130 | Frank Wilczek on the Present and Future of Fundamental Physics

What is the world made of? How does it behave? These questions, aimed at the most basic level of reality, are the subject of fundamental physics. What counts as fundamental is somewhat contestable, but it includes our best understanding of matter and energy, space and time, and dynamical laws, as well as complex emergent structures and the sweep of the cosmos. Few people are better positioned to talk about fundamental physics than Frank Wilczek, a Nobel Laureate who has made significant contributions to our understanding of the strong interactions, dark matter, black holes, and condensed matter, as well as proposing the existence of time crystals. We talk about what we currently know about fundamental physics, but also the directions in which it is heading, for better and for worse.

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Frank Wilczek received his Ph.D. in physics from Princeton University. He is currently the Herman Feshbach professor of physics at the MIT; Founding Director of the T. D. Lee Institute and Chief Scientist at Wilczek Quantum Center, Shanghai Jiao Tong University; Distinguished Professor at Arizona State University; and Professor at Stockholm University. Among his numerous awards are the MacArthur Fellowship, the Nobel Prize in Physics (2004, for asymptotic freedom), membership in the National Academy of Sciences and the American Academy of Arts and Sciences. He is the author of numerous books, most recently Fundamentals: Ten Keys to Reality.

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0:00:00.1 Sean Carroll: Hello, everyone, welcome to the Mindscape Podcast. I’m your host, Sean Carroll. And as many of you know, I’m a physicist by training and occupation. And the kinds of physics I do, cosmology, relativity, gravitation, spacetime, particle physics, field theory, all this stuff sometimes goes under the name fundamental physics. It’s a somewhat contentious name because fundamental sounds more important, and that’s not what it’s supposed to mean. At one point in my life, I tried to start a campaign to rename fundamental physics elementary physics, because elementary sounds a little bit less pretentious and it’s about the same idea, the underlying stuff, right, the most fundamental bottom layer stuff out of which everything else is made. Anyway, that never caught on.

0:00:44.1 SC: That never does catch on. Changing names is very hard. Fundamental physics is what we call it. As many of you know, fundamental physics is in a weird phase right now. In some sense, it’s a little bit stuck. Not completely stuck. But let’s say progress is slower than it used to be. In some sense, that’s because we were spoiled by the first half of the 20th century where we invented quantum mechanics and relativity and found the expanding universe, and it was just an amazing change every decade. So maybe now we’re in a more natural state of being. But still, we long for the good days when we could revolutionize physics every so often. And right now, our theories are just too good. We have standard models of gravity, of particle physics, of cosmology, and they fit a wide variety of data.

0:01:29.3 SC: Under that kind of circumstance how do you make progress? How do you move forward? So I can think of no one better to talk to about these kinds of questions than today’s guest, Frank Wilczek, who is a professor of physics at MIT, Nobel Prize winner. Back when he was a graduate student he helped understand quantum chromodynamics and the phenomenon called asymptotic freedom. That’s what he won the Nobel Prize for. But since then, he’s done many other interesting things on the Higgs boson, he co-invented the particle called the axion, which is still hypothetical, but might be out there. He invented a new kind of particle called anions, which can exist in two plus one dimensions. He’s the pioneer of time crystals, a whole bunch of other things. And Frank has a very specific, very grounded point of view on fundamental physics.

0:02:16.2 SC: He wants to push it forward in interesting ways, but he worries that it has become a little bit separated, disjointed from the data, the experiment, the phenomena on the ground. And that’s not because he’s just a crotchety old man getting grumpy about the kids today. He’s very willing to think about black hole information, the multiverse, the anthropic principle, but at the end of the day he wants to account for what we see. And he worries, I think with some good reason, that if you lose sight of that prize, you can sort of wander out in the wilderness without making substantial progress.

0:02:50.3 SC: Easy to say, of course. So how do you make substantial progress? Well, that’s a good question. And we’ll talk about that too. So Frank has a new book out called Fundamentals. It’s about… It’s a very popular little book, accessible to everyone, about all the big ideas in physics. And we talk about in this podcast both what those ideas are, and then maybe how we can move beyond them. If we can’t just speculate what is the right way to make progress. Nobody has any easy answers there, but there are some hot takes. There are some saucy opinions given in this podcast. So I think you’ll like it. Let’s go.

[music]

0:03:41.5 SC: Frank Wilczek, welcome to the Mindscape Podcast.

0:03:42.4 Frank Wilczek: It’s good to be here.

0:03:45.9 SC: So yeah, here in some virtual sense, of course. I haven’t actually seen anyone in person to do a podcast in… For many months now. But…

0:03:52.8 FW: In our minds…

0:03:53.9 SC: Exactly. What is the difference, really? Spatial locality. Speaking of which, you’ve written a new book called Fundamentals: Ten Keys to Reality. And I thought that would be just a good theme for a podcast interview because, obviously, fundamental physics is something we both care a lot about, try to move forward. But it’s also something that is in a… In a fun… Funny situation right now. And so I thought I would just start with your opinions about the use of the word fundamentals. Some people don’t want to call certain kinds of physics fundamental and not others. How do you conceptualize what that means to say fundamentals?

0:04:34.6 FW: Well, I guess, I’m in favor of a broad interpretation. Although it varies with context, of course. But as far as calling things fundamental or not within physics, I think it’s fair to call things like the second law of thermodynamics pretty fundamental or emergent phenomena that don’t… In principle, could be explained from more basic principles, can if they’re sufficiently wide-ranging, non-obvious, I don’t begrudge them the label fundamental. But on the other… If you say what you mean by fundamental, and if by fundamental you mean things that can’t be deduced from other things, that’s also a legitimate use of the word, and sometimes handy. So I… As long as… To me, it’s not so much the word but the meaning in context. How it’s… Of it. And…

0:05:48.3 SC: Yeah, and I think you probably have more credibility on this issue than many people, since you’ve done both fundamental particle physics, but also work in condensed matter physics and more complex systems.

0:05:58.3 FW: Yeah… Yeah. And I’m proud of both. And I do not have the… Well, I don’t have the prejudice that things in physics that in principle could have derived, or otherwise, that could have been derived from more basic principles in principle don’t… Somehow are a lesser achievement.

0:06:24.6 SC: Doesn’t make them more important, right.

0:06:28.6 FW: No.

0:06:28.7 SC: That’s right.

0:06:28.9 FW: Not at all. And in fact, right now, we have a pretty good account… A pretty good understanding of fundamentals that are necessary to explain chemistry, biology, almost all of astrophysics and so when… And I think also a great challenge, understanding how mind emerges from matter. And to me, trying to add to those principles is certainly… And get them from more basic foundations is a worthwhile enterprise. But I don’t think it is, in any sense, more privileged or better than the great enterprise of getting on with it, so to speak. Any more than… Take another… An example, which… In geometry, there are attempts to develop new systems of geometry or to discover new theorems. And at the frontiers of well-developed subjects like algebraic geometry or algebraic topology and so forth and so on. And I don’t think that that kind of activity is any less basic than the examination of whether you could get by with fewer axioms.

[laughter]

0:07:55.9 FW: And on the contrary, when a subject becomes mature then it’s not so interesting to look for fewer axioms anymore. The growth point is not that. The growth point is the exuberant flourishing of a structure that’s very rich already.

0:08:13.0 SC: There’s an obvious question that I haven’t quite figured out how to ask, but maybe you can just answer whatever you feel is best. So for the people out there who are not physicists, what are the fundamental laws of physics? This is a several hour lecture, I know. But how do you summarize it?

0:08:28.1 FW: Well, you wrote a book about it.

0:08:31.4 SC: I did.

0:08:32.5 FW: And do you wrote a t-shirt about it. It…

0:08:36.3 SC: Yes.

0:08:36.4 FW: The fundamental laws of physics, as we understand them today, are pretty well encoded in what’s called the standard model or the core, which includes includes a framework in which you state the laws which involves relativity and quantum mechanics. And I don’t… In the case of relativity, I know what the… I could state it axiomatically. And quantum mechanics is a little more tricky to say exactly what the axioms are. But, okay. We know in practice well enough. And then there are four forces. And there’s a small number of forces and a small number of ingredients, and you can write down very compact equations that as far as we can tell are an accurate description of all the behavior that is simple enough to study that we can… And fully document that we’ve observed… The interactions among elementary particles or interactions that are simple to analyze in cosmology and in astrophysics. And that’s it. I so you can write… And I can in principle write a computer program that has these laws. I could explain it to a computer and that that’s… Those are the fundamental laws right now.

0:10:06.3 SC: Maybe this is a good jumping-off point for what I thought would be much later in the conversation, but the fact that we know so much… This core theory that I’ve made t-shirts of. You’re the one who labeled it the core.

0:10:20.1 FW: Right.

0:10:21.5 SC: How do you express this amazing fact that if you care about biology or psychology or something like that, we know the laws of physics, that we’re not going to discover new particles, new forces that are relevant to biology. That’s hard to get across to people who say, “Well, there’s a lot of things we don’t know.” So how do you know?

0:10:39.5 FW: How do I know what? How do I know that those are… Those laws are good enough?

0:10:45.4 SC: Yeah, that’s right.

0:10:45.5 FW: Well, it’s hard to… You can’t be absolutely sure, but what is true is that these laws have been checked, tested as vigorously as people know how in very extreme circumstances. At high energies, at low temperatures, with great sensitivity and people have high motivations to try to find deviations. There have been been Nobel prizes and all kinds of glory. But so far, they’ve stood up to all these tests. Now, this is all in the framework of the program of, I guess, you could call reductionism of saying that if we understand the laws as they act between a few simple objects then we can build up mathematically to a description of more complicated things. In principle, there could be forces that involve many bodies at once or are outside the framework of quantum mechanics. In principle, it’s… The evidence is empirical that it works.

0:11:55.9 SC: Yeah.

0:11:56.5 FW: Yeah. And it seems very… It’s been tested so exact… So precisely, and under such extreme conditions, such a variety of conditions that it easily encompasses the degree of accuracy and the domain of conditions that one encounters in chemistry, biology and almost all of astrophysics… And in fact, all known astrophysics and… Yeah. And well… So at some point the burden of proof is on some… To show how it doesn’t work.

0:12:41.0 SC: Right, right. I mean…

0:12:42.4 FW: There are plenty of loose ends, there are definitely things we don’t know, and I’m sure we’ll come to discuss those, but none of them, as far as I can tell, seem likely to affect biology, even psychology, how the mind… Or chemistry or astrophysics, geophysics or any branch of engineering, it seems unlikely that significant changes in those equations are going… Significant enough to change the applications are going to occur.

0:13:25.6 SC: Have you ever played around? I know I haven’t, I’ve sort of failed with trying to modify the standard model or quantum field theory in some way that there’d be new forces that arise when matter arranges itself in certain complex ways, sort of try to make a little bit more careful the idea of some kind of strong emergence.

0:13:47.1 FW: Yeah, well, it’s sort of been on my agenda for a long time, but it’s one of those things I never really get around to. We work, as you know, in the context of effective field theory, and then you can discuss effects that involve many, many bodies at once, that do not succumb to this program of reduction where you study… You build up from interactions involving just a few bodies, and that’s sufficient. Now we have a rationalization, an understanding of how that could work in terms of the successive interactions among many bodies getting systematically smaller, drastically smaller than the interactions, the fundamental interactions among small numbers of particles. But maybe that breaks down at some level, and maybe there are 27 body interactions…

0:14:43.3 SC: Exactly, right.

0:14:44.6 FW: That somehow kick in or 10 to the 27 body interactions that somehow kick in and the drastic decrease with the number of particles somehow reverses. So that’s the most concrete idea I’ve had along those lines, but if the laws break down, I would be very surprised if that’s the way they break down.

0:15:07.5 SC: It’s true. It’s true. It’s hard to imagine it being that way.

0:15:10.1 FW: Of course, another thing that’s more or less plausible and you can play with is interactions that like gravity build up from being very small, because they’re long range and additive, like gravity that could build up. And not only if I play with that… I think that those are quite plausible, axions give you something like that. But people… There are constraints on how powerful those effects can be, people have looked for so-called fifth forces, and so far come up empty, and so again, they are highly constrained and seem unlikely to affect any practical application.

0:15:57.1 SC: Yeah, it’s an amazing fact about the theory as we currently understand it. On the one hand, there’s the four forces you mentioned, right, the weak force, the strong force, electromagnetism, gravity. On the one hand, it looks like kind of a mess, there’s various forces doing various things, there’s the difference between right-handed and left-handed particles, but on the other hand, it’s quite compact compared to the scope of what it is attempting to explain or account for. Should we count the core theory as simple or as complicated?

0:16:28.3 FW: Well, it’s both, but I think we understand or have a good guess about how that happened. If you look at the four forces at short distances, it seems that a lot of the complications and the apparent differences between the forces melt away, they come to look very much more like each other. Certainly the three of the four look very much like each other, they’re all mediated by photon-like particles, gluons, W bosons and so forth, which have the same… Are governed by the same sort of principles in remarkable detail. And even the strength of the different interactions comes together if you extrapolate the known laws to high enough… To small enough distances. So that’s, on the one hand, a lot of these complications melt away if you go to short distances. On the other hand, we understand that as you go to long distances from simplicity, complications can set in due to what we call symmetry breaking, roughly speaking.

0:17:40.3 FW: You know this, but for the audience, condensations occur that can muck up the appearance of the laws, and so from an underlying simplicity, if you can’t get to look at what’s going on at very small distances, it can look much more complicated. And that thought works out remarkably well, certainly in the weak interaction where there’s an abundance of evidence for symmetry breaking and the strong interaction where there’s an abundance of understanding of confinement and symmetry breaking also. And so we know of mechanisms, that we have a lot of evidence for this general picture and the unification of forces that small distances works pretty darn well, so I think it’s both simple and complicated. It’s simple, fundamentally, so to speak, at short enough distances, but its own internal logic tells you that it could look, and in fact, it does look more complicated phenomenologically when you view it at long distances. So it’s… The complexity emerges, I guess, is the two-word summary of…

0:19:00.7 SC: Yeah, and…

0:19:00.8 FW: Complexity emerges from simplicity is the four-word summary.

0:19:03.5 SC: And I can imagine that when this is first put together, the standard model in the early ’70s, obviously, you were a major player here…

0:19:11.2 FW: Yeah.

0:19:12.2 SC: It was… It must have just been overwhelmingly compelling to say, “Well, this idea of unifying all the forces and simple structures is so good, let’s keep doing it.” And we tried to do that with grand unification, but we haven’t found the evidence for it yet. Are you… I know that you’re a fan of the idea of grand unification. Have you… Has your Bayesian prior waivered a little bit?

0:19:35.8 FW: Was there my Bayesian…

0:19:38.2 SC: Yeah, have you decrease your freedoms just a little bit?

0:19:39.9 FW: Well, I think the general… I think there are very strong general conclusions that… Or circumstantial evidence, pieces of circumstantial evidence that support this view. There’s, first of all the primary fact that all the forces are governed by mathematically very similar structures, gauge theories. And then if you examine in detail the particles that actually exist, they fit together into a structure that allows you to imagine that all these theories that are… All these apparently different forces at long distances actually represent one unified force at small distances where all of them become equal and are on the same footing, and are all symmetric with each other. That is not at all automatic. It requires very strong conspiracies among the different particles that… As to their quantum numbers, as to how they behave under these different interactions, if it were different, then you couldn’t put them together.

0:20:51.6 FW: So that’s circumstantial evidence. The other circumstantial evidence is that if apparently… As we see them, the coupling strengths are very different, but if you extrapolate to short distances or high energies, again, to get to the basics, stripping away complications, they appear to become numerically equal to a pretty good accurate. So that’s two. And then another aspect of that is that at the scale at which they come together where things… Where the veil is lifted and you should see the ultimate simplicity, you find that gravity also, which appeared to be very, very different than strength, is approximately the same.

0:21:39.1 FW: So I think we have strong encouragement from nature to think along these lines. One last piece of evidence is more complicated to explain, which is that neutrinos have been found to have small but non-zero masses. So much sort of anomalously small compared to all other masses that are not zero. And that can be understood in terms of these unified theories where tiny effects that come from short distances trickle down to give neutrino masses, but would otherwise appear very mysterious. So to make… To boil all that down into a couple of sentences, there are many aspects of our scattered theories within the core that appear to be anomalous or lopsided or somehow unsatisfactory, that become much more ideal, much more… In the platonic sense much more beautiful in this larger framework.

0:22:44.5 FW: Now, there’s one piece of evidence… One piece that’s missing here that’s really tantalizing, which is that besides the small neutrino masses kind of the other new prediction that’s not just aesthetic and… Well, and not just numerical. I don’t know why we say just numerical. Numerical is pretty impressive.

0:23:05.6 SC: That’s pretty good.

0:23:05.7 FW: The… Of the unification… The… Is… But a spectacular new phenomenon that’s promised by these unified theories is the decay of protons. Or violation of baryon number, we say. And people have looked very hard for that. And so far they haven’t found it. And our only answer is, well, maybe you haven’t looked hard enough…

0:23:32.2 SC: Yeah.

0:23:32.3 FW: So… The limits are not powerful enough to really decisively rule out the picture, but it certainly would be better to have positive evidence on that front. And that would really, to me, lock it up. So that there’s one missing piece that we need to find.

0:23:55.9 SC: I know a lot of people who think along these lines are also fans of supersymmetry as an extra ingredient.

0:24:01.5 FW: Yes.

0:24:02.2 SC: That we haven’t seen evidence for. And I think that probably for the people out there listening they might not quite understand the distinction between superstring theory and supersymmetry and things like that. So what is your attitude towards the current state of that idea?

0:24:15.3 FW: Okay. Well, supersymmetry makes this unification of couplings work better quantitatively. So that’s… I was hoping that we would see supersymmetry at the LHC on the… At the…

0:24:30.7 SC: Maybe explain to us what it is, ’cause…

0:24:33.9 FW: What’s that?

0:24:34.0 SC: Maybe explain what it is. What supersymmetry is for the people out there.

0:24:36.1 FW: Okay. So supersymmetry is an extension of the idea of… Well, you can phrase it in various ways. But maybe the nicest way to phrase it is it’s an extension of the idea of spacetime to include not only the ordinary kind of dimensions of space and time that we’re familiar with that are based on numbers that satisfy X times Y equals Y times X, but into what are called quantum dimensions that are governed by numbers that satisfy X times Y equals minus Y times X. Now, this sounds very mystical and at that level it is, but it turns out that if you want to unite two big classes, two kingdoms of particles that people know about, so-called fermions and bosons, or more generally, if you want to consider making symmetries that relate particles of different spin, you have to go beyond the framework of ordinary space and time, and it turns out that it’s very difficult to do that in a mathematically consistent way, but using these quantum dimensions is one way you can do it.

0:26:00.4 FW: So you have motions in space and time, and boosts in space and time, special relativity that relates to space and time. And the natural extension of that, and really the only one that’s known how to do it is to add these quantum dimensions. And then if you do that, though, you need to account for what happens to particles when they move into these quantum dimensions. And they don’t change where they are, because they’re… In the sense of space and time, because those are the ordinary dimensions. If you move into the quantum directions what happens is you change into a different kind of particle that has a different spin, a different character, so if it’s a force or a boson particle that turns into a substance or a fermion particle and vice versa, and a different mass, but many of the same properties, many of the same electric charge, the same color charges, the same kind of behavior under weak interactions, all the standard… All the basic interactions of the standard model are common between these particles.

0:27:12.0 FW: So these are called superpartner and they are the price you have to pay if you want to have supersymmetry, you have… Because no particles with the requisite properties have been observed, no particle that we’ve actually observed has a candidate superpartner that it could be… Is a candidate for what happens to it when it moves in superspace. Nevertheless, it’s just as we think about higher symmetries that are obscured by condensates, as we talked about in our earlier discussion, here too, you apply the same idea that the underlying supersymmetry at short distances is hidden from us by spontaneous breaking of the symmetry, so that the basic laws would have this feature, but they’re obscured from view. And then it becomes a question of how obscured, how heavy are these particles? And again, detailed considerations indicate that if we want to preserve this wonderful improvement in how the couplings unify, the new particles shouldn’t be too heavy.

0:28:30.5 FW: And some of us were optimistic, some of us were… Not me, some of us were super optimistic that they would be light enough to be observed at the Large Hadron Collider. That is still not completely ruled out, but certainly the most straightforward searches have not found evidence for any of these particles, which now it… The data is very complicated and difficult to analyze, so it may be that it’s somehow hiding in the data. But more likely, in my view, is either that super… The whole line of thought is somehow flawed or the particles are much… Or the particles are just a little too heavy to have been observed at the LHC, unfortunately, and it would take a bigger and better Collider to start producing them.

0:29:28.8 SC: Yeah, maybe you can opine on this, but the way that I like to say it is, we could, in the space of all possible worlds that we live in, only one of them, we could have found supersymmetry already at the LHC very easily, but the fact that we haven’t doesn’t mean it’s not there. Maybe it’s less likely that it’s there, but it’s easy also to imagine theories where supersymmetry is real, and we just haven’t seen it yet.

0:29:53.1 FW: Right. So supersymmetry, as I said, there have to be… For supersymmetry to be valid, there have to be these superpartner particles that are the particles that the particles we know about turn into when they move into the quantum dimensions, but we don’t know what their masses are. We know some of their properties, but not their masses, and they could be very heavy. If they’re very, very heavy, we lose the benefit of improving… The benefit that supersymmetry would otherwise give in improving how the couplings unify, but okay, maybe that was a cruel joke on the part of nature. I want to think not, but the alternative is that they’re just a little bit too heavy to have been produced easily and identified easily at the LHC, and we just have to work a little bit harder and spend a little more money on…

0:30:56.5 SC: A little more.

0:30:56.6 FW: A scale of national debt to make bigger and better accelerators, and do bigger and better searches. And I should say, another aspect of this is that many implementations of supersymmetry produce a candidate for the dark matter, usually called a neutralino or… And that people have searched also very hard for that and have not found it. So that’s also somewhat discouraging, although that’s even more indirect, I would say, because there are cosmological considerations that come in and the abundance might just be smaller than is necessary to produce the dark matter or the particles might not be stable after all on cosmological scales. There are many outs for that, but nevertheless, it was something that might have been observed and hasn’t, so there it is, there it is.

0:31:55.9 SC: And maybe it’s also worth mentioning something that I think focuses the attention of a lot of particle theorists in this era, which is the hierarchy problem, this mismatch apparently between the energy scale of the weak interactions in the Higgs boson versus the energy scales of grand unification in gravity, and I think that for a lot of us, definitely for me, even without the specific solution to that problem in mind, that was a big motivation for thinking that when the LHC turned on, we’d see a bunch of new particles one way or the other.

0:32:27.7 FW: Well, I did not really share that intuition, other… To me, the likely alternatives were either supersymmetry showed up or something close to minimal, which is… It’s turned out to be very close to minimal implementation with just a Higgs particle and nothing else, but we’ll see… I still would be hopeful that at somewhat higher energies supersymmetry would come roaring in and lots of stuff, but… Well, I don’t know how much you want to enter into the technicalities, probably not very much, but…

0:33:05.8 SC: Well, probably not very much, but I just…

0:33:07.9 FW: The smallness of radiative corrections of various kinds of specially CP violation indicated that a pretty close to minimal structure or else a highly symmetric structure were the only consistent possibilities phenomenologically.

0:33:26.5 SC: So I guess I want your feeling on how big the hierarchy problem is as a problem, so to just let people know, we have the mass of the Higgs boson, which is a certain number, and then we have the Planck scale or the grand unification scale way, way bigger than that… And to many of us, this is a mystery, but other people think, well, you know, it’s just a number, we measured it. It’s not a mystery at all. So is this a motivating feature of the data for you?

0:33:52.6 FW: Yeah, well, it’s a motivation, but it’s a very broad motivation, and to me, it’s kind of been superseded by this unification of couplings, which is much more quantitative and sort of a much more positive clue. So no, the hierarchy problem, I think they go together, but to me, the unification of couplings is stronger as a guiding principle.

0:34:18.5 SC: So do you take then the unification of couplings… I think you answered this, but just to make sure that I didn’t misunderstand, do you take that as a good motivation for thinking that if we do keep pressing on to higher energies, either at the Large Hadron Collider, or at a new accelerator, we still have good reason to believe we’ll find some new particles?

0:34:38.5 FW: Yes, yeah. I’m a supersymmetry diehard, so I would be full of hope. Of course, it’s easy for me to spend other people’s money, but I would be pretty hopeful about that. In a similar vein, I’m hopeful about pressing proton decay experiments harder because the existing constraints are just kind of at the edge of compatibility with unification ideas and a little harder pressing might get us there. So if I were a young experimentalist, neither of which is true, I would not be discouraged. I think there are… And if I wanted to make an impact in fundamental physics, I think it’s not out of the question that there’d be breakthrough on either of those fronts, and either of them would be very rich.

0:35:46.4 FW: If proton decay is discovered, there would be many, many follow-up studies of how it decays, what it decays into, is it CP violating, blah, blah, blah, and similarly, certainly if supersymmetry is found, there’d be zillions of new particles and phenomena to study. We’d be able to get a much more detailed picture of how unification works. It’s very mysterious exactly how this supersymmetry gets spontaneously broken, so that would be a real gold mine of information and also really would bring the prospects of making contact between… Making detailed contact between quantum mechanics and gravity much closer.

0:36:31.0 SC: Yeah, that’s certainly true, but there does seem to be the possibility, just to play devil’s advocate here, as a middle-aged theoretical physicist, that we don’t find anything, that we don’t find dark matter directly, that we don’t find new particles at the LHC, and then what in your mind is the best strategy for moving forward, if that’s what happened.

0:36:52.0 FW: Well, we’ve had kind of a several decade experiment along these lines, and… Well, there are several possible reactions. One is to try to push aesthetic considerations of how an ultimately unified theory should look, and this leads us into superstring theory as probably the most ambitious and accomplished of those attempts. So that’s one attitude. Another attitude is to try to help the experimentalists make new kinds of experiments that will give further guidance to sort out LHC’s signals better or just astrophysical consequences of unified theories or if we did better experiments to look for axions and dark matter, there are a whole variety of different experimental, quasi-phenomenological approaches to improving the fundamental laws.

0:38:05.8 FW: And then there is another alternative which says, no, do something else. No one puts a gun to your head and says, you have to work on quantum gravity or you have to try to improve the core theory. There are great problems having to do with applications of the physics we’ve understood, which maybe sounds less glamorous, but is every bit as glamorous and rewarding and fundamental, in the good sense. Or there are problems like understanding how mind emerges from matter, there are many, many other things you can do that are entertaining and useful to mankind and rewarding, so if fundamental physics seems too arid to you, then do something else.

0:38:58.1 SC: My own idiosyncratic minority view is that there’s still a lot to be learned by thinking about the foundations of quantum mechanics. I think that we have not really mastered that yet, which is a bit of an embarrassment.

0:39:10.3 FW: Yes, well, and that has become to me a much more attractive thing to do now, because the technology of looking at the really basic processes of quantum mechanics where you probe consequences of entanglement and superposition, and of course now the prospect of quantum information processing, has made these formerly kind of academic questions that were divorced from experimental and technological practice, now they are at the forefront of experimental and technological exploration. And the questions get posed in really new ways that to me are much richer than philosophical discussions of the measurement problem, which is… I always used to warn students away from, I still do.

0:40:14.9 SC: I warn them toward it.

0:40:15.5 FW: But in this new form of real real questions that arise in practice and real opportunities, I think the same sort of questions arises much more fruitfully…

0:40:31.5 SC: I guess an operational way of putting it is given that we haven’t yet seen new particles at the Large Hadron Collider, or we haven’t yet seen dark matter, how much of our effort should be in sort of taking a step back and saying, well, maybe it’s just quantum field theory is not the right paradigm to think about things for moving forward or something even deeper?

0:40:51.6 FW: Well, that could be, but… How should I… Saying maybe this is all wrong somehow is not, doesn’t tell you what to do when you wake up in the morning. And we’ve discussed previously the tremendous success of our core theories, and so if something else is going on at a deeper level that has… The first job has to be to explain why the framework of quantum field theory looks so good. And well, okay, maybe that can be done, but that’s what you have to… That’s what you’re up against. What I thought you were going to say was that if you’re not optimistic about finding new elementary particles or new phenomena at accelerators to think about other worlds that can be realized in a laboratory.

0:41:57.9 FW: To me… It’s kind of a realization, an epiphany, that I had sort of subconsciously for many years, but that just came to me when I was thinking sort of philosophically in connection with the book, which is that every material defines a world in itself. You can imagine yourself living inside that material, and nowadays, with strong AI, there are even ideas about how in certain kinds of materials intelligences could live and the laws would look quite different and that particles would be quite different, and that to me, gives a whole new dimension to the idea of the issues of what’s fundamental and what are fundamental particles and so forth.

0:42:47.6 FW: So I was really ecstatic in the last few months, and in the midst of the COVID and all, a tremendous bright spot has been finally the observation of anions, the clear observation of things that are not bosons and not fermions that we’ve been playing with for almost 40 years now, and now, there’s beautiful experiments that show their behavior and that also, that happened because a lot more attention has been focused on these things because of their potential use in quantum technology. So that’s to me a beautiful example of how physics grows and the questions that are its most fruitful focus change over time.

0:43:42.2 SC: And this is I think something that will be worth… We should do this a little bit more justice because it’s a different way of thinking that I think a lot of people aren’t familiar with, like everyone knows you can go look for new particles that you haven’t found, everyone knows these days that there could be other universes out there, cosmologically, very, very far away. But this idea that you can make a material and essentially conjure into existence effectively new laws of physics in that material, explain that a little bit more ’cause people are less familiar with that.

0:44:14.2 FW: Yeah, well, I like to talk about quasiworlds. There’s a concept that’s come to dominate our understanding of matter at the quantum level called quasiparticles. And what quasiparticles are, are concentrations of energy that behave like particles but inside materials, and they can have quite different properties from the particles outside of materials. For instance, one example that’s changed the world is inside semiconductors, the particles that carry positive charge, which are called holes usually, can have quite a small mass, much smaller than the mass of the proton, more comparable to the mass of the electron and… Or even smaller.

0:45:07.4 FW: And this makes it possible to manipulate charge, create charge, and use it in ways that are practical and very versatile. And this is what led to the discovery of the transistor, similarly, and the properties of superconductors, we can understand as photons getting a mass inside the superconductor. So the particles, the behavior of particles inside matter can be quite different from outside matter, you can also have qualitatively different emergent kind of phenomena. So, quasiparticles are a very popular concept, but even if you take away the particles and think about what they’re embedded in, that’s a quasiworld, it’s a world with its own particles, its own laws of physics, and each one of them deserves to be given its respect.

0:46:05.8 FW: And that to me has been an epiphany, as I said, and really a wonderful stimulus to imagination. What would it be like to live in these worlds? What could you do to… What sort of questions would you ask? How strange would the world we ordinarily experience in empty space look? Yeah, so that’s another example of evolving the meaning of what it means to be fundamental, what it means to understand things, and just to have fun and use your imagination.

0:46:46.6 SC: And just to be clear, these are not purely imaginative, these are things that are technologically explorable?

0:46:51.6 FW: Oh, yeah, absolutely. And by imagining worlds that have wonderful properties, wonderful possibilities, you can then help experimentalists to do new things, explore these possibilities. So yeah, it’s, how should I say? If you’re frustrated with high energy accelerators, if you’re willing to go to a quasiworld, then the energies don’t have to be so high, and you can have experiments which take on, in favorable cases, on the timescale of days rather than years or decades. Sometimes, as I said, with anions, it did take decades, but at least it’s happened, and… Yeah, so… And it also… Just to complete the circle on this, it raises also a question, which is whether our world should be considered as a quasiworld. So…

0:47:53.0 SC: I knew you were going to go there.

0:47:57.1 FW: Yeah, so that, as we’ve discussed, the laws that we actually observe are very much conditioned by condensates and spontaneous symmetry breaking. We know that often in physics, when symmetries break, they can break in different ways, and also that materials, for instance, that define worlds can also be layers within materials that… Within different materials or be riddled with impurities. So it raises questions about whether our world can be fruitfully considered as a representative of a much wider class or something that’s embedded in a larger scheme, and even the question of whether… Of what kinds of defects there might be that would reveal that kind of possibility. So, it’s a… I love this idea of extending quasiparticles to quasiworlds, and…

0:48:58.0 SC: Yeah, I mean…

0:48:58.8 FW: To me, they’re… It’s not even speculative, it’s… Or I should say it’s not purely speculative, it has many contacts with reality, with real experiments and with technology, and not exotic technology. The understanding of quasiparticles was absolutely basic to microelectronics and the kind of thing that makes it possible for you and me to be speaking right now in very different locations and recording it and sharing it eventually with many other people. That all comes from basic quantum mechanics and understanding quasiparticles and semiconductors.

0:49:36.8 SC: I’ve done recent podcasts both with Nick Bostrom, who is famous for the simulation argument, and with your MIT colleague, Max Tegmark, who thinks that all possible universes exist. Is it even in your mind worth thinking about this question of is our universe part of some way bigger structure.

0:50:00.5 FW: Oh, it’s worth thinking about, but it doesn’t take much thought to discredit it, in my opinion. The laws that we observe just don’t look like a competently programmed simulation. They don’t.

0:50:16.9 SC: In what sense?

0:50:18.2 FW: Well, I guess most profoundly, they have a lot of hidden complexity, so when you dig deeper, you find that there’s hidden structure that’s not used for anything. Why would you do that if you were simulating a world? Also, the laws are very constrained, they’re local, they don’t change in time, they don’t change in place. In a programmed environment, there’s no reason to obey any of those constraints, and people who build computer games typically trash them all, so… So, it just doesn’t look that way, and then there’s the embarrassing question of, “Okay, if this is a simulated world, what is the thing in which it’s simulated made out of? What are the laws for that?”

0:51:02.3 SC: Yeah.

0:51:03.1 FW: So it begs the question, and then when you think about it, put off that question, which is… But if you even think about it coherently, it just doesn’t seem at all likely. Now, a very different question is whether it’s logically possible, and I do think it’s logically possible, and I think it may even be the future. I think… I like to say that a natural intelligence is a special case of artificial intelligence, because I think we understand the laws well enough to say that we can make sufficiently complicated structures that can carry out intelligent operations. I mean, we make computers that do fantastically difficult things and play chess better than I can or anyone can, and so forth.

0:52:00.2 FW: And so there’s every reason to think that intelligence can be embodied materially, and furthermore, that within our brains, people have studied how the nervous system works in great detail, how signals are transmitted, and never had to get beyond the framework of the laws of physics that we know. On the contrary, there are very many non-trivial applications of the laws of physics into designing instruments that probe the brain and also to making models of what’s going on inside the brain. Now, our understanding is far from complete, but so far, there’s been no need, despite tremendous amounts of research, some of it very delicate, to change the laws of physics.

0:52:50.5 FW: So for instance, to think of if there was a power of mind that was separate from the laws of physics, you would expect that we’d have to somehow shield delicate experiments from that power of mind, but that’s never been true. You have to shield from vibration and temperature variation and all kinds of things, but not where… You never worry about what the guy in the next office is thinking… And that hasn’t been necessary. It might have been necessary, but it’s not necessary, fact, it’s not.

0:53:21.7 FW: So there’s every reason to think that, every reason to believe, even if it’s not fully established, that mind as we know it emerges from matter and that we know the matter, the laws of matter sufficiently well to stimulate them on a computer so that we should be able to in principle simulate the operation of the mind, of a brain, on a computer. So in that sense, I think it’s hard to avoid the conclusion if you think hard about it, that natural intelligence is a special case, or artificial intelligence, and that means that you can imagine… Producing human level or better intelligence within a computer of some kind.

0:54:15.2 FW: It’s a quantitative question, how fast and big the computer has to be, maybe it has to be a quantum computer. But in principle, it is. And then you could imagine intelligences that live in a simulated universe, so that they are not experiencing the external world as we know it, but just a stream of data that’s inside this machine. So I think it’s logically possible, but I don’t think our… I don’t think that I work that way, and I don’t think our world works that way, because the way our world works doesn’t conform to reasonably, to any kind of reasonable programming practice, it’s too constrained and too wasteful of resources on hidden complexity.

0:55:02.5 SC: I think I’m on your side as far as our world is concerned, but the argument for the simulation argument at least bears a family resemblance to arguments that are used in cosmology with the multiverse and the anthropic principle and so forth, and if I’m remembering correctly, you are sympathetic to that kind of reasoning, whereas I know that some of our colleagues think that you’re just giving up on science entirely if you invoke a cosmological multiverse.

0:55:28.2 FW: Oh, no, I think it’s not only a logical possibility, but in some form it’s true. I don’t want to get into the technical details, but if you believe in axions, which I do, and you believe in inflation, which I do, and if you believe that inflation occurred after the Peccei-Quinn transition, then different parts of the post-inflationary universe, which we would call a multiverse, have different, vastly different amounts of dark matter in the form of axions, and so they have significantly different physical behavior. So you can construct very concrete models that are models that we use for good reasons in other contexts that do embody a multiverse, and… So I’d better not have a prejudice against it, because I work with those models all the time, that so they’re logically possible and I think very natural.

0:56:38.8 FW: They are, however, sort of, let me say, epistemologically dangerous, because they encouraged giving up. Theoretical physics and especially fundamental physics can be very difficult, you ask, because we know so much, as we’ve discussed earlier, it’s very hard to make improvements and we look for aesthetic flaws, we look for loose ends, of which there are plenty, but it’s been very difficult to make improvements. And one way to say… One way to get off the hook, so to speak, and say, well, why aren’t you improving the laws of physics, is to say, well, it’s not because I lack cleverness, it’s because it’s impossible, it’s because different laws of physics are realized in different places, and it’s just an accident which ones we happen to observe here.

0:57:32.0 FW: Okay, so maybe that’s right, but it’s too easy, it’s much too easy. So that’s the epistemological danger, I would say. Now, of course, if there is a multiverse with enough loose ends, so to speak, so the laws do change drastically, not just in the amount of dark matter, but in many ways from place to place, then in fact, it’s going to be impossible to improve the standard model or the core to the bitter end, so to speak, ’cause some of it will be accidental, but I think it’s way premature to give up on that. Way premature. And it’s too easy and it lets people off the hook. Yeah.

0:58:25.6 SC: Well, it brings us to the fact that many…

0:58:28.1 FW: It’s not my fault that I can’t do better. It’s God’s fault, and I don’t like that.

0:58:34.8 SC: Well, some of this worry has come out of string theory, many of our colleagues for the last several decades have pointed to string theory as the most promising way forward. As far as I know, you have not done a lot of work directly on conventional string theory. What is your feeling about that approach to moving beyond quantum field theory?

0:58:54.8 FW: Well, I think it has produced a lot of attractive work that’s intellectually rich and has spun off into fertile mathematics, but I don’t see that it’s been converging towards informative assertions about the physical world…

0:59:18.7 SC: That’s very elegantly stated, actually, yes.

0:59:21.0 FW: That you can check. And for me personally, I’ve kind of voted with my feet, I think there are more promising things to think about, that’s partially a sociological statement, but I think it’s, string theory is getting plenty of attention, it doesn’t need me. I’m happier doing things that other people aren’t doing, but that’s a personal statement, and so far, I haven’t regretted my choice, but I watch what people… I watch the subject and I watch what people are doing and I wish them good luck, and if and when things that I think are promising insights into the physical world emerge, I will pay a lot of attention.

1:00:12.3 SC: Sure Do you think that the rest of the field has voted with their feet in a slightly too uniform way, do you think that too much of our intellectual effort is going in that particular direction?

1:00:21.8 FW: I do, but I might be wrong, so I don’t want to discourage. Plenty of people are doing other things, so it’s not as if the rest of the world is feeling the lack of input from people who are working on string theory, it’s fine, people can work on string theory and it doesn’t hurt anything. I feel… Well, it’s going to sound, I don’t want to be patronizing, the people who do it are mostly adults and they know what they’re doing, but students and people who are thinking about what they’re going to do should go into it with open eyes. They should realize that the prospect of making an impact in our understanding of empirical science or technology are not… The prospect that you’ll make impact like that is probably not optimized by going into string theory.

1:01:20.8 SC: Yeah, no, actually, I think that we’re in exact alignment here. I feel a need to defend the string theory against unfair criticisms, but I do worry a little bit about the fact that it seems hard these days to connect it directly to empirical reality.

1:01:36.2 FW: Yeah, well, some nice ideas are coming off, as coming out as spin-offs, very, very clever people do string theory and they do clever things. So as I said, there’s been a lot of fruitful mathematics, there have been new techniques that have proved somewhat useful in condensed matter, although certainly not proportional to the amount of effort that’s going into it, and the future may look different, they may be real breakthroughs that come out of string theory that wouldn’t have come otherwise. But so far, the amount, I would say, other people may disagree, and I might be very unpopular among some of my colleagues for saying this, but I think the output compared to the input has been pretty disappointing on the empirical side.

1:02:33.5 SC: Right. You have been involved in productive ways on the black hole information problem, which a lot of string theorists care about. What is your feeling these days, ’cause I just did a wonderful podcast with Netta Engelhardt, another of your MIT colleagues. What is your current feeling on the state of that problem? Do you think we’re making real progress?

1:02:50.8 FW: I think progress is being made in the sense that more intellectually coherent pictures are being drawn and some surprising connections to error correction and really interesting new chapters of quantum theory are emerging. On the other hand, it is a very esoteric problem, nobody’s going to produce… I don’t see a way, but who knows, but nobody has produced an experimental system to which these ideas apply in any reasonably direct way. So what does it mean to solve a problem like that? I’m not even sure what it means, where you can’t check. Many hypotheses go into it, the distance between the models and actual black holes that were phenomena you can observe are vast and many things could go wrong along the way in making these models.

1:04:04.7 FW: So I guess, yeah, it’s wonderful that people are making progress at the field, they’re making progress and have a literature that they enjoy, and it really is interesting from any point of view, it’s good, and maybe I should leave it at that, but how should I say? I don’t think it’s… I don’t think it’s the pinnacle of physics, let me put it… Let me put it that way.

1:04:35.0 SC: Well, I think that’s a perfectly good attitude in the sense that, like you say, we have this enormously successful set of ideas and the core theory, etcetera, and unlike the 1960s where there are these big puzzles given to us by the data, it’s harder to figure out where to best put our intellectual efforts, and it’s hard to judge what the good work is, sometimes, like it is it just what the smart people say, ’cause no one is predicting a new experimental result at the moment.

1:05:06.3 FW: Well, I did, with the anions.

1:05:09.8 SC: In materials, sure, but that’s the point, right?

1:05:11.5 FW: Of course, that was a long time ago, but other things happened, time crystals and so on. And people vote with their feet. So I’m… For instance, I’m spending a lot of time now really working with experimentalists and even engineer, quasi-engineers to design better axion detectors, and that’s an activity that calls on very different kinds of expertise. It’s really fun, and the chance that we’ll succeed, of course, is not enormous, but it’s not zero either, and that’s one reaction is to try to devise new kinds of experiments to think about other worlds, in the sense that we talked about before, there are many creative things you can be doing. And I think it’s wonderful that people do different things.

1:06:15.7 SC: Well, let me make a terrible confession here, because you’ve done so many interesting things that I completely forgot about the time crystals. Why don’t you tell us a little bit about time crystals, what they are and what the state of the art is there?

1:06:28.1 FW: Yeah, well, I guess the best non-technical way to describe it is to say that… Well, maybe I should step back a little. What is a crystal? A crystal is an arrangement of atoms in a regular system, from the point of view of basic description, fundamental physics, if you like, is an arrangement of atoms in a definite pattern in space that is not fully translation-invariant in the sense that if you move it a little bit, it’s not quite the same thing, but if you move it by a finite amount, it becomes the same thing if you translate it, so that one atom goes into the next atom. And we are accustomed in physics to making analogies between space and time these days, they just re-use many of the same concepts for both, they’re continuing dimensions, and so it’s natural to ask, and I raised this question in a serious way in 2012, whether you can have materials that are like crystals, but in time.

1:07:37.3 FW: So this of course wouldn’t be atoms that come to exist and pass away, but it could be some kind of phenomenon, some kind of marker that spontaneously arises within a physical system that has less than full time symmetry, and so is something that doesn’t settle down in a normal way into a boring equilibrium, but somehow in its preferred ground state, or maybe you have to generalize the definition of ground state slightly, into something that has non-trivial behavior in time, and this was very controversial when I first proposed it, but now some beautiful experiments have discovered this phenomenon in a very convincing way, and it’s become a thriving field.

1:08:34.7 FW: And another way to phrase it that’s more colloquial is to say that these are materials that spontaneously form clocks, they’re sort of… It’s like the blind watchmaker, right, you just wind these clocks lying around and under the right circumstances, if you find the right material and at the right, at low temperatures, and this and that, and maybe you have to sculpt it a little, that it forms a clock with very little prodding. And so that’s basically what a time crystal is. There are many variants, but that’s the basic idea and the basic hope for the field, I think… There are many hopes because it’s a whole new kind of states of matter, but to me, one very tangible possibility that would be extremely important is if these kinds of ideas about things that form clocks spontaneously would let us build better clocks.

1:09:38.6 FW: Forming accurate clocks is a frontier of physics. As you probably know, people are working very hard and constantly trying to push the accuracy of clocks because they’re very useful. It may seem absurd, for instance, to try to get clocks that would lose one second over the whole lifetime of the universe, but that’s the frontier right now, and it’s very valuable to have those things, because it allows you to look for very small effects, like the effect of passing gravitational waves for scientific purposes, so if a clock slows down a little bit due to a passing disturbance in the gravitational field, you can hope to sense it, but another use is in GPS-like systems, because small errors in measurements of time translate into much larger errors in estimates of distance, just because the speed of light is so big, so that’s why… So getting accurate cheap clocks is a frontier of physics that’s very, very important, and I’m hoping that thinking about time crystals and maybe some kind of hybrid between a time crystal and atomic clocks would allow us to push the frontier of accurate time timekeeping further.

1:11:06.9 SC: Well, this has been great, but I think you’ve been giving us a lot of inspiration and evidence for the idea that fundamental physics is in an exciting phase, there’s a lot of good questions that we’re struggling with and we’re going to learn a lot in the years to come.

1:11:20.0 FW: Yeah, including what fundamental means. To me, and time crystals was a good example of this, the meaning of words evolves and sometimes the most fruitful meaning of a word emerges from its use, we find out what the… Energy was like that. And I think as physics advances and we conquer more and more new frontiers of knowledge, what we regard as fundamental advances and fundamental questions will also evolve, and it’s a never-ending frontier as far as I can tell.

1:12:02.0 SC: That’s what makes it fun to me. So Frank Wilczek, thanks so much for being on the Mindscape Podcast.

1:12:06.4 FW: Okay. Thank you very much, Sean, it was fun.

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