## Oops, they did it again…!

(Well, not really oops… It’s deliberate.) LIGO has announced another gravitational wave detection from a black hole merger! This time the black holes were 14.2 and 7.5 times the mass of the sun, and merged to form a new black hole of mass 20.8 times the mass of our sun, releasing a burst of energy […] Click to continue reading this post

## What Fantastic News!

This is an amazing day for humanity! Notice I said humanity, not science, not physics – humanity. The LIGO experiment has announced the discovery of a direct detection of gravitational waves (actual ripples in spacetime itself!!), opening a whole new window with which to see and understand the universe. This is equivalent to Galileo first pointing a telescope at the sky and beginning to see things like the moons of Jupiter and the phases of venus for the first time. Look how much we learned following from that… so we’ve a lot to look forward to. It is 100 years ago since gravitational waves were predicted, and we’ve now seen them directly for the first time!

Actually, more has been discovered in this announcement:- The signal came from the merger of two large (stellar) black holes, and so this is also the first direct confirmation of such black holes’ existence! (We’ve known about them […] Click to continue reading this post

## News from the Front, XII: Simplicity

Ok, I promised to explain the staircase I put up on Monday. I noticed something rather nice recently, and reported it (actually, two things) in a recent paper, here. It concerns those things I called “Holographic Heat Engines” which I introduced in a paper two years ago, and which I described in some detail in a previous post. You can go to that post in order to learn the details – there’s no point repeating it all again – but in short the context is an extension of gravitational thermodynamics where the cosmological constant is dynamical, therefore supplying a meaning to the pressure and the volume variables (p,V) that are normally missing in black hole thermodynamics… Once you have those, it seems obvious that you can start considering processes that do mechanical work (from the pdV term in the first law) and within a short while the idea of heat engines in which the black hole is the working substance comes along. Positive pressure corresponds to negative cosmological constant and so the term “holographic heat engines” is explained. (At least to those who know about holographic dualities.)

So you have a (p,V) plane, some heat flows, and an equation of state determined by the species of (asymptotically AdS) black hole you are working with. It’s like discovering a whole new family of fluids for which I know the equation of state (often exactly) and now I get to work out the properties of the heat engines I can define with them. That’s what this is.

Now, I suspect that this whole business is an answer waiting for a question. I can’t tell you what the question is. One place to look might be in the space of field theories that have such black holes as their holographic dual, but I’m the first to admit that […] Click to continue reading this post

## One Hundred Years of Certitude

Since the early Summer I’ve been working (with the help of several people at USC*) toward a big event next Friday: A celebration of 100 years since Einstein formulated the field equations of General Relativity, a theory which is one of the top one or few (depending upon who you argue with over beers about this) scientific achievements in the history of human thought. The event is a collaboration between the USC Harman Academy of Polymathic Study and the LAIH, which I co-direct. I chose the title of this post since (putting aside the obvious desire to resonate with a certain great work of literature) this remarkable scientific framework has proven to be a remarkably robust and accurate model of how our universe’s gravity actually works in every area it has been tested with experiment and observation**. Despite being all about bizarre things like warped spacetime, slowing down time, and so forth, which most people think is to do only with science fiction. (And yes, you probably test it every day through your […] Click to continue reading this post

## Tales from the Industry XXXXI – Puppet Black Hole

Yeah. Not sure how to best title this post or fully explain the picture [edit: Picture taken down temporarily until the show is ready to be promoted]. Let’s just say that I spent a bit of this afternoon explaining some of the science of the Large Hadron Collider to a bright orange puppet that was determined to not believe whatever I told him/it. It was fun, and was done to camera at Los Angeles Center Studios downtown. (I was actually speaking about things that intersect with the subject of yesterday’s post, if you’re interested.) It is for a new show on a channel that I can’t mention yet*, and I’ll let you know as soon as I know what the air date is, etc.

Well, one more thing, in support of the old “It’s a small world after all” saying. I noticed from the call sheet that this morning they were shooting a fun segment that was hosted by my friend Hal Rudnick the host of Screen Junkies! (Have a look at some of the science-meets-movies things we’ve done together here, here and here.) Also, a friend I’d not seen in […] Click to continue reading this post

## ‘t Hooft on Scale Invariance…

Worth a read: This is ‘t Hooft’s summary (link is a pdf) of a very interesting idea/suggestion about scale invariance and its possible role in finding an answer to a number of puzzles in physics. (It is quite short, but think I’ll need to read it several times and mull over it a lot.) It won the top Gravity Research foundation essay prize this year, and there were several other interesting essays in the final list too. See here.

## Black Hole Slides!

Since people were asking for copies of my slides from my colloquium chalkboard-style talk on black holes and the things I call “holographic heat engines” last month at Harvey Mudd College, I decided to export them as a movie. You can find it on YouTube. Link below. It was a 50 minute talk, but all the builds are compressed down to a 6 minute file! I try to keep the bulk of the narrative in my head and speak it with the slides as visual aids (instead of writing everything on the slides as is often the practice) and so I do not know […] Click to continue reading this post

## Interstellar Science at Screen Junkies!

As promised on Tuesday, below you will find my Screen Junkies interview where I chat with Hal Rudnick about some of the science in Interstellar. We covered a lot of topics and went into a lot of detail, but a lot of that is on the cutting room floor in order to make a svelte (but relatively generous) ten minute cut. I hope you enjoy it. (See my earlier thoughts on why I think scientists need […] Click to continue reading this post

## 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