And so it began… All this butter had to end up inside this little bit of flour/water mix. But in lots of tasty layers of deliciousness! (And I’ll do this four times, and so that’s my morning.)
More later. I love puff pastry!
–cvj
A newly computed spectral density function for a new example…
(* Update: “not quite right” means, for example, that the large E behaviour should have been E^(3/2), generalising the E^(1/2) of the previous example, but it wasn’t for a bit, because of a normalization issue. All fixed now!)
To (most of) you it’s just a bunch of dots and wiggles and that’s ok. To me is some very nice physics that connects things I’d been thinking about back some 30 years ago with things I’m thinking about now. More later. Sorry if this is annoying, I’m just very pleased with the result.
-cvj
Oh, this was a recent post I did on social media that I think ought to go here too. I will try to write a longer post about the physics behind this (in involves black holes, quantum gravity, string theory, quantum chaos, and a host of other things):
A spectral density function of particular interest to me and a recent project on 2D quantum gravity.
Were you looking for more reasons to read my recent paper on JT gravity? This figure from it is my favorite reason. It summarizes a lot of the abstract – identical perturbation theory (right) but better non-perturbative regime (left). Go! https://arxiv.org/abs/1912.03637
-cvj
Panorama showing snow on the mountains and hills near Los Angeles. (Click for larger view. Taken 30th November 2019)
Speaking of returns, I’m back! You’ll have noticed that I’ve been bit quiet here on the blog, for which I apologise. I hope that you’ve been following my (quite regular) microblogging on twitter, instagram and Facebook, however – It is not the same as some of the longer in-depth posts I do here, but it might still help you keep in touch, so do follow on those platforms if you wish. Links in the sidebar.
This semester has been a very busy one, in which I’ve felt rather pulled apart. New routines due to various things too tedious to mention have meant that the places in my schedule where I might usually find time to do a longer thoughtful blog post were harder to come by, and by later in the evening when I’d time to take a breather, it would be the wrong part of the day to pull together coherent and interesting thoughts. But I am working back to a new approach and/or routine where there is some time for that. This includes the fact that it has occurred to me that since I use dictation more in place of lots of my typing (particularly on smaller devices like phones, iPads and watches), I might start banking snippets of thoughts when I can, using dictation, and then edit and concatenate later. We shall see if that helps me create more, er, “content”. Either way, do look out for posts in the coming days and weeks.
I’ve actually a LOT of things to tell, since I’ve been doing a lot – fascinating meetings and conferences, and research projects of various sorts. So it isn’t for lack of material that I’ve not said much since August, but quite the opposite. More to come!
-cvj
Avengers cosplayers in the audience of the Friday panel.
Day 1 – Friday
I finalized my schedule rather late, and so wasn’t sure of my hotel needs until it was far too late to find two nights in a decent hotel within walking distance of the San Diego Convention Center — well, not for prices that would fit with a typical scientist’s budget. So, I’m staying in a motel that’s about 20 minutes away from the venue if I jump into a Lyft.
My first meeting is over brunch at the Broken Yolk at 10:30am, with my fellow panellists for the panel at noon, “Entertaining Science: The Real, Fake, and Sometimes Ridiculous Ways Science Is Used in Film and TV”. They are Donna J. Nelson, chemist and science advisor for the TV show Breaking Bad (she has a book about it), Rebecca Thompson, Physicist and author of a new book about the science of Game of Thrones, and our moderator Rick Loverd, the director of the Science and Entertainment Exchange, an organization set up by the National Academy of Sciences. I’m on the panel also as an author (I wrote and drew a non-fiction graphic novel about science called The Dialogues). My book isn’t connected to a TV show, but I’ve worked on many TV shows and movies as a science advisor, and so this rounds out the panel. All our books are from Click to continue reading this post
Diamond maser. Image from Jonathan Breeze, Imperial College
Now let us turn to de Sitter black holes. I mean here any black hole for which the asymptotic spacetime is de Sitter spacetime, which is to say it has positive cosmological constant. This is of course also interesting since one of the most natural (to some minds) possible explanations for the accelerating expansion of our universe is a cosmological constant, so maybe all black holes in our universe are de Sitter black holes in some sense. This is also interesting because you often read here about explorations of physics involving negative cosmological constant, so this is a big change!
One of the things people find puzzling about applying the standard black hole thermodynamics is that there are two places where the standard techniques tell you there should be a temperature associated with them. There’s the black hole horizon itself, and there’s also the cosmological horizon. These each have temperature, and they are not necessarily the same. For the Schwarzschild-de Sitter black hole, for example, (so, no spins or charges… just a mass with an horizon associated with it, like in flat space), the black hole’s temperature is always larger than that of the cosmological horizon. In fact, it runs from very large (where the black hole is small) all the way (as the black hole grows) to zero, where the two horizons coincide.
You might wonder, as many have, how to make sense of the two temperatures. This cannot, for a start, be an equilibrium thermodynamics system. Should there be dynamics where the two temperatures try to equalise? Is there heat flow from one horizon to another, perhaps? Maybe there’s some missing ingredient needed to make sense of this – do we have any right to be writing down temperatures (an equilibrium thermodynamics concept, really) when the system is not in equilibrium? (Actually, you could ask that about Schwarzschild in flat space – you compute the temperature and then discover that it depends upon the mass in such a way that the system wants to move to a different temperature. But I digress.)
The point of my recent work is that it is entirely within the realm of physics we have to hand to make sense of this. The simple system described in the previous post – the three level maser – has certain key interconnected features that seem relevant:
My point is that these features are all present for de Sitter black holes too, starting with the two temperatures. But you won’t see the rest by staring at just the Schwarzschild case, you need to add rotation, or charge (or both). As we shall see, the ability to reduce angular momentum, or to reduce charge, will be the work channel. I’ll come back to the maximum Click to continue reading this post
Hubble photo of jupiter’s aurorae.
It is an idea I’d been tossing around in my head from time to time over years, but somehow did not put it all together, and then something else I was working on years later, that was seemingly irrelevant, helped me complete the puzzle, resulting in my new paper, which (you guessed it) I’m excited about.
It all began when I was thinking about heat engines, for black holes in anti-de Sitter, which you may recall me talking about in posts here, here, and here, for example. Those are reciprocating heat engines, taking the system through a cycle that -through various stages- takes in heat, does work, and exhausts some heat, then repeats and repeats. And repeats.
I’ve told you the story about my realisation that there’s this whole literature on quantum heat engines that I’d not known about, that I did not even know of a thing called a quantum heat engine, and my wondering whether my black hole heat engines could have a regime where they could be considered quantum heat engines, maybe enabling them to be useful tools in that arena…(resulting in the paper I described here)… and my delight in combining 18th Century physics with 21st Century physics in this interesting way.
All that began back in 2017. One thing I kept coming back to that really struck me as lovely is what can be regarded as the prototype quantum heat engine. It was recognized as such as far back as 1959!! It is a continuous heat engine, meaning that it does its heat intake and work and heat output all at the same time, as a continuous flow. It is, in fact a familiar system – the three-level maser! (a basic laser also uses the key elements).
A maser can be described as taking in energy as heat from an external source, and giving out energy in the form of heat and work. The work is the desired Click to continue reading this post
I’m quite excited because of some new results I got recently, which appeared on the ArXiv today. I’ve found a new (and I think, possibly important) instability in quantum gravity.
Said more carefully, I’ve found a sibling to Hawking’s celebrated instability that manifests itself as black hole evaporation. This new instability also results in evaporation, driven by Hawking radiation, and it can appear for black holes that might not seem unstable to evaporation in ordinary circumstances (i.e., there’s no Hawking channel to decay), but turn out to be unstable upon closer examination, in a larger context. That context is the extended gravitational thermodynamics you’ve read me talking about here in several previous posts (see e.g. here and here). In that framework, the cosmological constant is dynamical and enters the thermodynamics as a pressure variable, p. It has a conjugate, V, which is a quantity that can be derived once you know the pressure and the mass of the black hole.
Well, Hawking evaporation is a catastrophic quantum phenomenon that follows from the fact that the radiation temperature of a Schwarzschild black hole (the simplest one you can think of) goes inversely with the mass. So the black hole radiates and loses energy, reducing its mass. But that means that it will radiate at even higher temperature, driving its mass down even more. So it will radiate even more, and so on. So it is an instability in the sense that the system drives itself even further away from where it started at every moment. Like a pencil falling over from balancing on a point.
This is the original quantum instability for gravitational systems. It’s, as you probably know, very important. (Although in our universe, the temperature of radiation is so tiny for astrophysical black holes (they have large mass) that the effect is washed out by the local temperature of the universe… But if the univverse ever had microscopic black holes, they’d have radiated in this way…)
So very nice, so very 1970s. What have I found recently?
A nice way of expressing the above instability is to simply say Click to continue reading this post
(The following post is a bit more technical than usual. But non-experts may still find parts helpful.)
A couple of years ago I stumbled on an entire field that I had not encountered before: the study of Quantum Heat Engines. This sounds like an odd juxtaposition of terms since, as I say in the intro to my recent paper:
The thermodynamics of heat engines, refrigerators, and heat pumps is often thought to be firmly the domain of large classical systems, or put more carefully, systems that have a very large number of degrees of freedom such that thermal effects dominate over quantum effects. Nevertheless, there is a thriving field devoted to the study—both experimental and theoretical—of the thermodynamics of machines that use small quantum systems as the working substance.
It is a fascinating field, with a lot of activity going on that connects to fields like quantum information, device physics, open quantum systems, condensed matter, etc.
Anyway, I stumbled on it because, as you may know, I’ve been thinking (in my 21st-meets-18th century way) about heat engines a lot over the last five years since I showed how to make them from (quantum) black holes, when embedded in extended gravitational thermodynamics. I’ve written it all down in blog posts before, so go look if interested (here and here).
In particular, it was when working on a project I wrote about here that I stumbled on quantum heat engines, and got thinking about their power and efficiency. While working on that project, I had a very happy thought: Could I show that holographic heat engines (the kind I make using black holes) -at least a class of them- are actually, in some regime, quantum heat engines? That would be potentially super-useful and, of course, super-fun.
The blunt headline statement is that they are, obviously, because every stage Click to continue reading this post
About 2-3 (ish) years ago, I was asked to visit the Disney/Marvel mothership in Burbank for a meeting. I was ushered into the inner workings of the MCU, past a statue of the newly acquired Spidey, and into a room. Present were Christopher Markus and Stephen McFeely, the writers of several MCU projects including Infinity War and Endgame, as well Anthony Russo, of the directing team, and several producers. The infinity gauntlet was literally on the table.
We were there to talk about lots of aspects of the science of the MCU and in particular those two movies. They laid out many confidential aspects of the story, and I tried to help where I could, giving lots of advice and suggestions, and following up later with extra notes. (You’ve seen the results of some of this on screen in those movies – also others, like Thor: Ragnarok.)
Obviously I’ve told nobody about the details ever since, even declining to be interviewed generally about the use of a certain aspect of science in movies lest it be interpreted as an Endgame spoiler by those who know I sometimes consult for MCU. But now the movie has been out for a week, and they and others of the MCU creative team are discussing the details in interviews, so I think it is safe to be involved too. So, here is a very nice SPOILER-FiLLED interview with the writers where they talk about the choices they made, with guidance from some physicists. I am one of the physicists they mentioned – yes, they talked to other scientists too, which makes a lot of sense*.
(No, I did not wait to see if my name shows up in the credits. That hardly ever happens. 🙂 )
Enjoy!
-cvj
*The excellent Science and Entertainment Exchange was crucial in making some of this contact.
Well I did not get the special NYT issue as a keepsake, but this is maybe better: I got to attend the first presentation of the “black hole picture” scientific results at a conference, the APS April meeting (Sunday April 14th 2019). I learned so much! These are snaps of moments from talks by Shep Doeleman and Avery Broderick.
-cvj
Yes, it’s a black hole all right. Following on from my reflections from last night, I can report that the press conference revelations were remarkable indeed. Above you see the image they revealed! It is the behemoth at the centre of the galaxy M87! This truly groundbreaking image is the result of international collaboration involving hundreds of scientists – another wonderful tribute to the power of scientific cooperation.
There’s more at the Event Horizon Telescope website here, and a Guardian article here, among many other sources. (update: Excellent Physics World piece here.)
-cvj
Well, I’m off to get six hours of sleep before the big announcement tomorrow! The Event Horizon Telescope teams are talking about an announcement of “groundbreaking” results tomorrow at 13:00 CEST. Given that they set out to “image” the event horizon of a black hole, this suggests (suggests) that they are going to announce some landmark on that stated mission. We shall have to see what they say. See here.
I’ve been asked a lot by press people for my thoughts over the last couple of days. I’ve no inside knowledge, and do not want to over-promise on what the results could be, so I’ve not said much. I just sent off some remarks to one outlet, and since they are lengthy and late I expect they won’t be used. So I’ll record them here. Also, it’ll be amusing to see them alongside whatever it is I think once the results are announced! So here goes:
I was asked why the project is important. My response:
While we confirmed the existence of black holes and studied their properties in so many ways, nothing beats a direct observation. The Event Horizon project aims to effectively image the horizon of a black hole on the background of the glowing matter that surrounds it! It’s rather like seeing the shots fired as well as the smoking gun.
What would we learn from an image of a black hole’s horizon?
The shape of the horizon itself is important to determine. We know what it should be like if our best theory of gravity -Einstein’s General Relativity- is true. Deviations from that would be a stunning result. But before even worrying about that, it is important to check whether the sharp region corresponding to the edge of the horizon is even there. Establishing that something less than sharp is there could also spark a revolution in our understanding of what lies in those strong gravity regions we believe to be black holes. Could there be some alternative that acts like a black hole, being massive and compact, but deep down is quite different – no sharp horizon? This will help rule that possibility in or out.
What would it mean for the field? For my own research?
The answer to those questions depends upon what is announced! Either way there’ll be genuine and justified excitement. We’ll either have confirmation of the core property of a black hole -the existence of and nature of its horizon- confirming General Relativity (GR) or we’ll have a result that suggests something new and surprising about the massive compact centers of the galaxies being studied (our Milky Way and our neighbor M87) – maybe telling us that black holes behave differently from what we know from GR. Either way it will be a huge result.
A lot of my research time is spent thinking about black holes and understanding their properties in various situations in theoretical physics. They are central to a lot of core ideas. More confirmation that they are real objects with the expected properties is great to have. On the other hand, I also hope that black holes can one day provide a window into how the physics of space and time works beyond where Einstein’s GR can go. Work such as string theory (one of the things I work on) tries to go beyond Einstein in seeking how to combine quantum physics and gravity. Such a combination will ultimately produce deviations from what Einstein’s theory tells us. But the issue is that we do not know robustly where such deviations would show up. The theory is not predictive enough in that regard. Deviations from GR beginning to show up near a supermassive black hole (although I’d expect it to be a long shot that this is the announcement!) would be a huge boost for that thread of investigation, and may provide much needed clues for how to tackle the whole “beyond Einstein” program.
-cvj
It was Chutney time here! Well this was a few weekends back. I’m posting late. I used up some winter-toughened tomatoes from the garden that remained and slowly developed on last year’s vines. It was time to clear them for new planting, and so off all these tomatoes came, red, green, and in between (over the course of a few days).
Turned out very well, it transpired. There’s nothing better in this world with a sharp cheddar cheese than a good chutney (and a simple cream cracker). Grommit.
-cvj
The petition to revoke article 50 (stopping the UK jumping off a Brexit cliff) passed 5 million signatures sometime on Sunday! My colleague Nick Warner provided the screen shot of the counter just after it passed 5M (2019-03-24 at 3.18.25 PM, Parisian time). Thanks Nick! (Click for larger view.)
I normally keep politics off this blog, but I’m making an exception and providing the link to the place where you can sign, if you want to. If you’re a UK citizen abroad, know that you do not need to be resident to sign! If like me you’ve been feeling powerless, watching from abroad as a beloved country tears itself apart, this is a way of contributing to something that could well make a positive difference.
This link is here.
-cvj
