Last night I graded the exam for my advanced string theory course, computed the final grades, and will enter them into the System today. Hurrah! It was a fun class to teach, maybe even to learn (you’ll have to ask them). The final exam took me most of Sunday to write and LaTeX carefully, checking for typos since there were a few “show this” type questions where I gave a complicated expression that they had to derive. In […] Click to continue reading this post
I had an early rise this morning, to make it down to campus early enough to set up (with the help of my co-conspirator Tameem) the room for the all day meeting I mentioned earlier, in order to start at 9:00am. All worked well… And things are progressing nicely (see photo of some of us in the lovely room we’re using) with local participants from USC, UCLA, UCSB, and even Stanford! It is excellent to see such support and enthusiasm for this semi-annual event!
-cvj
We’re hosting the next Southern California Strings Seminar here at USC. It will be on Friday 6th May. I’ve been working on it a lot over the last several days* and put the finishing touches on the arrangements just this morning.
There’ll be no Saturday morning component this time, since there’s yet another huge event down at USC this weekend, and things will be rather disrupted, I understand, as there’ll be lots of people, street closures, and so forth**. (It is the Revlon Run/Walk event in exposition park, across the street from us, an excellent event!)
So I’ve packed five exciting talks into Friday, and I think it is going to be fun! Here’s the schedule:
[…] Click to continue reading this post
I’m lounging around. In Cincinnatti Ohio. Why? Well, it has been a long day, and I’m tired. I spent the whole day yesterday traveling to get here, changing planed in Chicago, eventually getting to my hotel at 10:00 pm, having arisen at 6:00am. I spent the flying time thinking about physics and writing the talk I was invited to give here as part of the regional SPOCK meeting at the University of Cincinnatti. The letters stand for something, but I can’t recall what it is right now. It does not mean, as the title might suggest, that several emotionally-challenged and otherwise tv-cliche-handicapped individuals were meeting to discuss arcane matters in a room somewhere. It was a meeting of several people from the region’s string theory community to discuss…. Ok, ok, Ok…stop giggling!
I got up at 7:00am -really 4:00am my time- transferred all my pages of notes to the iPad, and got ready, leaving to do the short (I assumed) walk to the physics department in the wind and rain. I enjoyed that bit… I had however mis-estimated how long it would take me to get to the Physics department building, largely due to […] Click to continue reading this post
It was a fun week in the string theory class this week, as we got to some major landmarks that are always fun to teach. We’ve uncovered the extended objects called D-branes (see numerous previous posts for how useful and important these objects are in string theory research) in all their glory in the lectures before, and deduced lots of their properties, such as the form of the action that determines how a D-brane moving in spacetime responds to the various fields (including the geometry) created by the string theory. That’s all fun, but then the key thing to do next is to compute the mass of these dynamical objects, or the mass per unit volume – the tension. Computing it fully, with no hand-wavy factors. Your mass measures how strongly you interact with gravity. So you can measure it by studying the gravitational interaction between masses. (You do that when you step on a scale to measure your weight… well the scale does it by showing how much force it takes to stop you from falling through the floor toward the center of the earth…)
So in class this is when we go all Polchinski and unpack the tension computation, stopping to admire the various features of string theory you learn along the way, and seeing how simply beautifully the various basic features of the superstring theories that we’ve met in the last few lectures encode themselves in one nice object – the vacuum amplitude [tex] {\cal A }[/tex] from the cylinder diagram representing either exchange of closed strings (including quanta like the graviton – this is what you focus on to learn what the mass is) between a pair of D-branes, or an open string with its ends tethered to a pair of D-branes going in a closed loop. That there are two ways of looking at the diagram, an open string way (running time around the cylinder) and a closed string way (running time along the cylinder) is a hugely powerful thing, and is at the heart of so very much of what we do in string theory these days especially – including a lot of what I’ve told you in previous posts (see e.g. here and here) about applications to things of interest in current experiments.
One of the fun things about all this is that the answer is actually [tex]{\cal A}=0[/tex]. It is zero because all the infinite modes of oscillation of the string gather themselves up nicely to give a factor:
[tex]
q^{-\frac{1}{6}}\prod_{n=1}^{\infty}(1+q^{n-\frac{1}{2}})^8-q^{-\frac{1}{6}}\prod_{n=1}^{\infty}(1-q^{n-\frac{1}{2}})^8-16q^{\frac{1}{3}}\prod_{n=1}^{\infty}(1+q^{n})^8=0\ .
[/tex]
Think of [tex]q[/tex] as a book-keeping device that lets one track energy contributions (in the power of it that appears in a term if one expanded this expression), and how many […] Click to continue reading this post
A student asked a question in the string theory class today to which my answer was a suggestion of how to think about the issue raised in order to go about answering the question themselves. There’d be a few minutes of diagram drawing, and all would (hopefully) be clear. I thought that might not be an unreasonable thing to ask of a student, particularly in a graduate class, where they are ultimately trying to develop skills to do research. Well, it all went a bit pear-shaped as the student seemed to get quite strongly annoyed by this suggestion. I’ve still no idea why.
A
nyway, on the bus home I thought I’d do some idle doodling, and ended up doing the exercise I’d suggested… and sure enough what I suggested doing works nicely and does (I think) make it clear.
I’m sure it was all a misunderstanding… Probably my fault.
Over late night dinner just now, since I can’t put down this lovely brush pen I’ve been drawing with recently, I scribbled the figures out from the notebook for you to look over my shoulder, as it were, and see what we’re up to in the class. Don’t worry so much about what it all means. It is sometimes nice to just look at the shapes. (Actually, one of the students brought his mother to visit the class today. She sat through the whole hour and fifty minutes of the lecture. That was nice. I hope she enjoyed it all!)
I find these diagrams and the computations they represent rather pretty.
Wednesday will be the big climactic lecture of a sequence I’ve been leading them […] Click to continue reading this post
This always catches people off guard (myself included), so I thought I’d post a little reminder. The deadline for applications to the Summer workshops at the Aspen Center for Physics is January 31st. That’s coming up soon, so to physicists interested in doing a research stay, start thinking about the dates you want to attend, finding funds for support, planning for things like childcare or summer programs for children if you have any, and so on and so forth. There’s a wide variety of excellent […] Click to continue reading this post
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
I’m feeling strangely cold, although the heating is on and I’ve got a jumper (“sweater” to readers in the States) on. It has been this way all day, so I suspect it is something to do with my frame of mind. I’m feeling a bit reflective with it too, so I’ll think out loud (as it were) a bit before going to bed early.
Well, it is almost time to start another round of teaching. This semester, starting Monday, it is a graduate course that I’ll be teaching, the second part of a year’s sequence of string theory that we teach from time to time. My focus will be non-perturbative issues, focusing on much of what has been forming the foundation of research in various areas of string theory since the 1990s. Should be fun. Some of the material will come from my book, D-branes, that was published back in 2002. It seems so long ago now. I actually looked in it today, as I was discussing a research issue with a colleague, and could not recall some details. Happily there was a chapter with it all in there. That’s rather nice. The book serves me well as a personal reminder of things I used to have at my fingertips all the time back then, and as a bonus, lots of people around the world still use it as a handbook/guide/intro/etc, I hear.
I joke, of course. The cart and the horse are the other way around.
Speaking of books, I’ll be doing my best to continue working on the current book project, with all the excitement and adventure in developing it. (And the occasional […] Click to continue reading this post
Ah. This is just perfect. I actually looked into my Institute For Advanced Study news magazine this time around and noticed a gem I’d like to share. Edward Witten gave a lovely talk entitled Knots and Quantum Theory for a (sort of) general audience, and there is video of it available. Ever wondered why mathematicians study knots? Why do physicists care? What do they bring to the table? Well, this could be a talk for you to take out a bit of time to watch.
Not long ago I wrote a post about Ed, his huge influence on the field of theoretical physics, and some of his role in my own development as a physicist during my time […] Click to continue reading this post
It is midnight and I really should get to sleep in order to wake up and work some more on editing the final exam for my class so that it can go to the printer by noon. But I’ve got several pokes from people clamouring to find out what The Project actually is, and I promised yesterday I’d start to spill the beans. Thanks for the interest! I think I’d better get at least some of it out there or I’ll have an angry mob by morning! So here goes. I will drag out the draft I sketched yesterday and beat it into shape:
So, as you may have guessed, The Project, which I’ve been mentioning here since a post way back in February, is a writing project, but it is somewhat different from what you might expect. The bottom line is that I hope that at some point into future you will be able to purchase a copy of your own, and that you will find it instructive, exciting, and enjoyable. At least.
Yes, it is a book about science. However… Well, here’s the thing. Over many years, people (friends, colleagues, potential agents and publishers, blog readers, etc) have been asking me when I am going to write my book. You know, the popular-level book that every academic who is interested in the public understanding of their field (as you know I am from reading this blog) is expected will write at some point. To be honest, I have given it some thought over the years, and it has been something I figured I might do at some point. In fact, several different ideas have occurred to me over the years, and I may well implement some of them at some point.
But a major thought began to enter my mind well over ten years ago. In my field, there is a rather narrow range of models for the shape of such books, usually involving about 80% of it being a series of chapters covering all the standard introductory material (some relativity, some quantum mechanics, and so forth) for the lay reader, before culminating in a chapter or two of what the researcher really wants to tell them about: some aspect of their research. This is a fine model, and it is great that people continue to write such books, and I will no doubt use that model one day, but to be honest, I don’t think there is any urgency for me to add to the canon yet another one of those books. Moreover, if you line examples of that type of book up against each other, you see that the […] Click to continue reading this post
Jacques Distler pointed out that ALICE has just published their first paper, only a little over a week after the beginning of the heavy ion phase of the LHC at CERN! Moreover, they are way ahead in energy of the previous heavy ion collision experiment (RHIC) and have verified “elliptic flow” (the main sign that the quark-gluon plasma behaves like a strongly coupled fluid, the big experimental surprise of some years back, with properties of a type that can be nicely captured using string theory models). Have a look at Jacques’ post here, and go directly to the paper of ALICE here.
Hurrah!
Please see my post from last week (“Experimental Excitement” was the apt title) about why I’m excited about all this!
Well, 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
Oh! 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
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
