Part 8! 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)
Perhaps the worst insult you can possibly give a physicist when they present their idea is attributed to none other than Wolfgang Pauli. He's one of the greats when it comes to quantum mechanics, and I'm sure we'll talk about him more someday. We met him in the last episode when his response to conformal field theories was to say, shut up. And there are a few stories of when listening to a colleague or reading a paper, and I can totally see this. You present your ID.
You say it to him. In, you know, Wolfgang Volley is he's poly. You want his opinion. You want his approval. So you're like, hey.
You you shoot some ID at him, and he simply sighs, shakes his head sadly and says, this is not even wrong. The point of science is to generate theories that are testable. Not even wrong means that the idea is so far beyond reasonable science that it can't even be tested. It's nonsense. It's nonverifiable.
A testable but wrong theory is preferable to this. At least if your theory is wrong, at least you're still doing science, and that's one possible benchmark to use to evaluate string theory. Is it even good enough to be wrong? As you can imagine, this is a somewhat contentious subject, and so we've spent the past few months exploring string theory. I mean, give yourself a pat on the back for slogging through s matrix and conformal field theories and supersymmetries and dualities.
Like, it's been this, like, sixty year journey in our understanding of what we now call string theory, and we've dug in deeper than any other topic we've dug into on this podcast, at least in one continuous series. I think I've done more than eight episodes on black holes. Does last episode count as an episode on black holes? Maybe. Anyway, you've done it.
Like, you're here. Like, what we've explored in the past seven episodes is the growth and evolution of our modern understanding of string theory. That's where we are as of today, whenever today is, probably even if you're listening to this thirty years from now. And we've explored in string theory its colorful history, its development, its features, its failures, its almost theirs, and its cheerful, dogged determination to actually get something done. We've seen it grow from some cool math trick to tackle the strong nuclear force into a model for the fundamental constituents of all matter and all forces.
We've seen quantum gravity naturally hidden inside of it. We've seen how extra dimensions aren't so weird after all. We've seen how supersymmetries and dualities can link ideas together into a consistent whole. We've seen the rise of the landscape and the multiverse of possibilities where all of your dreams can come true. And we've seen a new beginning with Kaluza and his extensions of general relativity.
It's been a journey. We're tired. We're hungry. Our brains are fried. But hopefully, we understand string theory a bit better than when we started, and we know where it's positioned now today.
Like, what does string theory mean today? We've seen how much work and time and money and brainpower has been poured into string theory over the course of decades, and we're wondering if we should continue to invest. Have we gotten a decent return for our money? Do we think some big breakthrough is right around the corner, or is string theory not even wrong? String theory has been saying the check's in the mail since the nineteen eighties, so they partly have only themselves to blame for our continued disappointment.
Like, it's it's just year after year of string theorists writing papers, giving talks, and saying, yeah. We're almost there. We're almost there. We're almost there. And when popular science books come out and take hold of the public imagination like Elegant Universe did for me and many other people in 1999, well, then you better make sure you can cash those checks that you're writing.
If you're going to write a book and say, You know what? We haven't solved everything in string theory, but it's coming soon. When is soon going to happen? In our evaluation, our final judgment, our Armageddon of string theory, we should take a look at the current state of play. And I'm going to say something that I think is going to be quite shocking to you because when I realized it, I was absolutely dumbfounded.
Mostly because I don't pay super close attention to string theory papers and I don't get invited to string theory parties, so I am a little bit out of the loo, but it still came as a shock. It seems, based on everything I've read and the articles that are coming out and what the topics of conference presentations are, that by and large the majority of string theorists have abandoned the quest for a theory of everything. That's right. The raison d'etre of string theory, the motivation for the whole shebang since 1974 and the top of the list for reasons why we should continue investing, that string theory is a candidate theory for unlocking quantum gravity and uniting the forces of nature, is largely ignored by the majority of scientists and mathematicians who call themselves string theorists. Most talks, most papers, most researchers don't even bother question to a reason why most people have abandoned the quest for the holy grail.
It's because it's too dang hard. They recognize we see it from the outside. Like, man, why haven't string theorists, like, figured out string theory yet? And the string theorists are wondering the exact same thing, and so they stop trying. That's not a judgment call.
I could be wrong about this. I think this is just an observation. Over the past ten to fifteen years, most string theorists have given up trying to figure out string theory and make it a theory of everything. The mathematical techniques used in string theory is something called perturbation theory, where you build ever better but ever harder to solve approximations. But we don't know if those approximations hold because we don't have the real deal.
To be fair, this isn't unique to string theory. Our understanding of the electroweak force, the merger of the electromagnetic and weak nuclear force is in a similar tough spot. So it's not alone in that. So we don't have the real string theory. We just have approximations to string theory over what we think might be approximations to string theory.
And even those approximations, the mathematics is deeply, viciously horrendous. You've got all these extra dimensions. They're curled up in weird geometries. They affect the strings. It's it's hard to understand how the strings even behave and how that gives rise to things that we call particles and forces and fields.
String theories, ideas are elegant and fun to say. It's an elegant universe after all, but the actual math and structure is a nonstop express train to Migraine Town. And the whole thing with the Calabayao manifolds and how the extra dimensions are curled up in on themselves that determine the physics, just the sheer variety of possible solutions is literally overwhelming. Like 10 to a hundred thousand different possible universes allowed by string theory and we only get one of them. The situation got so bad that string theorists just gave up trying to solve it.
Let that sink in. They stopped trying. They stopped trying to pick which universe is ours of all these possible Calabrio manifold and instead appeal to the landscape. This idea that all bajillion possible universes exist and we just happen to live in one of them, and you need the anthropic principle to pick ours, the notion that we're here with this set of physics because if physics were different, we wouldn't be here. Now you can form your own opinion here about the anthropic principle, but this is my podcast, and so I get to sell you mine.
The anthropic principle makes me angry and a little bit sad. Or maybe it's sad and a little angry. I'm not exactly sure if the ratio is here, but there's definitely a mix of anger and sadness. When I first encountered the landscape idea years ago, my first reaction was, you've got to be kidding me. This is it?
This is what we're calling science? This is the big achievement? The main result? We've been working on string theory for decades and this is it? That you've determined after years and decades of of blood and sweat and toil and tears is that that the universe is the way it is because it's the way it is?
And if it wasn't, we wouldn't be here? The anthropic principle is fine, I guess, as a philosophical argument, but physics isn't philosophy. Well, physics is a branch of philosophy, but we're supposed to do things differently than, say, normal philosophy. We're supposed to have evidence. We're supposed to use observations to guide our thinking.
We're supposed to be empirical. That's the whole game of science. And yes, there are a lot of philosophies that underlie science and guide our thinking. Should the anthropic principle be one of those things that guides our thinking in trying to understand how the universe works? How in the world do you even begin to test it?
How do you how do you verify that the anthropic principle is a good way to think about the universe? Because we only have the one universe, and it's the only universe we're only ever gonna have. It's the one we live in whether we like it or not and whether this kind of universe is natural or not or unique or not. This is it. So how can you make statements?
Of course, we live in the universe. How can you possibly move past that as some sort of guiding principle? Basically, of all the possible universes, you can either attempt to argue that kinds of universes that look a lot like ours are rare and we're just lucky to be here, but really we had no other choice because it's the only universe that looks like ours that could give rise to us to be able to ask these questions. And you just say, you know what? It's just rare.
We're one of a kind. We're the only universe in the landscape that gives rise to intelligent life and so we just happen to get lucky. Because that was our only choice. Because you're not gonna be born in another universe with a different Galabio, Manivold. That one's lame.
Lifeless. This one's all fun. This is where the party is. But it's the only one it's the only one we're ever gonna have. Or you can try to find ways to argue that universes like ours are common.
Like, oh, oh, oh, there it's actually this value of the electric charge appears in, say, 80% of all Calabrio manifolds. And so, we naturally expect life bearing universes to fill up most of the landscape. This was the approach taken early on after the development of the landscape idea, saying, like, okay. Yeah. Yeah.
No. There's a landscape. We can't pick which one. But maybe just like our set of physical parameters, our electron charge, our speed of light, our value of dark energy, just happen to be common, and so we wouldn't shouldn't be surprised that we live in our universe. This line of thinking didn't really last long because it's really, really, really hard, I guess, to make universes that look like ours.
Our universe does look relatively rare in the landscape. But we know so little about the development of life and intelligence and all of that that we can't hope to ever make probabilities. Like the common argument is, oh, if you change something about the physics, we're going to make gravity stronger. And then you can run through that thought experiment and say, oh, if gravity's stronger, you know, on that particular Calabrio manifold where gravity's stronger, it's just, you know, then, you know, stars are gonna burn out too quickly, and so you can't get planets and you can't get life forms appearing. I mean, I guess you can say that.
You can say that, but you don't know if you're right because we basically don't know anything about what it takes to make intelligence. We're the only example that we know of in this universe, let alone another one. Sure. You can mess with physics, and you won't get intelligent life on a relatively wet surface on a relatively rocky planet around a relatively fuse and burning star. But how are there any other paths to intelligence?
Is this the only one? And so we're gonna decide if universes can give rise to intelligence or not based on this one example in this one galaxy in this one universe? Really? That's what we're gonna go on? We're gonna discount a different universe, a different Calabrio manifold?
I'm sure if the landscape does exist, I'm sure we can point to some Calabrio manifold, some arrangement of physics, and say, oh, there's no way life could appear on that. Well, right now, there's some creature with some form of intelligence that arose in that particular universe doing these exact same calculations and looking at our Calabrio manifold saying, oh my gosh. Could you imagine that universe and how devoid of intelligence it would be? Maybe they're right. We don't know anything about what it takes to develop intelligence.
So how can even if this anthropic principle was a solid guidepost to help us navigate the landscape, how can we trust it if we don't know what it takes to make intelligence? All we know is about us. We're just guessing. And just random spitballing, is that how we're going to construct scientific theories It make decisions about the validity of scientific theories? Because you can say anthropic principle all day long.
That's fine. That doesn't make it a scientific theory. That doesn't make it testable. That doesn't make it useful. And this whole thing about the anthropic principle and the landscape, it assumes some knowledge of how the various parameters of various physical constants are spread throughout the landscape.
Remember, we don't know how a particular location on the landscape, which means a particular arrangement of a Calabrio manifold, gives rise to the physics in that universe because we don't have a string theory to tell us. So we can't say just how common or probable, is a particular value of the electric charge. Like, we have one value. How common is it across the landscape? Or we have one value of the speed of light.
How common is that across the landscape? Or our strength of gravity, how common is that across the landscape? We have no idea and don't listen to anyone who says that they do because they go in with assumptions. They say, oh, you know, our our value of electric charge, represents, you know, just point one percent of all possible electric charges. And if they're all represented equally across the landscape, then we have point 1% of seeing this electric charge in any particular that's that's they're just making it up, folks.
I swear it. Because we don't know because we don't know anything about the landscape. We don't know anything about what gives rise to physics, and we have no way of assigning probabilities to these constants. We have no way of saying if one is more common than the other. These approaches to thinking about our universe through the landscape, and, again, this is my opinion, and I know I'm laying it on thick, have such a rich amount of hubris that it's almost laughable to me.
That in order to make these arguments in any direction about the landscape and the anthropic principle, to put it on any kind of footing that actually has a chain of logic other than just say we're here because we're here and the universe is the way it is because the universe is the way it is. Anything past that, we have to start making assumption after assumption about the likelihood of certain constants and their values that we have no idea and, how different combinations of physics can give rise to intelligence which we have no idea. It's just there's no way to evaluate or test or calculate using the anthropic principle. It's simply a philosophical statement, which is fine for philosophy. I love philosophy.
Physics is a branch of philosophy. I think philosophy is a worthwhile pursuit and a beautiful, powerful way to think about some deep questions. It's also not even wrong. But that's the landscape. And there have been attempts to try to narrow down the landscape, asking the question, are there any possibilities of manifolds that simply aren't allowed, that give blatantly wrong physics even when looking through the murky mirror that is perturbation theory in our approximate methods?
This is not a new program. String theorists have been trying this for a few decades now. But in the past couple decades, the landscape gives them a new framework for asking this question. It's like, can we can we slice off whole chunks of the landscape and say, you know what? That's not valid.
The way they do it because we we can't talk about the physics in a particular landscape because we don't have the string theory, or we can't talk about the physics in a particular manifold, a particular point in the landscape because we don't have a full string theory. But we can ask if the mathematics and the string structures that appear in a manifold are at least internally consistent. And there are turns out there's some progress. There are regions of the landscape that are not internally consistent, which is important. To be a math theory, you can't ever contradict yourself.
You can't say one statement and then a few lines later say something that contradicts that. You know, that's that's just not allowed. Then you're not even a valid math theory. This is often cited as one of the major successes of string theory that it is internally self consistent. I remember after reading Brian Greene's book and being all excited about string theory.
I I was speaking to an adult colleague or a mentor of mine in high school and saying we're talking about string theory, and he was not a fan of string theory because he was older and wiser, I guess. And I said, oh oh, but it's but it's internally self consistent. And his response was golden right away. He said, Mormonism is internally self consistent, but it doesn't make it a valid theory of physics. No offense to Mormonism, but you're not predicting, you know, top quark behavior here.
Self consistency is necessary but not sufficient. If you have it, good. If you don't have it, goodbye. And it turns out some string theories in the landscape are not self consistent. These places in the landscape where the math goes haywire, where where the string theory says, no.
This isn't working out for me is called, and I'm not making this up, it's called the swampland. And this is a major focus of research. By finding out what string theory can't be, we might learn what it must be, maybe, but it's not looking so good. And the reason is supersymmetry. It's a lack of detection.
The lack of detecting supersymmetry, which is a major cornerstone of string theory, is a major, major problem for string theory. We haven't seen any hints of supersymmetry. We haven't ruled it out because there's more complicated supersymmetry models out there that we haven't touched yet with arc aligners. But the fact that we haven't seen any of the easy ones or the natural ones makes us question if this is really a valid research program in the first place, and it puts some serious doubts on string theory as a valid research program. And it's one of the reasons why a lot of people have simply given up looking at string theory as a theory of everything over the past few years.
There's also the problem of dark energy. Dark energy is the name we give to the accelerated expansion of the universe. And for various reasons that we don't fully understand this was a surprise, you know, discovered about twenty years ago. For various reasons that we don't fully understand, string theory has a really, really hard time with dark energy. It just has a really hard time constructing itself, being consistent with itself in a universe that's filled with just a tiny bit of vacuum energy that stays constant.
It just has a hard time. Remember last time in our discussion of the AdSCFT correspondence that AdS, the anti de Sitter space string theory, had a really good time. It was feeling good, feeling itself, feeling alive, ready to tackle the day in a universe that was almost literally the exact opposite of ours. If you try to put the exact same mathematical equations in a universe that does look like ours, it's it starts falling apart. I went to a talk recently entitled string predictions for cosmology, and it was presented by a string theorist.
And I'm a cosmologist. I study the universe, so I thought it would be interesting. It was annoying because the talk contained no predictions. But there was a surprising fact in that talk that I want to tell you about. It seems, maybe, and this is absolutely cutting edge work, so it may change.
This is new stuff. But it's possible that all the universes with dark energy live in the swampland. If this is true, it means that string theory is incompatible with a universe with dark energy in it. And since we have a universe with dark energy in it, string theory might be incompatible with our actual real living breathing universe. Now, there's a ton of caveats to that.
And so it's not a death knell. It's not the final nail in the coffin. The two main reasons why that's not a final statement is that we don't fully understand dark energy, and we also don't fully understand string theory. But it's a little bit worrying, isn't it, that dark energy poses such a challenge to string theory. Now, it poses a challenge to all of modern physics because we don't understand what dark energy is, but it doesn't cause an existential crisis to say the standard model.
It doesn't pose a potential guillotine to general relativity. There's a lot we don't understand about dark energy, but we're still pretty sure that general relativity and standard model are valid. String theory has a hard time with dark energy. It seems like string theory is in in a bit of a pickle. And for years, I think they've been overpromising and underdelivering.
And I've said it before, and I and I wanna reiterate it here. This is my opinion. I think if the graviton hadn't been found inside of string theory in the seventies, it would have just evaporated. People would have just forgotten about it. And if m theory wasn't proposed in the nineties, it would have ran out of steam then.
And if it weren't for the ADSCFT correspondence, then Brian Greene's book, The Elegant Universe, probably would have been the last book written on string theory ever. So it's one way to look at the mid eighties and the mid nineties is to call these the first and the second superstring revolutions and then the development of the ADS CFT correspondence. As the third superstring revolution, you can also completely validly look at those years, the mid eighties, the mid nineties, the mid two thousands, as a defibrillator shock on a dying patient where they, you know, they they they jerk around a little bit. You know, they they they spat the oh, there's a little bit of sign of life, but then it just flatlines again. The landscape itself, which has received a lot of attention over the past twenty years, may be fine for mathematicians, but it's a real tough pill for physicists to swallow, so it's not gonna make a lot of progress there.
It's just not. Not. Just most physicists are just gonna look they do. They just look at the landscape, and they just say, shut up. As for ADS CFT, that's what most string theorists work on nowadays, partly in the hopes that it might elucidate some aspects of string theory and partly because it might help in some real world physics applications.
So it gives them something to do. But let's fantasize for a bit. Right? String theories, you know, it it it's looking a little worrisome. In my opinion, it's on a little bit thin ice for a few reasons.
But let's say we cracked it. Let's say we did get a string theory. What would it give us? What would happen to general relativity and quantum field theory? Would the bending of space time still be a thing?
Would quantum fields still be a thing? Would these cornerstones of how we understand and view the universe still exist? What but what happens to them? Would they still contribute to Patreon? Would they go to patreon.com/pmsutter and contribute as little as $1 a month to help keep these shows alive?
I don't know. Would you? I don't know. The answer is, sort of, if we did have a string theory, an ultimate theory of everything, general relativity and quantum field theory would go away in a sense. They'll still be there as low energy approximations of a more complete picture.
Just like Newton's gravity is a low energy approximation to general relativity, relativity. Newton is still handy for a lot of applications except when he isn't. And we know that general relativity is always working behind the scenes to make gravity be gravity, but say, for throwing something at your face, I don't have to go through all the headache of solving Einstein's equations because, for that case, they'll give the exact same result as Newton. But, even though I can use Newton's laws to calculate trajectories, I can still visualize the world as, you know, bending space time and all that. I can still think of it at that higher general relativity level even as I'm using Newton's math.
And it's a similar case if we did have a string theory. General relativity and quantum field theory will still be useful in the vast majority of cases, but there will be some other more powerful, more fundamental view of reality that sits behind them. What does that view look like in string theory? Would it resemble quantum fields and excitations? Would it resemble the bending, the geometry of space time?
We don't know. I suppose it would involve strings, I guess, and extra dimensions, which we've covered. But other than that, we have no idea because we don't have a full string theory yet and we may never get one. So why have so many people worked so hard on string theory over the decades? This is more of a sociology of science question, but it's interesting to look at because it shows just how science works.
Why has string theory been so tenacious over the past few decades? Well, one is that, you know what, it is a promising candidate for a theory of quantum gravity, and that's something that people really want. There's a a richness to the mathematics, so there's a lot of room to play. There's a lot of ways to make a name for yourself as a young researcher. There's just so many hidden corners to string theory that we haven't fully worked out yet because it's so hard.
But if you can work out some little corner of it, even if it doesn't get you quantum gravity but it gets you some other understanding, well, you know, you'll make some progress and there's a lot of fertile ground there. But over the decades, there's also things like entrenchment. You know, if a few key researchers start working on it, make it popular, they start to get grants, and they hire students. What do those students do? They work on what their mentors want them to work on.
And so they go on to graduate, and they get positions elsewhere, and then they continue those lines of research. And then they get students of their own, and then they get students of their own. And over the course of decades, more and more people get attracted to string theory simply because there aren't many other alternatives. String theory just won early on as a slightly more popular idea. It's almost like evolution.
It had some trait that made it more popular, and so it could pass on its genes to another generation when other ideas, other competitors couldn't. And so, if you're interested in the problem of quantum gravity, there's string theory and then there's other. And string theory sucks up the vast majority of resources simply because it's already entrenched. It's already there. There's already a huge body of work.
There's already an annual conference on this subject, and there's plenty of colleagues that you can work with, and they can write letters of recommendation, and you can advance your career. Science is made of humans. Humans change fashions. They want to be a part of what's it, And especially if there's really high profile like Nobel Prize winners talking all about string theory. You bet you're gonna sign up for some of that so some of that shine rubs off on you.
It's cool. Oh, it was cool. String theory is cool. Look, all the cool kids are doing string theory. Come on, man.
There's wishful thinking. Oh, yeah. Like articles, presentations, popular press counseling whereas right around the corner, just a few more weeks oh, just a few more years until we've cracked string theory and we've made that major revolutionary leap in understanding. There's a something called status quo bias, where it's like, well, we've been working on string theory for decades, and so it's just what we're gonna keep working on. And related to that is is this idea the sunk cost.
Like, man, we've already spent so much look how far we've come with string theory. And to be fair, we have learned a lot about string theory, and it gave us some really powerful ideas like supersymmetry. And so, man, wouldn't it wouldn't it be really lame if we had to start again with some brand new theory and would spend another fifty years? Like, what if that one's wrong? And if instead you gave us fifty more years of string theory, we'd have cracked it.
Sunk cost fallacy. You can and sometimes it's just inertia. Like, you just get stuck in a career path. Senior researchers like this idea. If they like string theory, then they're gonna be more interested in working with and writing recommendation letters for younger people who are working in the same field.
And then once you get going in that, by the time you're like a post doc or like a junior faculty, like, this is it. This is your research program. Changing research programs mid career is incredibly challenging and very rare. So if you were an up and coming string theorist in the nineties, chances are you're still a string theorist today, assuming you have a career in science. And then there is that whole public imagination thing, this whole fascination that people have with string theory.
What's been fascinating to me, as I've done my podcast, I've done as I've done my outreach, more people are aware of string theory than they are of quantum field theory. Even though quantum field theory is actually well tested and forms a cornerstone of the stand like, it is the standard model in how we understand how the universe works, and it's so well tested. It's the most well tested theory in all of science. And it says some really wild stuff about the nature of reality, like, with all these quantum fields and everything. Hardly anyone knows about that compared to the people number of people who know about string theory or at least heard about string theory.
And so a lot of young people, like me, read books when they're teenagers and get excited about string theory and want to go in that direction. Helps fuel the fire. But there's another major reason the string theory has had the staying power that it's had, and that's because it's interesting. Really interesting. There are all sorts of little nuggets and corners and hints that we simply don't get from any other theory of quantum gravity.
And maybe that's why we haven't worked as hard on other theories because they're not as interesting. For example, string theory has no tunable parameters that we need to shove in to make it work. Like, in order to make quantum field theory work, you have to shove in the electron mass and just there you go. Take it. There's the electron mass.
And now do your thing. String theory doesn't mean that. There's no tunable parameters, except for maybe the strength of the string interaction. That's up for debate. We don't know if that's tunable or not or if it's fixed.
String theory really does naturally include quantum gravity without even breaking a sweat. It's just there, and it's been there since the beginning, which no one else can claim. The ADS CFT correspondence might end up being a very valuable useful toolkit. Maybe if we keep working on plugging away, we might be able to solve some pretty challenging problems in physics using this toolkit. There's lots of neat math.
I know this sounds snarky, but math for the sake of math is still a virtuous endeavor that I will always support, like art for the sake of art or cheese for the sake of well, I guess you eat cheese. Never mind. But, like, you know, lots of cool math is is fun and awesome, and we like that, and we can we can support that. String theory might be telling us something really interesting about the nature of black holes in the information paradox, which, you know, that's worth paying attention to if we can understand black holes a little bit better. Its approach to quantum mechanics is different enough that just being different might be valuable.
Just having a completely different approach to the nature of forces and particles and interactions, just having that contrast, just being there and being different is just useful in general just because it's something else that we can always turn to to try. The string theory gave us supersymmetries. And even though supersymmetries aren't looking so hot nowadays, you know, it has formed a major component of extensions of moving past the standard model And the concept of dualities that it applies in string theory is very, very rich and very, very intriguing and very, very powerful, and those techniques might be applied elsewhere. So string theory hasn't been fruitless. We've definitely gotten stuff out of string theory.
At the very least, we've gotten some new conceptual ideas that help contrast and frame some other successful ideas, but what we're judging is whether it should continue. Is string theory even science? I know some folks scoff at the question itself. Some say, of course, it's science. It's speculative.
It's hypothetical. It's judged on merits other than the evidence, like beauty and symmetry in the equations rather than the output of a particle collider. But we scientists have a long and storied tradition of sophisticated spitballing, tossing out ideas, fleshing them out, following our guts, and sometimes guts grand unified theories, sorry, nerd joke, looking for ways in. I mean, look at what Einstein did with general relativity. He just followed his intuition.
He just did thought experiment after thought experiment for seven years before he spit out general relativity. And then when the experiments came back confirming it, he's like, yeah. Of course, they did. So we can't necessarily knock a theory for not making contact with experiment yet Because, you know, we do that. Theorists physicists do that.
I mean, I've written papers that don't make contact with experiment and have no intention of making contact with experiment. I've written papers just for the fun of it because it was like some interesting angle about cosmology. So, you know, can't point the finger too much, but it's been a while, hasn't it? It's been a few years. It's been a few decades.
At what point do we stop being patient? At what point do you say, you know what? You you you folks have worked hard enough. We're gonna need some results, or we're gonna cut your funding. And in the question of is string theory even science, yes, science has a long tradition of just going with ideas and being very speculative and hypothetical, but is string theory even testable?
Is it falsifiable even in principle? The thing is, unless string theory collapses from the inside like, if if if we were to find out that every possible arrangement of manifolds is incompatible with, say, dark energy in the observed universe and there's just no way forward for string theory to explain our universe. If unless that happens, string theory is going to stick around for a very long time because there is no such thing as string theory. It's not done. It's not done, so we can't test it.
We can't test it until it's done. Even if we had the universe's most powerful collider that could reach the Planck scale and Planck energies and the regime of quantum gravity and all that, we still couldn't test string theory because we don't know what it predicts at those energies. We don't know what string theory predicts. At this present moment, string theory is untestable, even in principle. Could it someday become testable and falsifiable?
Sure. Even if it's testable but not feasible, like, you need the universe's most powerful particle collider looping around the Milky Way to even approach the energies needed to get a width of string theory. That's a different case. But right now, it's not even testable in principle because string theory isn't done. We don't have a string theory.
We have ideas that we hope approximate string theory or approximate end theory if that's the direction you want to go in. So, yeah, research programs all the time work kinda in the shadows, in the background, not yet making contact with the experiment. But then someday, they do have to make contact with the experiment because there's this is the whole point of what makes science science. So should we be giving string theory the old side? I'm being a little bit suspicious of it because it hasn't produced experimental results.
For a while, you get a pass, but is it time's up? That's what we're asking. Are we saying it's just time to let it go, man? Like, you've had a good run string theory, but we've gotta move on. How long can a research program go on for without connecting to any evidence?
Evidence? And even now appearing to stop pretending that they even care, most string theorists appear to have abandoned the search for a theory of everything and have focused on correspondence principles. How long can you keep that up and still be called science instead of, say, I don't know, mathematical philosophy? Do you get ten years? Do you get a hundred years?
It's impossible to answer this. And so I come back to where I was all those episodes ago. Remember those days? We were so young then. My verdict when it comes to string theory, and and please feel free to have a different opinion, is that there are interesting avenues worth continuing to explore in string theory.
But we need to foster a diversity of ideas and really start investing and opening up some other approaches to quantum gravity. We don't have to kill string theory, but maybe we should take a break before we really do decide that it's not even wrong. Thank you, everyone. This, as you can imagine, was a monumental undertaking. This is the most I've talked about a single topic over the course.
It was it's been eight episodes. Four months worth of Ask a Spaceman. That's a third of a year we've been exploring string theory. I sincerely hope you found it useful. I sincerely hope you found it enlightening.
I sincerely hope that you came out of this podcast series knowing more about string theory than perhaps you thought possible and perhaps more than you ever wanted to know in your life. Maybe, hopefully, over the next few years, as articles and news stories about string theory and information paradox and holographic universe and correspondences and dualities come up, that you'll be able to better understand the context for those news stories. That's what I really hope. I really hope you've learned something. I really hope you had fun.
I really hope you enjoyed this. I really hope you consider contributing to Patreon you know I couldn't resist. Patreon.com/pmsuder, especially my top Patreon contributors this month, Matthew k, Justin z, Justin g, Kevin o, Duncan m, Corey d, Barbara k, Neuter Dude, Chris c, Robert m, Nate h, Andrew f, Chris l, John, Cameron L, Nalia, and Aaron s, and all the other people that hundreds of people that contribute as little as a dollar a month to keep this show going. There's also all the people I've got questions lined up. I have a backlog, by the way.
Questions that you guys send me, I put on a little spreadsheet. I have a backlog of over 500 questions. And I just noticed a lot of people were asking about string theory. And I'd like to thank John C. On email, Zachary H.
On email, editroom on Twitter, Matthew Y. On email, Christopher L. YouTube, Sion p on YouTube, Neha s on Facebook, Zachary h on email, Joyce s on email, Mauricio m on email, at Srinik Schra on Twitter, Panos t on YouTube, Drove r on YouTube, Maria a on email, ter b on email, Oi Snowe on YouTube, Evan t on Patreon, Dan m on Patreon, some unknown human being, maybe from a different part of the landscape on the website, John t on Facebook, TW Blanchard on Twitter, Ori on email, Christopher m on email, at UnpluggedWire on Twitter, g u como s on Facebook, and Gully Foyle on YouTube. It was these questions about string theory, about supersymmetry, about extra spatial dimensions, about unification that led to this series. And so I can't thank you enough.
I can't thank the listeners enough. I can't the people who ask questions. I can't thank you enough. If you review on iTunes, I can't thank you enough. If you tell someone about this show, I can't thank you enough.
If you've enjoyed this journey of the past eight episodes that we've gone on, I can't thank you enough. And I'll see you next time for more complete knowledge of time and space. And I hope I don't have to ever talk about string theory ever again.