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Part 6! 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)
I think it's fair to say and I hope you can agree with me, that our journey in this series, which we are now six episodes deep, and our journey to explore the origins and nature and complexities of string theory with the hopes of eventually judging if it is going to be worth it to continue has been a pretty wild ride so far. And we've covered so many cool angles of how scientific theories are built and imagined and expanded and worked on. We've seen symmetries and the role of symmetries and unification and dualities, like the work of Einstein with general relativity and quantum electrodynamics and and its role in supersymmetry and the connection between fermions and bosons in our universe and how that underlies string theory. We've seen just random ideas come in like, hey, everyone. How about m theory?
Maybe all these different string theories that we have are just like little corners of a much bigger idea. We've seen desires and hopes and dreams like they're just driving passion, this concept. There's a lure that we think there ought to be a quantum theory of gravity because there doesn't necessarily have to be a quantum theory of gravity. We could just be way off. It could be just gravity doesn't allow a quantum description of it, and we're gonna be forever stuck.
But we just hope there is, and so we're gonna work on. So the fact that string theory allows for a quantum description of gravity makes us really interested in it. We've seen a lot of mistakes and blind alleys, like Kaluza's original concept of adding dimensions to unified gravity and electromagnetics. That by itself was a dead end, but it had a useful idea buried in it. Or the whole thing about tachyons in the sixties and seventies, how string theory predicted the existence of particles that could travel faster than the speed of light, which is not a thing, and how we're able to eventually resolve that by including supersymmetry.
And we've seen how string theory has deep roots, like any major theory, like any major idea, has deep roots going back nearly a century, maybe over a century depending on when you'll actually listen to this, and it's been slowly built piece by piece over the course of decades. And it's done all of it almost in the shadows, really. Yeah. There were brief spurts in the mid eighties and the mid nineties of intense interest in string theory where everyone's like, oh, string theory. Okay.
I guess I'll dig into it and write a couple papers. And most theorists, high energy theoretical physicists, you know, dabble here and there in string theory or something related to string theory. But in those same decades, in the sixties and seventies and eighties and nineties, we had this huge growth of quantum field theory leading to an explanation of the strong nuclear force in something we call quantum chromodynamics, leading to what we now call the standard model, which is a single theory that describes three of the forces of nature and the interactions between those forces in the building blocks of matter. And while the standard model is imperfect, we talked about this in the first episode, you have to put in a lot of things by hand. Some of it's a little bit janky.
Some of it's a little bit cranky. Some of it's just a little bit awkward. It's a little bit unsatisfying, to be fair, but at least it's functional. It it it ain't looking pretty, but at least it works, and it could produce things that we can compare against experiment. And tons of people, for all the talk I've given about the history of string theory, I'll repeat something I said in the first episode.
Most physicists and most theoretical physicists simply don't work on string theory. That was true today, and that was true in the seventies, and that was true in all the times in between. Even during the superstring revolutions in the eighties and nineties, most physicists were working on standard model stuff and, I mean, just other things that were more interesting to them. So string theory has really been always a little bit in the background. There's been periods where there's been a little bit more light on it than average, but still, the minority of scientists and physicists and theoretical physicists have worked on string theory, but those little rascals have managed to make a lot of progress.
Even though there haven't been a lot of them, they still got a lot of ideas out there. They've built a pyramid of ideas culminating in what we call m theory. And remember, you get to decide what the m stands for in m theory. It's a hypothetical idea that links together the five string theories as tiny little corners of a much vaster, less well understood concept. It needs 11 dimensions, 10 of space, one of time, but that eleventh dimension only appears at the most extreme energies, and usually, it looks more like 10 dimensions where the five string theories do their things.
It's like a The way that the strings vibrate lead to the variety of forces and matter particles that we see around us, including gravity. And in m theory, I need to say that these are really called brains, not not the zombie kind, but brains as in b r a n e, which is a higher dimensional string, but most of the work seems to be done by the strings themselves, and we really don't understand how those brains work in m theory. String theory requires all these extra dimensions and those extra dimensions are curled up in configurations called Calabi Yau manifolds. There are about a bajillion possible configurations of those curled up dimensions. Each one of those makes the strings vibrate in a unique different way, leading in each of those possible manifolds to a different population of forces and particles, essentially different universes.
And we don't know which one is ours because we don't know exactly how the strings respond to the different arrangements of dimensions. And using what's called perturbation theory, we have an approximation method for string theory, but we don't know if it's right or not because we don't have the full theory. That's the best summary I can give of the current status of string theory. But we have to ask. It's been a long time, folks.
Even though we don't have the full string theory, like, okay. You get a pass because you're not done yet. We have a thing that we can point to and say, we have a collection of ideas and approximations related to string theory. At least we have this family of ideas that we're going to call string theory. Surely.
I get it. You're you're not done yet. You can keep working, but is there anything at all that we can test? Is do at least one of these ideas and concepts that are being explored in string theory that have been bouncing around in the string universe, do any of them make contact with our universe where we can test it in an experiment? Well, here is a complete and exhaustive list of all the direct tests for string theory.
There have been no direct tests of string theory. And see you next week, folks. Don't forget to contribute to Patreon. That's patreon.com/pmsutter to keep this show going. And see you next time for more complete knowledge of time and space.
Right. This this episode isn't quite over. We do not have any direct tests for string theory. I'll say it again. We do not have any direct tests of string theory for two reasons.
One, we're talking about quantum gravity here, folks. This isn't backyard physics. This isn't high energy collider physics. Our most powerful atom smashers are a millionth of a billionth of times weaker than what's needed to directly probe the energy range where quantum gravity effects appear. So technologically, it's just not happening even with something really awesome like a collider looping around Jupiter.
We just can't do it. We can't access the energy scales where quantum gravity matters, where string theory matters. And, b, we don't have a direct test of string theory because we don't have a string theory. We only have approximations of what a string theory might be, and we don't know if those approximations are any good or not. So don't believe any sensationalist news headlines you might read that say string theory finally tested.
We we don't. It just doesn't exist. Period. Still, we have begun to probe some ideas and concepts that are either related to string theory or underlie string theory in some important ways. So we haven't tested string theory itself, but we can test string theory adjacent ideas and that's worth a look.
Let's start with one of the cornerstones of string theory, which is supersymmetry. Remember supersymmetry is there's this weird quantum thing, property of particles called spin. Some particles have spin half or three halves. We call them the fermions. These are what you're made of, electrons and the quarks and the neutrinos.
Other kinds of particles have spin zero, spin one, spin two. These are called the bosons, and these are the force carriers. And supersymmetry is an idea that maybe the fermions and the bosons aren't as different as they seem, that they're linked together through some complicated mathematical framework that makes it turn out that they're related. Supersymmetry is an integral part of string theory. It's what allows it to describe all the things because originally string theory was just designed to work on the strong nuclear force and then from there just the bosons.
Supersymmetry allows it strings to underlie all of reality, but it's also been folded into non string theory work to potentially resolve some problems with the Standard Model, like the hierarchy problem of why gravity is so weak, why the mass of the Higgs boson is so small. So it's just overall an interesting idea. Supersymmetry is very loosey goosey and open ended. It's just it's just an idea. It's like, hey.
What if the fermions and bosons are linked somehow? It's a very broad general idea that that says this connection can happen, but it doesn't necessarily say how. So there's a bunch of supersymmetric theories out there that work out the details. They know it works like this. Know it works like this.
Know it works like this. So don't think of supersymmetry as a theory, but as a family of theories. And the absolute simplest bare bones version of string theory pairs every particle with, and it says here in my notes to give an audible sigh, a sparticle, superpartner particle, with this exact same mass and charge but a different spin. This obviously doesn't work because we see no sparticles in the universe. Like, we don't see the superpartner of an electron, which is called a selectron, with the exact same mass of the electron, the exact same charge, but a spin one.
We just don't see it. This means that supersymmetry only appears at high energies and that the masses of the particles are very, very high, which explains why we don't see them laying around anywhere. We can only see them in giant particle colliders. But if you have a powerful enough collider, you can run it hard enough, you'll see supersymmetry appear, and then all the particles will start popping out. But what does hard enough or powerful enough mean to make supersymmetry happen?
In good question, there's about a thousand answers because each individual supersymmetry theory makes its own predictions, and their own predictions give different families of sparticles with different kinds of masses. And so the hope is that we can run the experiments, and we can either find or not find the particles, and we can decide, we can use experiments to decide which supersymmetry theory is right. There are certain kinds of supersymmetry models, the simplest and most natural ones, that say that if the Higgs boson is in a certain mass range, then some of the particles should be roughly in the same mass range. It's not every supersymmetry model says this, but some of them do and the ones that claim to be the simplest and most natural say this. Hence, the Large Hadron Collider, designed and built to do two things.
One, find the Higgs boson and its predicted mass range and find some particles in that same mass range. I guess it also does other things, but that's not important for our discussion right now. So we spent a bunch of money. We ran the Large Hadron Collider for a really long time, and drum roll, please. We found the Higgs boson.
Hooray. Nobel prizes for everyone. And we didn't find any hints of supersymmetry. We didn't find a single stupid sparticle. Over the course of a few years of running the Large Hadron Collider, the simplest supersymmetry models were simply wiped off the map.
They were ruled out. They were invalidated by experiment. If nature does have supersymmetry, it doesn't have simple kinds of supersymmetry that we can access with Large Hadron Collider. Well, where does this leave supersymmetry after a few years of not finding any evidence for super supersymmetry? Well, the only remaining models are the weirder ones that have high enough mass super particle partners because I'm sorry.
I'm I'm kinda done saying sparticle by now. Where the super partners have high enough mass that they are conveniently out of the detection range of the Large Hadron Collider and probably whatever collider we could feasibly build in our lifetimes. It doesn't necessarily mean that supersymmetry is on the chopping block as a whole, but it does make things extremely uncomfortable when the simplest, most natural models that solve the bunch of other issues is dead in the water because then you have to start adding complications, and the complications start to look as troublesome as the problems you were originally trying to solve with the whole supersymmetry thing. And where does that leave string theory? Well, string theory needs supersymmetry to let it explain all the particles, so I'll just leave that hanging for now.
Okay. We can't reach high enough energies in our colliders to directly probe quantum gravity, and we don't even know what we'd find because we don't have a string theory yet. But we're just puny humans. Right? And and we've invalidated some supersymmetry models, but there are more complicated ones out there.
And if nature is complicated, nature is complicated. We're puny humans. We can't reach the energies, but the universe once had those energies. Right? Way back in the early days of the big bang, it was crazy energetic back then.
It was like the wildest dreams of the theoretical physicists. This is where the universe it just acted like this. If our universe was supersymmetric, it was supersymmetric back then. So maybe by studying the big bang, we can learn about fundamental physics, which means we can learn about something about string theory, maybe. Two ways.
One is through cosmic strings. Remember when I did the episode on cosmic strings, which are these cracks, these flaws in space time that might be roaming around the universe, and I said in that episode that the cosmic strings were unrelated to the superstrings from string theory, well, they can be stretched out versions of superstrings where there's a superstring just hanging out in the early universe minding its own business, and then the universe gets bigger and it somehow catches that that string and stretches it out to be macroscopic and then, like, so large it's intergalactic. And what we think of as a cosmic string might be just a giant intergalactic superstring from string theory that's all vibrating like crazy? Well, it could be. People have worked on this.
So if we found a cosmic string, it wouldn't prove string theory because the cosmic strings can be made other ways, but it would, a, be really sweet, and, b, maybe help the case for string theory because maybe we could study that cosmic string and decide if it's a giant superstring or a crack in space time or something else. But we haven't found any cosmic strings. The other thing is inflation. Remember when I did that series on inflation theory? No.
That's fine. Inflation is our concept of the very early universe where there is something called a scalar field that evolved and really made the expansion of the universe go haywire for a brief moment that completely just just the universe is bigger by a lot in a very short amount of time. Okay. If you don't remember that, just remember what I just said. Now do you remember a few episodes ago when I was talking about extra dimensions and Kaluza's idea, Theodore Kaluza's idea of adding an extra dimension and seeing what happens to general relativity.
You don't remember that either? Fine. I just recapped it for you anyway. I briefly mentioned that adding the extra dimension, like Theodore Galuza did, you get general relativity, you got electromagnetism, and you got an equation for a scalar field. That scalar field for various reasons of string theory is called the diletton because why the heck not?
And wait a minute. Did you think did I just say scalar field in that it's automatically included in string theories? Oh, well, Kaluu's theory is is dead on arrival, but string theory is still kegging, and it contains these scalar fields called dilettantes. Maybe that could be the thing that made inflation happen. And we totally have evidence for inflation, but we also have non string theory explanations for it.
So until we know more about inflation, we don't know anything else about string theory. But, you know, that's today. Twenty years from now, we might know more about inflation, and we might be able by using the universe itself as a giant particle collider, we might learn a little bit about string theory, but not yet. Okay. Okay.
Okay. How about another track? You know those tiny curled up dimensions? We don't know how big they are. We assume that they're the Planck length for working in string theory because Planck length is the quote unquote natural scale for quantum gravity, and that's where we like to play is at the quantum gravity scale.
But they could be larger because why the heck not? The universe can do whatever it wants, and we can't say no. If the extra curled up dimensions are larger, say, a nanometer across or a micrometer or a femtometer across or a picometer across, it could explain why gravity is so weak. Because, for some reason, gravity fills all the dimensions, even the curled up ones. And so it just gets so diluted, and that's what makes it weak, whereas the other forces don't care about the other dimensions for some reason, and so they don't have to be diluted.
But curled up dimensions of that size, say a nanometer across, would really mess up a bunch of subatomic physics, the kind we see in supercolliders. And in our particle colliders, we don't see any messed up subatomic physics. We only see the standard model as we predicted it. It's just good old reliable standard model all the way down as far as we can see. We have no evidence for large extra dimensions.
Large extra dimensions aren't totally ruled out because large means anything bigger than the Planck length and smaller than what we observe, so there's just a tiny bit of wiggle room. But no evidence for it makes it really hard to buy in because the smaller the dimension you have of these curled up dimensions, the smaller you get, the harder it is to get your weak wimpy gravity because there's less to dilute. So large extra dimensions as an explanation for weak gravity is just not looking so hot. Is there anything else? There are a few other little edge cases here and there where some calculations adjacent to string theory make some some somewhat fuzzy predictions, but all of them haven't panned out, or I should say none of them have panned out.
And remember, string theory isn't fully developed, so there's no direct way to test it. Instead, these ideas are sponsored by string theory. Yeah. So welcome to supersymmetry brought to you by the good people at string theory. Like, it's stuff like that.
So even if we were able to test these ideas, it wouldn't it would might bolster a case for string theory, but it doesn't say one thing or the other definitively about string theory itself. These results that I've talked about don't rule out string theory because there isn't anything to rule out because there isn't a string theory yet, but it does make it harder and harder to buy into some of the key ingredients like supersymmetry. And without supersymmetry, it's very hard to get yourself motivated in the morning to get up and start working on string theory. And even if some of these experiments found something interesting like a super partner particle or a cosmic string wiggling around, it doesn't prove string theory. It just bolsters the case.
And there were a lot of high hopes for the Large Hadron Collider to find something, anything new, but it didn't and now we're trying to figure out where to go next. And we're forced to ask a very difficult question: does any idea in string theory make any connection at all to the real world so we can at least see and test that thing, even just one little corner? Can we get a little nibble? Well, let's talk about those extra dimensions. There are a lot of them, like 10 to the hundred thousand, different ways to arrange the small curled up dimensions, each one creating essentially a different universe with different set of particles and forces and fields and strength and interactions and masses and charges and all that, which one of those shapes is ours?
Can we pick which way of arranging the curled up dimensions is ours? This is an awesome way to test. Like, if we could figure out which configurations of curled up dimension string theory prefers remember, it's a theory of everything, so we shouldn't have to put anything in by hand. We shouldn't be telling the string theory which one it prefers. It should tell us.
Then even if just approximately we can make some predictions for what the ideal string theory universe would look like, we can compare it to our universe and see how well we did. That's, you know, a program. Like, okay. Let's just test a bunch of string theories and or find a way for string theory to tell us which curled up dimension arrangement it prefers, and, well, look at that. Well, after decades of work, string theorists couldn't find a way for string theory to pick which arrangement of dimensions it liked best.
So the conclusion is that it likes all of them. Every single one. Equally. We just happen to live in one of them. This is called the landscape.
It's the set of all possible universes allowed by string theory, which is not a tiny number. And yes, Dorothy, this is a version of the multiverse, where our universe with our physics and our parties and forces and laws and varieties of cheese is just one amongst countless many all out there in some sense of time and or space, living their lives with their physics and their particles and their forces and their laws and their varieties of cheese. And there's a lot of universes and each one is realized by string theory. There isn't one arrangement of Calabi Yau manifolds, of compact extra dimensions all curled up on themselves leading to our universe. There's all the arrangements with all the universes, and we live in this one.
But how did we end up in this one? The one with electrons and gravity and a little bit of dark energy and gruy air. How did we end up here? Well, imagine if anything about our universe was different. Say you change the mass on the electron or change the strength of gravity or notch up dark energy just a teensy bit more.
What would happen? Well, the universe as we know it surely wouldn't look like the universe as we know it. Stars wouldn't form. Nuclear fusion wouldn't be efficient. Light wouldn't propagate.
Molecules wouldn't bind together. There are so many catastrophic ways to have a really miserable universe that isn't fun for life at all. So this argument goes, instead of making one single universe, string theory makes all the universes possible, all 10 to the hundred thousand of them, and we just happen to find ourselves in this one with these physics because otherwise we wouldn't be alive to be asking these kinds of questions in the first place. This is called the anthropic principle, that the universe is the way it is because if it wasn't we wouldn't be alive to be asking these questions. As you can imagine, this line of thinking is contentious.
Is it even a theory? Is this even a prediction? Is this really the end result of decades of work in string theory? I'll explore this in more detail in the last episode in this series, but I wanted to plant the seed here so we have some time to come to terms with it because there is no string theory to directly test. We just have approximations of it.
Some of the underpinnings and some of the thoughts that lead us to string theory can be tested, make contact with experiment, but those experiments haven't panned out. Not exactly ruled out, but they're not looking so hot. And so we'd at least have want to have a way for string theory to predict which universe we live in. But now with this landscape concept, which was born about twenty years ago, With this landscape concept, the argument is, oh no, there isn't one universe predicted by string theory. String theory makes all the universes and we live here because we do.
And I want to plant this seed so we have some time to come to terms with it before we chat more. Because, you know, this is a weird place to be in for evaluating, the string theory. We wanna know if it's worth it. And if this is the final answer, we need to decide if this is satisfying or not. So I want you to just keep living your life, doing your thing.
And while you go to work, go to school, go to the grocery store, while you take a shower, I want you to think about the string landscape. And I want you to think if you believe it. I want you to wonder if there's anything, anything that can rescue string theory. Thank you so much to all of my top Patreon contributors. Remember, you can go to patreon.com/pmsutter to keep this show going.
This is my income. This is my job. This is my life, and I really, really appreciate it, especially 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, I don't know why I start singing when I do this, John, Cameron L, Nalia, and Aaron s. Lots of great people. Asked tons of great questions over the course of years that led to this episode, this series of episodes.
John c on email, Zachary h on email, at ed room on Twitter, Matthew y on email, Christopher l on Facebook, Chrisna w on YouTube, Sian P on YouTube, Niha s on Facebook, Zachary h on email, Joyce s on email, Mauricio m on email, at Srinick Schra on Twitter, Panos t on YouTube, Dravar on YouTube, Maria a on email, Terbii on email, Oi Snowy on YouTube, Evan t on Patreon, Dan m on Patreon, unknown on my website. Whoever you are, it can be all of you. John T on Facebook, at TW Blanchard on Twitter, Ori on email, Christopher Ham on email, at Unplugged Wire on Twitter, Giacomo s on Facebook, and Gully Foyle on YouTube. I love questions. I love your curiosity.
Keep throwing your curiosity at me, hashtag ask a spaceman. Follow me. I'm at paulmattsonter on all social channels. Ask a spaceman dot com. Ask aspaceman@gmail.com.
Just give me your questions. And in this series, we are exploring string theory, which wants to be the complete knowledge of time and space, a theory of all of time and space. So I will see you next time for more complete knowledge of time and space.