(A relatively technical post follows.)

So imagine the following:

You’re walking along the street, minding your own business, and somebody walks up to you and tries to sell you a string theory. So you stop and examine the goods, since you’re in the market for string theories, on the lookout for any that might be novel, useful, bright, or shiny, etc. You never know when one or other property might come in useful.

Question: How do you know that it is a string theory? Let me be sure to point out that it comes with a lot of the defining path integral done for you. In other words, you don’t have to do the integral over string world sheet metrics and world sheet fields. This was done in the factory for you. What you have access to are parameters such as the coefficients of the operators in the theory, and you can also adjust the value of the string coupling.

So a lot of the stuff you would recognize as a string theory in your typical string theorist’s notebook have been cleaned up. They’ve been integrated over. The observable physics actually never cared about them (the technical details of summing over metrics – slicing up the moduli space of inequivalent metrics properly at each genus, etc etc…. all done), assuming you’ve done the integrals properly. The factory did it all for you.

So what criteria do you use to decide that it is a string theory at all? Actually, this is not an idle question. Think about the issue in the context of trying to understand some phenomenon or phenomena in Nature. How would you know you had a string theory description underlying the physics?

Well, what we might start doing at this point is start listing various things we’ve learned about strings that we think are rather spiffy about the theory that make them different from what we’ve seen before. I’m sure you have your favourites.

A word or more of caution though. From my new paper:

With a few notable (and highly instructive) examples in D ≤ 2, string theory still lacks a satisfactory and well–understood non–perturbative definition. It is fair to say that while strings have marvellous properties that may prove a great boon for studying Nature, we have not been learning about these properties systematically, but instead by following the theory into regimes which have become accessible to us by various techniques. As a result, it is not clear what the big picture is —certainly not clear is the complete list of phenomena we should expect from string theory.

My point? Do we really know enough about what string thory is to decide when to rule out something as being stringy or not. How do you know when to hand over the cash to the person on the street trying to sell you one?

Ok, so you’re thinking: What’s he getting at? What’s in this paper?

Well, one of the things of which we are all very proud -that we show off at theorist parties to all our theorist friends from other fields, you’ll admit- are branes, right? We’ve spent a huge amount of effort on them in the last decade or so expecially, and Click to continue reading this post →