Never toured the inside of the Houses of Parliament before, seeing all the red and green colour coded areas (lords and commons – look at the benches next time you see debates in either place) and busts and statues of some of the shapers, for better or worse, of much of the fabric of UK democracy. (Thanks to my sister for this wonderful opportunity on Friday!) BTW, most of the interesting stuff I saw was off limits to photos, sorry!
–cvj
During the pandemic shutdown I regularly ran these london streets and bridges -virtually- on a treadmill watching a YouTube video of such a run.
This morning (actually 8 days ago since I see now I forgot to hit “publish”) was the first time I did it for real! I wonder if any of the many other runners I saw were on the video?
(Do have a look, if you wish, on my social media accounts for other posts from this London visit.)
–cvj
For some weekend listening, there’s a fun and informative podcast for youngsters called Tumble Science Podcast. I learned of it recently because they asked to interview me for an episode, and it is now available! It is all about time travel, and I hope you (and/or yours) have fun listening to it. The link is here.
There’s an accompanying blog post here.
More generally, listen to more episode on Tumble’s website, or at the pod link here.
Enjoy!
–cvj
This morning I had a really fantastic meeting with some filmmakers about scientific aspects of the visuals (and other content) for a film to appear on your screens one day, and also discussed finding time to chat with one of the leads in order to help them get familiar with aspects of the world (and perhaps mindset) of a theoretical physicist. (It was part of a long series of very productive meetings about which I can really say nothing more at the current time, but I’m quite sure you’ll hear about this film in the fullness of time.)
Then a bit later I had a chat with my wife about logistical aspects of the day so that she can make time to go down to Los Angeles and do an audition for a role in something. So far, so routine, and I carried on with some computations I was doing (some lovely clarity had arrived earlier and various piece of a puzzle fell together marvellously)…
But then, a bit later in the morning while doing a search, I stumbled upon some mention of the recent Breakthrough Prize ceremony, and found the video below (one of several). It’s as though I fell asleep at my desk and was having one of those strange dreams where two parts of your life that have little to do with each other get intertwined: Robert Downey Jr (RDJ) and Da’Vine Joy Randolph (DJR) doing a “bit” about statistical physics, quantum field theory, and symmetries, and then John Cardy and Alexander Zamolodchikov come on stage…
-cvj
Since you asked, I should indeed say a few words about how things have been going since I left my previous position and moved to being faculty at the Santa Barbara Department of Physics.
It’s Simply Wonderful!
(Well, that’s really four I suppose, depending upon whether you count the contraction as one or two.)
Really though, I’ve been having a great time. It is such a wonderful department with welcoming colleagues doing fantastic work in so many areas of physics. There’s overall a real feeling of community, and of looking out for the best for each other, and there’s a sense that the department is highly valued (and listened to) across the wider campus. From the moment I arrived I’ve had any number of excellent students, postdocs, and faculty knocking on my door, interested in finding out what I’m working on, looking for projects, someone to bounce an idea off, to collaborate, and more.
We’ve restarted the habit of regular (several times a week) lunch gatherings within the group, chatting about physics ideas we’re working on, things we’ve heard about, papers we’re reading, classes we’re teaching and so forth. This has been a true delight, since that connectivity with colleagues has been absent in my physics life for very many years now and I’ve sorely missed it. Moreover, there’s a nostalgic aspect to it as well: This is the very routine (often with the same places and some of the same people) that I had as a postdoc back in the mid 1990s, and it really helped shape the physicist I was to become, so it is a delight to continue the tradition.
And I have not even got to mentioning the Kavli Institute for Theoretical Physics (KITP) Click to continue reading this post
(A more technical post follows.)
By the way, in both sets of talks that I mentioned in the previous post, early on I started talking about orthogonal polynomials 
Someone raised their hand and ask why it truncates to three terms and on the spot I said that I’d forgotten the detailed proof (it has been many years since I thought about it) but recall that it follows straightforwardly from orthogonality. Lack of time meant that I did not want to try to reconstruct the proof on the board in real time, but it is a standard thing that we all use a lot because it is true for all the commonly used families of polynomials in lots of physics, whether it be Hermite, Legendre, Laguerre, Chebyshev, etc. Anyway, I finally got around to reminding myself of it and thought I’d record it here. Now all I have to do in future is point people here as a handy place to look it up. ([Although you can find equivalent discussions in several sources, for example this nice YouTube lecture here, which is part of a lovely series of lectures on polynomial approximation of functions, which is a fascinating topic in its own right.]
Ok, I should quickly remind what the setup is. The polynomials are normalised so that the 
So we have the inner product:
defining the orthogonality, where the 
Imagine there are terms in the recursion beyond the three terms. Let’s write these “remainder” terms as a linear combination of all lower polynomials up to degree n-2, so the recursion is tentatively:
Taking the inner product 
So 
Then you take the inner product
But this is also (by definition; I can let the lambda act in the opposite direction inside the integral) 
That’s it. All done. Except for the remark that given the expression for
Ok, one more useful thing: It is clear from the definition of the inner product integral that 
Hence we’re derived an important relation: 
(We essentially got this already in the earlier equation for 
–cvj
Well, that was my title for my seminar last Thursday at the KITP. My plan was to explain more the techniques behind some of the work I’ve been doing over the last few years, in particular the business of treating multicritical matrix models as building blocks for making more complicated theories of gravity.
The seminar ended up being a bit scattered in places as I realised that I had to re-adjust my ambitions to match limitations of time, and so ended up improvising here and there to explain certain computational details more, partly in response to questions. This always happens of course, and I sort of knew it would at the outset (as was clear from my opening remarks of the talk). The point is that I work on a set of techniques that are very powerful at what they do, and most people of a certain generation don’t know those techniques as they fell out of vogue a long time ago. In the last few years I’ve resurrected them and developed them to a point where they can now do some marvellous things. But when I give talks about them it means I have a choice: I can quickly summarise and then get to the new results, in which case people think I’m performing magic tricks since they don’t know the methods, or I can try to unpack and review the methods, in which case I never get to the new results. Either way, you’re not likely to get people to dive in and help move the research program forward, which should be the main point of explaining your results. (The same problem occurs to some extent when I write papers on this stuff: short paper getting swiftly to the point, or long paper laying out all the methods first? The last time I did the latter, tons of new results got missed inside what people thought was largely just a review paper, so I’m not doing that any more.)
Anyway, so I ended up trying at least to explain what (basic) multicritical matrix models were, since it turns out that most people don’t know these days what the (often invoked) double scaling limit of a matrix model really is, in detail. This ended up taking most of the hour, so I at least managed to get that across, and whet the appetite of the younger people in the audience to learn more about how this stuff works and appreciate how very approachable these techniques are. I spent a good amount of time trying to show how to compute everything from scratch – part of the demystifying process.
I did mention (and worked out detailed notes on) briefly a different class of Click to continue reading this post
It has been extremely busy in the ten months or so since I last wrote something here. It’s perhaps the longest break I’ve taken from blogging for 20 years (gosh!) but I think it was a healthy thing to do. Many readers have been following some of my ocassional scribblings on social media (see sidebar), and will guess from those that the main news to report is that I’ve been getting on with the usual practices of my job (research, teaching and mentorship, science communication, etc) and life, including getting settled into a new city and a new working environment. The latter has all been rather fantastic, I’m happy to say! I hope to say a bit more about all these things at more-than-social-media length.
Let me end this little update with something juicy to dig into: Last week I gave a public lecture at the Kavli Institute for Theoretical Physics (KITP) here at UCSB. Its title was “Living in the Matrix – Recent Advances in Understanding Quantum Spacetime” and you can see a recording of it here. It is a little bit of an update on some aspects of research into understanding black holes at the quantum level using random matrix model methods to perform the gravitational path integral.
I put a lot of preparation into it, trying to motivate the research and give some ideas about how and why it proceeded the way it has. I was trying to do a bit more than just show a lot of pretty pictures and talk over them. I wanted to convey to the audience member a little bit of the sense of what it is like to think about some of the issues involved, and how a theoretical physicist tackles them. So you get to look over the shoulder of physicist as they write in their notebook.
I learned afterwards that people seemed to enjoy the talk, and so maybe you will too.
Enjoy.
–cvj
There’s not much in this post, but I wanted to mark a significant date. It is the first day of the rest of 2023, but in addition, it is the beginning of a new chapter for me. Yesterday was my last day as an employee of the University of Southern California, after 20 years (!) and today is my first day as an employee of the University of California, Santa Barbara. (I mentioned this was coming up in a previous post, and some of its significance is explained there.)
Of course, since I’m still on a research retreat at the Aspen Center for Physics, none of this seems entirely real just yet. Electronic signatures winging back and forth over the web don’t really substitute for the physical business of changing where you show up for work, who you meet there, and so forth, and so that will have to wait for little while. That having been said, lots of electronic welcomes and so forth did help a lot with that “new job smell”, if that’s a thing. But I’ll be showing up in person soon enough, and it’ll be fun to begin to construct a new routine.
Sadly I’ve not had a lot of time to properly sit and contemplate and let this all sink in, to be honest, so I’ll have to find time to do it later. (That photo, above, of me at the top of Aspen mountain is nice, but I was in that spot for about 5 minutes!) Right now, there’s travel, and then sorting and packing and more packing to do, along with family matters, and all that entails. I’ll have to leave the contemplation for a bit later.
Perhaps during a nice long run along the beach…!
-cvj
A lot of us have been waiting for a long time to hear this news! The NANOGrav collaboration has announced strong evidence of a background of low frequency gravitational waves emitted from supermassive black hole mergers. Their detection methods are pulsar timing arrays (still one of those fantastically simple, cool ideas I still wish I’d thought of). There’s a New York Times article by Katrina Miller here (“The Cosmos is Thrumming with Gravitational Waves…”), and here’s Yale’s Chiara Mingarelli (one of the team) describing some of what this means in simple terms:
-cvj
About two years ago I wrote a post entitled “A Return”, upon moving to Princeton for a year (I was a Presidential Visiting Scholar at the Physics department). I reflected upon the fact that it was a return to a significant place from my past, where I’d been transformed in so many ways. Princeton was the first place I visited (not counting airports) in the USA, the location of my first postdoctoral appointment (at the Institute for Advanced Study (IAS)), and its was there that I did a deep enriching dive into the hubbub of Theoretical Physics, at one of the very top places in the world to do so.
After that, I moved West, to Santa Barbara, where my next postdoc position was at the Institute for Theoretical Physics at the University of California Santa Barbara (UCSB), now called the KITP. I was very lucky to be able to go from one top place to another, and (as I’ve recently talked about in a BBC interview here) additionally, my field was in a delicious turmoil of activity and discovery. I was able to be a part of the maelstrom (the “Second Superstring Revolution”, and all the gifts it gave us, including better understanding of the role in quantum gravity of extended objects beyond strings (such as D-branes), the physics of quantum black holes, the tools to unlock the holographic nature of quantum gravity more generally (through AdS/CFT), and so on. (I’ve blogged about many of these topics here, so use the search tool for more.)
I’ve been known to say that Princeton was the place where I found my physics voice (Edward Witten was a key guide at that time). Well, to continue the theme, Santa Barbara (with its wonderful research group made up of people from both the KITP and the wider Physics Department) was the place where I started to learn how to use that voice to sing (with the guidance of Joe Polchinski (who sadly passed away a few years ago)).
Well, as you may be guessing after that long introduction, I’m doing “A Return” again, but this time not with some boxes and suitcases of things for a year’s stay: I can now announce that I’ll be leaving the University of Southern California (USC) and (as of 1st July 2023) joining Click to continue reading this post
After doing a night bottle feed of our youngest in the wee hours of the morning some nights earlier this week, in order to help me get back to sleep I decided to turn on BBC Sounds to find a programme to listen to… and lo and behold, look what had just aired live! The programme that I’d recorded at Broadcasting House a few weeks ago in London.
So it is out now. It is an episode of Jim Al-Khalili’s excellent BBC Radio 4 programme “The Life Scientific”. The show is very much in the spirit of what (as you know) I strive to do in my work in the public sphere (including this blog): discuss the science an individual does right alongside aspects of the broader life of that individual. I recommend listening Click to continue reading this post
I’m sitting, for the second night in a row, in a rather pleasant restaurant in Oxford, somewhere on the walk between the physics department and my hotel. They pour a pretty good Malbec, and tonight I’ve had the wood-fired Guinea Fowl. I can hear snippets of conversation in the distance, telling me that many people who come here are regulars, and that correlates well with the fact that I liked the place immediately last night and decided I’d come back. The friendly staff remembered me and greeted me like a regular upon my return, which I liked. Gee’s is spacious with a high ceiling, and so I can sit away from everyone in a time where I’d still rather not be too cavalier with regards covid. On another occasion I might have sought out a famous pub with some good pub food and be elbow-to-elbow with students and tourists, but the phrase “too soon” came to mind when I walked by such establishments and glanced into the windows.
However, I am not here to do a restaurant review, although you might have thought that from the previous paragraph (the guinea fowl was excellent though, and the risotto last night was tasty, if a tiny bit over-salted for my tastes). Instead I find myself reflecting on Click to continue reading this post
As I mentioned in the previous post, I had business at BBC Broadcasting House this week. I was recording an interview that I’ll fill you in on later on, closer to release of the finished programme. Recall that in the post I mentioned how amusing it would be for me (or W1A fans), as a Brompton rider, to arrive (as in the show W1A) at the building on a Brompton. In the end I did not rent one, although it was tempting, but I was pleased to see that upon my arrival there was someone folding a Brompton, just like Hugh Bonneville’s character in the show. It was a welcome sight. See the multi-component photo. After the interview, the producer (Lucy) kindly let me look around some of the parts of the building that were featured heavily in the show, and it was fun to see them in real life!
-cvj
Brompton bicycle rental lockers.
-cvj
