Daniel and Kelly’s Extraordinary Universe - Must there be a single unified theory of physics?

Episode Date: October 16, 2025

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 for privacy information.

This is an I-Heart podcast. Hey, it's Ed Helms host of Snafu, my podcast about history's greatest screw-ups. On our new season, we're bringing you a new snafu every single episode. 32 lost nuclear weapons. Wait, stop? What? Yeah, it's going to be a whole lot of history, a whole lot of funny, and a whole lot of fabulous guests. Paul Shearer, Angela and Jenna, Nick Kroll, Jordan, Klepper.

Listen to season four of Snafoo with Ed Helm. on the IHeart radio app, Apple Podcasts, or wherever you get your podcasts. There's a vile sickness in Abbas Town. You must excise it. Dig into the deep earth and cut it out. From IHeart Podcasts and Grimm and Mild from Aaron Manky, this is Havoc Town. A new fiction podcast sets in the Bridgewater Audio Universe, starring Jewel State and Ray Wise. Listen to Havoc Town on the iHeartRadio app, Apple Podcasts, or wherever you get your podcasts.

I'm Jonathan Goldstein, and on the new season of heavyweight... And so I pointed the gun at him and said this isn't a joke. A man who robbed a bank when he was 14 years old. And a centenarian rediscovers a love lost 80 years ago. How can a 101-year-old woman fall in love again? Listen to heavyweight on the I-Heart Radio app, Apple Podcasts, or wherever you get your podcasts. Welcome to Decoding Women's Health. I'm Dr. Elizabeth Pointer, chair of women's health and gynecology at the Atria Health Institute in New York City. I'll be talking to top researchers

and clinicians and bringing vital information about midlife women's health directly to you. A hundred percent of women go through menopause. Even if it's natural, why should we suffer through it? Listen to Decoding Women's Help with Dr. Elizabeth Pointer on the IHeartRadio app, Apple Podcasts, or wherever you get your podcasts. 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 happened. 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 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, or maybe they'll tell us the whole project is hopeless. Either way, welcome to Daniel and Kelly's extraordinary unexplained universe. Hello, I'm Kelly Wienersmith. I study parasites and space. Hi, I'm Daniel.

I'm a particle physicist, and I really do believe there is a reason why things happen. 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? Oh, wow. Back in time to be present for a physics discovery. No, no, I'm going to flip the question. I want to go forwards in time.

No, no, Daniel. No, we're talking about going forwards. I'm specifically asking you a backwards-looking question. Okay, a 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. Also, I would have like

to bring him like a mug of hot cocoa because I think you got chilly up there in those evenings. That's really nice. Maybe he would have named something after you for that, you know? Exactly. Also, I wish I could have been there to point out to him that he discovered Neptune without realizing it. 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. So you actually want to go back and tinker with the past. That's interesting. I just meant observing. Anytime you go back,

you're going to tinker with it. It's all quantum mechanical. You can't observe it without interacting, Right. So, yeah. If I go back and chat with Galileo, then I'm going to change the course of history. 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. 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 the universe works this way. And we use it as the foundation of basically the whole scientific method. And we're just going to keep going until it breaks down. Hmm. I feel like I'm still not totally convinced you haven't just said like a hand-wavy guruy.

Everything happens for a reason. Well, you know, if you buy my crystal and hang it around your neck, then you can control those reasons. But you know what people should buy? Do aliens speak physics? 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 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 out November 4th? 11 out of 10 or 6 out of 5 stars. Best book ever. Amazingly good reviews from the parasitologists

who've read it. Yeah, well, this parasitologist was very impressed with the deep dive and the entertaining and clear way it was presented. 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 in my 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. I don't think so because maybe some things are separate. Just the feeling that I have that the universe is like Russian Matryoska dolls, nested dolls, except that there's no final kernel at the end. Let's just keep going and going. 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.

It would be ideal to have a singly-fired 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. 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 latter, beyond what we can conceive?

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. 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, and those seldomly work out. So I vote no. 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.

As ever, amazing answers from the audience. 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 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 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. Oh, it's my pleasure to be here. Thanks for inviting me to an extraordinary conversation about the universe. I can tell you're going to fit right in already. 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? Well, 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? Great. 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're 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 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 can do all my quantum stuff for where is the electron? What's its momentum? Where is 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 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 these quantum 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? Why is there a

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. Well, that sounds pretty straightforward. Why haven't y'all like figured that out yet, says the biologist. 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 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 you came to the United States in the Pacific Northwest, prior to 1980, you would have discovered, oh, Mount St. Helens is known as the Mount Fuji of the West. Because 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 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 going to go for the big prize. I'm going to assume there's a unified theory. 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 make predictions that are way outside of what we can observe or measure. Their predictions are, you know, it's really an exercise in like, oh, no, companies coming over and I'm going to do like a cartoon and I'm going to 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 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 want to 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. 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? 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 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, 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 super symmetry and where are my extra Higgs bosons that the LHC should have found? Where's the lightest super symmetric 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 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 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.

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? Well, if you talk about a unified theory, the first job of any new 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. 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 going to 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's not how math works. You can't, 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 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. 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. We have to take a break. but when we come back, we're going to hear Kelly's question about unified theories in physics. Hey, it's Ed Helms, and welcome back to Snafu, my podcast about history's greatest screw-ups. On our new season, we're bringing you a new snafu every single episode. 32 lost nuclear weapons. Wait, stop? What?

Yeah. Ernie Shackle. and sounds like a solid 70s basketball player. Who still wore knee pads? Yes. It's going to be a whole lot of history, a whole lot of funny, and a whole lot of guests. The great Paul Shear made me feel good. I'm like, oh, wow.

Angela and Jenna, I am so psyched. You're here. What was that like for you to soft launch into the show? Sorry, Jenna, I'll be asking the questions today. I forgot whose podcast we were doing. Nick Kroll. This story is good enough to get you to toss that sandwich. So let's see how it goes.

Listen to season four of Snap-Fu with Ed Helms on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. There's a vile sickness in Abbas Town. You must excise it. Dig into the deep earth and cut it out. The village is ravaged. Entire families have been consumed. You know how waking up from a dream?

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And he got down. And I remember feeling kind of a surge of like, okay, this is power. Plus, my old friend Gregor and his brother tried to solve my problems through hypnotism. We could give you a whole brand new thing where you're like super charming all the time. Being more able to look people in the eye. Not always hide behind a microphone. Listen to Heavyweight on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. What's up, everybody?

This is Snacks from the TrapNer's podcast, and we're bringing you the horror every week all October long. Kicking off this month, I'll be bringing you all my greatest fear-inducing horror games from Resident Evil to Silent Hill. Me and Tony bringing back fire team on Left for Dead, too. And we're just going to be going over some of the greats. Also in October, we'll be talking about our favorite horror and Halloween movie and figuring out why black people always got to die first. The umbral reliquary invites any and all fooling, brave enough, to peruse its many curiosities. But take heed, all sales are final.

Weekly horror side quests written and narrated by yours truly. With a full episode read and a commentary special. And we will cap it off with horror movie battle royale. Jason versus Freddie. Michael Myers versus the 80 thing with the little tongue muster. October, we're doing it Halloween style. Listen to the Travener's podcast from the Black Effect Podcast Network on the IHard Radio app, Apple Podcasts, or wherever you get your podcast. Okay, we're back and we're talking to Dr. Ethan Siegel about unified theories in physics.

and I'm dying to hear what Kelly wants to know. I guess I was just going to 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? 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

electroweak theory. And so this is actually a part of the standard model. It says that 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 Tevotron, and now at the large Hadron Collider at CERN, you can actually say, oh, no, it looks like ElectraWeak unification does happen. And that the theory of Electro Week 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 W&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 a 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. 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. 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-Bome effect in the electro-week 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 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 extra components that needed to come out of it? Now, 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&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, bariogenesis, 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. So let me just summarize for the listeners where we are. You're saying that any time 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 the 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? 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. Would you disagree? I can accept that as a valid perspective, that maybe isn't the one I share, but 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 100 GEV of energy or higher. We live down in a millevee 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've 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 electro-week, which is, you know, electromagnetism and weak, unified, except broken by electro-week symmetry-breaking, right? The Higgs symmetry. 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 collusion Klein theory, which tried to unify Maxwell's electromagnetism with Einstein's general relativity, which produces an extra field known as a dilaton, 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 Barians and Maisons. 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 a lot of 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'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. So if I can try to, as the 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 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. I think 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.

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. Like the weak force, I consider electricity and magnetism to have been unified with the 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. Hey, it's Ed Helms, and welcome back to Snafoo, my podcast about history's greatest screw-ups. On our new season, we're bringing you a new snafu every single episode. 32 lost nuclear weapons. Wait, stop? What?

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the Gulf of Mexico. No, America. No, the America. The Gulf of Mexico, continue forever and ever. It blows me away how progressive Mexico was in this moment. They had land reform. They had labor rights. They had education

rights. Mustard seeds were so valuable to the ancient Egyptians that they used to place them in their tombs for the afterlife. Listen to Hungry for History as part of the My Cultura Podcast Network, available on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. So let's talk about the arguments for and against the existence of, 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 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. Now we're left, essentially, as you said, with quantum mechanics and general relativity. 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? 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. It's 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 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-week force unify. And so you really just start saying like, but it's pretty but I would like it if it did, but it works in the past. But I would like, okay, 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 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 a bit off-putting to me. So 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? I would say that no one should be believed. 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 countless millennia 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 mathematics theory. 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 ask 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 two.

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'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 just the possible space, 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. Believing requires evidence in a physical science. Wow, that's some hardcore skepticism. I love it. Is it skepticism? I feel like He just said you need data. Do you want to provide a counter argument, though? Like what would someone say who thinks that, no, Ethan, you're too cold in your skepticism here. Like, what would you say to argue against that? 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 okay to be motivated by, you know, aesthetic 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. 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. 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 borne out as a measurement, 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 really have to say, no, there's physical evidence that supports that this picture of reality. is true. 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? That as you say, you have to go and write down your theory and makes predictions,

you can test them, et cetera. 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. 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 we believe that math is discovered, or do you think that it's invented by humans. 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 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 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 on mathematical physics. And I was like, well, hang on. Someone is giving a talk on the E10 exceptional group. And I said, 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? 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 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, direct. 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 collider 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. I'm enjoying this gloves-off debate. Well, let me throw out in other pills, 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 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 it 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 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? Oh, my goodness. I mean, so I'm going to, I'm going to 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 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 probabilityness into the quantum operator? Right. This is, 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 the other aliens out there, they use springs, and it works just great. They just said, like, boing, and here we go, right? That's great. 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 that or not, that actually works. but, you know, it's not, doesn't lead to 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 S032 string theory and someone

goes, well, I've formulated E8 cross E8, super string theory. And someone says, well, my theory only has 26 dimensions. 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. this. And so, yeah. 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 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. 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. 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 that 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 you know, turtles all the way down, just effective theories forever. 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 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. All right. Well, thanks very much for answering all of our questions today and taking us down to the bottom turtle. We really appreciate you coming on. 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. Daniel and Kelly's Extraordinary Universe is produced by IHeart Radio. We would love to hear from you. We really would.

We want to know what questions you have about this extraordinary universe. We want to know your thoughts on recent shows, suggestions for future shows. If you contact us, we will get back to you. We really mean it. We answer every message. Email us.

us at Questions at Danielandkelly.org. 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. Don't be shy. Write to us. Hey, it's Ed Helms host of Snafu,

my podcast about history's greatest screw-ups. On our new season, we're bringing you a new snafu every single episode. 32 lost nuclear weapons. You're like, wait, stop. What? Yeah. It's going to be a whole lot of history, a whole lot of funny, and a whole lot of fabulous guests.

Paul Shear. Angela and Jenna. Nick Kroll. Jordan, Klepper. Listen to season four of Snafu with Ed Helms on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. There's a vile sickness in Abbas town. You must excise it. Dig into the deep earth and cut it out.

from iHeart podcasts and grim and mild from aaron mankey this is havoctown a new fiction podcast set in the bridgewater audio universe starring jule state and ray wise listen to havoc town on the i heart radio app apple podcasts or wherever you get your podcasts i'm jonathan goldstein and on the new season of heavyweight and so i pointed the gun at him and said this isn't a joke a man who robbed a bank when he was 14 years old And a centenarian rediscovers a love lost 80 years ago. How can a 101-year-old woman fall in love again? Listen to heavyweight on the I-Heart Radio app, Apple Podcasts, or wherever you get your podcasts. Welcome to Decoding Women's Health. I'm Dr. Elizabeth Pointer, chair of Women's Health and Gynecology at the Atria Health Institute in New York City. I'll be talking to top researchers and clinicians.

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