string theory directly, but would be folded into string theory like Kaluza and Klein and these extra dimensions and attempts at unification. And we found in the nineteen forties the story of Heisenberg in the scattering matrix theory that went nowhere, but how it was tried again in the nineteen sixties to explain this newfangled strong force that the kids are all talking about. And it kind of failed pretty hard on that count too for several reasons, but hidden in the math of the s matrix theory when applied to the strong force was strings.
Then the concept of strings required small, extra, curled up dimensions or whatever, I guess. That's just the way the universe is. And in the seventies, we invented supersymmetry to be able to incorporate fermions, which are the building blocks of matter, into this theory of stringy goodness. And by the mid nineteen seventies, everything was pretty groovy, man. String theory could potentially explain the origin of matter and the forces of nature.
You know, maybe, hard to tell since it's still using perturbation methods. We didn't have a full string theory that we could point to. We just had a bunch of approximations that we hoped were somewhat close to the actual theory, and we couldn't really make predictions with this theory because it was too complicated, like, even with the approximations, And there's the whole business about the extra dimensions and how they're curled up and how they affect the physics of the strings, and we can really understand that connection. So, you know, we had some cool ideas in the seventies, didn't everybody? And I presented this as a somewhat linear story, but, of course, it wasn't.
It was full of the usual twists and turns and blind alleys of a typical of any research program. So it shouldn't be a surprise that 1974 came as a big surprise. And, man, 1974 was a pretty incredible year. Stephen King published his first book. The formal impeachment proceedings against Richard Nixon began.
The Cleveland Indians promoted a 10¢ beer night. The game ended prematurely when mayhem spread into the field, and string theory was found to also potentially be a theory of quantum gravity. That's right. Nineteen seventy four. Work in string theory nowadays is often motivated by the thought that might be the holy grail of physics, a theory of everything, a theory that's able to finally unify quantum mechanics and gravity.
But remember that it spent years in development with no such thought in mind. And it was found to be a potential theory of quantum gravity almost by accident. So how do we know string theory might be used to explain gravity? It's a good question. You're very inquisitive and I'm glad you asked.
Our current explanation for gravity is general relativity, and general relativity is classical. It doesn't know or understand or care about you. I mean, quantum physics, general relativity just doesn't know anything about quantum this or quantum that or wave particle blah, blah, blah, blah. It's all nonsense. Gravity is just like, no, I'm gonna be over here being gravity.
Gravity is carried, for lack of a better term. Maybe maybe mediated is a better word by the curvature of space time itself. You get some matter, you get some energy, you toss them around, they warp and flex space time around them. This warping and flexing of space time tells the matter how to move. Boom, gravity.
There's a little bit of math involved, and there's no quantum description of gravity. The other forces of nature do have quantum descriptions. And in those quantum descriptions, the forces are carried for lack of a better term. Maybe mediated is better by the bosons. Not the brozos, those are clowns, by the bosons.
The family of particles with integer spin. So if you have spin zero, spin one, spin two, etcetera, you are a boson. Each force gets a set of bosons. The electromagnetic force gets the photon, the original boson. Weak nuclear force gets three bosons, the z and the w plus and w minus.
And the strong nuclear force gets eight bosons. They're called the gluons. And as you might imagine, these bosons have various properties. They have different masses, like the photon and gluons are massless. They have different charges and on and on and on.
There's a whole family of bosoz bosons. Totally meant to say bosons. But they all have the same spin. They all have spin one. They don't have spin zero, spin two, spin three.
So the original string theory, the nineteen sixties string theory had a very simple job. It was only explaining the known bosons. It was only trying to explain originally strong nuclear and then slowly extended to try to include weak nuclear and electromagnetism. And all those bosons had the exact same spin. And we can ask if we had a quantum description of gravity in string theory or anything else if we had that, what would its force carrier look like?
What would its boson look like? How would it act? How would it talk? What would it smell like? That's a decent question.
First of all, if we're gonna have a boson to explain gravity, it needs a cool name because without a cool name, we can't get any funding. What should we call it? What should we call it? Albertan? No.
Einsteinian. Gluon has taken gravity, graviton. Yeah. I like it. Graviton.
Graviton will be the name of the force carrier in quantum mechanics of gravity. Yes. What would a graviton look like? Well, gravity has infinite range and travels at the speed of light. And back in the funky seventies, this was just assumed, but since then we've tested it.
And so it has to be like light. It has to be massless. Can't have any electric charge because gravity doesn't care about electric charge. Okay? And gravity is the special kind of theory.
It's something we call a tensor theory. I don't wanna get too much into the weeds of this, but you know how everything is a field, like, just everything in the universe is a field where it's just the number associated with every point in space time, that's called a scalar field. If there's little arrows, both a number and a direction, associated with every point in space time, that's called a vector field. The electromagnetic field is a vector field. And if there is, a two dimensional arrow, yeah, it's a thing.
We call them tensors. If that's associated with every point in space time, then you have a tensor field and gravity is a tensor field. Don't worry if all of this is just too much to handle right now because it's way too early in the morning. It's fine. Don't worry too much about it.
For various mathematical reasons, and I hope you can see me waving my hands as I skip over this part, this means that the graviton needs to have spin too. It has twice as much spin as a photon. Okay. That's what a quantum carrier of gravity should look like. But why should we believe this until we have, you know, an actual theory of quantum gravity?
Because every time we try to make a quantum description of gravity, the whole thing blows up. So why should this be a guidepost? The answer is because if we had an actual living breathing graviton right there in the quantum mechanics, it would behave as gravity. So if we are able to find and tame a graviton, we would by default have a quantum theory of gravity because we are defining gravitons as the quantum thing that does what gravity does. So that's why even though we don't have a quantum theory of gravity, we're gonna make a quantum theory of gravity and it's gonna start with a graviton.
If this doesn't feel all that convincing or good to you, that's fine. It's just the plan we have, so we're gonna move on. The big deal is that in 1974, we found out that string theory automatically includes, in a very generic way that's hard to get around, spin two massless bosons. It automatically includes gravitons. Gravitons are just there inside of string theory, as in it's baked into the math.
The very way that strings are just a feature of the mathematics, hence the name string theory, some strings just happen to vibrate in just the right way as to look like a spin to massless boson. Some strings just look like gravity, and it seems to be pretty generic Then no matter what kind of approximation method you technique or approach you take, it's just right there, the same way that strings are right there. The way strings have to vibrate no matter appears this appears. We're not exactly sure yet, so I have to add that caveat. It appears that no matter the shape of the curled up extra dimensions, no matter the number of extra dimensions, or what strings look like, whether they're open or close it doesn't matter.
They seem to always have gravitons or at least act like gravitons. In the words of Edward Witten, who is a pretty famous string theorist who we'll meet again later, he said something along the lines of string theory predicted gravity. Would have been way cooler if string theorists had come around about four hundred years ago, but, hey, a win is a win, and it's just hard to downplay just how interesting this is. String theory appears to naturally include gravity and potentially include the other forces and all the fermions just naturally. This is what makes string theory so fascinating.
There's no other theory of physics out there where gravity is just there hanging out. We always start like like Newton's theory is just like, okay. I'm gonna assume that gravity exists and I'm gonna try to describe it. General relativity is I'm gonna assume that gravity exists. I'm gonna try to describe it.
String theory is I'm gonna work on the strong force of the hey. Wait a minute. There's gravity over here. It's interesting. Even without a full solution, even without full understanding, you can see why this has captured the imagination of everyone.
Honestly, this is my own opinion. With all the other challenges facing string theory, not knowing if perturbation theory is appropriate, not knowing the shape of the extra dimensions, etcetera, etcetera, I think that if we hadn't found gravitons, then string theory would have died on the vine in the seventies. I think it's the existence of the gravitons inside string theory that has made it last for these, like, nearly fifty years. And if they weren't there, people have been like, oh, this is an interesting theory, but it's too hard. We can't make it work, so we're just gonna move on.
But it does contain gravitons, and people got interested. And you know what? Between the mid nineteen seventies and mid nineteen nineties, basically nothing happened. Well, okay. I shouldn't say nothing happened for twenty years.
In fact, the mid nineteen eighties is often called the first superstring revolution. Superstring, remember, is the proper name for the theory because it's string theory, which was developed in the sixties, plus supersymmetry, so superstring theory. But I have a really hard time saying that, and I really can't explain why. So I'm just gonna go with string theory. In the mid nineteen eighties, there was just a huge surge of interest and tons of papers.
It was triggered by a few things. They figured out a way to remove those nasty tachyons, those particles that could go faster than the speed of light. They found this whole graviton thing, and they started to take it seriously. Supersymmetry was more fully incorporated. Like like, the ideas that were planted in the seventies took a few years to germinate.
And some high profile theorists like Edward Witten would just not shut up about it. And if you're a young student and Edward Witten is talking about string theory, you're thinking, oh my gosh. If Witten is interested, then it must be something. I'm gonna work on it too. So tons of research in the mid eighties.
But after the huge surge of excitement and interest, and we're talking hundreds of papers and conferences and updates and proceedings and symposium, cloaking, emails and even letters, a just serious investment of human time and energy, and lots of cool things happen. Lots of the guts of string theory were fleshed out and expanded and argued about and brought up for consideration and swatted down for being dominant. It was just lots of activity. But after all that effort, nothing happened. There was movement in activity, but there were no results that we could point to in the real world.
There was no theory of gravity yet. There was no string theory yet. It was expanding and growing bigger because the more people working on it, the more interesting mathematical questions you can pose and answer, the more corners to uncover. Nothing happened. But, I mean, this is true of anything.
Even with so called complete theories, this is an ongoing process. You know, compare what we know about general relativity in 1917 when Einstein published a single paper on it to what we know about it now with, like, black holes and cosmology and gravitational waves and neutron star mergers. Like, there's a lot that we've learned even with a complete theory, and it's taken us a hundred years to learn it. And since string theory is so huge, it's a potential theory of everything. Of all the things, there's a lot to expand into.
There's a big domain. Like if your domain is all the physics, then there's a lot of corners to uncover and flush out and try to understand. And yet after all that flurry, after all that interest, after all those papers, there wasn't anything to connect to experiment. There wasn't any concrete predictions. There weren't any new insights into the universe.
There were a lot of insights into string theory, but nothing that said anything about the universe that we actually lived in. So we were learning about the theory, but we weren't learning about our universe and string theory still wasn't in full physical theory. It wasn't solved. People were still working in this perturbation mode, approximation mode. They didn't have the complete thing.
There was nothing to print on a t shirt and say, yep, that string theory, which you can do for electromagnetism, which you can do for general relativity, which you can do for quantum electrodynamics, which you can do for quantum chromodynamics, which is how we understand the strong force, but you can't do it with string theory. So after the mid eighties, after all that interest, the seeds were planted in the seventies. It grew, it grew, it grew. And finally there was a critical threshold in the mid eighties. Lots of people were working on string theory and they just lost steam.
People would work on it for a few years, get lost and confused and sad, and then move on to other fields of physics so they weren't so lost, confused, and sad anymore. And what's worse, you know, I even use hesitate to use the term string theory because we don't have a proper string theory, a single set of equations that we can print on a t shirt and say this is what we're using to explain the universe. We just have sets upon sets upon sets of approximations that we hope get close to the real thing, but we don't know what the real thing is. So I even hesitate to use the phrase string theory, but that's just a matter of linguistics and jargon. But if instead we call the set of approximations that we have as string theory, in the mid nineteen eighties, we didn't have one.
We had five string theories. And you know what? We still have five string theories. What how does five I don't it's how could you possibly have five theories that are all different and all claiming to be string theory? This is Highlander, folks.
There can be only one. We don't have five descriptions of general relativity that all look different. We don't have five descriptions of the weak nuclear force that all look different, but all claim to be the quantum theory of the weak nuclear force. What's going on? String theory had an embarrassment of riches before.
Before in the sixties, they had so many strings that could produce all the particles and more, but we turned that into a win by finding the graviton, finding that this thing really is that powerful. And here we are again with an embarrassment of riches. We had five theories for the price of one. Can you blame people for getting a bit turned off? The main problem, the reason that we have five string theories is that is supersymmetry.
Supersymmetry unlocks string theory to encompass both the bosons and the fermions, both the force carriers and the building blocks of matter. But supersymmetry itself is kind of vague about how you go about the procedure. Remember, it's just a family of theories. It's a family of ideas that say, hey, every fermion should be paired with some sort of boson. There are different ways to achieve supersymmetry.
And since it's a hypothetical idea, there's nothing in an experiment or collider, especially back in the eighties and nineties, you can point to and say that's the supersymmetry you want. That's the super way that nature does this whole supersymmetry thing. That's the one she picked. So you just kind of try all of them and hope it works out for the best. So supersymmetry is in kind of a similar boat.
Like we don't know which supersymmetry idea is right, if any of them are. And since supersymmetry is a cornerstone and a building block of string theory, there are different string theories. There are different ways of approaching this whole theory of strings. The five string theories do share some commonalities. All involve strings.
No points awarded for that. All of them have 10 dimensions. All of them contribute to Patreon. Go to patreon.com/pmstar where you can join the five string theories in their contributions to support this show. There's a lot of ugly details in the differences between these five string theories.
I won't get into all of them because this series is all about ugly details. Short version, some of the theories only have closed strings. Some allow open strings. Some allow the vibrations in the string to travel in one direction. Some allowed to travel in the other direction, and then there's a various combinations like, okay, we're gonna have only closed strings, but the vibrations can travel in one direction, or we're going to have closed strings, the vibrations can go in all directions, and we can have open strings because we feel like so there's various combinations of these ideas.
For reference, here are the names of the five string theories, and I challenge you to name your five children after these. Type one, type two a, type two b, SO 32 heterotic, and e eight times e eight heterotic. I will also accept those as pet names. Let's pause and take stock. What do we got?
Strings are tiny. They wiggle. Their wiggles make them behave in certain ways, becoming the fermions and bosons of our world, the building blocks and the force carriers of our world. There are extra dimensions. They're all curled up, and how the dimensions are curled up determine how the strings wiggle.
Supersymmetry is needed to expand string theory to explain both bosons and fermions. Without it, it would just be the bosons. It would just be the which would be cool, but not theory of everything cool. String theory automatically includes the graviton, which is why everyone's so nuts about it. We don't have a solution for string theory, And there are five different approximations of string theory, all claiming to be the approximation to string theory.
Like, come down this road, and you'll make it all the way. Everyone else is liars. So now what? In the mid nine in the 1990s, people just gave up. It was too weird, too hard, too confusing.
And the methods of quantum field theory for describing what we now call the standard model were so fruitful, it was hard to pass up even though there's all this initial excitement in string theory in the eighties. But at least the standard model, which was a little bit more awkward and a little ungainly, at least it was producing results. At least it was getting the job done. You may not like how it does it, but it still gave you an answer at the end of the day and so you can run experiments and you can make a name for yourself. And so a lot of people just drifted away from string theory because it just wasn't it wasn't producing.
And then 1995 happened. And more specifically, Edward Witten in 1995 happened. At a conference, he got up, gave a talk, and he said, look. Hey, guys. Listen.
I got this crazy idea. And I'm super smart, Edward Winton, so listen to me for a second. I know we've got five string theories, and it's lame. Right? Well, what if just for fun, hear me out, we assume that they're really the same theory, but just written in different ways.
Wouldn't that be cool? And everyone was like, well, if Edward Witten says so, I guess it must be true. I'm not trying to be uncharitable here, but I can't help but poke a little bit fun at the history of string theory. Because at the end of the day, Edward Witten's idea might just be right, and that's a big deal. We've talked a lot about symmetry.
Here's how he might be right and what he was thinking. We've talked a lot about symmetry. Symmetry this, symmetry that, supersymmetry, regular symmetry, beautiful powerful results, blah blah blah. Here's another cool word, duality. And it's kind of a synonym for asymmetry.
Sometimes physicists use the word symmetry and sometimes they use the word duality. Supersymmetry can be viewed as a duality between fermions and bosons, for example. Dualities like symmetries have to do with the math, that if you change something in the math, at first glance, you have something completely different, but, really, you're describing the same fundamental thing in a different way, like electromagnetism. Electricity and magnetism, there's a symmetry there. They are duals of each other.
You can write everything from an electric point of view, or you can write everything from a magnetic point of view and you can flip between them, whichever is easiest in the moment. The math looks different. The way we describe magnetic fields is very different than the way we describe electric fields, but they end up giving you the same answer in the end because they're duals. And Edward Winn and others found dualities in the string theories. They found some symmetries in the string theories.
Remember the question of whether perturbation theory was valid? You know, we only have approximations of string theory, and we don't really know if these approximations strings actually interact in real life. And string theory isn't telling us how strongly the strings interact, so we don't know. If the strings only talk to each other weakly, like, then our approximations are just fine and we're close to a real string theory. But if they interact strongly, like, like, hey, then we're way off base.
Our approximations aren't even close, and we just don't know how strongly the strings interact. It turns out that there's a duality between some of the five string theories. A strong, like a hey version of one theory turns out to be the same as a weak, like an version of another theory and vice versa. They're duels. You can flip between them.
And there's another duality that has to do with strings wrapping themselves around one of the itty bitty curled up dimensions. It turns out that wrapping around a dimension in a certain direction in one theory is the same as running free in another direction. In another theory there's dualities there. So even though these five string theories look completely, totally radically different, you can flip between them with these dualities. So if you're like working in a weak version of one theory or a wound up version of one theory, you can just translate that to a strong version of another theory or a not wound up version of another theory.
It looks like you're looking at the same thing through different windows. You can build duality links between all five theories showing how they're all describing the same underlying thing just in different ways. It's like speaking different languages but expressing the same ideas. What Edward Whitten gave everyone in the mid-1990s was a translation service. So you've got these five languages, and each one is claiming to be the language of string theory.
Well, look. Look. I can translate between them. What we're really interested in is the ideas behind the language. But if we have five languages that are all trying to express the same underlying idea, what is the underlying idea?
Whitten called it m theory, and it stuck. Nobody knows what m stands for. And the joke is that if we figure out the theory, then we actually get to name it. My vote is Manchengo theory. We can find hints and pieces of this m theory by looking at all the possible dualities, trying both the strength of interaction duality and the winding around dimensions duality on each of the theories.
Sometimes when you try duality, you connect it to another of the string theories. And sometimes when you try duality, you go somewhere new, somewhere and explore something that's not represented by any of the five theories. It's hypothesized that this unexplored place is the m theory or at least a part of it. The old view in the early nineties is that we thought we had five different planets, each planet claiming to be the string theory, but we didn't hadn't mapped out the full planet. We just had little islands here and there on each planet that we were working on.
But within theory, it turns out that all the islands are on the same planet, It's just the planet is much larger than we thought before. What is M theory? Well, nobody knows. It's kind of a common theme in string theory, but we think we, it has 11 dimensions and not 10. The five theories that we call string theory are just approximation of the real, and this time we mean real theory.
So instead of one of the five being right and being a close approximation of the real string theory, all five are right in their own ways and are just approximations of the actual M theory. The 11 dimensions are important here. In the places where these five string theories live, the eleventh dimension just goes away. It's a it's an in the approximations that they're using, there's just no need for that eleventh dimension. But at full power, at full glory of of string theory, whatever that means, that eleventh dimension appears.
Maybe it's at super high energies. Maybe it's in really strange combinations. But, like, really, there's that eleventh dimension, but most of the time, we don't have to care about it. The icing on the cake with m theory here is that because of that extra dimension, strings aren't just strings anymore. There can be two dimensional sheets.
There can be three d blobs. There can be six dimensional, hyper blobs. These things do all the things that string do. They wiggle. They wave.
They vibrate. They cause the particles and forces that we're familiar with, but just in more dimensions. And in general, they're called d brains, not brain as in, you know, what's inside your skull, but brain as in b r a n e, and the d is for dimension. And that's m theory, which is believed to be the real string theory. That was the mid nineteen nineteen nineties, and that's where we are today.
Over twenty years since Ed Witten's launch of what's called the second superstring revolution, or maybe it should be called the first brain revolution. But anyway because after that announcement in the mid nineteen nineties, everyone's like, oh my gosh. He's so smart. Duh. All these five string theories are just aspects of the same overarching theory.
So now we're really on to something. We should get it any day now. A few years later, Brian Greene, not Brain Green, wrote the elegant universe that I read, and I'm sure a bunch of you have read. And string theory captured the public's imagination, including mine, in a way that it never had before because it seemed like we were on the cusp that string these five string theories that were really, really confusing had a bunch of really powerful properties, like including gravity automatically by including both fermions and bosons automatically by having, like, this very consistent coherent framework. But there was this niggling problem that there were five different theories all competing with each other.
Oh, but it turns out they might actually all be the same, just little tiny corners of a much larger, much more complex theory, m theory. And then twenty years happened. And today, m theory is still m theory. 11 dimensions, d branes, which are just higher dimensional strings. It contains gravity, contains all the forces, contains all the building block particles.
It's quantum, and we don't have an M theory. It doesn't exist. It's an idea. We think because this isn't proven. We think that m theory is the fundamental theory that all the individual string theories are probing at the edges, and it has one extra dimension that the string theories don't.
But with this extra dimension, the strings can wiggle in as many dimensions as they want, and that makes it even more complicated. Like, you thought, you thought string theory was complicated with all the compact dimensions and all the different ways the strings can wiggle and how these could give rise to the properties of the world that we see it. You thought that was coming? Well, how about another dimension? And how about making the strings multidimensional?
Now have fun. You thought it was complicated. You had no idea. And so, yeah, there hasn't been a lot of progress. I mean, there's been progress.
There's been interest. There's been exploration, but there still hasn't been anything that's, you know, connected to experiment, at least not directly. There are ways of probing a string theory, of trying to get a handle on string theory through experiment, not directly because we don't know what string theory actually is. We just have our approximations of what we think it is. But now we're at the point where we actually have our modern day string theory in our discussion.
Like, m theory is it, is the string theory or at least what we think the string theory is supposed to be. And that was proposed twenty five years ago, almost twenty five years ago. And so, yeah, let's and, yeah, let's see if there's a way to test it a little, and that's where we'll go next week. Thank you so much for listening. Please don't forget.
Please go to patreon.com/pmsutter and keep these episodes going. We've been on this journey for such a long time. It's been such a fun journey. Thank you so much for going on this string theory journey with me. I hope you over the course of these episodes, you get a better understanding of the guts of string theory and the history of string theory and why it's so dang hard.
And where we're gonna go next is even weirder. Trust me. It's gonna be fun. Thank you to my top Patreon contributors this month, Matthew k, Justin z, Justin g, Kevin o, Duncan m, Corey d, Barbara k, Nuder Dude, Chris c, Robert m, Nate h, Andrew f, Chris l, John, Cameron l, Nalia, and Aaron s. It's them and all of you that are keeping this show going.
I really appreciate it. Also, don't forget to go to iTunes and drop a review, tell some friends, shout outs on social media, all of that really helps. And, of course, thanks for all the amazing questions everyone sends me, especially the ones that led to this amazing series, this huge adventure that we were on. John c on email, Zachary h on email, at edit room on Twitter, Matthew y on email, Christopher l on Facebook, Krisnet w on YouTube, Sion p on YouTube, Neha s on Facebook, Zachary h on email, Joyce s on email, Mauricio m on email, at Shrennik Shaw on Twitter, Panos t on YouTube, Dhruv r on YouTube, Maria a on email, Tara b on email, Oi Snowy on YouTube, Evan t on Patreon. There's so many people.
Dan m on Patreon. I love it. Unknown on the website, John t on Facebook, at t w blanchard on Twitter, Ori on email, Christopher m on email, at unplugged wire on Twitter, Giacomo s on Facebook, and Gully Foil on YouTube. Hit me up with more questions, hashtag ask a spaceman, ask a spaceman dot com, or askaspaceman@gmail.com. And I will see you next week for more complete knowledge of time and space.