Part 1! 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 have a complicated relationship with string theory. And you know what? To be perfectly honest, I haven't exactly asked everyone, but just the sense I get. Maybe I'm just projecting myself, but the sense I get is that most physicists have a complicated relationship with string theory, and and I'm pretty sure that most string theorists have a complicated relationship with string theory. My complicated relationship with string theory began, as most complicated do in high school.
I remember in 1999 when Brian Greene's Elegant Universe came out, and it's a wonderful book. If you haven't read it, it's it's still good. It's still relevant, still packed with tons of cool physics. I remember reading it in high school. I had been hearing about string theory.
I had known vague things about it, but that book just, man, like, chapter after chapter just blows you away with relativity and quantum fields and then little tiny vibrating strings and then extra dimensions and then and just, oh, unified physics, the holy grail. This is it. Like, wow. I remember being so intensely fervent for strength. Like, you read that book, and it just feels like we are almost there.
Like, any day now, the new story is gonna come out that some mathematician or some physicist figured it out, solved string theory, and we're all good to go, and we can enter the new era of physics. That was twenty years ago, and that revolution hasn't happened. It's been two decades. And so now I'm older. I won't claim to be wiser, but I know a little bit more.
And when I think about string theory, I think about how excited I was as a teenager, how hopeful I was in the early two thousands, and now I'm just kind of jaded and bitter about it. To be fair, I think most physicists and scientists, as they get older, they become more and more jaded and bitter about their own work, let alone, you know, the work of somebody else. So, you know, maybe that's just a general trend. This is how scientists evolve. But and here's the but, the one we're gonna explore.
Yes. I'm jaded and bitter about string theory. I'll be upfront about that, but, it's an intrigued jaded and bitter. Like, every once in a while, something pops out of the string community that makes you go, Scratch your head a bit and wonder if there's something deeper going on, and then you move on in life. And then you look up the result a few years later and find that it either didn't pan out or people just stopped working on it, and so you go back to being jaded and bitter.
But then something comes back up, bubbles up, you see a news story, and you're like, I wonder if they're onto something, and and, like, this cycle repeats. On one hand, it's just so tantalizing. A theory of everything, a single physical theory, a set of mathematical equations that describes all of this, the holy grail. How beautiful, how powerful, how remarkable, what an achievement that would be. And so, yeah, of course, people get excited about string theory because it's a candidate theory of all the things.
And yet, it's been a few decades. Decades. Even by 1999, even by Brian Greene's Elegant Universe, which throughout this story, I'm gonna use as kind of a a landmark post, like a good pivot point of what came before and what came after. But even at that point, it had already been a few decades. In fact, the roots of string theory go back a century, a hundred years of working on ideas related to string theory.
And there have been interesting hints and new developments and revelations, and yet there is no actual theory. There's no actual set of equations that attempt to explain the universe. In fact, now that I mention it, we should probably define this. Like, what is string theory? At one time, it was considered, like I said, a candidate theory of everything, and it still is today.
But today, it's much, much more than that. The best definition I can come up with for what is string theory is it's a loose collection of related ideas and mathematical constructs that might or might not have applications to the real physical world in various ways. Was that too vague? Was that too uncharitable? Some may agree, some may not.
That's that's gonna be the definition I'm gonna work with, and we're and we're gonna explore this. Don't worry. And one thing that I want to make very clear, and I will come back to this later, several times, is that the vast majority of physicists do not work on string theory, even theoretical physicists, even high energy physicists, even theoretical high energy physicists. It's just not interesting to most physicists. You get the impression that because it's a theory of everything and, like, physicists are kind of obsessed with the concept of a theory of everything, and it's very popular outside of physics, outside of scientific circles, and, like, in the popular imagination, string theory is it.
So you might get the impression that, like, everybody's working on string theory at least part time, but the truth is the vast majority of physicists do not even touch string theory with a 10 foot pole. But some do work on string theory, and it has captured the public imagination. So, ultimately, what I wanna answer or the question I'm gonna pose to myself, and because you're listening to this, you will also get to hear the question. You'll get an opportunity to ponder it as we go on this journey, is this. Is string theory worth it?
And worth it, I mean, should we keep spending time and resources, that means dollar signs, pursuing it? Or should we give just give it up and try some other things? You know, like, maybe it's just had its moment in the sun, gave it a good shot, guys. We tried. But you know what?
Sometimes you just you gotta know when to hold them and won't know when to fold them. And maybe with strength there, we gotta fold them. And, yes, this question of should we keep doing it, is it really worth it? I'm not a string theorist, and I'm not gonna pretend to be a string theorist. I'm not gonna play a string theorist on TV, but we're we're gonna talk about string theory.
And so, yes, you can rightly ask the same question of my own discipline, cosmology, you know, theoretical physics, anything. We're allowed to ask this question like, hey. Hey. Hey. What does what does Paul Sutter work on?
Oh, he works on works on cosmology. Is cosmology worth it? Well, that's fair. That's fair. Maybe we can do an episode on that, and I will rightly defend myself.
But if we're gonna ask the question, you know, string theory has been around for a few decades. It's not done. Should we keep paying for it, or should we try to grow and explore in other directions? Is string theory worth it today? In order to answer this question, we have to really dig into string theory, and I mean really dig into string theory.
Most pop side descriptions of string theory are honestly, they're fine. I don't really have a big problem. There's a couple little niggling problems that that we'll explore in a little bit, things I don't like about the the popular descriptions of string theory. But for the most part, they're just fine. I'm not gonna knock what other videos or podcasts or articles or blog posts do.
Most descriptions of string theory, unless you're gonna read a book, If you're watching a YouTube video, reading a blog post, or an article, it's gonna be a pretty brief and cursory overview of string theory, which is fine. But this is a podcast, and so we have a beautiful opportunity to dig in deeper. It's like a it's like a book that I'm reading aloud to you, and the book doesn't really exist. It's coming from my own head. That's basically what a podcast is.
Right? And in this podcast, we're trying to answer this difficult life changing question of is string theory worth it. And so I don't think a broad overview is gonna cut it because the super simple broad over, like, the most highest level summary I could possibly come up with is that, we used to think particles were made of points, now it's made of strings. This solves, the problem of quantum gravity. Also, there are extra dimensions, but they're tiny, so it's no big deal, and we're still working on it.
That's, you know, six decades of work in a nutshell. And string theory seems so elegant and simple and beautiful and unifying. Oh oh, it's so wonderful to say out loud, like, oh, everything's made of strings, and it's so cool now. But its beauty may be only skin deep, and the surface level, high level descriptions, which are fine, they make us sit back and wonder what's taking so long to wrap this whole thing up. Like, oh, okay.
Get it. Okay. If everything's strings and there's extra dimensions, so so what what's the problem? And if we're gonna evaluate string theory, we need to understand the problems. And if we need to understand the problems, we need to go into the details, The ugly details, the nasty details, the things that don't make any sense, the arcane jargon, the complicated math.
If we're going to evaluate string theory, then we need to do the whole hog and not just the juicy delicious bacon, but the hooves and the hair and the snout. We need the tools to make a judgment call. Also, string theory is occasionally in the news, and I hope these tools that we're gonna explore and expand upon help you interpret what comes out of of the news in string theory because, you know, people are still working on it. I know that this is an exceptionally long preamble to get started, but we're gonna slow down. We're gonna take our time.
Ain't no rush on this. No need to hurry, and we're gonna really feast on string theory. For example for example, you know, it's very tempting to just, you know, in the discussion in a high level overview, say the word supersymmetry and how it's important to string theory and just move on. But we really need to understand, what supersymmetry is, how it's a cornerstone of string theory, and how recent results about supersymmetry affect string theory. And that's gonna take a while.
How many whiles will it take? How many episodes? I'm not sure right now. Somewhere I'm telling you, it's somewhere between six and eight, making this the longest by far series I've ever done. I've done series on space time, on general relativity, on inflation theory.
Those, you know, those were nice healthy explorations. String theory is a different animal. And if you're listening in real time as these episodes come out, we're talking months of exploration of string theory, but I think it will be worth it. Not necessarily string theory itself. I'm not I don't know if string theory is gonna be worth it at the end of our exploration, but I think the discussion and what we're learning is going to be worth it.
Like, I want us all to come out of this understanding string theory a little bit better than we did before. Here's a brief outline. To me, string theory has three main threads, and that pun is very much intended and will be repeated. There's the thread of strings themselves as little physical objects. There's this whole concept of extra dimensions, and then there's supersymmetry.
You weave those three threads together, and you get a string theory. So we're gonna hit each of those topics hard. And then there's the concept of duality, which was like a turbocharged concept for string theory, and then plays a major role, so we need to hit that hard. Then we need to actually try to test some of these ideas, I guess. And we need to go over all that, and we need to hit that hard.
And then there are some more recent advances. And, yes, I said the word aloud in square quotes, and I'm gonna try to work very hard to go above my own bias and present string theory in as fair a light as possible. So in order to help that, I'll state my own conclusions, like, I will answer this question right now. Do I think string theory is worth it? I don't think string theory is necessarily a dead end.
There are definitely some intriguing structures in there and some concepts that are useful, but I do believe we need seriously entertain and develop alternative ideas. But don't ask me what those alternatives are because I'm just a poor country astrophysicist. That's my conclusion, and that's where I'm gonna go at the end of this series. You may go a different place, and I hope I am able to explain string theory in as fair a light as possible so that you can make your own conclusions about whether it's worth it. But I'm human.
Okay? And so there's my conclusion at the end, and there's gonna be little hidden unconscious, subconscious biases that are gonna, you know, make me present string theory to reach that conclusion. Anyway, the the advances, quote, unquote advances, things with wonderful names, like, are you ready for this? The ADS CFT correspondence, the landscape, the swampland. I'm not making any of this up, but we need to talk about it.
And then I wanna devote some time actually discussing the question, is it all worth it? And don't you dare skip any of this because I swear, if you're like, man, that episode looks like it's a boring title. I'm just gonna go on the next. You will be lost. I'm gonna do my best in every episode to, like, bring us back up to speed and keep keep the momentum going.
But I swear if you if you skip this, you're only hurting yourself, mister and or miss. But first, where I wanna start today after that very long preamble is we need to motivate string theory itself. Why do we care? Why do theoretical physicists care about string theory? What has driven hundreds of people over nearly a century to work on this?
And that's where we're going to start, at the beginning. And in the beginning, there was Newton. One of Newton's big deals was that he figured out how gravity is universal, how the exact same force pulls an apple from a tree, and the exact same force keeps the planets in orbit around the sun, the moon in orbit around the Earth. Like, it's a universal force. It applies equally everywhere.
This is a big deal. This was our first shot at something we call unification. Newton unified the physics of the ground, our earthly experience with the physics of the heavens. And unification was such a revolutionary concept, such a powerful concept, such a good idea that since then, nobody's really had a better idea. For example, in the eighteen hundreds, we had all these crazy experiments about electricity and all these crazy experiments about magnetism, and it was all super cool and funky and weird, and people were coming up with descriptions of of of just what you can do.
Like, oh, if I play around with electric charges like this, I can get an electric field, but sometimes I get a magnetic field, and sometimes I can oh, wow. It was so fun and confusing. I can't wait to tell that story. Thank you for those of you who have already asked that question. Please feel free to to keep asking.
I would love to tell the story. And it was Unified by James Clerk Maxwell in a theory of electromagnetism. He unified these concepts. There's, oh, there's electricity, there's magnetism, and there's light. Turns out they're all aspects of the exact same thing.
There were concepts also in the eighteen hundreds about thermodynamics, about heat and temperature and entropy, and then there was this growing sense of microscopic motion, our understanding of how little itty bitty pieces of gas called atoms and molecules behaved, and we're able to unify these concepts into something we call statistical mechanics that allows us to explain thermodynamics in terms of microscopic actions. That was a unification. In the early nineteen hundreds, Einstein attempted to unify the gravity of Newton with the electromagnetism of Maxwell because for various reasons, these two views of the universe were incompatible. So Einstein tried to bring them together. When his first shot, he ended up with neither because he made a special relativity.
Special relativity turned out to be a cornerstone, almost like a metatheory. Special relativity tells us about space and time and motion and energy, and those constants must be baked into every other physical theory. But even special relativity itself unifies, say, space and time. A few years later, as we've explored before, Einstein tried again with the whole unification thing, and he did end up unifying gravity with special relativity, with electromagnetism, and ended up with general relativity. General relativity is our theory of gravity.
There were some attempts to unify to keep this program going. Like, we're not done yet. To unify general relativity and electromagnetism. Like, oh, maybe these are these two theories are like two sides of the same coin, and there's some master meta theory above them that didn't go anywhere. And we'll get to this in more detail, like, a month from now.
And things seem to be going pretty okay, and then quantum mechanics happened pretty much accidentally. Nobody wanted it. Everybody hated it. It was weird and awkward and confusing. There are these wave particle dualities, this quanta of energy.
There was this concept of spin. There was uncertainty principles. There was probability of measurements. Oh, gosh. Quantum mechanics.
What a nightmare. We'll do we'll do some episodes on that someday. Don't worry. But the universe demanded that we understand quantum mechanics, and so we did. We came up with a theory of the microscopic world.
And then so so are we done yet? No. Of course not. We have quantum mechanics. We have electromagnetism.
We have special relativity. We have general relativity, like, in the early twentieth century. Like, that that that was physics. That's what we had. So the next step was to unify quantum mechanics with special relativity because quantum mechanics as it was originally formulated didn't care about these deep and powerful relationships between space and time and all that.
And so you have to bring quantum mechanics into agreement with special relativity. You have to rewrite it, and that's how you get quantum field theory. Many discussions of string theory start off by saying that the fundamental object, the thing that we care about in the subatomic world is a point like particle. That's that's wrong. That's wrong.
The the fundamental object, the physical thing that makes up our universe as we experience it in quantum field theory is just that it's the field. In quantum field theory, you have space time. It's right there. It's just hanging out and being its own thing. And then on top of that, you have a bunch of fields.
These fields permeate all of space time. There's a field for every kind of particle. There's an electron field. There's a top quark field. There's a photon field.
That's called the electromagnetic field. Example I like to use is, like, when you take a piece of bread, the bread is space time, and you dip it in oil and vinegar. And so there's oil seeping through the bread, and there's vinegar seeping through the bread. These fields seep through space time. In quantum field theory, space time is the stage, and the fields are the actors.
And the fields are the thing. The universe in quantum field theory, the universe is made of fields. So quantum field theory is just a broad class. It's like anytime you try to bring quantum mechanics together with special relativity, you get quantum field theory, but there has to be specific applications of these quantum field concepts. And so there are different quantum field theories, and the first one was called are you ready for this?
And trust me, this entire series is just gonna be jargon after jargon. We're we're not just gonna get used to it. We're gonna enjoy it for what it is because we're we're learning vocab words. The first quantum field theory was called quantum electrodynamics, QED. QED is a unification program.
It unifies quantum mechanics, special relativity, and electromagnetics. So there like, three. You wrap three concepts together, electromagnetic, special relativity, quantum mechanics, and you get quantum electrodynamics, which is a kind of quantum field theory. And the field is the important thing. There's an electron field.
There's a photon field. These fields interact and play with each other and give rise to the richness and variety of physics that we see in the universe, at least when it comes to the electromagnetic force. But why do people, when they we talk about string theory, talk about the importance of of point like particles? I mean, what what is a particle in the sense of a quantum field theory? What we call a particle is just a local patch of the field that has gotten all excited.
Like, these fields are everywhere. They fill up space time. And every once in a while, a little part of the field can just get, like, really, really, really intense, really into it, and that's what we call a particle. But local is a kind of a tough word because I mean, what do we really mean by local? The the particles of quantum field theory have no spatial extent.
If a piece of the electron field gets all excited, excited enough for me to start calling it an electron, like, oh, look at that. I'm holding an electron in my hand. I'm holding a piece of a quantum field in my hand. It's a particle. That particle takes up no volume.
It is point like, a geometric point. It has no spatial extent. It has no volume. You guys, what's the what's the volume of electron? Zero.
This is kinda hard to think about, but it's also not hard to think about. The strange thing about it is, like, the immediate question you have is, like, how can a thing I'm holding an electron in my hand, how can it have no spatial extent? How can an electron even exist if it doesn't take up any volume? The not strange part is like, wait. Wait.
Wait. Take just take a deep breath. What we call a particle, that electron, is just a part of the field that got all excited at a particular location in space time. Like, that little that location in space time where that field exists, and the field exists everywhere, but that location got really, really excited, and so we call that excited location, that excited place an electron. That's why it's a point because it's just a location.
Either way, it's, you know, you know, kinda challenging to think about, and so, you know, it's it's hard for me. Don't worry. It's hard for me to wrap my head around it. However you decide to think about it, the particles have no spatial extent. An electron has no spatial extent.
A neutrino has no spatial extent. A top quark has no spatial extent. But they do have other properties. They have mass. They have charge.
They have spin and so on. And through those properties, they can interact with everybody else. For example, an electron has an electric charge. That charge gives rise to an electric field. The electric field talks to other charges, and so that electron can interact with other electrons.
It can talk to everybody. Even though it takes up enough space, it can still talk. The I think this needs to be said because so many popular descriptions of string theory start off with, oh, the the fundamental I I don't know whose voice I'm doing. The fundamental element of physics prior to string theory was the point. No.
The portrait painted by quantum field theories is where the physical universe is comprised of overlapping and interacting fields, and it's the most well tested theory in the history of well tested theory. Seriously, we've nailed quantum field theory, quantum electrodynamics. Like, we know this, And in that theory, in that model, the field is the primary thing. It's the thing that the universe is made of. But and I know you're waiting for a but.
There are some caveats, which we'll get to, But first, we have to talk about gravity. You'll notice in this story of quantum field theory, quantum electrodynamics, all that, I've mentioned special relativity. I've mentioned quantum mechanics. I've mentioned electromagnetism. More work in the twentieth century would discover the strong and weak nuclear forces, you know, find out that, oh, wow.
We got two more forces that we didn't ask for, but we would be able to fold these forces into the quantum field theory thing, and so we would have quantum field theory descriptions of those forces. But what about gravity? Well, gravity is weird, man. It's its own animal. It's all thanks to Einstein who had to run around being a super genius and come up with a completely, totally, radically different way of describing reality that nobody else can, hundred years since him, can think of a better idea.
Thanks, Albert. General relativity is a completely different story of the universe. There's still a field. This is interesting to me. There's still a field.
Fields are to physicists. Like, cheese is to me. It's just like it's an ingredient of everyday life. There's still a field. There's a gravitational field.
I generate a gravitational field, and I can interact gravitationally with other things. The the Earth generates a gravitational field. It's what keeps me here. The sun generates a gravitational field. That's what keeps the Earth in orbit, etcetera, etcetera.
Gravitational fields are still fields, and this field tells matter how to move through space time, but it's not a quantum field. In the quantum field universe, in that way of describing reality, the field is there and then little bits of it get excited, and that's what we call particles, little local excitations of the field. In general relativity, the gravitational field is is the geometry of space time itself. In general relativity, space time plays two roles. It responds to matter.
It bends and warps and flexes in the presence of matter and energy, and it tells matter how to move. It's both the stage and the actor. It's all geometry. In general relativity, everything, what we call the force of gravity, is really just the act of geometry. If you're curious, the the full word for general relativity, the mechanics of it is called geometrodynamics, geometry dynamics, geometrodynamics, which you should, whisper in your lover's ear the the next time you get a chance.
But in this story of quantum field theory on one side and general relativity on the other, you should smell something fishy. In quantum field theory, the actors in the stage are separate. The actors are the quantum fields, the electron field, the photon field, and the stage is the stage. Stage is just space time. It's just there.
It's just hanging out. And in general relativity, the stage is also an actor. Trying to mesh quantum field theory with general relativity just doesn't work. We we have this unification program that we've been on. It's been a good ride since Newton, and we've unified so many cool concepts.
General relativity is a unification of special relativity and gravity, Newton's gravity. Quantum field theory, quantum electrodynamics that unifies quantum mechanics and special relativity and electromagnetism. And so we're like, okay. Let's let's take the next step. Let's make the big jump.
Let's unify quantum field theory in general relativity, our description of the three forces of nature, and let's unify with our description of gravity. Doesn't work, and we've been kind of stuck on this problem for a hundred years. One major headache in quantum field theory is, well, you know, the quantum nature of the fields. Like, it's it's that's the whole point is that these fields have a quantum nature, and that's what makes it such a headache. Not impossible, not impossible, but quantum field theory is challenging to work with mathematically because things never like to stay still and they never like to maintain their identities.
For example, for example, let's say you have, two electrons bouncing off each other. Like, you shoot an electron at another electron and they go ping, and then they go off in some other direction. They do this. They interact through electric fields. That's the electromagnetic force, so photons need to be involved.
So for this picture of two electrons bouncing off each other, you need some electrons and some photons to to make this story happen. They're the characters in the story. You can envision this as one electron shooting a photon at another electron. So you the electrons come close, a photon bounces between them, and then they go off in their other direction. So very simple, very easy, straightforward.
Like, this is how they bounce, by shooting a photon at each other. And it's almost right. But the thing is, because quantum mechanics is quantum mechanics and things are never the same thing, while that photon, while that bit of light is en route to the other electron, it can, for reasons known only to it, spontaneously become a pair of electron and positron. Why? Don't ask silly questions.
This is quantum mechanics, and everything that can happen will happen. It'll just stop being a photon and just become an electron and a positron. As an analogy, and this is, like, the worst analogy possible, but it's the only one I could think of, if you send me a letter, dear Paul Sutter, when are you gonna end this series on String Thru? And you write me a letter, and while en route so you're a human being, I'm a human being, and there's a letter between us. While en route, the mail carrier, that thing that's transmitting the letter spontaneously becomes two more human beings.
And they travel parallel paths for a while, and then they come back together and they recombine to form, like, a single mail carrier, like a a single letter again. This is just weird, and it sounds like nonsense, but it's what happens in the microscopic world. But that's not the only possibility of just the photon becoming electron positron and then recombine to be a photon. It can do this a couple times. It can do this 47 times.
It can split into electron and positron, then those can split into other things. Those can become photons of their own, or they can become, say, neutrinos, or they can become top quarks. And then they all recombine. And at the end of the day, the you you just have all these possibilities of what actually goes down between two electrons when they get close to each other. Technically, there's an infinite number of ways that two electrons can interact, and all of them happen.
For the curious, what's actually happening is that the electron field is interacting with the photon field in a very complicated way, but this is commonly visualized as diagrams with, like, a bunch of lines and loops that look like particles. But it's not really particles. It's all fields. And I know that's a little bit confusing, but we're just gonna breathe through it and move on. The point is that when two electrons interact, they can do so in an infinite number of ways.
What saves quantum field theory? What allows us to actually make progress is that the more complicated ways of interacting, like, oh, okay. Now we're involving, like, 20 positrons and electrons and, like, some photons and a few top cores. Like, it's a very, very complicated process. The more complicated the process, the less and less important it is.
So what you can do is a very, very tricky mathematical procedure developed by people like Richard Feynman where you gather up all the potential infinities because you're like, man, I'm never gonna get done counting up all the ways and including all the ways that these electrons can interact. There's an infinite number of ways. This is gonna take literally forever, but you can play some mathematical games where you gather up all the potential infinities. And since you know that the more complicated interactions are less and less likely to happen or they're just not as common or they don't contribute as much to the process of two electrons bouncing off of each other, you can just take all those infinities and all those headaches and just sweep them under the rug and pretend they don't exist, and voila, you have a theory of physics that can predict how electrons interact. It turns out that just this super simple case of one photon going from one electron to another is the largest contribution, and then any other possible variations on top of that just contribute less and less, and so you can gather them up into a nice, neat pile and just say, I'm done.
This is the guts of quantum field theory. This is how calculations are actually made in quantum field theory. But then when you try to do the same thing with gravity, when you try to make a quantum field theory of gravity, then you allow for gravity to do whatever it wants. So it's not just all these crazy interactions between electrons and photons doing whatever they want, but at least the floor is fixed. At least the stage is fixed.
It's just there. It's just space time. It's not doing anything. But once you allow space time to also be a player like, hey, guys. Can I be on your team?
And it starts doing whatever it wants. Imagine that process I just described with all these electrons and positrons and photons and everything, all the complicated possible interactions. And the space time between the electrons gets to do whatever it wants to. There's an infinite number of ways for the space time between those electrons to do whatever it wants. And so you go to try to add up all of that infinities, all the different infinite ways that the space time can just do whatever it wants, and you get infinity, which is the usual answer in quantum field theory.
But then when you try to play your mathematical games to sweep them under the rug, you end up getting more infinities, and they can't go away. And you end up being able to write down a theory of quantum gravity, but you can't actually use it to make any predictions. You can't actually perform any calculations. You can't say, okay. If if two electrons bounce off of each other and there's also gravity involved, you don't know the answer, which is a very frustrating place to be in because you can sit there and stare at the equations.
Like, you're right there. You can write down quantum gravity, but they don't do anything or at least we can't figure out how to get them to do anything. So it's like a stubborn dog that you can't get to come outside, and you're just, like, tugging on the leash. You're like, come on. Come on, boy.
Come on. Come on. And it the dog's just like, no. No. Thank you.
Some string theory as a little aside here, some string theory introductions talk about the roiling of space time. Like, if you get down to the what's called the Planck scale, this very, very tiny scale in our universe. If you zoom all the way in, then general relativity says that space time has to be smooth, but quantum mechanics says it has to be roiling and frothing. That's not necessarily wrong, but let's just say we don't really understand what happens to our concepts of space and time at the Planck scale at very, very, very, very tiny length scales. I mean, we barely managed to define space and time at atomic scales.
Like, we have difficulties there. So I shy away from that description because the truth is we don't know what happens to space time at very, very tiny scales. And so that's not a very compelling argument because it assumes if you say, oh, general relativity says space time is smooth. Quantum mechanics says space time is frothing madness, seething mass, and these are incompatible. Well, we don't really know that.
So you can't really base, you know, an argument for string theory. What they're getting at is the discussion I just had that when you have particles interacting and you try to include gravity, the equations go nuts and you have no idea what's going on. The end result is quantum mechanics and general relativity can't get along. Fortunately, this hardly ever matters. Basically, nobody cares.
Quantum field theory works in space in flat space time just fine, and we have all sorts of quantum field theories describing three forces of nature. And, hey, three out of four ain't bad. Quantum field theory works too if you just take the space time and slightly curve it, but keep it fixed, and then let quantum field theory do its thing on top of that. That sometimes works. Yeah.
We we have some some good examples of that working. Quantum gravity does work. We can have quantum theories of gravity when the gravity is really, really, really weak, like, so weak that nobody cares and it doesn't participate in the interaction. So that's, you know, not as impressive as it sounds. This is a problem in theoretical physics.
This is a mystery in theoretical physics, but not really, quote, unquote, practical physics. This is a problem that appears at what we near what we call the Planck scale, which is the scale where you're very, very, very small, like quantum mechanically small, and gravity is very, very strong. That's where we have the difficulties, the strong gravity at small scales. And this means there are only two places in the entire universe where quantum gravity matters. Seriously, like, you just run around the whole universe.
Quantum gravity just doesn't matter. You just don't need a theory of quantum gravity to live your life, to have your rocket ships, to have neutron stars, to explode supernova, to have stars born, to to have neutrinos, like, all that. You just don't need quantum gravity. There's only two places in the universe where quantum gravity matters, and it's just two very minor, inconsequential, unimportant topics, you know, places, black holes, and the big bang. Okay.
Maybe those are kind of interesting places, and, you know, maybe we should work on a little bit more. Black holes and the big bang contain singularities. They contain points of infinite density. At the center of a black hole is a place where all the matter has collapsed down to an infinitely tiny point. The earliest moments of our universe, quote, unquote, began in a singularity, in a place of infinite density, a single geometric point.
These are called singularities. These are predicted by general relativity. Like, we're using the math of general relativity. You're like, wow. There's a singularity at the center of a black hole, and there's a singularity that that started our universe.
We know those are wrong. We know that singularities don't actually exist, but they signal. This is g r. This is general relativity saying, hey. Hey.
Hey. I know I just said that there's a singularity at the center of black hole. What I meant was we don't understand what's at the center of black hole because that's when quantum mechanics starts to matter, and that's when you need that's when you need some quantum gravity theories. And I'm just I just can't do it. I just can't do it.
I don't know. Alright? Replace singularity with question mark. Replace singularity with, wow. We really need a quantum theory of gravity if we're gonna understand what's going on there.
So that's two reasons why we think modern physics is just incomplete. We have no theory of quantum gravity and because the singularity is appearing in g r. If general relativity never had singularities, we'd be like, okay. I guess general relativity is just able to completely and totally explain the universe. But general relativity itself is saying, hey.
I don't think we're quite done yet because there are places in the universe that I don't understand. And then the quantum field theories are saying the exact same. They're saying, hey. Whenever I try to under whenever I try to fold gravity into my framework, I break down crying, so I don't think I'm done yet. We don't think modern physics is done.
Besides that, there are a bunch of, like, just mysteries of the cosmos that are just begging for an explanation. Like, in quantum field theory, the fields just are. They just exist, and they're different fields because they're different. There's just so many different fields. There's an electron field, a top quark field, a neutrino field, a photon field, a z boson, a Higgs boson, and on and on and on and on.
It's a madhouse. Why? Why are there different particles at all? By the way, throughout this entire series, I'm gonna be very loose with my use of the word particle because I'm just lazy like that. You know what I mean.
When I say a particle, I mean electrons, which are really represented as excitations in the electron field, but we're just gonna call them particles for the sake of convenience. But why are electrons different than photons? What makes the electrons the way they are? What makes quarks the way they are? Why are there four forces of nature?
Why do they behave the way they do? Why? Why? Why? We don't know.
In order to make quantum field theory work, to actually crunch through the mathematics and make predictions, we have to plug in some values by hand about 18 numbers that the theory doesn't predict, that we just have to go and experiment like, okay. Let's figure out what the mass of the electron is, and we can plug that into the theory to make other calculations and start turning the crank. Does anybody have an answer for that? Anybody? No.
That's what I thought. And speaking of the four forces of nature, why is gravity so weak? Man, it is like you think gravity is weak, but no. It's even weaker than that. Why?
And don't even get me started on dark energy. Seriously, who asked for that? Why is it so weak? What's going on? Is anybody out there?
Is anybody going to support me on Patreon? That's patreon.com/pmsuder. It's how you keep this show going. I really do appreciate the contributions every single month. But is anybody gonna do it?
Mysteries of the cosmos. We need some answers. As you can see, there are some unanswered questions in modern science. Can we have a unified picture of quantum mechanics and gravity? Can we end up explaining the singularities in general relativity?
Can we explain why we have four forces in nature and not some other number? Can we explain why matter and photons and the forces have the properties that they do? So many different questions, and they don't all need to have the same solution. You can have a quantum theory of gravity that isn't unified. I mean, we've been following this program for a hundred years of unifying physics, and it's been pretty awesome, but it could be the end of the line.
You could use quantum field theory to explain the three forces and then something else, whatever it is to explain gravity in a quantum way, but it's not a quantum field theory, and then you're just done. But some physicists are searching for a toe, t o e, a toe. They're searching for a theory of everything. They want a unified framework. They wanna get rid of the singularities.
They wanna explain the electron mass and the range of the photon. And if you want a theory of everything, then you have to explain everything, the forces, the matter, the properties, the constants, the big bang, dark energy, black holes, why two pieces of cake at that party, the hierarchy problem, which is why gravity is so weak, all of it. String theory is an attempt to do it. And next time, we'll see how the string party got started. And how it got started might just surprise you.
Thank you so much to all the people that have asked me questions about string theory through all the years, and, yes, I will thank you in every single episode. John c via email, Zachary h via email, at edit room on Twitter, Matthew y on email, Christopher l on Facebook, Krisna w on YouTube saying p on YouTube, Neha s on Facebook, Zachary h on email, Joyce s on email, Mauricio m on email, x Shrenik Shrod, Twitter, Panos t on YouTube, Dhruv r on YouTube, Maria a on email, Maria a. Again, two questions. That's cool. Good for you, Maria.
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It's not over yet, folks. Thank you so much for listening. You can ask questions, ask a spaceman, hashtag that up, askaspaceman@gmail.com, ask a spaceman dot com. If you want, just go to the website. Or if you're just bumping in me on the street, just ask me a question, I'll answer it.
Thank you so much, and I'll see you next time for more complete