Matrices and Gravity

So I have a confession to make. I started working on random matrix models (the large $latex N$, double-scaled variety) in 1990 or 1991, so about 30 years ago, give or take. I’ve written many papers on the topic, some of which people have even read. A subset of those have even been cited from time to time. So I’m supposed to be some kind of expert. I’ve written extensively about them here (search for matrix models and see what comes up), including posts on how exciting they are for understanding aspects of quantum gravity and black holes. So you’d think that I’d actually done the obvious thing right? Actually taken a bunch of random matrices and played with them directly. I don’t mean the fancy path integral formulation we all learn, where you take N large, find saddle points, solve for the Wigner semi-circle law that the Dyson gas of eigenvalues forms, and so forth. I don’t mean the Feynman expansion of that same path integral, and identify (following ‘t Hooft) their topology with a tessellation of random 2D surfaces. I don’t mean the decomposition into orthogonal polynomials, the rewriting of the whole problem at large $latex N$ as a theory of quantum mechanics, and so forth. No, those things I know well. I just mean do what it says on the packet: close your eyes, grab a matrix out of the bag at random, compute its eigenvalues. Then do it again. Repeat a few thousand times and see that all those things in the data that we compute those fancy ways really are true. I realized the other day that in 30 years I’d never actually done that, and (motivated by the desire to make a simple visual illustration of a point) I decided to do it, and it opened up some wonderful vistas.

Let me tell you a little more. […] Click to continue reading this post

A Dialogue about Art and Science!

On Saturday (tomorrow), I’ll be talking with science writer Philip Ball at the Malvern Festival of Ideas! The topic will be Science and Art, and I think it will be an interesting and fun exchange. It is free, online, and starts at 5:15 pm UK time. You can click here for the details.

I’ll talk a little bit about how I came to create the non-fiction science book The Dialogues, using graphic narrative art to help frame and drive the ideas forward, and how I really wanted to re-shape what is the norm for a popular science book, where somehow using just prose to talk about serious scientific ideas has become regarded as the pinnacle of achievement – this runs counter to so many things, not the least being the fact that scientists themselves don’t just use prose to communicate with each other!

But anyway, that’s just the beginning of it all. Philip and I will talk about […] Click to continue reading this post

Full Circle

snapshot of paper

Yesterday I submitted (with collaborators Felipe Rosso and Andrew Svesko) a new paper to the arXiv that I’m very excited about! It came from one of those lovely moments when a warm flash of realisation splashed through my mind, and several fragments of (seemingly separate things) that had been floating around in my head for some time suddenly all fit together. The fit was so tight and compelling that I had a feeling of certainty that it just “had to be right”. It is a great feeling, when that happens. Of course, the details had to be worked out, and everything checked and properly developed, new tools made and some very nice computations done to unpack the consequences of the idea… and that’s what resulted in this paper! It is a very natural companion to the cluster of papers I wrote last year, particularly the ones in May and June.

What’s the story? It’s all about Jackiw-Teitelboim (JT) gravity, a kind of 2D gravity theory that shows up rather generically as controlling the low temperature physics of a wide class of black holes, including 4D ones in our universe. Understanding the quantum gravity of JT is a very nice step in understanding quantum properties of black holes. This is exciting stuff!

Ok, now I’ll get a bit more technical. Some background on all this (JT gravity, matrix models, etc), can be found in an earlier pair of posts. You might recall that in May last year I put out a paper where I showed how to define, fully non-perturbatively, a class of Jackiw-Teitelbiom (JT) supergravity theories that had been defined in 2019 in a massive paper by Stanford and Witten (SW). In effect, I showed how to build them as a particular combination of an infinite number of special “minimal string” models called type 0A strings. Those in turn are made using a special class of random matrix model based on […] Click to continue reading this post

The Notebooks

[caption id="attachment_19724" align="aligncenter" width="499"]A montage of notebooks A montage of some of my notebooks. Click to zoom in![/caption]

This is a quick montage of a selection of my notebooks over the last few years. As you may know, I often carry a little (usually black) notebook with me whenever out and about in the world (in normal circumstances at least). It is useful for jotting down or working through ideas, doing computations of research ideas, writing to-do lists, and -very importantly- it is an especially good means of reminding me to grab a moment to do a sketch. As a result, they’ve become a record of what I’ve been thinking about in certain periods, what I might have seen on the way to work (back when I was sketching faces on the subway), and also an interesting combination of marks on paper that I actually simply like just looking at.

On Thursday I’ll be taking part in a big event at the Getty […] Click to continue reading this post

Shaping the Future of Scientific Conferences

This year’s big annual flagship conference in String theory, Strings 2020, ended two days ago. It was a massive success, and it was held entirely online. There were more than 2000 registered participants from all around the world, with sessions where a large portion of that number were engaged simultaneously! This conference’s attendance more usually ranges at around 300 – 400, as far as I remember, so this was a spectacular change. The success was made possible by -most importantly- the willingness of many people to take part and engage with each other to a degree that was foreign to most participants, combined with smart and tireless effort by the team of organizers in Cape Town, where the conference was originally going to be held physically. There were excellent talks (selected by the programme committee) and many illuminating discussions.

Due to the pandemic, the conference was originally going to be cancelled (or at least postponed to much later in the year), but organizer Jeff Murugan announced at relatively short notice that they were instead going to attempt to do it online on the original dates, and it is wonderful that so many people around the world engaged, instead of just shrinking away into the Covid-19 gloom.

The other major component of the success is what I want to discuss here. It was the use, sometimes in concert, of tools such as Zoom […] Click to continue reading this post

Spectral, II

plot of spectral density of (2,2) JT SupergravityWhat’s that now? You want more physics teases? Ok. That dotted line is a (known) JT gravity Schwarzian spectral density. That red line? It’s the fully quantum corrected result! To all orders in topology and beyond. See my paper that appeared today on the arXiv.

(For experts: The red line is made up of about 2000 points for each of which I know the energy, and the full wave function for an associated problem. Using those I can compute lots of things, to good accuracy. One example is the full non-perturbative spectral form factor, that I showed last post.)

-cvj Click to continue reading this post

Pausing “Business as Usual”

This Wednesday (10th June), in support of #BlackLivesMatter and the demonstrations taking part worldwide, there will be a day of action in various parts of academia to simply stop doing “business as usual” while the horrors of what is routinely done to black people at all levels of society continue.

What people choose to do on that day is up to them, but there are suggestions as a number of websites. I encourage you to go there and read what they have to say, and make up your own mind. A good start is the ShutDownStem site, and search under #ShutDownAcademia, #ShutDownSTEM and #Strike4BlackLives on social media for chatter, activity, and more resources. The Particles For Justice group, led by people in or close to my field, have also joined in to lead and encourage, and their site is here, again with lots of suggestions for types of action to get involved in.

Frankly, having seen and heard […] Click to continue reading this post

Network Improvements

The desire to have glitch-free online teaching and business meetings at home has driven me to do some infrastructure improvements I should have done years ago: extending the Ethernet backbone of the home network. Connecting the jacks (ethernet connectors) is a tad fiddly (but trivial), but the results are worthwhile! … Click to continue reading this post

Online Teaching Methods

[caption id="attachment_19517" align="aligncenter" width="499"] Sharing my live virtual chalkboard while online teaching using Zoom (the cable for the iPad is for power only).[/caption]It is an interesting time for all of us right now, whatever our walk of life. For those of us who make our living by standing up in front of people and talking and/or leading discussion (as is the case for teachers, lecturers, and professors of various sorts), there has been a lot of rapid learning of new techniques and workflows as we scramble to keep doing that while also not gathering in groups in classrooms and seminar rooms. I started thinking about this last week (the week of 2nd March), prompted by colleagues in the physics department here at USC, and then tested it out last Friday (6th) live with students from my general relativity class (22 students). But they were in the room so that we could iron out any issues, and get a feel for what worked best. Since then, I gave an online research seminar to the combined Harvard/MIT/USC theoretical physics groups on Wednesday (cancelling my original trip to fly to the East Coast to give it in person), and that worked pretty well.

But the big test was this morning. Giving a two hour lecture to my General Relativity class where we were really not all in the same room, but scattered over the campus and city (and maybe beyond), while being able to maintain a live play-by-play working environment on the board, as opposed to just showing slides. Showing slides (by doing screen-sharing) is great, but for the kind of physics techniques I’m teaching, you need to be able to show how to calculate, and bring the material to life – the old “chalk and talk” that people in other fields tend to frown upon, but which is so essential to learning how to actually *think* and navigate the language of physics, which is in large part the diagrams and equations. This is the big challenge lots of people are worried about with regards going online – how do I do that? (Besides, making a full set of slides for every single lecture you might want to do For the next month or more seems to me like a mammoth task – I’d not want to do that.)

So I’ve arrived at a system that works for me, and I thought I’d share it with those of you who might not yet have found your own solution. Many of the things I will say may well be specific to me and my institution (USC) at some level of detail, but aspects of it will generalize to other situations. Adapt as applies to you.

Do share the link to this page with others if you wish to – I may well update it from time to time with more information.

Here goes:
[…] Click to continue reading this post