Category Archives: research

Strings Talks

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20140627-080640-29200119.jpgThe conference is really rather good, with a varied program involving topics and speakers from all over the map. This includes the parallel sessions we had on Wednesday, which were held down at the Institute. Those were a lot of fun, because of the dodging back and forth between different auditoria at the IAS to get to talks of interest. I was chairing one of the sessions, and so did not get to dodge about in the first 90 minutes, and had to miss some interesting talks, but did a little talk-surfing in the second 90 after the break.

It had been many years ago now since I began to talk about there being a need for parallel sessions at strings conferences. Some people would object to them, saying that it would somehow be damaging to the field’s connected nature, where everyone is following many strands and topics in the field. To me that concern was always balanced by the problem of only having a small cluster of people and ideas represented each year due to the constraints of only having five days to present the activity of such a diverse population of researchers in the field. The main objection to having parallel sessions were, to my mind, based on a view of the field left over from when the field was smaller in terms of both people and thriving ideas. I think the conference organizers this year found a way of combining the two models rather well, with the single afternoon of parallel sessions, along with well chosen sets of half hour talks for the main sessions where we all sit together, roughly grouped by themes. There were three one hour big marquee plenary/summary talks. Theses are really useful. If I were to make a change, I’d perhaps have four or five of those, putting the two half hour talks that were displaced into the parallel section.

There is a two hour session of “Vision Talks” this afternoon. Should be interesting to hear what is said. We will perhaps get some good discussion going about where various ideas may be headed. I hope there is a lot of audience participation.

Poster sessions and the “gong show” were also great things to have as part of […] Click to continue reading this post →

Strings 2014 and a Return

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20140623-114053-42053836.jpgToday is day one of Strings 2014, this year’s version of the official annual conference about the latest research in string theory. There’s a feeling that there is a buzz of excitement in the air, in part because (I’m guessing): (1) Well, it is the annual conference, you’re going to find out more about what’s been going on in the various corners of the field, and (2) everywhere you look there walks a giant of the field, and (3) more generally, people just like you who “get you”, and whose papers you’ve read that you’ve spent a good portion of your life thinking about, so it would be odd if you were not excited, and (4) it is in Princeton, which is sort of equal to Mount Olympus in our field, where a lot of the giants live, if you’ll permit me to mix metaphors a bit, and (5) apparently this is the largest Strings meeting since Paris in 2004. (I’ve heard that it is maybe 600 people registered, making it the biggest Strings ever?… Not sure.)

I could go on guessing about the buzz felt by others, but instead I’ll mention […] Click to continue reading this post →

Swoosh!

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swooshWell, I sort of disappeared there for about a week. I got lost in some really interesting physics and had a lot of fun doing it.

I kept walking away, and it kept bringing me back. There’s that fun groove one can get into that other theorists will recognise: You hit an interesting vein where you can calculate interesting results in a particular model, and you just can’t help yourself computing more and more […] Click to continue reading this post →

Stockpiling Notebooks

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notebook_supplyAs you know from my writings and sketches, I like to carry a notebook. People often ask me what types I use, or assume that I use the (increasingly fashionable) Moleskine books. I like Moleskine books (the little 3inx5in ones for example), and have used them a lot in the past, but actually I prefer the books by HandBook Journal Co., (made by Global Art Materials). The surface of the paper is more flexible, in my opinion. It has a little more tooth than standard Moleskine, which makes mark-making with pencil much surer, and it also takes wetting better,airline_sketches_9th_july_2013 so you can work with a little wetness as well, such as lots of ink, or watercolour (paint- or pencil-based). That allows for crisp drawings like the one on the right (click for larger view – more about these sketches here), right alongside physics research musings and computations in pen and ink line on the pages shown below on the left (those notes pertain to the paper I discussed here.)

I tend to carry one of the 8.25inx5.5in landscape ones (although I love the 5.5in square ones too). (See more chat about them here.) They allow a good […] Click to continue reading this post →

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

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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 [tex]\Lambda[/tex] 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 [tex]p = -\Lambda/8\pi G[/tex], (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

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world_engineThis 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!

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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 heat_enginedo 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 [tex]\eta[/tex] 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 →

Discovery Clarification

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[Update: Over the months following the announcement, doubt was cast over exactly what BICEP2 saw, and now it seems that the signal announced by BICEP2 is consistent with polarisation produced by galactic dust. See here.]

I’m actually in hiding and silence for a week. It is Spring Break and I have locked myself away in a seaside town to do some writing, as I did last year. But I must break my silence for a little while. Why? Well there’s been a really great announcement in physics today and while being very happy that it is getting a lot of press attention – and it should since the result is very important and exciting – I’ve been stunned by how confusingly it has been reported in several news reports. So I thought I’d say a few things that might help.

But first, let me acknowledge that there’s a ton of coverage out there and so I don’t need to point to any press articles. I will just point to the press release of the BICEP2 collaboration (yes, that’s what they’re called) here, and urge you once you’ve read that to follow the link within to the wealth of data (images, text, graphs, diagrams) that they provide. It’s fantastically comprehensive, so knock yourself out. The paper is here.

I keep hearing reports saying things like “Scientists have proved the Big Bang”. No. The Big Bang, while an exciting and important result for modern cosmology, is very old news. (You can tell since there’s even a TV comedy named after it.) This is not really about the Big Bang. This is about Inflation, the mechanism that made the universe expand rapidly from super-tiny scales to more macroscopic scales in fractions of a second. (I’ll say more about the super-tiny below).

I also hear (slightly more nuanced) reports about this being the first confirmation of Inflation. That’s a point we can argue about, but I’d say that’s not true either. We’ve had other strong clues that Inflation is correct. One of the key things that pops out of inflation is that it flattens out the curvature of universe a lot, and the various observations that have been made about the Cosmic Microwave Background over the years (the CMB is that radiation left over from when the universe was very young (about 380,000 years old – remember the universe is just under 14 million years old!)) have shown us that the universes is remarkably flat. Another previous exciting result in modern cosmology. Today’s result isn’t the first evidence.

So what is today’s exciting news about then? The clue to the correct […] Click to continue reading this post →

Wormholes!

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“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.”

wormhole_sketchThat 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 →

Take Part in the Festival!

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I’ve just returned from a rather wonderful two rainy days in Santa Barbara celebrating the work of Joe Polchinski. (See my previous post for more about this, including a few reflections.) It was a combination of high school reunion, group hug (with Joe in the center), and serious reflection about physics, now, back then, and to come. Now the great news is that pretty much everything was recorded on video, and so you can take part in it by settling down in front of your computer (or other device – those of you in the further (but pre-singularity) future can just instruct the appropriate plug-in from [Cyberdine systems ] [Tyrell Corporation] Google to stream directly to the vision centres of your brain) and view the various excellent talks and panel discussions here.

I had the honour of chairing (and contributing to) one of the panel discussions reflecting on D-branes (as I promised last post). The title was “D-Branes: Tools of the Revolution” and it went very well thanks to my three excellent panelists (Greg Moore, Andreas Karch and Samir Mathur) and many members of the gathered audience who contributed to the free-form discussion in the 15 minutes at the end. Have a look at that right along side the really interesting and lively discussion that Steve Shenker chaired at the end of the conference (which sadly I had to miss because I had to get back to LA through the rainstorm for another engagement). The idea there was to speculate a bit about the future of physics and thereby “Planning for Joe’s 90th Birthday“.

Enjoy.

-cvj Click to continue reading this post →

Festival of Joe

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20140226-214941.jpgEarly evening. Cocktail (made with Hendricks gin, muddled tangerine, and basil…). Roast pork on the way. Old haunt.

Where am I? At Roy’s, in Santa Barbara. I’m here for a two day celebration of the work of Joe Polchinski, one of the giants of my field. It all begins tomorrow, and I am taking the opportunity to have a quiet bit of time in an old haunt. I was a postdoc of Joe’s back in the mid 1990s, just when the world of theoretical physics was waking up to the awesome power of D-branes. D-branes are a special type of dynamical extended object in physics, and Joe had discovered their importance for string theory just around that time. Roy’s opened around that time too, if I recall, and a group of us became regulars, helping it along in those early days when it was smaller than it is now. (That small group included my friend and fellow postdoc Andrew Chamblin, who passed away some years ago.)

A D-braneSo I am here to help celebrate Joe’s work on the occasion of his 60th (hard to believe that number, frankly), and it will be good to see all the people who show up, and of course it’ll be excellent to see Joe. Part of my help in the celebrations is to organize and run a panel about D-branes, which will be on at 11:00 tomorrow. I’ll be reflecting a bit on the good old days when D-branes really broke, and turned out to be the key tool of the Revolution that took place in the field. In lectures and writings from that time and long after I used to refer to them as the Heroes of the Revolution, and in honor of that and of Joe I have named this session D-Branes, Tools of the Revolutionary, or something like that. Joe helped bring about the revolution, and his tools were D-branes, you see.

I was lucky to be here as a postdoc at that time, and happily I had the good sense to be quite sure that it was going to be important to quickly spread the […] Click to continue reading this post →

Summing Numbers in a Small World

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So here’s a slightly weird thing. So there’s been all this excitement over the web about the old old “shocker” that the sum of the positive integers is -1/12. You know, not even an integer, and not even positive. Apparently there have been articles in the New York Times and Slate and goodness knows where else… and I’ve been ignoring it all since I’m tired of what it often leads to: People wilfully using it as a device to manipulate people’s ignorance about subtleties with infinite processes to make the tired point that string theory is somehow wrong since it is based on “funny math”. I called Lawrence Krauss (who should have known better) out about it some years ago when he did that at an event I happened to attend. It’s a bit tedious, not the least because it is actually part of a wonderful field of mathematics that can get misrepresented, and of course because it has nothing to do with string theory.

So I ignored it all. Then some students in my class asked me about it. And I explained why it is interesting and so forth… Then I carried on ignoring it all.

Then a day or two ago a mathematician colleague emailed me to ask what […] Click to continue reading this post →

Hawking an Old Idea

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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.

light cones in spacetime with a compact mass - a black holeWhat’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…

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black_hole_phil_club_talkSo 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 →

Weinberg on Physics Now

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I just spotted (a bit late) that Steven Weinberg (one of the giants of my field) has written a piece in the New York Review ofBooks entitled “Physics: What We Do and Don’t Know”. I recommend it. He talks about astronomy, cosmology, particle physics, and by casting his eye over the arc of their recent (intertwined) histories of ideas, experiments and discoveries, tries to put the Standard Models of particle physics and of cosmology into perspective.

The article is […] Click to continue reading this post →