Category Archives: string theory

Back to Basics

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Well, today was the first lecture of the string theory course (part 2) that I mentioned in the previous post. And I applied the “when they think you’re going to zig, you zag” principle. They have been expecting me to dive into the whole business of open strings and D-branes and so forth (the subject of the book), and I did not. Sure, that will come, and sure, we’ll explore what they mean and what they can tell us about string theory beyond perturbation theory and so on and so forth. But I want first to spend a couple of weeks on getting to the heart of the matter. They made several standard choices along the way in doing their first semester of study of string theory. What did they mean? Why did they work? Were those the only choices? What is underlying a lot of it all, and what, when stripped down to the essence, is at the core of string perturbation theory and beyond? In other words, let’s look more closely at the path integral definition (such as it is) of a string theory (slightly schematically):

[tex] Z=\int [{\cal D}g {\cal D}X] e^{-S(X,g)}\ ,[/tex]

and make sense of all the bits. (Er, for the two of you still reading, [tex]S(X,g)[/tex] is an […] Click to continue reading this post →

Experimental Excitement!

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alice_frist_run_dataWell, this week is a big week, in some ways. The Large Hadron Collider has gone into a new phase! For a while, the experiment has turned aside from the task of searching for the origin of mass (the Higgs Particle, or whatever it is that mediates the generation of the masses of elementary particles – see earlier posts, and features like this, etc) and is turning to heavy ion collisions. Rather than studying processes in which only a few particles at a time are interacting at super-duper-uber-high energies, the experiment will instead collide together the nuclei of lead atoms, so that you get lots of particles colliding together and creating a messy “soup” of high energy stuff all together. The goal is to understand the constituent nuclear particles (quarks and gluons) working collectively at high temperature (and low to moderate density), instead of focusing on issues concerning individual fundamental particles. Today (starting late yesterday, actually) is an exciting day because it marks the first step on the journey to probe deeper into this physics. The ALICE experiment has started looking at these collisions. See top right for some screen-shots of the mess of particle tracks that are left after the soup flies apart. The trick is to analyze all these tracks from millions of such collisions to work out the properties of the soup.

As you perhaps know from reading this blog, while of course I’m interested in the behaviour of fundamental particles and the origin of mass, and so on and so forth, I’m very interested in this nuclear issue too. Some of the most interesting work that […] Click to continue reading this post →

Southern California Strings Seminar

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scss_studentsOh! I almost forgot to remind local readers that the next Southern California Strings Seminar is happening this week. Friday and Saturday. It will be at UCLA, and there’s a great line up of speakers. As I’ve said here before, graduate students are especially encouraged to come…

…since special effort is made to make sure that each talk begins with a pedagogical portion to help non-experts in that subfield navigate and see the motivation.

The speakers and talks are:[…] Click to continue reading this post →

Too Little, Too Much, or Just Right?

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Charles Day on the Physics Today blog asks an interesting question: Why has physics today’s news coverage of string theory been so sparse? I must admit that I had not noticed what the level of coverage is, and so the matter had not sprung to mind, but it is an interesting one. He looks back at the number of major articles written about the subject in recent years (he was a feature editor, and so shepherded some of them through), and concludes that the numbers are low, and he may well be right, but I am not sure I know what the best measure is.

How do we measure the appropriate coverage level? Is it by the number of people working in a sub-field as a percentage of the overall field of physics? If it were possible to break things down that way (I’m happy to see that we have come to a point in the field where I’ve no idea how to define what a “string theorist” is, per se, any more than I know what a “field theorist” is. There are simply people working in various fields who use both as tools to make progress) I wonder what fields would end up appearing under or over covered?

Perhaps a large part of it it is the issue of how many articles on the topic can be written that are of general enough interest, and finding the people who are both able and willing to write. This is a tricky issue, and depends on a combination of the topic in hand and the knowledge and writing skill of the author. There are two extremes that would be a factor here. On the one hand, you have the obvious difficulty of a non-expert journalist either not being able to see the general relevance or value of new results in such a specialist field, or if having seen it, not being able to explain it […] Click to continue reading this post →

I Know What You Did Last Summer

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That was the unmentioned title of yesterday’s group meeting. We try to do this every year, and so one of the first meetings of the year is us sitting together, eating our lunches, chatting about ideas. We basically catch up, and chat about things we learned about from Summer travels, perhaps from conferences, schools or workshops. We also mention what we’ve been up to in terms of projects worked on, work in progress, etc. I find the latter especially useful from the standpoint of student development, since it is often the first time that a student gets to tell a wider audience (but in a safe environment) a little about what they are doing, often their first projects. Even a few minutes summarizing what you have been doing can be daunting at the early stages of your research career, so this is a good place to start.

Talking about things that we’ve learned are going on in the larger community (or […] Click to continue reading this post →

Witten at Your Fingertips

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l-333-194-98891247-3696-41e6-97c7-386cb6dbeef9.jpegI just noticed, via this article at the Guardian, that not only has Edward Witten been awarded the Institute for Physics’ Isaac Newton medal, but his lecture at the ceremony has been posted online for all to see! See the link below. It is not often that you get an opportunity to see Witten, one of today’s giants in theoretical physics, on the public stage, to the extent that most people outside the field have not heard of him at all. The public view of who is driving forward and massively contributing to the field is rather skewed as a result of the number of appearances and grand pronouncements (often on subjects they’d be better shutting the hell up about!) from certain other renown theoretical physicists. So here’s an opportunity to hear from one of the true masters of the field.

When you speak to sensible people in the subject, whatever their own work is about and whether they like some of the things he works on or not (such as string theory), they will quite readily concur that he has done a huge amount for the field, driving forth many powerful ideas, sharpening the way we think about certain kinds of problems, conceiving of new ideas, and overall strongly influencing much of the basic manner in which practitioners think about the physics they are doing. Often, even if he is not the originator of a particular […] Click to continue reading this post →

Emerging Gravity

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In the New York Times this week there’s an article* by Dennis Overbye on Erik Verlinde and his paper on the idea that gravity is not a force at all, but a consequence of thermodynamics. You can think of it as an extreme take on one of the directions a lot of the research (that I’ve mentioned a number of times has been going on in string theory) has been pointing, although I think it is safe to say that there’s a lot to be done on making the statement a concrete one that you can do physics with. I think it contains the germs of the right thing we’re all reaching for, but does not quite get there yet. We’re now quite routinely formulating some of the key physics of gravity entirely as physics of a completely non-gravitational dual theory – this is the content of what we call holography – and in particular the quantum physics of black holes in those settings get holographically mapped to the thermodynamics of the non-gravitational physics. This is the basis of the tools that we’ve been applying to studying aspects of phenomena showing up in various experimental systems in nuclear physics and cold atomic physics (and studies of phenomena relevant to various condensed matter systems are also being done). I’ve told you about a lot of this in various posts. (Some of them are listed below.) Running this the other way, the model non-gravitational systems (certain gauge theories at strong coupling and with large rank gauge groups) can be thought of as examples of how gravity (and space-time itself) is really an emergent phenomenon, appearing simply as […] Click to continue reading this post →

Scenes from School

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unam_astro_math_mexico_2Well, my work at the Quantum Gravity school over (see previous posts here, here and here), I hopped on a plane yesterday, in order to return to Los Angeles. It was an excellent time. I enjoyed being in Mexico again, and seeing a new part of it (see posts to come), and the school was well organized and very enjoyable. The students were very receptive overall, and I spent a huge amount of time chatting with them about physics between sessions, over lunch, and on the bus back and forth between the campus and the city (and on the excursion on Sunday for a while). A number of students seemed very interested in some of the quantum gravity phenomena that are possible to describe using string theory – surprised in most cases (since the background independence issue that keeps being waved about by some as a naive (in my view) and bluntly used principle seems to have served as a block to many) – and it was a pleasure to be able to open them up to new physical ideas. It worked pretty well because I gave them three lectures on the basics of string theory, and so could build on that material in the one to one discussions to explain more detail.

lunch_waiting_unam_mexicoDynamical changes of spacetime dimension is one thing that captured a lot of interest, as well as string theory’s various ways of showing in model examples how spacetime is an emergent, classical approximation to an underlying quantum description that does not have manifest spacetime geometry. The latter is something that all approaches to quantum gravity hope to realize in one way or another (since those sorts of ideas and fantasies about what quantum gravity should be appeared in the mid 20th Century), and exactly how it emerges in string theory is a beautiful story. Who knows, perhaps they will take these hints from string theory about how quantum gravity can work and develop them […] Click to continue reading this post →

Minority Report

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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 →

Arrival

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(Anyone remember Mike Oldfield? I recall a lovely piece of music of his with the same title as this post and it has now been playing distantly in my head as I type…very pleasant.)

Well, I’m sitting at an outside table across from the main square with the cathedral and having a simple lunch. It is hot, and so I have a glass of cold pale beer in front of me, and everyone around me is speaking Spanish. This feels like it could be that I am back in Madrid, but in fact I am in Morelia, Mexico. (I can tell because, among other things, I’ve never been offered a shoe shine quite so many times in the space of 15 minutes…) It is quite a lovely place, as far as I’ve seen so far (I’ve not explored much yet). I’m resting after a very early morning travel schedule which saw a slight panic (er…long story) at LAX to catch my flight which left just before 1:00am. The seats were fixed to the bolt upright position the whole flight and so the two and a half hours I’d planned to snooze were not so […] Click to continue reading this post →

Summer School Prep

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So, it is that time of year.

I’ve got to give three introductory lectures on string theory in a Summer school somewhere in Mexico next week. So I must break away from intense work on The Project (which is going well, so maybe you will hear about it sometime soonish(ish)….) and focus on how to cram all the cream into three comprehensible and useful lectures. I hate overdoing the “it can be shown” line that ends up making the whole thing seem like magic (as is so often done), so this is always a challenge. Here’s how my preparation usually goes (and indeed it went this way this week): […] Click to continue reading this post →

The Search For Perfection…

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…is all very well, but to look for Heaven…is to live here in Hell*.

may 2010 physics today coverOne of the things I worked a lot on in earlier months this year (and late ones of last year) was the lead article in a cluster of articles that has appeared in the last few days in May’s special edition of Physics Today. They are sort of departmental-colloquium-level articles, so for a general physics audience, more or less. It’s about some of the things I’ve told you about here in the past, concerning exciting and interesting applications of string theory to various experiments in nuclear physics, as well as atomic and condensed matter physics (although we do not have an article on the latter in this cluster). I had a fun time working with Peter Steinberg on the article and remain grateful to him for getting us all together in the first place to talk about this topic way back in that AAAS symposium of 2009. It was there that Steven Blau of Physics Today got the idea to approach us all to do an article, which resulted in this special issue.

My article with Peter Steinberg, “What black holes teach about strongly coupled particles “, is here, and the pdf is freely downloadable (update: alternative link here). It is mostly about the theoretical aspects of the whole business and the relevance of the string theory […] Click to continue reading this post →

New LHC Physics Phase Begins!

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cern-first-physicsWell, here we go. It has been a little over 20 years since I’ve been actively working in this field and have been hearing about the promise of this machine, the Large Hadron Collider (LHC), and now it is really here, working, and colliding protons at an energy much higher than any previous experiment, promising us to a glimpse of new aspects of how the universe works. It is not guaranteed, of course, but there’s a great deal of hope, and so much of what we know strongly suggests that there’s going to be some exciting things to learn. See the list of related posts below for several bits of background on the LHC, or go to CERN’s website. [Image above right -click for larger view- is a CERN-supplied montage of data/images from the various experiments at the LHC. Caption: 7 TeV collision events seen today by the LHC’s four major experiments (clockwise from top-left: ALICE, ATLAS, CMS, LHCb).]

Two of the things foremost in people’s minds are on one hand the Higgs (the particle or particles that ultimately give masses to the elementary particles that make up the […] Click to continue reading this post →

News From The Front, VIII: One Down…

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work_snap…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 →

Here, There, and Everywhere

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british_library_1Well, it has been quite the week so far. I’ve been mostly in England. First I spent Tuesday getting over the main effects of jetlag and a short but strong cold (both more or less gone now), and then Wednesday I went to King’s College London to give a seminar to the three groups in the Triangle series of seminars – King’s, Imperial, Queen Mary are the three places the participating research groups in theoretical high energy physics come from. It was excellent to see so many old friends and colleagues, meet some new ones, and chat physics at the pub and over dinner later on. The seminar seemed to be well received, although I know I was far from my best, given jetlag and cold. The next two days saw me saying hi to family and friends at coffee and dinner in the evenings and visiting at Queen Mary and Imperial for the day, and hiding in the British Library for most of Friday, writing.

What am I writing? Four lectures on D-branes and string theory and M-theory, with a focus on some of the fun and instructive applications (and potential applications) of […] Click to continue reading this post →