Must there be a single unified theory of physics?

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28:40okay 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 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 percent 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?

29:13You 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 here at our low energies, we see one, two, three, four separate forces, gravity, strong nuclear, weak nuclear, and electromagnetic.

29:57But if you go up to high energies, like the types of energies they've reached at the Large Electron-Positron Collider, at Fermilab's Tevatron, and now at the Large Hadron Collider at CERN, you can actually say, oh no, it looks like electroweak unification does happen. And that the theory of electroweak Symmetry breaking is where the Higgs sector and the Higgs boson comes from. It's why we have the WNZ bosons be very massive instead of massless, because when the symmetry breaks, there are degrees of freedom that get eaten by, well, they're not displacement currents like Daniel talked about. They're different types of currents that arise in physics, but they get eaten by those directional degrees of freedom.

30:45And 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, 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.

31:22And 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 Aharanov-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.

31:59So 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 extra form of unification and that things are not unified now, where are those extra components that needed to come out of it?

32:57Are they around somewhere? Do we need to figure out how to detect them? Are they hiding because they get eaten or subsumed into particles that exist like the W and Z bosons? These are the sorts of things that we need to ask ourselves. But I sort of look at it as the, you know, it's great to be like, I want to look and see what's on the top of the mountain. I think that's maybe a little too ambitious to be connected to reality. I just want to know what direction should I go take my next step in if I want to get towards this goal? Or is any step at all futile? And have we already discovered the most unified version of the universe that there is?

33:41And this is it. 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. 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.

34:16We unify electromagnetism with a weak force. we see the Higgs boson and the WZ sector we see these things but that recent efforts to try to unify everything else together has made predictions that we haven't been able to verify and so the question is like well how do you construct this new complex theory of everything with all these extra moving pieces and then somehow make it so we don't see them in the universe to be consistent but I also wanted to clarify one thing which is the definition of what we're talking about for a unified theory, because you said a couple of times, we don't live in a universe with a unified theory.

34:50And 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 those pieces are still playing out in different ways today.

35:28Would you disagree? I can accept that as a valid perspective. That maybe isn't the one I share, but 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 milli-EV 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.

36:18It 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 electroweak, which is, you know, electromagnetism and weak unified, except broken by electroweak symmetry breaking, right, the Higgs symmetry.

37:08We 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 Kalusia 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. 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.

37:49And those are brilliant ideas that turn out to not be reflected in reality. So I think it's very important to say, you know, yeah, we have we have a lot of different ways of or ideas of going about unification or grander theories or more comprehensive explanations. But the ones that disagree with reality, we were smart enough to throw away. And the ones that agreed with reality, we kept and we say, look at those successes. But don't forget that the history of science was not just success, success, success, success. And now we're like, oh, I don't know where to go next. At no point did we know where to go next.

38:28We 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.

39:04So 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 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.

39:40Yeah, 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, 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.

40:14We 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.

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44:20So let's talk about the arguments for and against the existence of a unified theory. for a lot of people. And for me, one of the strongest arguments is that it's been working so far. If you just assume that the universe makes sense, that there is a single reason why things happen out there, you know, why a particle goes this way or not that way, or, you know, why things interact at this level and not at that level, um, 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?

44:56That over time, we've harmonized various kinds of phenomena into smaller numbers of things. And now we're left essentially, as you said, with quantum mechanics and general relativity. And we're, 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 is like, this is great for addressing yesterday's problems.

45:37This 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 electroweak force unify. And so you really just start saying like, but, but it's pretty, but I would like it if it did, but it works in the past.

46:21And I'm 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.

47:02So 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 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 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.

47:49I know in my gut, I know in my heart, I feel the instinct. This is not how we do science. This is fine if you want to say, oh, I can write down a mathematic theory and 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, 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.

48:30Why is he trying to trick me? And I I would say, well, it could be 2 or it could be negative 2. And you would go, oh, yeah, that's right. Square roots can have plus or minus solutions. Then I say, but that's only true mathematically. If I say I'm going to throw this ball and the ball is going to land at the square root of 4, you can go measure that ball and find if it's at plus 2 or minus 2. Our universe gives one answer to physical questions. And we can only find that out by measuring it. So as a theorist, your goal is to provide not just the possible spectrum of explanations. You want to be able to have someone who uses your theory predict the answer.

49:18And until we get concrete, unique predictions that we can test against reality, we only have ideas. We don't have something that's worth believing. Got it. Believing requires evidence in a physical science. 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.

50:00I 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.

50:42You 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. 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. Of course. 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.

51:37Absolutely. 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.

52:12We're discovering it. it's out there. The universe itself is mathematical. There's math that runs as sort of the source code of the universe. And if that's true, then there has to be some math that describes the universe, even if we haven't found it yet. What would you say to that argument? Do you believe that math is discovered or do you think that it's invented by humans? I mean, for me, I look at math as math 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?

52:54Like, 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.

53:41So 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. We can invent all sorts of mathematics that cannot correspond to reality. We know 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 you can't have more than E8. You're not a group anymore.

54:25And 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.

55:01Like, this is nuts. Like, I don't want to be hampered by reality. But I say you do, because if you have, for example, a theory or a framework that predicts the presence of a large number of flavor changing neutral currents in particle physics, like that predicts I can go from a heavy, unstable quark directly to a lighter quark with the same quantum numbers, except that's a lighter flavor that has the same electric charge, but a lighter flavor. We have enormous constraints from reality, from experiments, from observations, from collider data that we know that doesn't happen to a shocking degree.

55:45Almost 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 another philosophical argument at you, Ethan. And I'm happy to take either side of this.

56:25So 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 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 schmings or something. And it also explains everything. Do you think that's possible?

57:03Or 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 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.

57:47I'm just selecting out which aspect of the wave function is most accurately represented by our universe. Or, you know, do I take the uncertainty away from the particle entirely and move all of that uncertainty and probabilitiness into the quantum operator, right? This is, it's sort of like you're asking me which interpretation of quantum mechanics is right. And I'm going to tell you they're all equally right because they all give you the same answers. I don't think there's any way to tell them apart. So if you're telling me like, oh yeah, well, we use strings, but we, the other aliens out there, they use springs and it works just great.

58:30They 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 it or not, that actually works, but it doesn't lead you 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.

59:17Many 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 super string theory. And someone says, well, my theory only has 26 dimensions. They go, well, mine only has 10. It's 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. All right. So I thought you were arguing on one side. I thought you were going to say, look, it doesn't matter.

59:51you 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.

1:00:28All 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?

1:01:06Or do you think there's a risk that it's like, 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.

1:01:44If 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.

1:02:19All 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.

1:02:44Daniel and Kelly's Extraordinary Universe is produced by iHeartRadio. 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 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.

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