From Hourglass to Black Holes: A Conversation with Mark Mezei, Interview by Zohar Komargodski

Zohar Komargodski, physicist, is a permanent faculty member at SCGP. Mark Mezei, physicist, is the inaugural research associate professor at SCGP. All previous postdoctoral positions at SCGP have been research assistant professors.

To watch the video, please visit:

An excerpt, as published in SCGP News Vol XIV, can be found below:

Mark Mezei. Photo: Joshua Klein

ZK: Congratulations on becoming the first Research Associate Professor at the Simons Center! Very well deserved. We have a few questions for you now. When and why did you become interested in physics?

MM: I became interested in physics in high school, around junior year. I had long been interested in math, but in high school, math was somewhat easy, and I had a physics teacher who, whenever he had a good day, taught really challenging things and posed really challenging problems. Basically, it was the challenge physics problems posed that got me interested. And then came the book by Brian Greene, The Elegant Universe. My mom’s boss, an American, gifted it to me and I read it. It was fascinating and got me really interested in fundamental physics. Then I started reading popular books.

ZK: How was your English at that point?

MM: I knew enough English to read that book. I learned it at school and after-school programs. By that time, I was pretty good, not so much in speaking but in reading.

ZK: Then you participated in the Physics Olympiad?

MM: Yes. The challenging problems led to the Physics Olympiads, which are taken very seriously in Hungary. There were weekly preparations taking place in my high school, for all of Budapest. Everybody who was interested in physics problems came to this after-school two-hour weekly club, including from the Hungarian-speaking parts of Slovakia. It was a big deal. They took the train for two hours just to participate in it. And there were fun problems—not really Olympiad-style which are boring and long—but tricky, fun ones.

ZK: In my time they also used to be short and tricky but now they are boring and long.

MM: Yes, the fun ones were tricky, Russian-style or Hungarian-style problems. And then we always got a long problem sheet that we had to solve for the following week—somebody presented it and it was a lot of fun. So, there were these preparations for the Olympiad, and I did it for two years. And that’s how I learned physics, really, on the high school level.

ZK: Do you remember some particular puzzles from that time that you still think are beautiful or worth sharing?

MM: I might not remember the solution.

ZK: That’s fine, we can look up the solution.

MM: One that was really fun was to estimate or give a dimensional analysis type of an equation for the time it takes for sand to run through an hourglass, as a function of different parameters of the hourglass. This question seems very hard at first. It’s amazing that you can answer it based on dimensional analysis. There seems to be so many parameters. I think the trick is that you shouldn’t include the size of the grains of sand in your dimensional analysis.

ZK: Very nice. In your more recent career, you have written a few seminal papers on the connection between some ideas in high energy and condensed matter physics, such as quantum criticality, and various kinds of black holes. Can you speak to us about what are the most important lessons from the work that you have done for black hole physics, and what are the most important lessons for condensed matter physics?

MM: The underlying ideology behind much of my work is to use AdS/CFT or Gauge/Gravity duality, which is an exact equivalence between gravitational theories and strongly correlated quantum many-body systems. The idea behind applying this framework to problems in strongly coupled physics is that you ask what kind of phenomena are generic in the gravitational system, which might provide interesting or surprising dynamical phenomena in the many-body language. So, my work on quantum criticality and black holes was driven by the realization that there are generic black holes in string theory, which have a certain kind of geometry, and this geometry implies or predicts some exotic phenomena in strongly coupled systems, e.g., scaling symmetry in the time direction, but fairly local physics in space. We are used to critical points where space and time play similar roles. In these types of examples, their roles are very dissimilar because in time you have very long correlations, but in space, very short ones. And such phenomenological models have been written down in condensed matter before. But the fact that this is so generic in gravity was a realization that we made. We speculated that this represents some universal intermediate energy liquid-like phase of matter that might be somewhat generic in strongly coupled systems.

ZK: What is the gravitational dual of this?

MM: These are AdS2 throats with some transverse space. And this work I did in the early 2010s and

ZK: The correlations are local in space, so you can reduce some space and it is just quantum mechanics?

MM:The way the physics works is that in the time direction the scaling exponents are dependent on the wave number in the transverse space, so it’s somewhat dependent on the spatial structure. The way to visualize it is these clusters next to each other that are only coupled in some way that the correlation length is finite. So, that’s almost individual clusters but they still talk to each other.

ZK: It’s like stacks of things in space.

MM: Yes. But AdS2 physics kind of made the reappearance recently in the context of this SYK model. And a good model of these phases would be to consider SYK models coupled to each other in some spatially local way. I’ve been following this recent work on AdS2 because of my past with these phases.

ZK: More recently you also contributed a lot of work to the subject that goes by the name TT deformations. Can you tell us a little bit about why that’s interesting and what are the main questions in that field?

MM: There are two reasons, somewhat independent, for why TT is interesting. One—it provides the first example known to me where you start from a long-distance infrared description of physics, then you learn a little bit of the remnants of the UV physics, the technical term is that you know what irrelevant operator appears in the low energy, long distance effective action. And just from this information you can reconstruct the microphysics and learn what the ultraviolet behavior of the system is. This would be completely out of question in one of the familiar examples of a
non-trivial… Renormalization group flow here in the theory of strong interactions at long distances we have some theory of mesons and perhaps baryons and in terms of those degrees of freedom it seems completely infeasible that you would be able to describe the microphysics of quarks and gluons. But then in TT, at least in one way of thinking about it, just knowing the infrared degrees of freedom and that you are working with this very special theory, you can learn about the microphysics.

ZK:What people did in real life was they observed hadrons, i.e., mesons, baryons and they reconstructed the quarks. In fact, they sort of actually did that.

MM: Well. But then they didn’t just do very low energy experiments. To discover partons they really had to go through…

ZK: Yes, a hypothesis based on low energy experiments.

MM: Correct. But then the dynamics of quarks and gluons was really only revealed by high energy scattering. Is that fair?

ZK:I think so, yes.

MM: That is one reason why TT is interesting and the other reason again comes from the connection between gravitational theories and strongly coupled systems. And that is where we, with Lauren McGough and Herman Verlinde, made a proposal for what the gravitational dual, gravitational equivalent description of the TT deformation might be in two-dimensional boundary theories/three-dimensional pure gravity. And the prescription is that you implement a sharp cut-off on the bulk geometry at finite distance and impose Dirichlet boundary conditions. This prescription produces gravitational physics that is mirrored in the field theory in the TT deformation. Thus, we have this very nice holographic theory of quantum gravity, but it’s very hard to reconstruct local gravitational physics from it. And so our proposal is an interesting step in that direction where you can “detach” yourself from the asymptotic boundary and study more local bulk physics. This has generated a lot of interest in the field and people are trying to exploit this new way of thinking about local bulk physics. It’s a deformation of the boundary theory by an irrelevant operator.

ZK: So the bulk physics doesn’t exist before you do the deformation?

MM: It does exist. It’s encoded in a very nonlocal way in the boundary degrees of freedom. And by doing these deformations you can make the encoding somewhat simpler.

ZK: So you’re saying that the holographic information is encoded more obviously?

MM: Yes.

ZK: What a wonderful creation! Last question. Do you have any advice for researchers of your age and stage on how to work out the work-life balance, especially if people have young children? You have been tremendously, as they say nowadays, “productive.” Perhaps you can share with us your experience on how to strike a healthy balance between work and family life?

MM: If you look at my pattern of publications you will notice that when my son was born it went down drastically for at least half a year and then it picked up again. Some of my recent productivity is basically just projects that were started before my son was born and are just being finished. I mean, one important advice is not to kill yourself. As probably every parent knows, during the first half-year you are just a zombie. So you can’t do much about it. You should accept it. And when you get your energy levels back you can catch up. One nice thing in theory is that even if you missed some time, some months, a year, you can get back into it. It’s not like working in a lab where your mice die or your bacteria don’t evolve. It’s very easy to get back into research. And the other thing that was very helpful for me is to start working more extensively with collaborators who are perhaps younger, perhaps students who can do some calculations that you would also be able to do, but you don’t have enough time. So for me it was, I think, broadening a little bit the pool of people that I collaborate with was certainly helpful.

ZK: Well, you seem to have recently collaborated extensively with some students at Stony Brook. Was it useful?

MM: Yes, I found it very useful for the reason I talked about. I think I have enough experience that I can see what problems are feasible and then the students can be extremely useful in figuring out the details. And there are some excellent students in Stony Brook, so it’s very helpful. Yes.

ZK: Very good. Thanks so much for the interview!

MM: Thank you. •