Must there be a single unified theory of physics?

Daniel and Kelly talk to Ethan Siegal about whether we should expect physics to be explained by a single unified theory. See omnystudio.com/listener https://omnystudio.com/listener for privacy information.

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:00Speaker 00

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• 0:40Daniel Whiteson?

  I got into physics because I want to know how things work. What's going on? What are the rules of the universe? Because I figured there must be rules and there must be reasons why this happens and not 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. They're two sides of the same coin. And this all makes sense if there is, in fact, a single reason why things happen, a unified

• 1:23Daniel Whiteson?

  theory to explain it all. We might be discovering pieces of it and then snicking them together. We're making zigzagging progress towards this final 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?

• 2:06Daniel Whiteson?

  Or maybe they'll tell us the whole project is hopeless. Either way, welcome to Daniel and Kelly's extraordinary unexplained universe.

• 2:28Kelly Weinersmith?

  Hello, I'm Kelly Wiener-Smith. I study parasites and space.

• 2:31Daniel Whiteson?

  Hi, I'm Daniel. I'm a particle physicist and I really do believe there is a reason why things happen.

• 2:38Kelly Weinersmith?

  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?

• 2:54Daniel Whiteson?

  Oh, wow. Back in time to be present for a physics discovery? No, no, I'm gonna flip the question. I want to go forwards in time.

• 3:01Kelly Weinersmith?

  No, no, Daniel. No, we're talking about going forwards. I'm specifically asking you a backwards looking question.

• 3:07Daniel Whiteson?

  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 in the cosmos and how it all works. Must have been mind blowing.

• 3:30Kelly Weinersmith?

  Yeah.

• 3:30Daniel Whiteson?

  Also, I would have liked to bring him like a mug of hot cocoa because I think he got chilly.

• 3:36Kelly Weinersmith?

  That's really nice. Maybe he would have named something after you for that, you know?

• 3:40Daniel Whiteson?

  Exactly. Also, I wish I could have been there to point out to him that he discovered Neptune without realizing it.

• 3:46Kelly Weinersmith?

  Oh.

• 3:46Daniel Whiteson?

  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 Galileo actually missed out on a great discovery.

• 4:03Kelly Weinersmith?

  So you actually want to go back and tinker with the past? That's interesting. I just meant observing.

• 4:08Daniel Whiteson?

  Anytime you go back, you're gonna tinker with it. There is, it's all quantum mechanical. You can't observe it without interacting, right?

• 4:15Kelly Weinersmith?

  All right.

• 4:16Daniel Whiteson?

  If I go back and chat with Galileo, then I'm gonna change the course of history.

• 4:20Kelly Weinersmith?

  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 puzzle at some point. And I, you know, I would like to go forward to that moment if I could.

• 4:37Daniel Whiteson?

  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, we assume

• 5:20Daniel Whiteson?

  the universe works this way, and we use it as the foundation of basically the whole scientific method, and we're just gonna keep going until it breaks down.

• 5:28Kelly Weinersmith?

  Hmm. I feel like I'm still not totally convinced you haven't just said like a hand-wavy, guru-y, "everything happens for a reason."

• 5:36Daniel Whiteson?

  Well, you know, if you buy my crystal and hang it around your neck, then you can control those reasons.

• 5:42Kelly Weinersmith?

  But you know what people should buy?

• 5:45Kelly Weinersmith?

  Do aliens speak physics?

• 5:47Daniel Whiteson?

  That's right. 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? Is that possible? 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 gonna touch on in today's episode. But there's a deep dive on all of that in my book, Do Aliens Speak Physics?, out November 4th.

• 6:31Kelly Weinersmith?

  11 out of 10, or six out of five stars. Best book ever.

• 6:35Daniel Whiteson?

  Amazingly good reviews from the parasitologists who've read it.

• 6:38Kelly Weinersmith?

  Yeah, well, this parasitologist was very impressed with the deep dive and the entertaining and clear way it was presented.

• 6:45Daniel Whiteson?

  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. Thanks very much. But today, we're not just here to sell my book. We're here to talk about the big theories of the universe. And so first, I asked our audience if they thought that there needed to be a single unified theory of physics out there for us to discover. Here's what folks had to say.

• 7:13Speaker 03

  I don't think so, because maybe some things are separate.

• 7:18Speaker 04

  Just the feeling that I have that the universe is like Russian Matryoshka dolls, nested dolls, except that there's no final kernel at the end. It'll just keep going and going.

• 7:34Speaker 05

  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.

• 7:45Speaker 06

  It would be ideal to have a single unified 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.

• 8:00Speaker 07

  The expectation has been, yeah, one ring to rule them all, and in the darkness, bind 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.

• 8:19Speaker 08

  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 for us.

• 8:33Speaker 09

  It's a human lust to have, like, simplified down to one. A single unified theory of physics sounds like the search for a very simple solution to a very complex problem.

• 8:46Speaker 09

  And those seldomly work out, so I vote no.

• 8:51Speaker 10

  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.

• 9:10Kelly Weinersmith?

  As ever, amazing answers from the audience.

• 9:13Daniel Whiteson?

  Absolutely. Thanks, everyone, for contributing your hilarious ideas. And so let's jump into the episode. Today, we're actually joined by a friend of mine, a fellow physicist and podcaster and science communicator extraordinaire Ethan Segal. 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 Webb Space 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 100 million years. Ethan, thanks very much for joining us today.

• 9:55Speaker 11

  Oh, it's my pleasure to be here. Thanks for inviting me to an extraordinary conversation about the universe.

• 10:01Kelly Weinersmith?

  I can tell you're going to fit right in already.

• 10:04Daniel Whiteson?

  So today, we're talking about the concept of a unified theory. Is it possible to have a unified theory of physics? What are the arguments for it and against it? But as usual, because it's a 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?

• 10:23Speaker 11

  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

• 11:07Speaker 11

  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're all quantum field theories that 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 does not, you know, if you say, I'm going to take an electron, I'm going to pass it through a double slit. You say, great, I 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 and give you all of that.

• 11:48Speaker 11

  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 I can't deal with that. I 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 it even goes beyond, I'm going to take general relativity, which I don't know how to do, and I'm going to make it quantum two and make it play nice with with these quantum forces.

• 12:33Speaker 11

  Or maybe I'll take the quantum forces and make them play nice on general relativity's footing. We 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. Thing would be some framework where all of these different questions were described

• 13:12Speaker 11

  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 and put them in a single framework where you can explain and derive everything about our universe.

• 13:45Kelly Weinersmith?

  Well, that that sounds pretty straightforward. Why haven't y'all like figured that out yet? Says the biologist.

• 13:51Speaker 11

  I know, right? It's sort of like it's sort of like the question of like if I 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 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 strato volcano shape. That's what Mount Fuji looks like. And if you came to the United States in the Pacific Northwest, prior to 1980, you would

• 14:36Speaker 11

  have discovered, oh, Mount St. Helens is known as the Mount Fuji of the West. Or because because you that that's us like we're the Mount Fuji on the other side of the world. And then in 1980, Mount St. Helens, 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?

• 15:19Speaker 11

  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 gonna assume there's a unified theory. I'm gonna work on that. And then you want to, as a physicist, as someone who's connected to reality, you wanna 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 observe or measure.

• 16:02Speaker 11

  Their predictions are, you know, it's, it's really an exercise in like, oh, no, company's coming over and I'm gonna do like a cartoon and I'm gonna lift up the rug and I'm gonna 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 wanna work to unify 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 in our universe.

• 16:47Speaker 11

  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 it spinning? It spins like your left hand's 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?

• 17:26Speaker 11

  And where are all the left-handed anti-nutrinos? 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, two 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,

• 18:08Speaker 11

  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.

• 18:50Speaker 11

  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.

• 19:32Speaker 11

  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, is that is that how we really do science? Is that something we would accept as like, oh, that's a good story for how our universe is? We really, we really demand something more than that.

• 19:54Daniel Whiteson?

  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?

• 20:12Speaker 11

  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.

• 20:24Daniel Whiteson?

  Right.

• 20:25Speaker 11

  And there's no simpler way, you can actually prove this. There's no simpler way to explain what 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,

• 21:08Speaker 11

  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 gonna make something simpler or that contains less and then the more stuff comes out of it. Like, that's, that's not how math works. You can't, you can't make a, 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

• 21:46Speaker 11

  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 will 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.

• 22:21Daniel Whiteson?

  I do. 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 clicked those together without creating 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.

• 23:07Daniel Whiteson?

  We have to take a break, but when we come back, we're going to hear Kelly's question about unified theories in physics.

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• 25:01Speaker 14

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• 26:19Daniel Whiteson?

  Okay, we're back and we're talking to Dr. Ethan Segal about unified theories in physics. And I'm dying to hear what Kelly wants to know.

• 26:27Kelly Weinersmith?

  I guess I was just gonna ask, are physicists really sure that all of the smaller pieces are separate? Like, 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 100% confident in all of the smaller pieces of the puzzle already?

• 26:45Speaker 11

  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 1960s, and I'm going to credit Shelley Glashow 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 electro weak theory. And so this is actually a part of the standard model. It says that

• 27:28Speaker 11

  here at our 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 Tevatron, and now at the Large Hadron Collider at CERN, you can actually say, oh, no, it looks like electro weak unification does happen. And that the theory of electro weak symmetry breaking is where the Higgs sector and the Higgs boson comes from. It's why we have the W and Z bosons be very massive instead of massless, because when the symmetry breaks,

• 28:12Speaker 11

  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 directions. Degrees of freedom and that produces three very massive bosons, the W and 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, uh, 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

• 28:56Speaker 11

  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. Cool.

• 29:02Daniel Whiteson?

  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. We see it.

• 29:11Speaker 11

  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 you said, displacement currents. In the quantum world, this leads to things like the Aharonov Bohm 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

• 29:51Speaker 11

  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 Nambu 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 extras form of unification, and that things are not unified now, where are those extras?

• 30:33Speaker 11

  Components that needed to come out of it. 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 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?

• 31:15Speaker 11

  And have we already discovered the most unified version of the universe that there is? And this is it. And and any new physics we have, dark matter, dark energy, baryogenesis. 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.

• 31:38Daniel Whiteson?

  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 W Z 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.

• 32:21Daniel Whiteson?

  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.

• 32:59Daniel Whiteson?

  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. Would you disagree?

• 33:08Speaker 11

  Um, 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 a hundred GEV of energy or higher. We live down in, in a milla EV universe. If you look at the background energies of the universe, we, 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.

• 33:51Speaker 11

  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?

• 34:35Speaker 11

  Worked for electricity and magnetism, which is now electromagnetism, worked for electro weak, which is, you know, electromagnetism and weak unified. Except broken by electro weak 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. Right? We have Kaluja Klein theory, which tried to unify Maxwell's electromagnetism with Einstein's general relativity, which produces an extra field known as a dilepton, 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.

• 35:18Speaker 11

  So I would say we have a lot of false starts, right? We have technicolor theory, we have the Sakata model for baryons and mesons. 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.

• 36:03Speaker 11

  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'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.

• 36:40Kelly Weinersmith?

  So if I can try to, as the biologist, summarize a couple of 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 it 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.

• 37:08Speaker 11

  I think that I want to let Daniel answer this because I, 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.

• 37:20Daniel Whiteson?

  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, they're 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 magnetism 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

• 38:05Daniel Whiteson?

  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.

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• 41:38Daniel Whiteson?

  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. There, you know, why a particle goes this way or not that way, or you know, why things interact at this level and not at that level. That it works. So 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.

• 42:23Daniel Whiteson?

  And we're, you know, 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?

• 42:32Speaker 11

  I mean, the, this, this is sort of like the argument of like, should we all be taking our shoes off at the airport? Because we, 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.

• 43:14Speaker 11

  That there's no evidence for proton decay, no evidence for extra dimensions, no evidence 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 electro weak force unify. And so you really just start saying like, but, but it's pretty. But I would like it if it did, but, but it worked in the past.

• 43:38Daniel Whiteson?

  But I would like it.

• 43:39Speaker 11

  These are great arguments for someone who's completely ignorant about the existence of experimental data-driven physics, which is not us. We, 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, uh, they don't want to talk about reality. And that's, that's, that's a bit off putting to me.

• 44:15Kelly Weinersmith?

  So like, again, the biologist 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?

• 44:27Speaker 11

  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, I think in my heart or in my gut that there's gotta 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 millennia. Since, since before we were writing down human history, I, I know in my gut, I know in my heart, I feel the instinct. This is not how we do science.

• 45:12Speaker 11

  This is, this is fine if you want to say, oh, I can write down a mathematic theory and, and isn't 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, uh, 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 two. And you would go, oh yeah, that's right. Square roots can have plus or minus solutions.

• 45:57Speaker 11

  Then I say, but that's only true mathematically. If I say, I'm going to throw this ball and the ball's 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-

• 46:25Speaker 11

  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.

• 46:46Kelly Weinersmith?

  Got it.

• 46:47Speaker 11

  Believing requires evidence in a physical science.

• 46:51Daniel Whiteson?

  Wow, that's some hardcore skepticism. I love it.

• 46:54Kelly Weinersmith?

  Is it skepticism? I feel like he just said you need data.

• 46:57Speaker 11

  Do you want to provide a counterargument, though? Like, like, what would someone say who thinks that, no, Ethan, you're you're too cold in your skepticism here? Like, what what would you say to to argue against that?

• 47:09Daniel Whiteson?

  All right, fair. 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 OK to be motivated by, you know, aesthetic preferences for various kinds of theories,

• 47:51Daniel Whiteson?

  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.

• 48:15Speaker 11

  But 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.

• 48:29Daniel Whiteson?

  Of course.

• 48:29Speaker 11

  And that if your theory does not give you something that's borne out as a measure. That's borne out by experiment, by observation, then you can't just accept it or believe in it because of, you know, aesthetic reasons or natural reasons or beauty reasons that you you really have to say, no, there's physical evidence that supports that this picture of reality is true.

• 48:54Daniel Whiteson?

  Absolutely, I agree with that. But then let me make a different argument then in favor of a unified theory, which is a little bit more flowery and philosophical. And that's this argument about mathematics that, you know, so far the universe seems to be well described by mathematical theories, right? As you say, you have to go and write down your theory and make predictions. You can test them, etc. And all those theories 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.

• 49:37Daniel Whiteson?

  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? Or do you think that it's invented by humans?

• 49:50Speaker 11

  I mean, for 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

• 50:35Speaker 11

  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 universe in this way. That of course, it's describable in terms of mathematics. But again, I want to, I want to strongly reinforce. Just because mathematics exist, doesn't mean that it corresponds to anything in physical reality.

• 51:18Speaker 11

  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, someone is giving a talk on the E10 exceptional group. And I said, but, but you can't have more than E8. 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?

• 52:02Speaker 11

  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, as you put it, Ethan has a more stringent requirement than, than some other physicists do. He's like, ah, you have to connect it to reality too. Like this is, this is nuts. Like, I don't, 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

• 52:44Speaker 11

  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 collider data that 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?

• 53:26Speaker 11

  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.

• 53:35Kelly Weinersmith?

  I'm enjoying this gloves off debate.

• 53:38Daniel Whiteson?

  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, they are confirmed. And we have this fantastic theory and incorporates 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 schmings or something. And it also, exactly. And it also explains everything. Do you think that's possible? Do you or do

• 54:20Daniel Whiteson?

  you think there's a demand that the universe has a single reason for everything?

• 54:25Speaker 11

  Oh, my goodness. I mean, so I'm gonna, I'm gonna answer your question by talking about something entirely different, which is, how do you interpret quantum mechanics?

• 54:37Speaker 11

  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 wave like 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 probabliteness into the quantum operator? Right?

• 55:22Speaker 11

  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 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 we the other aliens out there, they use springs and it works just great. They just said like boing and then here we go. Right? That's great. I put a dark, 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 believe it or not, that actually works, but you know, it's not, doesn't lead you anywhere, which is also a problem.

• 56:06Speaker 11

  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 I demand a unique thing is when you start devising ways to test Right. 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 formulated SO32 string theory and someone goes, well, I formulated E8 cross E8 superstring theory. And someone says, well, my theory only has 26 dimensions.

• 56:51Speaker 11

  They go, well, mine only has 10 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.

• 57:03Daniel Whiteson?

  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 the 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.

• 57:24Speaker 11

  My my argument is if they're not mathematically equivalent, if there are actually major differences between different theories that equally describe the universe, then

• 57:34Speaker 11

  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.

• 57:45Daniel Whiteson?

  All right, so that leads me to my 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?

• 58:28Speaker 11

  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 150 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

• 59:10Speaker 11

  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.

• 59:36Daniel Whiteson?

  All right. Well, thanks very much for answering all of our questions today and taking us down to the bottom turtle. Really appreciate you coming on.

• 59:43Speaker 11

  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.

• 1:00:01Kelly Weinersmith?

  Daniel and Kelly's Extraordinary Universe is produced by iHeartRadio. We would love to hear from you.

• 1:00:06Daniel Whiteson?

  We really would. We want to know what questions you have about this extraordinary universe.

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  We want to know your thoughts on recent shows, suggestions for future shows. If you contact us, we will get back to you.

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  We really mean it. We answer every message. Email us at [email protected].

• 1:00:25Kelly Weinersmith?

  Or you can find us on social media. We have accounts on X, Instagram, Blue Sky, and on all of those platforms you can find us at D and K Universe.

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  Don't be shy. Write to us.

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