Must there be a single unified theory of physics?

00:00:07
Speaker 1: I got into physics because I want to know how things work, what's going on?

00:00:14
Speaker 2: What are the rules of the universe?

00:00:16
Speaker 1: Because I figured there must be rules, and there must be reasons why this happens, and on that there should be an explanation out there for us to find. Right and over millennia and centuries, a pattern has emerged where humans encounter lots of different kinds of phenomena like lightning and magnets, and then later realize these are actually deeply connected. There are two sides of the same coin. And this all makes sense if there is in fact a single reason why things happen, a unified theory to explain it all. We might be discovering pieces of it and then snicking them together. We're making zigzagging progress towards this idea, but what guarantees do we have that it actually exists? I mean, we have been trying to unify quantum mechanics and gravity for a century without much success. What if they don't just play along? Could the universe be governed by more than one theory, or like a patchwork of theories for different regimes, or even weirder, could there be more than one valid theory of the universe. Maybe the human project of physics has just gone down the wrong path, and when the aliens come, they can provide us a reset. Or maybe they'll tell us the whole project is hopeless. Either way, Welcome to Daniel and Kelly's Extraordinary Unexplained Universe.

00:01:55
Speaker 3: Hello. I'm Kelly Widersmith. I study parasites and space.

00:01:59
Speaker 4: Hi.

00:01:59
Speaker 2: I'm Daniel.

00:02:00
Speaker 1: You know I'm a particle physicist, and I really do believe there is a reason why things happen.

00:02:06
Speaker 3: Oh that's very philosophical of you, Daniel. So my question for you today is if you could go back in time to be present at any physics discovery, what physics discovery would you want to be present for.

00:02:21
Speaker 1: Oh, well, back in time to be present for a physics discovery.

00:02:25
Speaker 2: No, no, I'm going to flip the question. I want to go forwards in time.

00:02:28
Speaker 3: No, no, Daniel, No, we're talking about going forwards. I'm specifically asking you a backwards looking question.

00:02:35
Speaker 1: Okay, backwards looking question. Moments of discovery. You know, I wouldn't have minded being on that rooftop with Galileo as he looks through the telescope for the first time and sees Jupiter and its moons. What an incredible moment to understand our place and the cosmos and how it all works must have been mind blowing.

00:02:57
Speaker 4: Yeah.

00:02:58
Speaker 1: Also, I would have liked to bring him like a mug of hot because I think you got chili up there.

00:03:04
Speaker 5: That's really nice.

00:03:04
Speaker 3: And maybe he would have named something after you for that.

00:03:06
Speaker 2: You know exactly.

00:03:08
Speaker 1: Also, I wish I could have been there to point out to him that he discovered Neptune without realizing it. Oh, if you go back and look at Galileo's original logbooks, because the dude kept great notes. You see Neptune in his notes. He didn't appreciate what he was seeing. And it wasn't until like a couple hundred years later that we discovered Neptune. So Galles actually missed out on a great discovery.

00:03:30
Speaker 3: So you actually want to go back and tinker with the past. That's interesting. I just meant observing.

00:03:36
Speaker 1: Anytime you go back, you're going to tinker with it. There is it's all quantum mechanical. You can't observe it without interacting, right, So yeah, if I go back and chat with Galleo, then I'm going to change the course of history.

00:03:47
Speaker 3: Okay, all right, Well, In today's episode, we're looking forward and we are asking if there's a single unified theory of physics that we might discover in the future, it will all of the pieces fit together like some giant at some point, and I, you know, I would like to go forward to that moment if I could.

00:04:04
Speaker 1: Yeah, Well, I also want to explain my earlier comment when I say I think that there's a reason for everything. I'm not being like mystical, like you know, there's a reason why children die of cancer or something horrible like that. I'm just suggesting that, you know, the universe is self consistent, that the universe follows some rules that when a particle goes this way instead of that way, that there's a reason for it, even if that reason is like, hey, it's sarcastic, it's random, but it comes from this probability distribution at least, you know, as we talked about on that episode with Sean Carroll recently. And you're right that this is a philosophical position. It's not a scientific position, right, It's just sort of like, we hope the universe works this way.

00:04:47
Speaker 2: We assume the universe works this way, and we use.

00:04:50
Speaker 1: It as a foundation of basically the whole scientific method, and we're just going to keep going until it breaks down.

00:04:56
Speaker 3: Hmmm, I feel like I'm still not totally convinced. You haven't just said, like a hand wavy guruy, everything happens for a reason.

00:05:03
Speaker 1: Well, you know, if you buy my crystal and hanging around your neck, then you can control those reasons.

00:05:09
Speaker 3: But you know what people should buy? Do aliens speak physics?

00:05:14
Speaker 2: That's right.

00:05:15
Speaker 1: In today's episode, we're digging deep into questions of physics and philosophy and wondering about is there a theory out there for us to discover? This is one of the questions I dig into in my new book Do Aliens Speak Physics? Which attacks some of these philosophical questions in a very concrete way. It imagines, Hey, aliens have just shown up here on Earth, and we're excited to talk to them about what they know about physics. What if they don't have a unified theory of everything because one doesn't exist?

00:05:43
Speaker 2: Is that possible?

00:05:44
Speaker 1: Or what if they have a different unified theory than the one we've been working on? Could you have two theories? Those are some of the questions we're going to touch on in today's episode, But there's a deep dive on all of that in my book, Do Aliens Speak Physics?

00:05:57
Speaker 3: Out November fourth, eleven out of ten or six out of five stars, best book.

00:06:02
Speaker 1: Ever, amazingly good reviews from the parasitologists who've read it.

00:06:06
Speaker 3: Yeah. Well, this parasitologist was very impressed with the deep dive and the entertaining and clear way it was presented.

00:06:13
Speaker 1: Thanks very much, And I do think that listeners to this podcast who are excited about physics and philosophy and the big questions of the universe will enjoy it. So please do me a favor and check it out.

00:06:24
Speaker 2: Thanks very much.

00:06:25
Speaker 1: But today we're not just here to say in live book. We're here to talk about the big theories of the universe. And so first I asked our audience if they thought there needed to be a single, unified theory of physics out there for us to discover. Here's what folks had to say.

00:06:41
Speaker 2: I don't think so, because maybe somethings are separate.

00:06:45
Speaker 4: Just the feeling that I have that the universe is like Russian Matrioska dolls nested dolls, except that there's no final kernels. And let's just keep going and going.

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Speaker 6: Maybe, if the universe is composed of two or more types of nature, a unified theory of physics might be impossible, But otherwise one seems fine.

00:07:13
Speaker 7: It would be ideal to have a single quiet theory, but I don't think it's necessary to have some progress or functional results out of that. So I don't think it's a bad idea to have competing theories.

00:07:27
Speaker 8: The expectation has been yeah, wandering to rule them all and in the darkness behind them. But maybe we should also consider that we're so far down the ladder of magnitude that we can't forge a united theory without comprehending the entirety of the universe and what may exist beyond it further up the ladder, beyond what we can conceive.

00:07:46
Speaker 9: No, I don't think so. I think physics is just the way that humans are trying to understand the universe, but it doesn't need to be unified or fully understandable.

00:07:59
Speaker 5: For it's a human lust to have like simplified down to one.

00:08:04
Speaker 10: As seeing the unified theory of physics sounds like the search for a very simple solution to a very complex problem, and those seldomly work out.

00:08:17
Speaker 2: So I vote, though.

00:08:18
Speaker 11: Given what we've already learned about how the universe works, I'm inclined to believe that there is a single unifying theory of kind of everything, and it isn't actually separated into quantum and classical. But we have to keep funding and celebrating science to find that out.

00:08:37
Speaker 3: As ever amazing answers from the audience.

00:08:41
Speaker 1: Absolutely thanks everyone for contributing your hilarious ideas, and so let's jump into the episode today.

00:08:47
Speaker 2: We're actually joined by a friend of mine.

00:08:49
Speaker 1: A fellow physicist and podcaster and science communicator extraordinaire, Ethan Siegel. He's a theoretical physicist and science writer. He's previously been a professor at Lewis and Clark and is now a prolific writer and podcaster. You can find him online at Starts with a Bang. His books include Infinite Cosmos, Visions from the James Webspace Telescope by National Geographic and upcoming is a new book in November, The Grand Cosmic Story, which tells the whole history of the universe, where each page is one hundred million years. Ethan, thanks very much for joining us today.

00:09:22
Speaker 5: Oh it's my pleasure to be here. Thanks for inviting me to an extraordinary conversation about the universe.

00:09:29
Speaker 3: I can tell you're going to fit right in already.

00:09:31
Speaker 1: So today we're talking about the concept of a unified theory. Is it possible to have a unified theory of physics? One are the arguments for it and against it? But as usual, because of the philosophical discussion, we have to start with some definitions. So what do you understand to be a unified theory When somebody talks about a unified theory of physics, what does that mean to you?

00:09:50
Speaker 5: Well, let's start. Let's start at the basics, right which is, where are we now and why don't we have a unified theory of physics right now? And that's because what we have is we have two very fundamentally different ways of making sense of the universe from a physics perspective. On the one hand, we say, oh, everything is made up of these tiny, tiny, tiny, little quantized packets of matter of stuff, whether it's matter or energy or antimatter or radiation. We have everything is discretized, everything is quantized into these little packets, and these packets obey the quantum rules of the universe, and we have the quantum field theories that describe the electromagnetic force and the weak nuclear force and the strong nuclear force, and these all play on the same footings. Even though there are different theories that describe these different aspects, they are all quantum field theories that do kind of fit together into our framework of the standard model. And then on the other hand, we have general relativity, which is our theory of gravity. This is our best theory of gravity. Now this is not a quantum theory of the universe. This is not you know, if you say I'm gonna take an electron and I'm gonna pass it through a double slit, you say, great, I can do all my quantum stuff for where is the electron, what's its momentum, where's it going to appear? And I can do my probabilistic calculations and give you all of that, and then you can ask a question like, yeah, well, what happens to the gravitational field of the electron as it goes through that double slit? And general relativity says, I do not know how to deal with that. I can't deal with that. I don't have an answer to that question. If we wanted to answer that question, we would need a quantum theory of gravity. So to me, a theory of everything would be not just taking well, I can take all the forces of the standard model and all the particles of the standard model and unify them together into the same framework, and it even goes beyond. I'm gonna take general relativity, which I don't know how to do, and I'm gonna make it quantum too and make it play nice with these quant forces. Or maybe I'll take the quantum forces and make them play nice on general relativity's footing. We don't know how to do that either. If we wanted a theory of everything, it would have to not just unify those known parts of the universe, it would also have to solve the currently unsolved problems of our universe, like what is dark matter? What is dark energy? How did we get to have more matter than antimatter in the universe? Why is there a matter antimatter asymmetry? So a theory of everything would be some framework where all of these different questions were described within the same framework in a unique and unambiguous way, where we had the same level of predictive power that we demand from general relativity and quantum field theory today, but where we had a unified structure that could solve all of these problems together. The idea of a theory of everything, or of a unified theory would take all of these things and solve them together and put them in a single framework where you can explain and derive everything about our universe.

00:13:13
Speaker 3: Well, that sounds pretty straightforward. Why haven't y'all like figured that out yet, says the biologist.

00:13:19
Speaker 5: I know, right, it's sort of like it's sort of like the question of like, if I'm down at the base of a pyramid, even if it's a foggy day, I can assume there's a summit to that pyramid, and why is it so hard to get to the top. And the answer is, well, first off, it's a foggy day down here. I'm not even sure this pyramid has a top, or ever had a top. I'm not sure that's what it looks like. It's sort of like, you know, that classic mountain shape, that classic stratovolcano shape. That's what Mount Fuji looks like. And if you came to the United States in the Pacific Northwest prior to nineteen eighty, you would have discovered, oh, Mount Saint Helens, is known as the Mount Fuji of the West or because because you that's us, like we're the Mount Fuji on the other side of the world. And then in nineteen eighty, Mount Saint Helen's, you know, famously exploded and now there's no top to it anymore. It does not look anything like Mount Fuji anymore. So what happened in our universe was there a unified theory at some point in the very very distant past, and and we just can't recognize it because it blew up in some spectacular fashion? Was there never a theory of everything? And we just have these disconnected parts of the universe. And so what we've attempted to do mostly is, yes, there are some people saying like, I'm just gonna go for the big prize. I'm going to assume there's a unified thee I'm going to work on that. And then you want to as a physicist, as someone who's connected to reality, you want to say, like, okay, well, well, what signatures would we see if that was true? And how could we observe or measure the universe in some way to reveal that this is what it's actually like. And it turns out that the very very unified theories, they they make predictions that are way outside of what we can observer or measure. Their predictions are. You know, it's it's really an exercise in like, oh, no, company's coming over, and I'm going to do like a cartoon, and I'm gonna lift up the rug and I'm going to sweep all the things I don't want under the rug and put the rug down, and hopefully they don't notice this giant bulge of dog fur underneath the rug. Right Because because when we actually go to do that, you can say, well, what are the things I can add in to unify my theory? You can add in, for example, if you if you want to work to unify by the three forces of the standard model together, you can make something called a grand unified theory. Grand unified theories all have extra predictions of things we should expect to see that we don't see.

00:16:14
Speaker 2: In our universe.

00:16:15
Speaker 5: For example, for example, we have our neutrinos in the universe. All the neutrinos seem to be left handed particles. Where if you watch a neutrino moving and you say what direction is its spinning, it spins like your left hand fingers curl around it. Meanwhile, all the anti neutrinos are right handed. They all curl in the opposite direction. These are not the same particles. They're not spinning in the same direction. If you have a unified theory. Unified theories are left right symmetric. So where are all the right handed neutrinos and where are all the left handed anti neutrinos? Why don't we have them? The universe would also be symmetric between electric and magnetic forces. We have electric positive and negative charges. We do not have magnetic north and south monopoles. We only generate magnetism through the motion of electric charges. So where are they. It also predicts a super heavy set of what we call bosons. It's a class of particles that would allow quarks and leptons to separate parts of the standard model to couple together through both of them. This has the advantage that maybe it could explain the matter antimatter asymmetry, but it has the disadvantage that it makes particles like the proton inherently unstable. So we build these big tanks of water and we say there's a bunch of hydrogen atoms in there with a bunch of protons for nuclei. Let's wait and see if any of them decay, and we don't see any proton decay. We don't see any proton decay for tens of thousands of times longer than we would expect the proton to decay if the simplest model of grand unified theory was true. So this is sort of why we haven't gotten there is we look at well, before we even go all the way up to the top of the mountain, let's try and take that next step up, and any direction that we try and take that next step, you try and add supersymmetry, And where are my extra higgs bosons that the LHC should have found. Where's the lightest supersymmetric particle that should be at about the same energy as the top quark? Not there? Where are my extra dimensions not there? Where's proton decay? Not there? So it's really hard to say, like, well, you know, maybe we're just not adding enough things and we should add more and more and more and more and more, and just say oh. It's sort of like if I imagine I have a giant mystery box and I stick the right key into it and the whole box explodes and crumbles away, and I'm left with like four little crumbs. Oh, and maybe this crumbs are what our universe is and all the other shrapnel it just disappeared somewhere that isn't here, and that's why we can't see it. It's it's kind of hard to say, like that is that how we really do science? Is that's something we would accept as like, oh, that's a good story for how our universe is. Really we really demand something more than that.

00:19:22
Speaker 1: So you're painting this picture of unified theories as requiring extra bits which we don't see in the universe, which we have to somehow explain why we don't see them. But why is that necessary? Why is it required? And you have a unified theory to have these extra mathematical machinery, which then you have to then do all this work to hide.

00:19:40
Speaker 5: Well, if you talk about a unified theory, the first job of any new theory you want to propose is it has to explain the things you already know to be true, right, And there's no simpler way you can actually prove this. There's no simpler way to explain what we we already know with fewer parts. You can't say like, oh, I'm gonna have a smaller group than the standard model that explains everything in the standard model. Like no, you can prove the standard model is the smallest representation of what contains the Standard model anything else any other way, I could also represent the standard model inherently has that much at least stuff or more if I wanted to do the same thing with general relativity. Now I'm saying like, it's basically asking how can I fit these puzzle pieces and these puzzle pieces together into a puzzle. Your puzzle, your whole puzzle, has to at least contain the pieces you know are present. That's sort of the basic explanation of how you do it. You can't say I'm going to make something simpler or that contains less and then the more stuff comes out of it like that. That's not how math works. You can't make a bigger thing out of a smaller thing. You have to at least include the pieces you already know are there. So if you're talking about unifying this, you're inherently going to say, I need to at least include what I already have, and then if I want to unify them because they're not unified. Now, I need some grander framework somehow that these pieces either are both embedded in or we'll both emerge out of. That's sort of the general picture of it. But Daniel, you're an expert on this just as much as I am, at least, like surely surely you have an opinion on this too.

00:21:49
Speaker 2: I do.

00:21:49
Speaker 1: I want to hear Kelly's question, but I also want to first provide maybe a helpful historical analogy. People might be thinking, well, what about like electricity and magnetism, Maxwell click those together without reading some big, complicated framework with all these extra moving bits he had to then handle. And the actual story is the opposite, right, He clicked together electricity and magnetism, but to do so he had to create a larger framework, and that framework contained pieces he wasn't familiar with, like the displacement current, which is necessary to put these two together and make everything symmetric. And then he went out and discovered, oh, it actually is out there in the universe. And so you know, that's actually an example of putting things together into a larger framework and then discovering that some of those pieces of the framework really are out there in the universe. We have to take a break, but when we come back, we're going to hear Kelly's question about unified theories in physics. Okay, we're back when we're talking to doctor Ethan Siegel about unified theories in physics, and I'm dying to hear what Kelly wants to know.

00:23:08
Speaker 3: I guess I was just going to ask our physicists really sure that all of the smaller pieces are separate, Like, could could you discover at some point that like, oh, some of these things we thought they were different, but actually it's the same thing, just under different conditions. Or are we like one hundred percent confident and all of the smaller pieces of the puzzle already?

00:23:26
Speaker 5: Oh, Kelly, that's actually a genius question. So it turns out like now I get to be the excited one to tell you, guess what you know? How I told you that over on this side we have general relativity, and over on this other side we have quantum field theory with the strong nuclear force, the weak nuclear force, and the electromagnetic force. One of the huge advances that was made in the nineteen sixties, and I'm going to credit Shelley Glashau for it, although there were others. Is this idea that the weak interactions and the electromagnetic interactions can be unified into a single framework, and this is electroweak theory, and so this is actually a part of the standard model. It says that here at are low energies, we see one, two, three, four separate forces gravity, strong, nuclear, weak, nuclear, and electromagnetic. But if you go up to high energies, like the types of energies they've reached at the large electron positron collider at Fermilabs tevitron and now at the Large Hadron Collider at CERN, you can actually say, oh, no, it looks like electroweak unification does happen, and that the theory of electroweak symmetry breaking is where the Higgs sector and the Higgs boson comes from. It's why we have the Wnz bosons be very massive instead of massless, because when the symmetry breaks, there are degrees of freedom that get eaten by well, they're not displacement currents like Daniel talked about. They're different types of currents that arise in physics, but they get eaten by those directional degrees of freedom, and that produces three very massive bosons, the wn z bosons that mediate radioactive decay, along with that one massless boson, which is the photon, which is why the electromagnetic force is a long range force and travels at the speed of light, whereas all the weak interactions are very short range because of the high mass of the bosons, and well, they're not going to reach very far. You know, it's not like what's happening in me is going to make a neutron inside you decay. The weak force isn't going to reach from me to you, but the electromagnetic force does, and that's why you can get the radio waves from me right now.

00:25:43
Speaker 1: And that's another example of bringing two ideas together which creates more theoretical machinery which turned out to actually be out there in the universe.

00:25:51
Speaker 5: We see it, and there are a lot of examples of this. You know, you go back to electromagnetism and you have the magnetic vector potential and the associated like said, displacement currents in the quantum world. This leads to things like the ahronav Bom effect in the electroweak sector. When you unify that, that's where the prediction of the Higgs boson came from and why we have the Higgs mechanism and the Higgs particle, and turns out we were able to find it at the Large Hadron Collider. So these were additional predictions that without that unification, and of course, because we don't live in a unified universe today, we see things are playing on these different footings today. It means those symmetries have to be broken. And there's actually a theorem, a provable theorem called Goldstone's theorem that tells you in particle physics, every time you have a symmetry that gets restored at some point and that symmetry then gets broken, there are essentially new particles that have to emerge, these Namboo Goldstone bosons that have to show up. So this is sort of one of these things we're looking for, is if there was some extra form of unification, and that things are not unified, now, where are those extra components that needed to come out of it?

00:27:17
Speaker 2: Now?

00:27:17
Speaker 5: Are they around somewhere? Do we need to figure out how to detect them? Are they hiding because they get eaten or subsumed into particles that exist like the W and Z bosons. These are the sorts of things that we need to ask ourselves. But I sort of look at it as the you know, it's great to be like I want to look and see what's on the top of the mountain. I think that's maybe a little too ambitious to be connected to reality. I just want to know what direction should I go take my next step in if I want to get towards this goal or is any step at all futile? And have we already discovered the most unified version of the universe that there is? And this is it? And any new physics we have dark matter, dark energy, barriogenesis, it isn't built on unifying the framework we already have. It's some new framework or phenomena that's outside of our current standard picture with general relativity and quantum field theory.

00:28:19
Speaker 1: So let me just summarize for the listeners where we are. You're saying that anytime you bring things together into a unified picture, it generates new theoretical predictions. There are new elements of that theory that we can go out and search for. And in the past that's worked, like electricity and magnetism unify. We see these other pieces we unify electromagnetism with a weak force, we see the Higgs boson and the WZ sector. We see these things, but that recent efforts to try to unify everything else together has made predictions that we haven't been able to verify. And so the question is like, well, how do you construct this new complex theory of everything with all these extra moving pieces and then somehow make it so we don't see them in the universe to be consistent. But I also wanted to clarify one thing, which is the definition of what we're talking about for a unified theory, because you said a couple of times, we don't live in a universe with a unified theory, and I think what you're referring to is sort of phase changes in the universe. That it might be that when the universe was hotter and denser, all these things which look like different phenomena now looked more similar. That electricity and magnetism were more similar, they were more closely connected with the weak force, that the weak force had the same strength as electromagnetism, for example. But I was thinking about it more philosophically, like, even if various parts of the universe broke off at different times and changed into very different kinds of phenomena. Now, if we can connect them theoretically, I would still say that's a unified theory of everything, even if you know those pieces are still playing out in different ways today.

00:29:48
Speaker 2: Would you disagree?

00:29:50
Speaker 5: I can accept that as a as a valid perspective. That maybe isn't the one I share, but it's it's it's just not how I choose to look at it, because I sort of say, like, well, today, in our low energy universe, the electromagnetic force and the weak force, they aren't unified. I would not say that they are unified today. I would say that they are broken today because we live in a low energy universe. We don't live up at one hundred GeV of energy or higher. We live down in a milliev universe. If you look at the background energies of the universe, we live in a very low energy state. So I would say that if there's a theory of everything out there, if there's a unified theory out there, it has to be hiding up at not just high energies, but higher energies than we've ever observed. It has to be hiding at higher energies than the highest energy cosmic rays we've ever detected, which themselves are millions of times higher than the highest energies we have ever created in the laboratory. So our universe today is a low energy, non unified universe. But it is possible that we do come from a unified theory or a theory of everything that is just I would say, very badly broken today. And I also want to say, just to give people a little more historical context, is when you talked about that this has worked in the past. Right worked for electricity and magnetism, which is now electromagnetism. Worked for electroweek, which is you know, electromagnetism in week unified except broken by electroweak symmetry, breaking right the Higgs symmetry we have we have examples of like this is where it's worked, but we also have plenty of historical examples where we tried to unify things in ways that did not work.

00:31:40
Speaker 2: Right.

00:31:40
Speaker 5: We have collusion Kline theory, which tried to unify Maxwell's electromagnetism with Einstein's general relativity, which produces an extra field known as a dileton, which doesn't appear to exist in the universe, which also predicts cross terms where electromagnetism and gravity impact each other, which they do not do. So I would say we have a lot of false starts, right. We have technicolor theory, we have the Sakata model for baryons and masons, and those are brilliant ideas that turn out to not be reflected in reality. So I think it's very important to say, you know, yeah, we have we have a lot of different ways of or ideas of going about unification or grander theories or more comprehensive explanations. But the ones that disagree with reality we were smart enough to throw away, and the ones that agreed with reality we kept. And we say, look at those successes, but don't forget that the history of science was not just success success, success, success, And now we're like, oh, I don't know where to go next. At no point did we know where to go next. We had lots of ideas, and the ones that agreed with reality were the ones we kept, and the ones that disagreed with reality we let fall by the wayside. Because no matter you know, I I'm a theorist by trade, and so are a lot of people who talk about unified theories. But in the end, physics is an experimental, observational, measurement based science, and if your theory does not agree with the measurements and experiments and observations you make in the real world, it's not going to be accepted as a physical theory.

00:33:21
Speaker 3: So if I can try to, as a biologist, summarize a couple different camps here, So there's a is there a unified theory is one question, and some people might say yes, probably we just haven't found yet. Others might say no, there's no peak at all. But then there's another axis along which there's a debate, which is is the universe unified now or was it only unified in the past? Is that right? Or does everybody agree there's nothing unified now, we're just trying to look into the past.

00:33:48
Speaker 5: I think that I want to let Daniel answer this, because you already know what I would say. I want to hear what Daniel has to say about that, because I don't know what his answer is going to be.

00:34:01
Speaker 1: Yeah, well, I think we're not unified on the question of what we mean by unified. To me, it would be sufficient to have a theory which explains all of our phenomena and to have that theory be self consistent, even if there are various parts of it, even if it has broken symmetries within it. You know, even if electricity and magnetism are different in that theory in the sense that you know, there are two sides of the same coin, but they're not the same exactly in the same way that like the weak force, I consider electricity and magnantism to have been unified with a weak force in the sense that we have a consistent, coherent theory. We have one prediction. It makes a single prediction for what happens. You know, when you collide particles, for example, you don't have to use the weak force and electromagnetic force separately in some way. So that's the question for me about unity. I think you have a higher standard, which is like earlier in the universe, at higher energy, does this theory become even simpler? Do these things all become one so we have like a single force in the universe to rule them all. And I think that's a beautiful, ambitious goal. But I think it's beyond even what I would ask for. So let's take a break and come back and talk about the arguments for whether or not. It's possible to have a unified theory, either one that meets Daniel's requirements or Ethan's higher level demands. So let's talk about the arguments for and against the existence of a unified theory. For a lot of people and for me, one of the strongest arguments is that it's been working so far. If you just assume that the universe makes sense, that there is a single reason why things happen out there. You know why a particle goes this way or not that way, or you know why things interact to this level not at that level, that it works. We have made great progress in discovering laws of physics through experiment and deduction and inference on all of this stuff, and that the answer keeps getting simpler, right that over time we've harmonized various kinds of phenomena into smaller numbers of things, and now we're left essentially, as you said, with quantum mechanics and general relativity, and we're stuck at this level so far. But you know, the trend of history seems to be with us. What do you guys think of that argument? Is that compelling?

00:36:29
Speaker 5: I mean, this is sort of like the argument of like, should we all be taking our shoes off at the airport because we know that sometimes there's a shoe bomber at the airport, and so this way, if everyone takes their shoes off, we won't repeat the previous mistake that we made by letting the shoe bomber on the plane. Is like, this is great for addressing yesterday's problems, this is not necessarily great for addressing today's problems, right, because everything works until it doesn't. And I would argue that the evidence we have today strongly suggests that all of the avenues we've been pursuing towards unification have been showing consistent with null evidence that there's no evidence for proton decay, no evidence for extra dimensions, no exvidence for grand unification, no evidence for quantum gravity, no evidence that gravity and the other forces unify, no evidence that the strong force and the electroweak force unify. And so you really just start saying, like, but but it's pretty but I would like it if it did. But it works in the past, and I would These are great arguments for someone who's completely ignorant about the existence of experimental data driven physics, which is not us. We know about that, and that's what we confront our universe with. So it's it's great for I have a motivation in the absence of any data, and then I say, okay, now, now come down off your theory cloud and come meet reality. And so what does reality say? And the same people who say those things that you brought up just now, Daniel, they don't want to talk about reality. And that's that's that's a bit off putting to me.

00:38:12
Speaker 3: So like again the biologists jumping in to see if she can summarize it in one sentence. So, are you saying that you don't believe in a unified theory because reality just suggests it doesn't exist, or we just haven't found the right unified theory yet.

00:38:25
Speaker 5: I would say that no one should believe in something that the data you know, you shouldn't. You shouldn't have beliefs in things that the data's butt can't cash as far as like a check goes. So like if you wanted to say, like, but I think in my heart or in my gut that there's got to be a unified theory out there because I just know it, Like that's great, but this is this is the same argument that has led us astray for for countless malays since before we were writing down human history. I know in my gut, I know in my heart, I feel the instinct. This is not how we do science. This is fine if you want to say, oh, I can write down a mathematic theory, and is this mathematics interesting? Sure, the mathematics is interesting, but mathematics is outstanding at taking physicists to worlds that never were Only one form of mathematics actually winds up reflecting the reality we live in. It's sort of like if I asked you, hey, what's the square root of four and you said, I know this one, it's two. Two is the square root of four? I would say are you sure? And you'd go, oh, no, he's trying to trick me. Why is he trying to trick me? And I would say, well, it could be two, or it could be negative too, and you would go, oh, yeah, that's right. Square roots can have plus or minus solutions. Then I say, but that's only true mathematically. If I say I'm going to throw this ball and the ball is going to land at the square root of four, you can go measure that ball and find if it's at plus two or minus two. Our universe gives one answer to physical questions, and we can only find that out by measuring it. So, as a theorist, your goal is to provide not just the possible spectrum of explanations. You want to be able to have someone who uses your theory predict the answer. And until we get concrete, unique predictions that we can test against reality, we only have ideas. We don't have something that's worth believing, got it? Believing requires evidence in a physical science.

00:40:48
Speaker 1: Wow, that's some hardcore skepticism.

00:40:50
Speaker 2: I love it.

00:40:51
Speaker 3: Is it skepticism? I feel like you just said you need data?

00:40:54
Speaker 5: Do you want to provide a counter argument? Though? Like, what would someone say who thinks that? No, Ethan, You're you're too cold in your skepticism here, Like, what would you say to argue against that?

00:41:07
Speaker 2: All right? Fair?

00:41:07
Speaker 1: I will play that role, you know. I think the argument is not we don't need data. We just have to think about things. I mean, we're not the ancient Greeks, right, But I think the argument is instead, we can be inspired by things like simplicity or beauty, or just random moments of inspiration. Science tells us we have to go out and test our theories. It doesn't tell us necessarily how we have to come up with those theories, right, So you could be like, you know, smoking banana peels and have an idea and then just go and test it and if it works, great, And so I think it's okay to be motivated by, you know, esthetic preferences for various kinds of theories, as long as it's not the only way you're looking around for ideas, and to be motivated by what has worked in the past. You know, I agree with you. Of course, you shouldn't believe things that don't have data to support them. But you know, I think what we're talking about here is more like what questions do you ask? What ideas do you try? And so I think it's valid to continue to try things which have worked in the past.

00:42:13
Speaker 5: You but you also agree that it's really essential to the scientific process that you do come back to data, that whatever theory you come up with, you do connect it with reality, with an observable, with a measurable thing, and that if your theory does not give you something that's born out as a measurement, that's born out by experiment, by observation, then you can't just accept it or believe in it because of you know, esthetic reasons or natural reasons or beauty reasons, that you really have to say, no, there's physical evidence that supports that this picture of reality is true.

00:42:51
Speaker 1: Absolutely I agree with that. But then let me make a different argument than in favor of a unified theory, which is a little bit more flowery. And and that's this argument about mathematics. That you know, so far the universe seems to be well described by mathematical theories, right that, as you say, you have to go and write down your theory and makes predictions, you can test them, et cetera.

00:43:14
Speaker 2: And all those theories.

00:43:14
Speaker 1: Are mathematical, and the mathematics is so powerful, so famously unreasonably effective, that it's not unreasonable to argue that, like, hmm, maybe mathematics is part of the universe. We're discovering it. It's out there. The universe itself is mathematical. There's math that runs as sort of the source code of the universe. And if that's true, then there has to be some math that describes the universe, even if we haven't found it yet. What would you say to that argument? Do you believe that math is discovered.

00:43:45
Speaker 2: Or do you think that it's invented by humans?

00:43:48
Speaker 5: I mean, for me, I look at math as math is the best language we have for quantitatively describing anything. As soon as you start asking the question how much in what amount? As soon as you go from asking, well, qualitatively, what's going to happen? Like will it exist or not? And you start asking how much? You need mathematics to describe it. And that's kind of the basics of what physics and physical sciences are is it is a quantitative science. We do care about how much, we do care about what amount, And so the idea that you would be able to describe that without mathematics is alien to our understanding of how nature works. The very fact that we dare to ask the question how much means it's like it's like tautologically means that we have to describe it mathematically, because if we want to know the answer to the question how much, a non mathematical description will not give you that answer. So it's because we chose to investigate the un verse in this way that of course it's describable in terms of mathematics. But again, I want to strongly reinforce. Just because mathematics exist doesn't mean that it corresponds to anything in physical reality. We can invent all sorts of mathematics that cannot correspond to reality we know of. Like oh, like I went to a mathematics conference when I was a grad student that on mathematical physics, and I was like, well, hang on, someone is giving a talk on the E ten exceptional group. And I said, but but you can't have more than E eight. You're not a group anymore. And they're like, oh, yeah, well we just ignore that and we just continue and write it down anyway, and we just apply the rules that used to work for the things that were groups and we apply them to this too, and we see what comes out of it. I was like, but can you do that? Can you rigorously do that? Can you do that in a logical, self consistent manner? And some people don't care, and they just do it anyway, regardless of what the answer to that question is. And you know, I would say, as you put it, Ethan has a more stringent requirement than some other physicists do. He's like, hey, you have to connect it to reality too, Like this is this is nuts. Like I don't want to be hampered by reality. But I say you do, because if you have, for example, a theory or a framework that predicts the presence of a large number of flavor changing neutral currents in particle physics, like that predicts I can go from a heavy unstable quark directly to a lighter quark with the same quantum numbers, except that's a lighter flavor that has the same electric charge, but a lighter flavor. We have enormous constraints from reality, from experiments, from observations, from colighter data that we know that doesn't happen. To a shocking degree, Almost any attempt towards unification that you can write down is going to have enormous numbers of these flavor changing neutral currents. So I would say if you want to take that approach, you immediately are faced with the problem of how do I suppress the extra ingredients I'm adding in that are inconsistent with already established reality. I'm not saying you can't do it. I'm saying that's a challenge, that's a hurdle you have to clear or your theory is dead in the water. From the start.

00:47:32
Speaker 3: I'm enjoying this gloves off de base.

00:47:35
Speaker 1: Well, let me throw another philosophical argument at you, Ethan, And I'm happy to take either side of this. So I'm curious what your thoughts are. What are your thoughts on the possibility that there could be multiple theories of everything? I mean, what if we eventually figure out quantum gravity and we have, you know, some theory of strings and it works, and you know, somebody comes up with a way to test it, and we do experiments and boom if they are confirmed, and we have this fantastic theory and incorporate dark energy and dark matter and all of our questions are answered. And then aliens arrive one day and they have another theory and it's not strings, it's shmings or something, and it also exactly and it also explains everything. Do you think that's possible or do you think there's a demand that the universe has a single reason for everything?

00:48:22
Speaker 5: Oh my goodness, I mean, so I'm gonna answer your question by talking about something entirely different, which is, how do you interpret quantum mechanics. What is your philosophy on quantum mechanics? Do you say, oh, you know, well, these things that we call particles, they aren't really particles. They're wavelike entities while they propagate, and only when you observe them do they actually interact like particles. Or you know, the particle is actually a wave function, and the wave function is what's fundamental and universal. And when I make a measurement, I'm not even collapsing the wave function. I'm just selecting out which aspect of the wave function is most accurately represented by our universe. Or you know, do I take the uncertainty away from the particle entirely and move all of that uncertainty and probabilitiness into the quantum operator. Right, this is if it's sort of like you're asking me which interpretation of quantum mechanics is right, and I'm going to tell you they're all equally right, because they all give you the same answers. I don't think there's any way to tell them apart. So if you're telling me like, oh, yeah, well, we use strings, but we the other aliens out there they use springs and it works just great. They just said like buoying and here we go, right that that's great. I put a dart. I put a harmonic oscillator into my Newtonian gravity term and boom, I get dark energy out. It's perfect spring theory. There we go, and I believe that or not that actually works, but but you know it's not doesn't lead anywhere, which is also a problem. I would say, yeah, of course, you can have many different equivalent mathematical formulations of the same theory, and they can all be equally correct. The only way you can say I demand a unique thing is when you start devising ways to test different predictions that arise from these different ways of looking at it against each other. Many mathematical theories are dual or holomorphic or isomorphic to one another. And so if someone says, well, I've formulated so thirty two string theory, and someone goes, well, I've formulated eight cross eight super string theory. And someone says, well, my theory only has twenty six dimensions, they go, well, mine only has ten. Is like, actually, I can show you that these are mathematically equivalent. Like they'll both yell how dare you? And then they'll be like, oh crap, there's also three others that are also equivalent to this, and so yeah.

00:51:01
Speaker 1: All right, So I thought you were arguing on one side. I thought you were going to say, look, it doesn't matter, you can have two different stories that explain the universe. But I think you're actually arguing that the contrary point of view. You're saying, if there are two theories that explain the universe, they're going to be mathematically equivalent. You can have a mapping from one to the other that you couldn't have two incoherent stories.

00:51:21
Speaker 5: My argument is if they're not mathematically equivalent, if there are actually major differences between different theories that equally describe the universe, then theoretically there's some difference between them that will manifest physically that you could go out test, measure and determine which one's right and which one's less right.

00:51:43
Speaker 1: All right, So that leads me to the last topic on this question, which is reductionism. Right, if we think that the universe is controlled by the microscopic reality, that everything bubbles up from what's happening at the smallest scale, which so far has seemed to work, then shouldn't there be a way to test the ultimate theory? Shouldn't there be one answer to what's happening down there? As you just said, there should be some physical consequence to this theory. What do you think about that? Do you think that we're guaranteed to have some fundamental layer of reality if we keep building bigger and bigger accelerators, eventually we will expose and probe the base layer of reality. Or do you think there's a risk that it's like you know, turtles all the way down, just effective theories forever.

00:52:26
Speaker 5: The fact that you use the word guarantee makes me say there's no way. There's no way I can guarantee this because we don't get to tell nature how it works. We don't get to say nature, this is how you have to work. You can say, Look, all I can do is say I've made a good approximation of reality. It works for now, and if I go down to the next level, the next level, the next level, each level I go to, I'm testing it. I'm testing is this approximation still good? At some point it might break down. If you had come to me one hundred and fifty years ago and said, Ethan, cause and effect, that's the way everything has to work, I would have said, yeah, I believe that. Because we hadn't discovered quantum mechanics, we hadn't discovered radioactive decay, We hadn't discovered probabilistic wave function behavior yet, but that exists. That's a part of nature. So I don't know what assumptions we're making today that are going to be proven that that's not congruent with reality in the future. I just know enough to keep an open mind that maybe some of these assumptions are not necessarily good. All the way down to the level of the bottom turtle.

00:53:34
Speaker 1: All right, well, thanks very much for answering all of our questions today and taking us down to the bottom turtle.

00:53:39
Speaker 2: We really appreciate you coming on.

00:53:41
Speaker 5: Thank you. It's been my pleasure to be here. And thank you Daniel. Thank you Kelly for hosting me and having an extraordinary conversation about yet another aspect of the universe we still aren't sure about.

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