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
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
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
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
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
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 and another parameter called “nut charge”, . 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
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
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 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 , (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
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
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 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
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?
Calc or trig?
What are you studying?
I’m not…studying. It is my research.
Silence from him for a few beats, then:
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
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:
…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
“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
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
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
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
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
Today (Tuesday) saw me up at 6:30am to prepare for an 8:00am call time for a shoot on a special episode of – wait for it – Deadliest Space Weather. It is original programming for the Weather channel, and before you dismiss it because of the title, it turns out that it is not a bad idea for exploring various scientific concepts. The first season ended a few weeks ago. I’d not realized it was airing until recently, and actually those recent demos I told you about were used in examinations of planetary conditions on Venus and on Mars. (Two separate episodes.) The idea seems to be to consider what it would be like on earth if the conditions were like those on Venus, or consider what what happen if you went outdoors on Mars.
So you might think it is silly, but if done well, it is actually an opportunity to
explain some science to an audience who might not have been the usual science audience…in which case I’m happy to be on board! In addition to spectacularly showing what happens when sugar and sulphuric acid meet, I got to show how to boil […] Click to continue reading this post
This is a quick update on the school. I’ve been trying to give the students some of the core concepts they need to help them understand what string theory is, how it works, and what you can do with it. Here’s the really odd thing about all this (and an explanation of the post title): While this is a school on Quantum Gravity, after talking with the students for a while one learns that in most cases the little they’ve heard about string theory is often essentially over 20 years out of date and almost always totally skewed to the negative, to the extent that many of them are under the impression that string theory has nothing to do with quantum gravity at all! It is totally bizarre, and I suspect it is largely a result of things that are said and passed around within their research community.. So there […] Click to continue reading this post
…more to go. I’ve finished one of the papers I’ve been writing (this one co-authored with my student, Tameem) after delaying on it for months. I’m not sure how things got quite this backed up in terms of things I have to do, but they have. I meant to start on a new, long project last week, and all my efforts these days have been toward clearing away all those things I want to get done and dusted before focusing on that. It is taking time, but gradually the clearing is happening. Two more manuscripts to complete.
This paper reports on the continuation of the work we’ve been doing over the years in understanding the physics of various model systems in an applied magnetic field. This is in the context of holographic models of important strongly coupled phenomena that are of considerable interest in lots of fields of physics (particle physics, nuclear physics, condensed matter physics, atomic physics). (Since I don’t want to explain holography and so forth every time I talk about it, see a post I did about some of that here, and related posts in the list at the bottom of this one, if not sure what I’m talking about.) (Hmmmm, I see from my SPIRES listing that I’ve got seven papers mentioning magnetic field explicitly in the title in the last three years, and three or four more of the rest are occupied in large part with the issue too. No, really, I’m not obsessed.)
The issue here is the study of structures that suggest themselves as earmarks of Fermi surfaces in strongly coupled systems. It has been a goal for a long time in the context of gauge/gravity duals to understand what the signals of a Fermi surface would be. Would it be some geometrical object in the dual gravity theory, perhaps? Access to a computationally tractable description of such an object would be rather […] Click to continue reading this post
I’ve been wondering why over the last day or two I’ve been getting email about various apocalyptic scenarios. I’ve now figured out why, I think. On Tuesday, several scientists, myself included, played with the idea of how to destroy the earth! Well, it was on the History Channel in an episode of the show the Universe, (it was recorded back in June and July) entitled “Ten Ways to Destroy the Earth”. Of course, these are not scenarios we envision happening any time soon, but rather an excuse to talk about various kinds of science (from spontaneous symmetry breaking and the early universe, through planetary science, solar physics, and of course black holes and more). We list various favourite ways that were chosen to be discussed, and each physicist (although they called me an astrophysicist) picks a favourite. Fun stuff.
I chose putting a huge amount of antimatter at the core of the earth and letting it […] Click to continue reading this post
Well, ok… Boom is not quite accurate, but the idea is that there will be ten kinds of blasts/explosions/major_energetic_events discussed tonight on the History Channel’s The Universe:
The Universe is full of explosions that both create and destroy. The Chicxulub impact on the Yucatan peninsula, which may have wiped out the dinosaurs 65 million years ago, was two million times more powerful than the largest nuclear bomb ever detonated on Earth. But guess what? That’s only good enough for the very bottom of the Biggest Blasts top ten list. This episode works its way up through supernova explosions and gamma ray bursts all the way to the blast that started it all–the Big Bang.
Sounds good doesn’t it? There’s a lot of good people contributing again, so I know it’ll have some good material and explanations.
In addition, I’m reasonably confident I play a role in this one…This one was a blast (sorry) to film. You saw some posts earlier reporting on some of the filming. Assuming they used the material I did with them, you’ll get to see why I was at the […] Click to continue reading this post
Forgot to report on this email exchange from last semester:
From one of the staff in the physics office:
Subject: 499 Syllabus
Date: Thu, 09 Jul 2009 14:01:38 -0700
To: Clifford V. Johnson
I was reading the syllabus you sent over for the 499 class. I am not sure if this is a type-o but in the Extra Books section it reads “/Black *_hoes_* and Time Warps: Einstein’s outrageous Legacy/” should it read
“/Black *_holes_* and Time Warps: Einstein’s outrageous Legacy/”.
My response: […] Click to continue reading this post
The College of Letters, Arts, and Sciences here at USC has built a new website, and gone quite far in including extra media, and links and portals on YouTube, Facebook and so on. One of the things they did was have a filmmaker make lots of videos. Lots. Things about faculty, research, teaching, learning, etc. All very exciting. Have a look, here, if interested. Mira Zimet, who makes the films, gave me a call and asked me if I’d like to contribute, and I agreed. I chatted on and on for about 45 minutes to an hour and she cut two short films out of it. The shorter one is on the site and has me saying some general things about research, teaching, science, and USC. Mira made the second because she thought it might be a nice extra video for the College’s YouTube portal. It has me talking a bit more about what string theory is and does, […] Click to continue reading this post
Ironically, the day after earth day, a TV program (part of the Naked Science series) entitled ‘How to Kill a Planet’ will air tonight (Thursday 23rd April) at 10pm on National Geographic Channel. It explores various scenarios for how the entire planet could be destroyed. Sensational, yes, but maybe an interesting way of exploring some interesting physics topics in an unusual way, assuming they don’t scare people unduly. I appear in it somewhere, I’m told. I’ll be talking about black holes, and may in fact be shown being swallowed by one, if I recall correctly what was planned. Some may rather enjoy that aspect! [Update: Phil’s on it too!]
You may recall me mentioning a shoot I did up near a dry lake (El Mirage) in the […] Click to continue reading this post
Well, the last couple of days events were tiring, but good overall. The Hawking event was a bit like a rock concert. I made a video of some of the long line for you, and also take you into Bovard Auditorium with me to see the rather nicely packed crowd (well the downstairs part) of about 1200 excited audience members. At the end of the video are a few stills, showing Dean Howard Gillman during his welcoming remarks to the College Commons event, Nick Warner introducing Stephen Hawking, the man himself, and some of the high school students and the undergraduate student, who won the prize to get to ask Stephen questions. (The high school students had all asked similar questions, and so were all asked up to ask one question.)
Here’s the video: […] Click to continue reading this post
Gosh, time flies!
I almost did not get to announce this before it was upon us. Tomorrow and the day after combine into a notable event in the College Commons series here at USC. Some of us have been working on this for quite a while. On Tuesday we have Stephen Hawking giving a big public lecture entitled “Out of a Black Hole”. Here’s the announcement. Note that general tickets for seats in Bovard Auditorium all went within hours of us releasing the tickets several weeks ago, but there is room in the two spill rooms that we have set up where there will be screens relaying the talk. Make a bit of an event of it and go with friends! [Update: I forgot to mention that we had a competition in local high schools and also at USC where the prize was to ask Stephen Hawking a question. People submitted questions over the last few weeks and we selected some of the best. There will be three undergraduates and three high school students coming up from the audience (we’ve a lot of high school students visiting us for the talk) to ask him a question each at the end. Should be fun.]
The day after, there will be a related event. Some of us from the physics department […] Click to continue reading this post
The usual answer you’ll get from the person on the street (as it were) includes lots of nice words about wiggling strings that look like particles, and so forth, and that’s fine. However, the [
informed] next level answer, when you’ve worked enough in the field, is that we don’t know. I’ve told you why (at least in part) in previous posts and so I’ll let you read them. We’re still working on it.
While we work, we’ve learned that it is a quite marvellous thing (from the bits of it we’ve come to grips with) that is teaching us a lot about all kinds of physics, and mathematics too. Some of this may be good for describing things about Nature, and we’re still working out lots of that (although see some of the exciting things I’ve been talking about in my previous post and the links therein).
So what do we put on the T-shirt? (You know, the analogue of Maxwell’s equations for electromagnetism – light, etc – that every physics undergrad likes to have on their T-shirt). Well, we don’t know yet.
But that’s all my opinion. Every now and again it is good to hear from one of the masters about what they think of string theory*, and what it is and so forth. Happily, Joe Polchinski has been known to agree to stand up and give an exposition on this […] Click to continue reading this post
I had quite a marvellous brainstorming session today. You’ll recall I was preparing a presentation for the February 15th symposium at the AAAS meeting the other day. Well, all of us concerned (the four of us presenting as well as various Brookhaven staff who will take part in the symposium discussion as well) got together over the phone for a teleconference today. It took almost three hours (but happily I was sitting in the sunshine while doing it, as you can see). We wanted to chat about the distribution of ideas and results that will be presented.
There are four of us presenting at the event, with only a short time each to get across some of the key ideas and so we need to make sure that we don’t do too much […] Click to continue reading this post
I noticed that Robert Helling shared some thoughts about thermodynamics and gravity today on his blog. He is understandably confused about several aspects of the issue, especially when applied to cosmological issues. (What is the entropy of our universe? Does the Second Law really apply? Does equilibrium thermodynamics even apply here?)
I’ve nothing remarkable to add to the discussion at this time except to note that a blanket statement that thermodynamics and gravity don’t seem to go together (which I don’t think he’s strongly saying) is not one I’d make, since we have a major class of working counterexamples.
The context is the gauge/gravity duals I’ve talked about here a lot, starting with AdS/CFT and beyond. There we know that the gravitational systems are essentially able to display the more garden variety thermodynamics by being immersed in the (regulating-box-like) anti-De Sitter type backgrounds. Then we see that black holes […] Click to continue reading this post