Category Archives: black holes

News from the Front, XVIII: de Sitter Black Holes and Continuous Heat Engines

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[caption id="attachment_19313" align="alignright" width="250"] Hubble photo of jupiter’s aurorae.[/caption]Another title for this could be “Making sense of de Sitter black hole thermodynamics”, I suppose. What I’m going to tell you about is either a direct correspondence or a series of remarkable inspiring coincidences. Either way, I think you will come away agreeing that there is certainly something interesting afoot.

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 →

News from the Front, XVII: Super-Entropic Instability

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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 →

News from the Front, XVI: Toward Quantum Heat Engines

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(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 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. It was while working on that project that 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 →

Event!

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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 … Click to continue reading this post →

Black Holes and Time Travel in your Everyday Life

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Oh, look what I found! It is my talk “Black Holes and Time Travel in your Everyday Life”, which I gave as the Klopsteg Award lecture at AAPT back in July. Someone put it on YouTube. I hope you enjoy it!

Two warnings: (1) Skip to about 6 minutes to start, to avoid all the embarrassing handshaking and awarding and stuff. (2) There’s a bit of early morning slowness + jet lag in my delivery here and there, so sorry about that. 🙂

Embed:

Abstract: […] Click to continue reading this post →

Futuristic Podcast Interview

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For your listening pleasure: I’ve been asked to do a number of longer interviews recently. One of these was for the “Futuristic Podcast of Mark Gerlach”, who interviews all sorts of people from the arts (normally) over to the sciences (well, he hopes to do more of that starting with me). Go and check out his show on iTunes. The particular episode with me can be found as episode 31. We talk about a lot of things, from how people get into science (including my take on the nature vs nurture discussion), through the changes in how people get information about science to the development of string theory, to black holes and quantum entanglement – and a host of things in between. We even talked about The Dialogues, you’ll be happy to hear. I hope you enjoy listening!

(The picture? Not immediately relevant, except for the fact that I did cycle to the place the recording took place. I mostly put it there because I was fixing my bike not long ago and it is good to have a photo in a post. That is all.)

-cvj Click to continue reading this post →

Science Friday Book Club Wrap!

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Don’t forget, today live on Science Friday we (that’s SciFri presenter Ira Flatow, producer Christie Taylor, Astrophysicist Priyamvada Natarajan, and myself) will be talking about Hawking’s “A Brief History of Time” once more, and also discussing some of the physics discoveries that have happened since he wrote that book. We’ll be taking (I think) caller’s questions too! Also we’ve made recommendations for further reading to learn more about the topics discussed in Hawking’s book.

Join us!

-cvj

(P.S. The picture above was one I took when we recorded for the launch of the book club, back in July. I used the studios at Aspen Public Radio.) Click to continue reading this post →

Radio Radio Summer Reading!

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Friday will see me busy in the Radio world! Two things: (1) On the WNPR Connecticut morning show “Where We Live” they’ll be doing Summer reading recommendations. I’ll be on there live talking about my graphic non-fiction book The Dialogues: Conversations about the Nature of the Universe. Tune in either … Click to continue reading this post →

News from the Front, XV: Nicely Entangled

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This is one of my more technical posts about research activity. It is not written with wide readability in mind, but you may still get a lot out of it since the first part especially talks about about research life.

Some years ago (you’ll perhaps recall), I came up with an interesting construction that I called a “Holographic Heat Engine”. Basically, it arises as a natural concept when you work in what I call “extended” gravitational thermodynamics where you allow the spacetime heat_enginecosmological constant to be dynamical. It is natural to associate the cosmological constant with a dynamical pressure (in the usual way it appears as a pressure in Einstein’s equations) and if you work it though it turns out that there’s a natural conjugate quantity playing the role of volume, etc. Black hole thermodynamics (that you get when you turn on quantum effects, giving entropy and temperature) then get enhanced to include pressure and volume, something that was not present for most of the history of the subject. It was all worked out nicely in a paper by Kastor et. al. in 2009. So…anyway, once you have black holes in that setup it seemed to me (when I encountered this extended framework in 2014) that it would be wilful neglect to not define heat engines: closed cycles in the p-V plane that take in heat, output heat, and do mechanical work. So I defined them. See three old posts of mine, here, here, and here, and there are others if you search.

Well, one of the things that has been a major quest of mine since 2014 is to see if I can make sense of the extended thermodynamics for quantum field theory, and then go further and translate the heat engines and their properties into field theory terms. This seemed possible to me all the way back then since for positive pressure, the cosmological constant is negative, and when you have gravity with negative cosmological constant you’ve got duality to strongly coupled field theories. So those heat engines must be some kind of special tour in the field theories. the efficiency of an engine must mean something about the tour. Moreover, since the efficiency of the engine is bounded by the Carnot efficiency, it seems we have a naturally defined dimensionless number that has a fundamental bound… Alarm bells ringing! – Opportunity knocking to learn something new and powerful! Maybe even important!

So I chipped away at this for some time, over years, doing various projects that […] Click to continue reading this post →

Frank Buckley Interviews…

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Turns out that Frank Buckley, the news anchor at KTLA 5, is not just a really great guy (evident from his manner on TV), but also a really excellent interviewer with a sharp curiosity that gives me hope that great journalism is still alive, well, and in good hands. I showed up at the station expecting to just have a pleasant chat around the book and be done with it, but I walked into the room and he’d done all his research and was sitting with extensive notes and so forth about lots of physics ideas he’d read in the book that he wanted to talk about! So we have a blast talking about the physics of our universe and the world around us in some in-depth detail. It was fantastic, and just the kind f conversation I hope that the book celebrates and inspires people to have!

Check out our interview here (embed below), and be sure to tune in to his […] Click to continue reading this post →

North Carolina Science Fair!

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Tomorrow I’ll be giving a talk at the North Carolina Science Festival! The talk will be about black holes, time, space, movies, and books, held inside the historic Morehead planetarium. I’ll sign the book for you after if you want.
Here is a link to the event.

(On the plane over, rather than doing my usual sketch-a-face-from-a-magazine exercise you might be familiar with from earlier posts, e.g. here, I made some new sketches for use in the talk. One of those is above.)

-cvj Click to continue reading this post →

Talk Nerdy!

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I was on the Talk Nerdy podcast recently, talking with host Cara Santa Maria about all sorts of things. It was a fun conversation ranging over many topics in science, including some of the latest discoveries in astronomy using gravitational waves in concert with traditional telescopes to learn new things about our universe. And yes, my book The Dialogues was discussed too! A link to the podcast is here. You can find Talk Nerdy on many of your favourite podcast platforms. Why not subscribe? The whole show is full of great conversations!

-cvj Click to continue reading this post →