## Hanging out at Screen Junkies!

Just finished another enjoyable hour of chatting about movies and science with the Screen Junkies guys! You’ll recall the fun results of the last two (see here on Time Travel and here on Guardians of the Galaxy). We were talking about… wait for it… Interstellar! Their legions of fans have been shouting at them to do something about the science in Interstellar for weeks now, and they heard them, and called me in to chat. In the course of an hour we talked about a lot of fun things, but remember – they’ll cut it all down to 5 minutes or so, and so we won’t get to a lot of things. I do not know what bits will be used… (It will be different from my spoiler-free recent Interstellar discussion.)

In my previous visits there I’d never got to see the famous Screen Junkies wall in front of which they have conducted so many fun interviews (see their site […] Click to continue reading this post

## Interstellar Discoveries

I’m a fan of Chris Nolan’s work so I’ve been looking forward to Interstellar. I’ve also been fascinated by the McConaussance – the transformation of Matthew McConaughey into an actor of considerable stature in a series of excellent films (Mud, Dallas Buyers Club, etc…), so I’ve been doubly interested in seeing how he works in a film under Nolan’s direction. Same for the always amazing Casey Affleck. All quite exciting to see.

But then to my surprise it turns out there’s another reason to be interested. Kip Thorne. Some years ago, at a party when I last saw him, Kip told me that he had been working on some film or other with a major studio, but I did not know of the details. Then I ran into a mutual friend a couple of months ago who said something a long the lines of “Kip’s movie is coming out soon…”, and I learned that it was something to do with Interstellar! But I did not know any details.

Then I got sent* this Wired story, and then** this story, and I finally got around to looking. The Wired story has a lot of interesting detail, including a special film (that I ought to look at at) with interviews and behind the scenes material (the still to the right is a screen shot from it). The film will apparently feature a black hole and a wormhole in some way (I don’t want to know more – I like films to unfold in front of me in the theatre). Kip has been working with the visual effects people to get right exactly how such objects really look, an issue that has not really been fully addressed, it seems. He, like a number of us interested in science and film, is keen to help filmmakers really do a good job of representing some of these fascinating objects as accurately as possible. (Not, in my view, in order to stifle filmmakers’ imagination, as it so often seems when you hear scientists out there pontificating about what’s wrong in one film or another, but because the actual science is so very often far more interesting and full of delights and possibility than a visual effects kluge can be…) So apparently he wrote down […] Click to continue reading this post

## Screen Junkies Chat: Guardians of the Galaxy

You may recall that back in June I had a chat with Hal Rudnick over at Screen Junkies about science and time travel in various movies (including the recent “X-Men: Days of Future Past”). It was a lot of fun, and people seemed to like it a lot. Well, some good news: On Tuesday we recorded (along with my Biophysicist colleague Moh El-Naggar) another chat for Screen Junkies, this time talking a bit about the fun movie “Guardians of the Galaxy”! Again, a lot of fun was had… I wish you could hear all of the science (and more) that we went into, but rest assured that they* did a great job of capturing some of it in this eight-minute episode. Have a look. (Embed below the more-click):
[…] Click to continue reading this post

## Triply Dyonic

I thought I’d mentioned this already, but I could not find anything after a search on the blog so somehow I think I must have forgotten to. It is a cute thing about a certain favourite solution (or class of solutions) of Einstein’s equations that I’ve talked about here before. I’m talking about the Taub-NUT solution (and its cousin, Taub-Bolt). Taub-NUT is sort of interesting for lots of reasons. Many, in fact. One of them concerns it having both mass $$M$$ and another parameter called “nut charge”, $$N$$. There are several ways to think about what nut charge is, but one curious way is that is is sort of a “magnetic” counterpart to the ordinary mass, which can be thought of as an “electric” quantity.

The language is based on analogy with electromagnetism, where, in the usual […] Click to continue reading this post

## It’s Dynamical Cosmological Constant Day!

As you may know from three previous recent posts on research (here, here, and here), I’ve been thinking and calculating a lot in the area of dynamical cosmological constant – concerning mostly (but not entirely) thermodynamics and quantum gravity. Specifically, the cosmological constant becomes the pressure variable in the thermodynamics. I think it is important, and will teach us something about things like gauge/gravity duality, string theory, black holes, and perhaps even cosmology, but I am not sure what yet. I’ve made some suggestions in recent papers, and computed some interesting things along the way.

Anyway, the larger community has not been following this story much, since: (1) It means a break with some powerful and still very fruitful frameworks where the cosmological constant being fixed is a given – like AdS/CFT – and it is not clear what that means yet, so the motivation is not super-strong; and (2) Let’s be honest, there’s no superstar working on it, so it is not going to get anyone any points. So I’ve been trying to shout about it in my little way from the periphery, as I think it might be useful, and since several people have been doing really good and interesting work on this issue for many years and it is worth more people seeing what they’ve been up to.

So imagine my pleasant surprise when I looked on the listing of new papers on the arXiv for today and saw three (!) papers on the subject, moving things forward in various ways. (They all seem to have noticed some of what I’ve […] Click to continue reading this post

## News From the Front XII: The Nuts and Bolts of Enthalpy in Quantum Gravity

So it happened again. I got musing to myself about something and decided to do a quick computation to check it out, and it took me down an interesting rabbit hole, which resulted in me writing a nice little paper at the end of last week that appeared today on the arxiv. I think the physics is really really nice. Let me tell you a bit about it. It is in the same area of ideas that I mentioned last time, concerning that paper I wrote last month. So let me pick up the story there, since I did not really touch on the core of the story. [Note: for non-experts, the following will get somewhat technical and full of terms and ideas that I will not explain. Sorry.]

One of the things that might have struck you (if you’re an expert in the area) from my proposal to make heat engines out of black holes that do real mechanical work like the engines you read about in physics textbooks is that there ought to be no actual mechanical work since there’s no pistons – no pistons changing volumes and so forth. That is (or rather, was) a missing ingredient in the standard thermodynamics of black holes in quantum gravity. Well, that all changed a short few years ago with the work of a number of authors, particularly with the clear suggestion of David Kastor, Sourya Ray, and Jennie Traschen, and work by Brian Dolan, with a fair bit of followup investigations by various other authors including some I’ll mention below. (Update: Two reviews, with different foci, can be found in here and here.) The general idea is that if you allow the cosmological constant $$\Lambda$$ to be a thermodynamical variable as well (and there is a long history of authors considering this in various contexts), where it naturally acts like a pressure $$p = -\Lambda/8\pi G$$, (G is Newton’s constant, and I’m setting various other constants to unity in the usual way) then you naturally include a conjugate to that variable that should be the pressure.

For a simple static black hole like Schwarzschild, the volume turns out to the the naive volume you get by taking the radius of the black hole and forming […] Click to continue reading this post

## Another Engine

This diagram is the cycle for another heat engine (using a black hole as the working substance) that I studied in the recent paper. It is a path made of two constant pressure legs (isobars) and two constant volume legs (isochores) that happen (due to the properties of static black holes) to also be adiabats. See the post.

It got included in the paper as another example where one could compactly write down something useful about the efficiency, since, as it turns out, you can write closed form expressions […] Click to continue reading this post

## News From the Front, XI: Holographic Heat Engines!

Yes, you heard me right. Holographic Heat Engines. I was thinking recently about black holes in universes with a cosmological constant and their thermodynamics. I had an idea, it led to another, then another, then some calculations, and then a couple of days of writing, calculating, and thinking… then a day to cool off and think about other things. Then I came back to it, decided it was still exciting as an idea and so tidied it all up as a paper, made some diagrams, tidied some more, and voila! A paper submitted to the arxiv.

I’m sort of pleased with all of it since it allowed me to combine a subject I think is really fun (although often so bleakly dull when presented at undergraduate level) – heat engines – with contemporary research ideas in quantum gravity and high energy physics. So I get to draw some of the cycles in the p-V plane (graph of pressure vs volume) representing the inner workings of engines of particular designs (just like you might have seen long ago in a physics class yourself) and compute their efficiency for doing mechanical work in exchange for some heat you supply. It is fundamental that you can’t do that with 100% efficiency otherwise you’d violate the second law of thermodynamics – that’s why all engines have to have some exhaust in the form of heat, giving an efficiency represented by a quantity $$\eta$$ that is less than one, where one is 100% efficient. The diagram on the left illustrates the key pieces all engines must have, no matter what working substance you’re using. The details of the design of the engine are what kind of cycle you taking it through and what the properties (“equation of state”) your working substance has. In the case of a car, for example, the working substance is cleverly mixed up with the source of heat – the air/gasoline mix forms a “working substance” that gets expanded and compressed in various ways (in the green bit of the diagram), but the fact that it also burns releasing heat means it is also the source of the heat that comes into the engine (the flow from the red bit) to be (in part) turned to work, and the remainder flowing out to the blue (exhaust). Very clever.

The cool thing here is that I’m using black holes as the working substance for […] Click to continue reading this post

## Harmonics Can Make Metals Flux

I’m sitting on the bus, on my way to the LA Times Festival of Books. I’ve opened my notebook to maybe sketch a face or two, but then I spot an algebraic error on a page from a few days ago, so I set about correcting it….(you know, either for those historians who will argue about my intentions while poring over these once-lost manuscripts…. Or more likely for me in a few weeks, likely to get confused by my own blunder-in-haste with a kindergarten level variation.)

A bearded fellow had boarded the bus and sat next to me while I was doing this. He eventually glances my way, then away, then he does a double take. I’ve been watching him out of the corner of my eye. He starts to speak to me.

Calc or Trig?
Excuse me?
Calc or trig?
Er…. Physics.
What are you studying?
I’m not…studying. It is my research.

Silence from him for a few beats, then:

Into what?
Charged black holes.

I should say at this point that I get this a lot. No matter who I meet out there in the random world, or what they see me writing, the assumption is always that I am taking a class in high school mathematics. Why is that? People, at least guess for some of the time that I might be at least teaching it, even if you can’t imagine people doing research in this area. The grey hairs in my beard are a guide to your assumptions.

Another gripe: if you see someone writing words, and want to engage them in conversation about it, you don’t assume they are practicing the basic structures… Imagine the analogous exchange: Are you practicing joined-up writing madam, or spelling? Neither, I’m writing poetry.

Anyway, back to the conversation. So, as you recall, I said Charged black […] Click to continue reading this post

## Exam Heat

We had a midterm in the undergraduate General Relativity class this week. Midterm II. We’d just come off a batch of lectures on black holes (focussing on Schwarzschild and Kerr), and so it seemed very natural to focus on that as the topic. Schwarzschild is the most basic (vanilla) black hole and Kerr is the case of having rotation for a bit of extra flavour. They are both hugely important in real physics – the former for the basic phenomena and then the latter since most objects out there in the astrophysical realm are actually going to be rotating to some degree (and after gravitational collapse, probably a high degree). So I focussed on those in class.

For the midterm therefore, I got the opportunity to have them discover properties of the (less astrophysically important perhaps) charged black hole – the Reissner-Nordstrøm solution:
$$ds^2=-\left(1-\frac{2M}{r}+\frac{Q^2}{r^2}\right)dt^2+\left(1-\frac{2M}{r}+\frac{Q^2}{r^2}\right)^{-1}dr^2+r^2\left(d\theta^2+\sin^2\theta d\phi^2\right)\ ,$$

…and do a few computations with it analogous to what they did for the other cases in class and in homework. I hope they had fun (like discovering that for a […] Click to continue reading this post

## Wormholes!

“So, you have a choice. We’ve ten minutes of class left. I can either finish early, or…. I can show you that there’s actually a wormhole living in this picture, or can I tell you about Hawking radiation.”

That was the choice I presented the students with after we’d spent some time together exploring the Kruskal-Szekeres extension the the basic black hole solution, in my General Relativity class today. You probably don’t know what all that is, and that’s ok. Suffice to say that you end up with a pretty diagram which looks like two everlasting black holes put together as in a sort of elegant trading card. Or perhaps a neat knot where one black hole is sort of upside down and neatly slots into the other one so that they hug each other into eternity, acting as each others’ past and future. Look it up and see.

Anyway, they went for the wormhole, with a chant “Worm-Hole! Worm-Hole!”. So I constructed the wormhole for them… the Einstein-Rosen bridge, as it is known, ending with the sad news that it is not a real traversable wormhole that […] Click to continue reading this post

## Hawking an Old Idea

On the one hand it is good to get members of the general public excited about scientific research, and so having some new excitement about something Stephen Hawking said, driven by gushingly written articles in the press and online, can be good. On the other hand, it is annoying that the thrust of the articles are largely that he’s stunned the world again with a brilliant and unlooked-for idea. People just lap this stuff up, unquestioningly. It is actually an old idea (and in fact one that is being mis-reported – see below). One’s instinct is to just say “Welcome, Stephen, we’ve been waiting for you to join us”, or “Come on in, the water’s lovely”, and just move on, but it seems so unfair. The thing that’s most puzzling in all of this is Hawking’s own paper (which is all of two pages of words – a transcript of a talk he gave in August), which makes no reference at all to (for example) Samir Mathur’s work, which has been explicitly saying essentially the same thing for well over a decade, with a very definite proposal for how it might work. That work has hardly been buried in obscurity. Samir and many other people who have liked his idea have been working out the consequences of the proposal in numerous papers for over a decade and reporting on their results at all the main conferences, and even talking to him about it (I note that Samir was in the audience during the August talk and even politely asked the speaker to compare and contrast the similar-sounding proposals). So it is puzzling that you get no hint from the paper’s citations that this is a well-considered and ongoing idea, even if (perhaps) in detail it may pan out differently from other suggestions.

What’s the idea?, you ask. Well, it is not, as you might get from most of the articles (somewhat confusingly), that black holes do not exist. It is that the black hole’s event horizon, thought of as a sharp “point of no return” boundary, may not exist. Instead, it is approximation or shorthand for the complicated physics (of both matter and spacetime) that happens in the vicinity of the black hole. Simply put, the horizon arises in classical solutions to classical (i.e. non-quantum) equations (such as in General Relativity) of gravity. (See an earlier post I did about them here, from which came the illustration […] Click to continue reading this post

## Ten Things…

So I’ve been a bit quiet as I’ve had a lot going on. This includes preparing ten interesting slides to use as props for a talk I gave this evening to the USC Philosophy Club. It was entitled “Ten Things YOU Should Know About Black Holes”. It started with the original idea by Michell in 1783 (yes, really, that early!) and ended with topics of current research (what is the fate of the singularity? What really happens at a horizon? Etc., etc…) I spoke for a while and then fielded tons of questions, and am now (I am writing a draft of this on the subway train home – uploading later) suffering from a rather broken voice due to too much talking and projection…. Gosh. But it was fun. A really […] Click to continue reading this post

## News from the Front, X: Smoother Transition

Yesterday I submitted a new paper to the arxiv. This is is my favourite curve from it. Some of you who follow the blog will recognize the blue circle-dots and guess that this is the output of the dot-generation I’ve been tinkering away at (and reporting on somewhat cryptically) since April (see e.g. here, here, and here). Correct. There are many reasons why that is the case. One of them might well be because it looks like a very comfy chair, and by time I’d submitted the paper, I was rather tired. I’d pulled an all-nighter to finish the paper because I wanted to submit it by noon yesterday, and the night before I had to spend several hours at a social event.

So once it was an appropriate time to leave the place I was at, I said my goodbyes, jumped on my bike, pedalled home, put on some coffee, some Ana Tijoux (through headphones, so as not to wake anyone. Why her? Kinetic energy was what I needed at that moment – her vocal style is full of that. Try “La Bala” or “1977”. It is in Spanish, but that’s just fine.), and from 10:30pm to about 12 hours later, ground out the paper. I had to do this since I took some time away from the research project for a week, and then on Tuesday evening noticed the title and abstract of a new paper on the arxiv that suggested some overlap with what I was doing. So I had no choice but to gather all the results I’d been gathering the last several weeks and write them up and get them out, putting off reading the other paper until afterwards, so as to remain independent. Hence the all-nighter to finish it all. It was a pretty easy paper to write since I’ve had the results for a while, knew what I wanted to say, and it was just a matter of pulling everything together and writing a lot of background to set the scene for the results. A fair amount of the time was spent fiddling with things like how to generate figures from Matlab that embed nicely into the text, and so forth. Technical tedium.

The physics? Another reason I like the above curve is because it examines physics from an old favourite phase transition I co-discovered almost 14 – gosh yes, cvj, fourteen! – years ago. To my knowledge it is perhaps the earliest example of a […] Click to continue reading this post

## TED Youth Talk – Hidden Structures of the Universe

You might recall that last year I gave a talk at TED Youth, in their second year of short TED talks aimed at younger audiences. You’ll recall (see e.g. here and here) I made a special set of slides for it, composed from hundreds of my drawings to make it all in graphic novel style, and somehow trying to do (in 7 minutes!!) what the TED people wanted.

They wanted an explanation of string theory, but when I learned that I was the only person in the event talking about physics, I kind of insisted that (in a year when we’d discovered the Higgs boson especially!) I talk more broadly about the broader quest to understand what the world is made of, leaving a brief mention of string theory at the end as one of the possible next steps being worked on. Well, they’ve now edited it all together and made it into one of the lessons on the TED Ed site, and so you can look at it. Show it to friends, young and old, and remember that it is ok if you don’t get everything that is said… it is meant to invite you to find out more on your own. Also, as you see fit, use the pause button, scroll back, etc… to get the most out of the narrative.

I’m reasonably pleased with the outcome, except for one thing. WHY am I rocking […] Click to continue reading this post