Category Archives: research

Tune In!

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Later today, there’ll be a joint seminar by physicists from ATLAS and CMS, the two experimental halls at the Large Hadron Collider (LHC) looking for evidence of the Higgs boson. This will be the first official announcement of the physics seen (or not seen) by the combined results from both independent searches. Neither search has enough data to announce a discovery of anything (as far as I’ve heard) and the combined results would not constitute one either, but people are hoping for at least some nice hints of something suggestive that support each other. We shall see! this is an exciting time, as you’ve read me say before, and so I recommend looking out for what will be announced. Even a negative result (e.g., “we’ve found nothing yet”) […] Click to continue reading this post →

Almost at an End

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I’ve had a fun time over the last few lectures with some more mature topics, pointing the students to things that they will see more (I hope) in the advanced class next semester. We covered the large N Gross-Neveu model in some detail, giving me the opportunity to give a glimpse of several important topics and techniques… at large N the 2 dimensional model’s solution is exact, and it shows important phenomena such as spontaneous chiral symmetry breaking, dynamical mass generation for the fermions and dimensional transmutation. These are all important phenomena shared by (the more difficult to study) quantum chromodynamics, the theory of the strong nuclear interactions. (See an earlier post about some of these properties and what they are… there’s also a mention of a new general level book that goes into some detail on the physics and the history.)

The other thing I took some time to explore was the diagrammatics of the model, and the interesting patterns that emerge […] Click to continue reading this post →

Infinity Coincidence

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So here’s an interesting sequence of events. On Tuesday in the QFT class I finished the lecture on Renormalization Group Flow, and the idea of a “beta function”, unpacking the results we’d accumulated from QED and quartic scalar field theory to use as illustration. The key result, for those of you about to scroll away (or the few of you who have not, but are hovering over the scroll bar), is as follows. Never mind what a beta function is right now. The issue at hand concerns whether it is positive or negative for a force of interaction being studied. A positive beta function tells you that the strength of the interaction between constituent things (particles, etc) gets weaker as you work at lower energies… This is an important result in understanding how Nature behaves in a variety of situations… one way of seeing variety is to look at different energy scales, and sometimes what seems familiar takes on different character. The converse is true… that positive beta function tells you that the interaction gets stronger at higher energies… Energy is also rather like the inverse of distance scale too, so high energy is akin to shorter distance scales (higher resolution), and low energy like longer distance scales (grainier resolution). In other words, looking at stuff in really tiny detail means using higher energy… and the nature of that stuff can change when you look at that sort of resolution since the way things interact changes… For electromagnetism, for example, we see that it gets stronger the closer we look, digging more deeply into the structure of the atom, say, probing the charged constituents of the nucleus once we’ve understood electrons. The result is that you see the electromagnetic interaction changes, ultimately turning into something else… (it merges with one of the nuclear forces, in fact…but that’s a story for another day)

So anyway one of the things I ended the class with was the idea that if you had a negative beta […] Click to continue reading this post →

QFT Rocks!

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I’m having a blast teaching the introductory quantum field theory class, as you may have gathered from several previous posts. It has been taking a lot of time, but I’ve been doing detailed computations with the students (and hence taking up a lot of preparation time) to make sure they really get how to compute in a quantum field theory (such as Quantum Electrodynamics (QED)) and see how it connects to the real world. Having spent time on the electron anomalous magnetic moment computation (I told you about that spectacular feature of QED earlier) we went back to basics to discuss in more detail the nature of the physics that is to be extracted from what are called one-loop diagrams – diagrams of the sort I drew before that have a closed loop in them.

This is where I get to try to clear up a huge conceptual issue that still afflicts most people’s popular-level knowledge of quantum field theory due to poor writing – the nonsense often spouted about there being “hidden infinities” and so on and so forth. Covering the poorly-named “renormalization” procedure and treating it in a more physical way to see that the whole “infinity” business is dreck can be fun, since it allows for an emphasis on a lot of the key issues I care about in the science that we do, such as the idea of effective field theory, the importance of separation of scales in your physics, and most importantly the reminder: Let’s not confuse the tools we use to describe Nature with Nature itself.

I like to use that phrase, and it comes back again and again, whether it is to do with […] Click to continue reading this post →

Southern California Strings Seminar

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The next Southern California Strings Seminar is on Friday 21st October! The website is here. It is going to be held over at UCLA this time, and I expect it’ll be fun and informative, as these regional meetings have proven to be. This time it is a one-day event again. You may recall that the last one, on May 6th, was over at USC. See here. There’s a random picture from it above left.

Apologies to anyone who was […] Click to continue reading this post →

Landmarks

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We reached certain key landmarks in the quantum field theory class last week. Not only did we uncover how the Dirac equation predicts that the electron’s spin couples to a background magnetic field, but that it is (to a first approximation) twice the coupling that its angular momentum couples with. This is a remarkable output (once again) from that wonderful equation, the result of putting together Quantum Mechanics and Special Relativity. (I’ve discussed other properties of it before, here.) Makes you wonder, doesn’t it? If finding an equation that puts together QM and SR results in so many marvellous things about Nature just falling out so nicely (recall that spin, and also antimatter are also inevitable predictions of the Dirac equation), doesn’t it make you just thirst to know what we’ll learn (predict, explain, discover) when we find how Quantum Mechanics and General Relativity fit together? This is one of the great motivations for the work my research community does, in case you sometimes find yourself wondering – the quest for finding how Nature combines the two things together to make what we call Quantum Gravity. We’ve found one way, on the page, that you can put together QM and GR for sure (you’ve heard of it, it is called string theory) but it’s not yet clear if this form of quantum gravity is the one Nature chooses to work with, and in any case we need to work on it a lot more to understand what we’ve got, as it surely isn’t complete yet. There are huge pieces of the puzzle missing.

Anyway, the next thing we did was the classic computation of the first quantum correction to the “twice” I mentioned above. This is the computation of the famous […] Click to continue reading this post →

AdS Vs CMT

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So there are two workshops going on here at the KITP in Santa Barbara that pertain to issues in condensed matter physics. (You may recall that I am here for some of the week, and in LA at USC for the rest of the week – adds up to a full week each week.) One is Holographic Duality and Condensed Matter Physics (often referred to as AdS/CMT), concerning applications (I’ve told you about this here before) of techniques from string theory to issues in condensed matter theory and experiment, and the other is Topological Insulators and Superconductors. there are people from both communities on both workshops, and so it is an exciting and interesting time.

The KITP Fall picnic was yesterday, and this means that there’s the traditional football (i.e. soccer) contest between workshops. Not to resolve physics issues, you understand, but to blow off some steam before getting down to the business of eating all the food… Well, at the top of this post and in the following, you can see some pictures of the process of the string theory crowd getting slaughtered for a change:

The score was 5-0.

The KITP Director, David Gross, who has presided over very many matches where the […] Click to continue reading this post →

Looking Back and Forth

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Somehow after Wednesday I lost track of time, in a sense, in the natural course of having another very busy week. There were several things competing for time, and some of them may be of interest to you. (Left: Some lovely pink gladiolus flowers that have sprung up in my garden.) The Nobel Prizes kept coming, of course, with some very interesting winners announced. In addition to the ones I mentioned already in two earlier posts, I’ve got to find some people among our faculty who’ll be willing to spend 10 to 15 minutes making some informal remarks about the Who/What/Why aspects of the prize at one of two lunches I’ll be hosting in the coming weeks about the Nobel Prize. I’ve mentioned this before. It is an annual event I’ve tried to get going as part of the Dornsife Commons (formerly known as College Commons) series. I’ve locked in Physics and Peace, and want to get people for all the others. This year I know that if I have problems with Chemistry, I can step in if need be, although I’d rather not have to do that – I want to broaden participation, not do everything myself. Look out for those lunches (see here) and come along!

Speaking of doing everything myself, I’ve been continuing the weeks long struggle to get support, interest, and participation for the Science Film Competition I told you about earlier. Having spent a lot of time meeting with many faculty and other parties to build support and understanding, getting lots of faculty to make announcements (one time even coming down from Santa Barbara to campus to give a ten minute announcement in a class at the film school and going up again after!) and so forth – and thanks everyone who has helped! – I decided to amplify my focus on tackling […] Click to continue reading this post →

Quasicrystals!

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Wow! WOW!

As you can tell, I am very pleased about the 2011 Chemistry Nobel Prize announcement. I’d love to tell you a bit more about why, but I’m supposed to be working on something urgently right mow, and so will try to do so later. The key thing is that I think the discovery of quasicrystals is fantastic and very visual example of how we can dream up a mathematical structure just because it is interesting and beautiful (Penrose tilings, independently discovered by Roger Penrose as the answer to a mathematical puzzle, (see two dimensional version on left) but also showing up in Islamic tiling patterns from much earlier) and then find later that Nature has exploited that same pattern to make something- a new form of matter (the quasicrystals themselves… The subject of today’s prize to Dan Shechtman – image below on right is of a silver and aluminium quasicrystal compound. Both images here are from wikimedia commons.).

I was in love with these things when I was an undergraduate, not long after their discovery, […] Click to continue reading this post →

Breakfast Guy

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I’ve no idea who he was, but he made for an interesting subject for several minutes*, sitting eating his breakfast with his (I think) wife. I was staying at a hotel and having breakfast, just North of Santa Barbara. The person I set out wanting to draw (very interesting face) was sitting right opposite me, at the same table as me, looking over regularly, and so it seemed a bad idea to try to sketch him. Also, he turned out to be a physicist also visiting at the KITP for a workshop, and so it could have ended up quite awkward.

It was a pretty good week at the workshop. I had a number of interesting conversations with young people trying out ideas and calculations, who’ve actually read (!) various papers of mine, and so had questions and […] Click to continue reading this post →

Heretic…?

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We had a really interesting discussion of the quantum physics of de Sitter spacetime yesterday here in Aspen, starting with a review of the behaviour of scalar fields in such a background, led by Don Marolf, and then, after lunch, an open-ended discussion led by Steve Shenker. This is all quite difficult, and is of course quite relevant, since a piece of de Sitter is relevant to discussions of inflation, which seems (from cosmological observations) to have been a dominant phase of the very early universe. As the most symmetric space with positive cosmological constant, de Sitter may also be relevant to the universe today, since dark energy (first recognized after 1998’s observations of the universe’s accelerating expansion) may well accounted for by a positive cosmological constant.

So we need to understand this type of spacetime really well… and it seems that we don’t. Now there’ve been a lot of people looking at all this and doing really excellent work, and they understand various issues really well – I am not one of them, as I’ve not worked on this in any detail as yet. Do look at the papers of Marolf, and of Shenker, and collaborators, and references therein, and catch up with what’s been going on in your own way. For what it is worth, the sense that I get is that we’re trying to solve very difficult issues of how to interpret various quantum features of the spacetime and getting a lot of puzzles by trying to make it look a lot like things we’ve done before.

Now, we may solve all these puzzles…. but my current take on this all is that we’re […] Click to continue reading this post →

Living in Interesting Times

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As far as particle physics and big questions about how the universe works, we are living in very interesting times, I’m happy to say. We’ve all been waiting for the Large Hadron Collider (LHC) for over two decades, and now it turns out (I’ve been hearing from a number of people in various conversations here at the center) that the machine is running really well – impressively so. That alone is great, but an interesting thing is that we are almost certainly going to know something significant before the end of the current scientific run next year, maybe even by the end of 2011!

Recall (see posts like this one) that the primary goal is to understand the physics responsible for the Higgs mechanism – the physics that gave mass to those elementary particles that have mass. The particle that does this is called the Higgs particle, and exactly how Nature implements the Higgs mechanism is what we hope to learn. There’s sort of a vanilla version of the story, that fits into the Standard Model of particle physics without any further adornment than just doing the basic job. Then there are more complicated versions of the story, where, in some cases, the Higgs comes as part of a bigger physics story that leads the way to what’s generally called Beyond the Standard Model Physics. As the machine searches toward higher and higher energy that probes higher and higher mass, the simplest possibilities will begin to fall by the wayside pretty soon if nothing is seen. An exciting thing is that it seems that whether or not we have the Standard Model Higgs might be known soon.

Now that’s exciting enough, but there’s more. Actually, a lot of people, for various […] Click to continue reading this post →

Workin’

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It was a quiet and good day today on the work front, here in Aspen. I started at the Center relatively early (having risen at about 7:00am today – a bit later than I have been) and the plan was definitely a vastly-mostly-physics day, digging into some of the things I’ve been puzzling over, and having some conversations with one or two people about their projects. The first order of the day, once arriving, was to turn off my phone and go dark on IM, and turn off my email… The plan for such days is to only check messages and so forth at lunchtime, and then much later in the afternoon, which is rather helpful, I find. (In fact, I ignore the phone for the entire day until the evening…)

I won’t give you a tedious blow by blow of my day, since it was mostly a mixture of sitting and reading or staring into space (along with some scribbling here and there), and so forth. I also went for some walks, down by the Aspen Institute (where there’s currently the interesting “Brainstorm Tech” conference sponsored by Fortune magazine going on) and of course over to the Music tent, where there were lots of people in the morning, since the Music Festival orchestra (and a lot of very young pianists – maybe from the piano competition? – taking turns) were rehearsing for tonight’s concert. I sent an email or two, I’ll admit, but they were specifically concerning a project I’m working on with a colleague back in LA, and so they count as ok.

I went for a walk to think and eventually returned to the tent after one or two […] Click to continue reading this post →

Bed Post

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I’ve a big meeting in the morning at 9:00am and have not the slightest bit of sleepiness in me at all, and it is 11:30pm. This may have something to do with me making a pot of coffee at about 9:00pm after deciding that it was way too early to go to sleep and I should do some work. I’ve done some, and now im in bed. Let’s see if a little blogging off the cuff about some internal stuff helps…

Well, sort of internal. Pretty sure you would not want full-on internal stuff. Even I don’t know what’s in there all the way down…

….hmmm, and now it is 13 minutes after midnight. The intervening time was spent prepping a draft of what I expect will be the next post, involving generating a video and uploading it to YouTube and so forth. Do have a look.

One of the more interesting things from today was getting excited about a paper I was asked to referee. It turned out that I really liked it a lot. I’d somehow missed it in the regular scheme of things, and so this time around I get to say that refereeing was really useful for me. I cannot tell you which paper it s, unfortunatley. It was actually about something that I’d planned to try to do myself, before I got distracted by another set of projects and never returned to the issue… So it was nice, a bit of the way through, to suddenly realize what was going on and sit up and take note. I’ve a few minor comments to the authors to encourage a bit of clarity in the presentation, but other than that my report will be “nicely done, chaps”. It has all served the purpose of reminding me to get back to some of those projects I had in mind three years ago. (Three years already! Geez…)

Was chatting with a bunch of different sorts of physicists today. It was enjoyable.[…] Click to continue reading this post →

Stick with the round balls, for now

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So, apparently, electrons are round. Very very round. So when drawing those terribly wrong but evocative pictures of atoms as a lump in the middle (the nucleus) with a collection of round balls in orbit around them (the electrons), go ahead and make them nice and round. Very round. How “very” are we talking about here? According to this report on the recent experimental measurements in the Guardian:

Were the electron scaled up to the size of the solar system, any deviation from its roundness would be smaller than the width of a human hair […]

So you’d have to be using a pretty impressively sharpened pencil to draw it that accurately round. But give it a try.

Ok, what’s the story here? Well, oddly, this seemed to be on a lot of news sources yesterday, and I’m not sure exactly why. Maybe because it mostly seemed to be pitched as a “back to the drawing board for the theorists” story (two major sources I heard had it spun this way), which editors seem to like running with. And the roundness? What’s that about? Well, what they’re taking about is the result of a long careful set of measurements done by Hudson et. al. at Imperial College (my […] Click to continue reading this post →