Theories of Everything with Curt Jaimungal - John Donoghue: The Physicist Who Says We've Already Quantized Gravity

Episode Date: March 11, 2026

Professor John Donoghue explains why quantum physics and gravity actually work perfectly together. He tackles quadratic gravity, effective field theory, and random dynamics, arguing that grand unification and naturalness aren't required for a theory of everything. As a listener of TOE you can get a special 20% off discount to The Economist and all it has to offer! Visit https://www.economist.com/toe SUPPORT: - Support me on Substack: https://curtjaimungal.substack.com/subscribe - Support me on Crypto: https://commerce.coinbase.com/checkout/de803625-87d3-4300-ab6d-85d4258834a9 - Support me on PayPal: https://www.paypal.com/donate?hosted_button_id=XUBHNMFXUX5S4 JOIN MY SUBSTACK (Personal Writings): https://curtjaimungal.substack.com LISTEN ON SPOTIFY: https://open.spotify.com/show/4gL14b92xAErofYQA7bU4e TIMESTAMPS: - 00:00:00 - Limits of Quantum Mechanics - 00:06:35 - Effective Field Theory - 00:12:24 - Gravity: Geometry or Force? - 00:18:46 - QFT and Gravity Tension - 00:24:59 - Quadratic Gravity Theory - 00:34:16 - Dueling Arrows of Causality - 00:41:57 - Random Dynamics and Anti-Unification - 00:48:13 - The Naturalness Problem - 00:53:40 - Questioning Hidden Assumptions LINKS MENTIONED: - John's lectures: https://www.youtube.com/@johndonoghue469/featured - John's papers: https://arxiv.org/a/donoghue_j_1.html - The Renormalization Group and Critical Phenomena [Paper]: https://www.nobelprize.org/uploads/2018/06/wilson-lecture-2.pdf - Anthropic Considerations in Multiple-Domain Theories [Paper]: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.80.1822 - Old "Ghost" Theory of Quantum Gravity Makes a Comeback [Article]: https://www.quantamagazine.org/old-ghost-theory-of-quantum-gravity-makes-a-comeback-20251117/ - Unitarity, Stability and Loops of Unstable Ghosts [Paper]: https://arxiv.org/pdf/1908.02416 - An Effective Field Theory of Gravity for Extended Objects [Paper]: https://arxiv.org/abs/hep-th/0409156 - Not Quite a Black Hole [Paper]: https://arxiv.org/pdf/1612.04889 - On Quadratic Gravity [Paper]: https://arxiv.org/pdf/2112.01974 - Quadratic Gravity [Paper]: https://arxiv.org/pdf/1804.09944 - Renormalization of Higher Derivative Quantum Gravity [Paper]: http://www.weylmann.com/stelle.pdf - Quantum Theory in a Nutshell [Book]: https://www.amazon.com/Quantum-Field-Theory-Nutshell-nutshell/dp/0691140340 - Field Theory: A Modern Primer [Book]: https://www.amazon.com/Field-Theory-Modern-Frontiers-Physics/dp/0201546116 - Origin of Symmetries [Book]: https://amzn.to/4qxnLzj - String Theory Iceberg [TOE]: https://youtu.be/X4PdPnQuwjY - Neil Turok [TOE]: https://youtu.be/zNZCa1pVE20 - Avshalom Elitzur [TOE]: https://youtu.be/pWRAaimQT1E - Sir Roger Penrose [TOE]: https://youtu.be/iO03t21xhdk - Ted Jacobson [TOE]: https://youtu.be/3mhctWlXyV8 - Leonard Susskind [TOE]: https://youtu.be/2p_Hlm6aCok - Jonathan Oppenheim [TOE]: https://youtu.be/NKOd8imBa2s - Peter Woit [TOE]: https://youtu.be/TTSeqsCgxj8 - Joseph Conlon & Peter Woit [TOE]: https://youtu.be/fAaXk_WoQqQ - Michael Levin [TOE]: https://youtu.be/c8iFtaltX-s Learn more about your ad choices. Visit megaphone.fm/adchoices

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It's time to turn those what-ifs into with Shopify today. Sign up for your $1 per month trial at Shopify.com slash tow. That's Shopify.com slash T-O-E. I think the popular phrasing is totally wrong. Quantum physics and gravity go perfectly well, as well as any other theory we know about. One of the biases of the field is that things, unify. And we don't really have any evidence for that. I'm actually a champion of a crazier theory.

You've heard it before. Quantum theory and general relativity are fundamentally incompatible. But is that actually true? Or is it something we just say so often we start to believe it? Professor John Donahue thinks this entire framing is misleading. Gravity is a field, the metric, so you quantize it like QCD. In fact, Feynman and DeWitt did exist. Exactly that several decades ago. So what's the actual problem? Donahue argues its hidden assumptions. Perhaps something like causality, supersymmetry, or grand unification, even so-called naturalists

could be a human bias rather than an objective law. Today we delve into quadratic gravity theory and another more speculative theory called random dynamics. On this channel, my name is Kurt Jymungle and I interview researchers about their theories of reality most often in physics. And today's a particularly technical talk, so be prepared. I'm excited because you'll see why John Donahue is a legend in the subject of gravity and its quantization.

We'll delve into effective field theory and learn why this professor's judicious restraint unnerves his colleagues. Professor Donahue, you're known in a sense for being radical, for not being radical. Explain that. Well, I am in a way quite conservative because I grew up as a phenomenologist where I learned to listen to what nature is telling us. And nature has told us gradually over time that the fundamental interactions are gauge theories and the gauge series are composed in particular ways. and we've learned to understand quantum mechanics and the fundamental interactions

through interactions with experiment, and so I tend not to deviate from that very much. You mentioned that quantum mechanics may fail at some point, and when we're thinking of quantum gravity, we're assuming that something about quantum mechanics, whether it's the direct von Neumann axioms, or something is held sacred, and then gravity has to bow, in a sense, to those.

But you said when we were, speaking off there that you don't think that's necessarily true. And I'd like you to comment more about that. Okay. So the point is that all our theories have limits. We've tested them in some range of energies and conditions. And we're used to think of other various interactions as heavy limits like the standard model we expect to be supplanted by other interactions. But the same should hold true, we should hold the same standard to quantum mechanics. The standard assumption is that quantum mechanics is valid at all energies, all scales, all, everything. Because we, as present, we don't really need deviations. But nevertheless, there could be

deviations both at high energies or on macroscopic scales. And so there are experiments testing quantum mechanics on macroscopic scales, and I think they're very interesting because that's a frontier for the limits of quantum mechanics, and perhaps the theory is we know now will be changed at some scale, some macroscopic scale.

The connection to gravity is that gravity may be the best place to test this because you can get macroscopic bodies out of gravity, whereas it's hard to get very macroscopic charges, for example. If you're trying to get large amounts of charge, it's hard, large amounts of mass is easy. So, sir, when popularizers talk about quantum mechanics and GR are incompatible, are they getting something wrong?

And also, I just said quantum mechanics and not quantum theory. So people talk about quantum mechanics, quantum theory, and quantum field theory. I don't know if you see those as different when speaking about general relations. and the combination of those two. Actually, I think the popular phrasing is totally wrong, that quantum physics and gravity go perfectly well, as well as any other theory that we know about. The quantum gravity involves a field, which is the metric.

That field is quantum. It was done by Feynman and DeWitt in exactly the same way we do QCD. There's no difference at all in the framing of it. So if you just take standard quantum theory and standard gravity, you have a quantum theory. The quantum theory is a little different than the standard model in that it's explicitly an effective field theory, which I can explain what it is,

but it's a quantum theory, and it can make quantum predictions perfectly well. That's one of the lessons of the last 30 years. Now, is anyone of your colleagues thinking that gravity as an EFT for low energies is wrong in some way? It's my understanding that no one would disagree with that. Yeah, I don't think anyone disagrees with it at this stage. It took a while for that,

the effective field theory viewpoint to percolate through the field because many of us grew up learning about renormalizable field theories as the only valid quantum field theories. But that's not true. We have other theories that are effective field theories that we could make perfectly good quantum predictions with and general relatively false in that same category.

Okay, so then are there any problems in quantum graph that are considered to be fundamental issues, like you mentioned, non-renormalizability, that you think this is a pseudo-problem, is not even a problem. Well, it's a problem in the long run, if you want a totally consistent theory at all scales. Do you want me to explain

effective field theory a little bit? Please. So, effective field theory is really just quantum theory with thinking about the scales done right. And the important ingredient is really the uncertainty principle. So in many cases, even in the standard model, there's unknown physics at high energy, so we just don't know what about because we've never tested it. But the uncertainty principle tells us that the effect of that is local,

because delta E is big, delta X is small then, and so it appears effectively local. And so all the unknown effects of very high energy appear as local terms in an effect of Lagrangian. In a Lagrangian. And they're just constants that we don't know. There's like the mass of the electron is one of those constants. The charge of the electron is another. The gravitational constant. These are terms that are local.

But nevertheless, the quantum effect, are involved dynamics at the energies you're working at, and those can be done with the degrees of freedom and the interactions that you have at those energies. They're not local in that sense. And so effective field theory basically just separates out the unknown effects from high energy from the known effects that the energy you're working at

and makes predictions. And in the end, we think of all of our theories like effective field theories. Now, you didn't mention the name Ken Wilson. But I think it's probably useful, too. So can you talk about how Wilson changed the understanding of what a non-renormalizable theory is? And also feel free to expand on what a non-renormalizable theory is as well, and what the view pre-Wilson versus Post-Wilson was. Right.

So Wilson was clearly important in this change in the viewpoint that his, the point of view that he espoused was that, there are various local terms, and the Lagrangian doesn't need to be restricted. So we derive our physics from an action from a Lagrangian. The Lagrangian for a normalizable theories has just a few terms, but in principle there could be terms of higher dimensions, which are, I need to explain dimensions, I guess. The Lagrangian that we write out could have, many terms. So, for example, you could have fields with two powers of the fields, four powers of the fields,

six powers of the fields, etc. And those are said to be increasing dimensions. And normally we limit ourselves to the lowest dimensions. Wilson told us that quantum effects generate the other ones also, and that we can include them in the fundamental theory, in the Lagrangian equally well, we just have to have a way of determining them, either by direct calculation or by measurements. I want to just clarify that the dimensions here, they're not spacetime dimensions, they're just powers of the field. The powers of fields. That's great.

Thank you very much. It's, so powers of the fields. And so the usual theories that we do, the renormalizable theories like the standard model, are very limited in the powers of the fields that they use. The effective field theories are more generous, they're more general. In 1994 or so, you calculated the correction to the Newtonian potential. Yes. Walk me through that.

So the interesting insight there is that the long-distance corrections to the Newtonian potential, the ones that fall like a power, since they're not local in the sense, because they're long-distance, are then determined not by physics from very high energies, but only by physics from low energies. And we know that physics. So we know that it's general relativity. We know the interactions of general relativity. We know the Feynman rules because Feynman and DeWitt told us those.

And so the long-distance correction to the Newtonian potential are rigorously calculable only knowing what we know at the present energies. And so that was a very explicit example of how effective field theories work. By knowing the low-energy degrees of freedom, we can make calculations that are valid at low energies without needing to know the high-energy theory. There's something that I was thinking that might be interesting to the listeners of just a perspective on why historically it was difficult to think

of gender relativity being a quantum theory. And this is because of the effect in this of the classical geometric picture. When you learn gener relativity, one of the interesting features that you first learn is that generativity doesn't look like a force, it's straight-line motion in a curve space time. And so you think of a manifold or space-time structure as being fundamental. And there's this sort of mantra that I think probably goes back to Wheeler. Gravity is not a force, it's geometry.

And if you come into the game thinking about geometry and not about forces, then gravity does look different than other theories. Other theories have forces and particles mediating the forces. But that's really a feature of the classical theory, and it's not really fundamental. You could equally well formulate general relativity as a theory with exchange particles, with waves on space time like gravity,

gravitational waves that we now detect in LIGO with exchange particles. And if you formulated it like a field theory, then the quantum theory looks very similar to quantum theories, other quantum theories. If you're formulating in the geometric picture, it looks quite different. And so I think historically some of the trouble has come just because the classical theory has been so successful using classical methods that it was hard to switch to quantum methods. Is there a reason that when I watch your lectures

that you start from the path integral approach, as far as I've seen, and not something else, like canonical? That's certainly true. I mean, it's certainly true also if you think of X commuted with P being IH bar and all that, it looks much more complicated when X suddenly is a general or altruistic coordinate. But all our fundamental theories are now only formulated in path integrals.

You can't formulate the standard model in a Hamiltonian theory and do it use any reasonably useful formulation. So all our theories are formulated that way. and general relativity makes the most sense that way. And this is a lesson that goes back to DeWitt in a way when Feynman, this is a bit technical, but when Feynman showed that you needed ghost fields to get the correct degrees of freedom in general relativity, DeWitt formulated in the Path Integral Framework,

and then the QCD is also formulated that way. So basically, you have to use the same formulation that you use for QCD, You would never do anything else for QCD except Pathonyl's. I'm sure you've taught quantum field theory. Yes. So do you teach canonical quantization at all? I do. You know, I think the place where in the framework

where you're progressing from quantum mechanics to quantum field theory, you need to go through a phase where you define quanta, basically. the original quanta of photons. And so doing second quantization for photons is a necessary step to introducing the idea of quanta. But then at some stage, and so yes, you do that, you define the fox space, you define one photon, two photon states.

At some stage, to do theories like QCD, the weak interactions, you have to get into path integrals and show that they have the same content. And if you start with path integrals to begin with, it's actually, there is a place where it becomes, there's a logical disconnect on what is the one quant of state. So there's, I mean, there's two very good books that do this.

So one is Tony Z's quantum field theory book, and the other is pyrr-a-romones. They start straight from path integrals, but there's a stage in there where there's this logical disconnect of how you define one quanta. And so, yes, I tend to go through the canonical and then show that the same results come out of path integrals. earlier you were saying that most people or most students have heard the geometric interpretation of general relativity

and that that can stymie them when thinking about making that compatible with quantum theory. So I'm wondering, are there any other myths or whatever you want to call them that you have to dispel to students? Well, I think that's the main one. I think if you start with general relativity, as a field theory, classical field theory, then you could merge into the quantum theory quite easily.

The trouble is it does take a while to get there because it takes a while even to get to the quantization of QCD. It's not the day two on a quantum course. It's a little further on. And you have to have that background. And so getting to path of integrals has been at least the way I teach it the best way to get there. I asked earlier about the tension between quantum mechanics and gravity.

Now, should I have said quantum field theory and general relativity, or do you even say aloud quantum mechanics and general relativity? I tend to say quantum field theory. I probably have not said quantum mechanics. I have said quantum theory, but quantum theory in any relativistic system has become quantum field theory. And so we're talking about quantum field theory, really, when we want to do this. So has there been any trouble making general relativity consonant with quantum mechanics as such?

Or as soon as you, well, if you're talking about GR, you're talking about special relativity as a special case, and as soon as you combine that with quantum mechanics, you get Q of T. So you do have to get to the quantum field theory just because for any relativistic field theory, any relativistic system, you need quantum field theory rather than quantum mechanics. Most of my best ideas don't happen during interviews. They come spontaneously, most of the time in the shower, actually,

or while I'm walking. Until I had plod, I would frequently lose them because by the time I write down half of it, it's gone. I tried voice capture before, like Google, home, and it just cuts me off in the middle. It's so frustrating. Most of my ideas aren't these 10-second sound bites. They're ponderous. They're long-winded. I wind around. They're discursive. They're five minutes long. Apple notes, even Google keep. The transcription there is horrible. But Plaude lets me talk for as long as I want, and there's

no interruptions. It's accurate capture. It organizes everything into clear summaries, key takeaways, action items. I can even come back later and say, hey, what was that thread I was talking about regarding consciousness and information? In fact, this episode itself has applauded summary below and I'm using it right now over here. My personal workflow is that I have their auto flow feature enabled so it sends me an email anytime I take a note. Look, the fact that I can just press it and it turns on instantly like right now it's starting to record without a delay is extremely underrated. This by the way is the note pro and then this is the note pin. I have both. Over 1.5 million people use plot around the world. If your work depends on conversations or the

ideas that come after them, it's worth checking out. That's plod.a.ai slash T-O-E. Use code T-O-E for 10% off at checkout. So how do you see then the tension between quantum theory and gravity? I don't see any tension at the... Yes, at low energies, sure. But the world is not just low energy. So at the moment, we know low energies. And so I wouldn't say there's any tension between the standard model in quantum mechanics. There are problems if you try to take the standard model to high energies.

For example, if you try to take the Higgs potential at very high energies, there's an instability. So at high energies, the standard model doesn't work very well. We expect it to be supplemented with something else. The same thing's true with general relativity. We expect a more complete theory, at high energies.

That more complete theory could have various characteristics, but when we think of an effective field theory, we always think of what the limits of the theory are, and the effective field theory tells us that there are limits to where we can apply the effective field theory at high energies. So the string theorists are concerned with UV completion,

and they believe that you would require some extended object. So do you disagree with them, or is your disagreement more empirical or philosophical, or what? I don't think there's a disagreement between the string theory approach and the effect of field theory approach. The string theory is one possible UV completion, and it's interesting because it's one where we have the most

calculational control. so it is useful to see what a complete quantum theory of gravity could look like. And as far as I can see, it should match onto the effective field theory in all the regions where we expect them to both overlap. And so I don't see any conflict there. There's something called the Swamp Land Conjectures, I'm sure you've heard of. So does EFT gravity or quadratic? gravity sit in the landscape or in the swamp land?

So, EFT has a set of parameters which are undefined by the EFT, and there is a portion of those parameters that live in the swamp land and a portion that live in the non-swamp land. So the Swamp Plan program basically divides the EFT parameter space into two regions, Swampland and Not. We can do quadratic gravity, if you want. Sure, I would love to hear about that. The Einstein's theory follows from an action that, if I, let me neglect the cosmological constant and all that follows, just follows from a very simple Lagrangian, which is just the scalar curvature.

So if you take the scalar curvature, it has to be a general covariant and Lorenzen variant, object, there is one simple one, which is just the scalar curvature by itself, that gives you ion sine gravity. However, consistent with all the things that I've just said, you can also take the scalar curvature squared. That also works. So that's what, when we say quadratic, that means curvature squared. And it turns out there's two terms, one there's the scalar curvature squared and the other is the richy curvature squared. Or the Vial tensor squared. There's two possible terms at quadrate squared.

I'll show the Lagrangian on screen. Okay, okay. Then the beauty of that is if you do that, the theory becomes renormalizable. And so it fits in and with all the other theories that we don't, as renormalizable quantum field theories. And that was showed by Steli back in the 70s. So it's singled out as a particularly nice theory, a very conservative theory where the metric is still the main degrees of freedom. The theory is renormalizable, and it's essentially unique in that.

So this is the theory that I have done some work on in the past 10 years or so with Gabriel Menna as my collaborator and others in the field that are also exploring. wearing this. There's a but here. There's a but. Okay. And the but is that the reason this works is that you, the curvature is, involves two derivatives of the metric. The curvature squared said involve four derivatives of the metric. And when you do go from two derivatives to four derivatives, things happen that aren't part of the usual axioms of quantum field theory. In usual axioms, the requirement of analyticity restricts you to two derivatives. When you go to four, then something falls apart. And there's been a lot of debate on what it is.

It could be unitarity, it could be causality, it could be stability. my sense is that at least in pathoidal quantization, the theory is unitary, it's stable, but it does give up causality at high energies. And the reason for this is that analyticity, this assumption of analyticity, is also tied into the usual requirements for analyticity. Wait, analyticity is required for analyticity? You mean for causality? Analyticity is required for causality.

Yes, I'm sorry. Okay. So are you referring to Austroxiety? Stragratsky or something else when you say that there is instability? So the stability is Ostergrotsky. So there's a classical theory if you put higher derivatives in exhibits and instability, which was shown by Ostergradsky almost 200 years ago. However, there's a reasonable number of ways that we feel that that is not true in the quantum theory.

Interesting. So I think that Ostergoyt's. regards, the instability is not an issue. This seems to be a stable theory. One indication is that these higher derivative theories can be simulated on a lattice without showing its stabilities. As far as I can tell, the theories seems to be stable against decays to be stable, to lower energy states. It does require a particular sign for a coupling constant. So there are three coupling constants in the theory. They have to take particular signs, and one of the signs is particularly interesting.

And the reason they have to take signs is partially the stability issue. There are new states in the theory, and if you choose the sign wrong, you get tachyonic states, and those ordinary matter will just decay into infinites. number of tachyons, and you'll be unstable. So there is a sign choice involved. Why don't you give the viewer an understanding and intuitive understanding of quadratic gravity? Because when you have just the richly scalar and you vary it,

you get the Einstein equations. And then that has some intuitive picture that people like to espouse of geometry and geodesics and so forth. So what changes when you add these quadratic terms to the intuitive picture people have? of what general relativity is? Well, so low energies would be the same picture. These quadratic terms are negligible at low energies.

So everything that we've tested experimentally would be the same. The trick only happens when the curvatures or the energies are very large. And then the fact that the curvature involves two derivatives, curvature squared is involved four, derivatives turn into energies, meaning that if you have high curvature or high energy, the other terms dominate. You would still have a geometric theory,

but it would be satisfied different equations. There would be fourth-order differential equations. For the quantum theory, the important part of that is that normally with two derivatives, propagators for particles go like one over the energy squared, four derivatives, it goes like one over energy to the fourth. And so what changes is that all the high energy stuff becomes better behaved because the one over energy to the fourth

kills off high energy divergences and makes things renormalizable. And so the fact that it has four derivatives is crucial to making the high-energy theory well-behaved. So, in contrast to general relativity, where the high-energy theory has bad behavior, here the high-n-g theory has good behavior. Hmm.

Just like QCD, for example. What about singularities? Does it give any insight into singularities? Yeah. Or black holes, or the Big Bang, or what have you? That's a good question. And I don't think the answer is completely known at this stage. it's not even clear that the question is well posed because at high energies it's not clear that you should be using a simple classical picture.

And the singularity theorems are all classical theorems. There are work by Bob Holden and Selly and collaborators that show that there are non-singular, black wholesalry, that come out of the quadratic gravity case. So there's not a clear understanding of which one should be selected, but certainly there are non-singular solutions to quadratic gravity. Interesting. Earlier when you said that general relativity can be formulated as a force theory, not just as a geometric theory, like Wheeler suggested,

are you referring to it classically or are you referring to it quantumly? Both. How can it make sense as a particle theory classically if it's a field theory? So for example, there's now a whole body of a whole subfield where they treat black hole mergers using quantum field theory techniques. And basically there's a classical limit that you can take of doing vitamin diagrams. And you can pick out the quantum effects of the classical. effects, I'm sorry, of black hole mergers. It started with work by Goldberger and Rothstein,

and has now become a much larger field, including some of the people that use standard relativity techniques for a long time. It very quickly caught up to the other techniques using differential equations. Equivalently, just as you said that GR can be formulated as a force theory versus curvature theory. I'm sure you've heard that you can add torsion and then formulate general relativity as a torsion theory. I'd like Carton theory, or you can turn on non-metricity with metricify. So there's all these different little tweaks that you can do to GR

to get an equivalent theory. Is there anything about quadratic gravity that would rule out torsion or non-metricity? No, I don't think so. I haven't explored the extensions that much, but the extensions using torsion and nonmetricity do exist, and people have done some explorations. I think it's great exercises.

For me, quadratic gravity is the simplest version, and it's in some ways the most conservative, because it doesn't do anything beyond what we know we need. We already need, we know that we would expect a curvature squared terms to be in the action. And so we're just using them in the minimal way. I'd like to talk about causality. Okay. So you have a phrase that they're dueling arrows of causality.

Right. That could be yours or your collaborator Menendez. What did you mean by that, that they're dueling arrows of causality? So this is what I referred to earlier in that you're, when you do this theory with four derivatives, any theory with four derivatives, you give up some portion of the usual axiomatic field theories. And it seems that at least with pathinaural quantization followed the usual pathway, that what you get is, you're giving up some aspects of causality. And that aspect is the following. There is an arrow of causality

that's built into our theories, and that's contained in the factors of I in the quantization procedure. If you do path integrals, it's E to the I-S, either the i types the action if you chose either the minus iS you would actually have the exact same theory

but just the different arrow of causality things in positive time would influence things later in negative time it basically time reversal changes i to minus i it's time reversals anti-unitary and so

if you take e to the i s and do time reversal on it, you get a time reverse theory that's exactly the same. And the nature of the theories with higher derivatives is that you get a massless in case of gravity, a light particle with the usual arrow of causality and a very heavy particle with the opposite arrow of causality. Okay. So if you produce this guy, it, if you come, come in with positive energy, it propagates not forward in time like normally does, but it propagates backwards in time for a short period of time. It decays. You then get normal particles out, but you've had a short period of time where this thing

propagated with a different direction in time, which takes a while to wrap your head around, but at least I've come to peace with it. Why does it take some time to wrap your head around? Well, so we're used to the idea that particles can propagate to the left, particles can propagate to the right when they're carrying positive energy. However, the standard theory is if you have a positive energy particle, it only propagates in one direction in time, propagating in the backwards direction in time

doesn't normally occur. And that is because we use E to the I.S as the in the path integral. It's equivalently if you do commentators with IH bar as opposed to minus IH bar. And so we're not used to seeing, in this case, when you decompose the interactions, you get the white particle with E to the I.S

and the heavy particle, the ghost particle, with E to the minus I.S. And let me make a caveat, just to be honest. The theory that has this seems to work. We've been able to do one-loop calculations in it. I think it's still not clear how what does higher loop calculations, and we don't know for sure that it works at all orders. which is one reason for exploring lattice calculations.

And so I shouldn't say that it's known to be without flaws, but it's known at some order in perturbation theory. Does the S matrix assume causality? So the S matrix defines a transition from states that, stable states in the past to stable states in the future. Right. Well, if light cones are somehow fuzzy, then that's what I'm wondering is what would become, what would be considered in versus out. Right. And how do you define the S matrix?

So in this case, all these heavy particles that have the funny propagation are unstable. And so they don't live in the distant past or the distant future when you define an S matrix. They just live in its intermediate states. So you go through a region. So you imagine doing a scattering. You take usual particles in. You scatter them. Some have residences that go to heavy particles that are normal particles.

Some have these funny winds that propagate backwards for a short period of time. But then you end up with all your normal particles in the future. So the proof of Unitarian that Gabriel and I did it makes a different, an important feature that only normal particles live in the distant path and the distant feature for the S matrix. Does this mean there is something non-local occurring at the Big Bang? I don't know how to answer that. I mean, I don't know the answer. We are trying to look at some of the implications for the early universe,

but it's not really clear how one defines the start of the universe in any theory. You know, we could describe it accurately after a certain phase where the physics that we know kicks in, but I don't know what starts the universe. You told Quantum Magazine that you're not convinced that quadratic gravity is the final theory. Right. But that's much like the standard model in the sense that I also don't, the standard model I don't expect to be the final theory. in the case of quadratic gravity,

it feels to me like the standard model. It feels like it's a renormalizable quantum theory that would kick in at some scales, much maybe the scale of inflation. And for those scales, it would be a perfectly fine theory. But at that level, we have the standard model, which has three separate theories involved, we have a renormalizable theory of gravity,

that doesn't feel like the final theory to me. It feels like something else is going to explain the origin of all of these theories. And so I expect, or I would naturally expect that there's some deeper theory that explains the standard model and gravity both. And string theory is a possibility. That's perfectly fine, but it could be something else.

I'm actually a champion of a crazier theory, which is Holgerneelson. Holger Nielsen had this crazy idea that at very high energies, everything happens. So basically you can imagine everything you can imagine happening. And it's sort of, he calls it random dynamics. and that the reason why at low energies you only see certain theories is that some theories are protected and are able to have massless fermions, massless gauge bosons. So gauge symmetry and chirality protect fields from generating masses.

So if you gauge bosons, it's probably well-known. Chirality is probably less well known that for fermions, If you have chiral fermions, you have a left field by itself, it can't have a mass. And so his argument was that the reason we see at low energies, the standard model with chiral fermions and gauge fields, are the only ones that are protected at low energies from generating large masses. And therefore you get U1, S.E2, SG3. And by the same logic, you could get a gravity theory that is protected by a general covariance from having larger masses.

And so this is sort of the anti-unification instead of unifying, you have everything happening at high energies, but only some things living to low energies. I just like it as an idea. Okay, this is super interesting, because you're so conservative in terms of you don't want to posit more than necessary, But then over here, you're extremely attracted to something that is quite zany. Well, this is a bit, yeah, it's a bit contrary, and I have to say it's, it's, it's, we, we, if you search yourself for biases, one of the biases of the field is that, that things unify. And we don't really have any evidence for that. There's no evidence for gauge unification at all. People normally talk about unifying electricity and magnetism,

but that's not really two gauge theories. What you've done is you've identified the single gauge theory that describes both those phenomena, U1. And the electro-week unification isn't really unification. You started out with E&M and a weak theory, and you've explained it by having two theories that mix. S.U.2 cross U1. They're not unified. They're just mixing.

So we've never really seen unification. So the idea of unification could just totally be a bias. It could be that the theory as we go up in energies is U1, SU2, SU3, SQ4, SU5, XU6, X37, or something else as you go up. It doesn't have to ever come together into a unified gauge theory. Yes. Okay.

So I like the random dynamics I did mainly just because it's an example of how our biases may be making us miss. what is the ultimate high-energy theory? When I'm deep in research, reading papers on, say, the philosophy of mind or quantum gravity, it may seem sterile, but I still don't want my ISP logging every single page that I visit. ExpressVPN prevents exactly that. Your internet service provider, your ISP, can see, they can log, and they can, in the U.S., legally sell a record of everything you do online. ExpressVPN encrypts all your traffic and reroutes it through secure servers so your browsing

history stays entirely yours. I travel for interviews and conferences constantly and connecting to hotel airport Wi-Fi without protection is something I'd often kick myself for afterward. ExpressVPN's encryption makes those public networks safe. It can be used to totally unlock geo-restricted lectures and content by changing online location to any of 105 countries. There's no compromise on speed either. It's consistently among the fastest VPNs out there. The frivolity of hunting for workarounds just vanishes. Find out how you can get up to four extra months by clicking the link in the description box below expressvpn.com slash theories of everything YT. That's expressvpn.com slash theories of everything YT. Okay, so at first when you were saying that there hasn't been

unification, you qualify that with gauge unification. Yes. So, because the standard folklore that people hear is, well, there was the heavens and the earth, and Newton unified that with gravity, then there was space and time, and then Einstein unified that with space time, and then there's electromagnetism, or electricity and magnetism, they're unified and so forth. But those are just identifying the appropriate interactions. That's not a unification of any interactions. You read, you know, E&M, electricity and magnetism look like different phenomena, but we've actually, Maxol, you find out that it's really the same

underlying theory, just one theory. So you're, you know, if you want to use the word unify, it's okay, but it's not unification in the sense that the particle physics field assumes this is going to happen at high energies. We assume that we're going to merge

into some SU5 or SG10 or E8 cross E8 or some some unified theory with more degrees of freedom, but which has a higher symmetry than what we have at the moment. That's a beautiful idea. It's a great idea. I'm not saying it's wrong. I'm just wondering if that's a preconception that keeps us from seeing other options. So you're not saying there's something inherently wrong with guts.

No, no. With grand unified theories. You're saying that there are alternatives. Or maybe you are saying there's something wrong with guts. Well, you know, one of the lessons of the LHC may be that there is something wrong with guts. Because the reason that we, as a field, were so convinced that something was going to happen at the LHC, basically went back to a conflict with the idea of grand unified theories. In grand unified theories, you have this very heavy scale,

and the symmetry is broken, and the need is to have the electro-week theory happen on a lower scale. But in a grand unified theory, that's a very unnatural thing to have happened because you have the naturalness argument says that the Electro-Weak scale

would naturally be very close to the Gran Unified scale because radiative corrections generate large effects and would normally bring it back up. So the naturalness argument led us to the believe that there was some mechanism that prevented, that protected the electric weak scale

and kept it at low energies. And this, for example, was supersymmetry. The argument for low energy supersymmetry basically came from trying to protect Grand Unified theories from large radiative corrections and allow the electric weak scale to be very much less than the Grand Unified Scale. However, we've learned from the LHC that that doesn't happen. There is no evidence of supersymmetry at the LHC. And so if we really believed our arguments about naturalness, the conclusion should be that there shouldn't be any theory, any high-energy theory that has a naturalness problem.

And so there shouldn't be Gran Unified theories. I'm just shocked because I entered into this conversation with you and probably you the same, thinking that this would be one of the most not ordinary, but

uncontroversial podcast that I've had or podcast guests. In fact, I even started it by saying that you're known in a sense for the radicalness of you, if there is any radicalness would be that you are not radical in a place where you can see your colleagues as engaging in some speculation where it may be unwarranted in your frame of mind.

Okay. And now we're getting to something that philosophers called entham memes. I don't know if you've heard this term, entom memes. Not particularly, but... An entham meme is an argument with an unstated premise. So it sounds to me like you're attacking some of these unstated or hidden. There are these assumptions that are so widely held, like, that grand unification is going to occur, that we have supersymmetry, and that unification, even that, the fact that you say that unification also may be something to be questioned, but you're talking about a particular kind of unification. That's super interesting. So I want to know... And this is something that we should be doing. And in a way, it actually relates to our starting

topic, which is one of the unstated assumptions for... decades was the general termian quantum mechanics were incompatible. You know, one of the things everyone should be doing it every step of the way is questioning these assumptions in seeing if they're based in fact or

something that's keeping us from progress. And in the case of the general relativity, the effect of field theory techniques told us that that assumption was in fact incorrect. And so we're just continuing to do that.

You know, the quadratic gravity is questioning whether an assumption of analyticity and causality is valid at all scales. So I'm doing it again with unification. What if a string theorist says, look, string theory uniquely gives a finite two-to-two graviton amplitude at all orders? Is that not evidence that string theory is on the correct track? Well, it does that open some assumptions. I think if it really was proven to be unique with absolutely no exceptions, then that's a powerful statement.

I think that's unlikely to happen. I mean, one of the assumptions of the string theory program is, causality holds at all scales, and so if I give up causality, I could probably get a perfectly fine amplitude at all energies. And again, some of the assumptions that we've made at high energies are built into the theories that we're constructing. They may be correct. I'm not saying that it's incorrect, but it's also possible that there are ultimately. ways of completing a high-energy theory that I would expect is not unique to the string theory. Do you have advice that you consistently give your students?

Most, the biggest advice I'd like to give by students was to read widely and think about things outside of their own research areas. In practice, what I say is through, you know, when you're reading papers, read the introductions and the abstracts so that you get a sense of why people feel things are important problems and what approaches they use. And then just to keep thinking about things outside of your own research area. You're not going to be doing your thesis topic, the rest of your life. You know, everyone is going to move on

from their thesis topic, and they have to find the richest pathway to having a long career. So I started working on the quark models, ended up on quantum gravity. Quite a distinction. So when you say read outside your research area, you don't mean to say that if you're

physicists start looking into biology? Well, if that interests you, I think you could. There are many people that have made interesting transitions from physics, theoretical physics, to biological physics. If that's an interest of yours, I think that's a wonderful thing to do. But I do tend to mean that if you're working on electro-week phase transitions that you should also read up on on grand unification

and string theory you get at least a basic understanding of the whole field as a whole your particular research topic you're required as a student

to become an expert on a very particular topic and you really need to do that but you also for future success

need to look outside of that and I think many successful theorists have to do that early in their career, too. One of the things that I've always done when I'm on search committees looking for faculty members is to see if they've done anything that's not exactly their thesis work. So they have also been are said to be one of the top people on subfield X, Y, or Z. But then many people also have a lot of also written papers or had little collaborations that are outside of that. And that's always a

positive sign because it shows that they're interested and able to contribute to more than one area. When I'm wrestling with a guest's argument about, say, the hard problem of consciousness or quantum foundations, I refuse to let even a scintilla of confusion remain unexamined. Claude is my thinking partner here. Actually, they just released something major, which is Claude Opus 4.6, a state-of-the-art model. Claude is the AI for minds

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I use Claude, actually live right here, during this interview with Eva Miranda. That's actually a feature called artifacts, and none of the other LLM providers have something that even comes close to rivaling it. Claude handles, interalia, technical philosophy, mathematical rigor, and deep research synthesis,

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slash theories of everything, all one word. Professor, are there any lessons that you wish you'd learned sooner in life? In some ways, I guess it would have been nice to put my own advice into action a little sooner and that working on some of the other areas earlier in my career, I think could have been fun. But then again, I don't regret anything particular. I probably could have done the effective field theory six or seven years earlier. I have gravity six or seven years earlier.

I basically knew the general principles at that stage from doing effective field theory work I just to do the correction to the toning potential I knew was going to be a very long calculation with a result that's not able to be measured and I was

pretty much a phenomenologist at the time and so I'd like to do measurable things so I put off doing it until until at one stage I decided I just needed to do it but I could have done that six six years earlier. Might have been fun.

What idea of yours has faced the most resistance? Well, I was on this anthropic bandwagon for a while. The anthropic stuff has tremendous resistance by large numbers of people. But you are no longer? I know you had a 1998 paper on it. No, no, I just don't see anything that I usefully can do at the moment. I sort of read out of useful things to do. I still think it's one of the pathways

to a fundamental theory that makes a good deal of sense. Again, it's part of this questioning the hidden assumptions that we're doing is that we grew up thinking that there is a unique theory, the grand unified theory in particular that was going to explain everything, give us all the masses and coupling constants of the theory that we see in nature. And the anthropic ideas make that actually seem very unlikely, just such a small portion of parameter space that is consistent with existence of stars and atoms, that it seems to be almost a very unlikely possibility.

Speaking of hidden assumptions, are there any hidden assumptions that your colleagues or perhaps even your students have questioned that you said, that's a bit too far? I wouldn't question that. My attitude on that is I would probably never say it, but I'd ask if they could do anything useful with it by questioning it.

You know, if they would like to give up four dimensions, fine. Do something with it. So I guess I wouldn't ever say, Not out loud. Don't go there, but I'd say whether it's a profitable thing to do to suggest. Sir, it's been an honor to speak with you. Thank you. Very good.

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