Category Archives: science

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 →

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 →