String theory and all that

I was asked to give a talk to the physics students here about string theory. Now, I'm not a string theorist, but am probably the closest thing on campus with the possible exception of a guy in the math department.

I emphasized that quantum gravity is perhaps the most conceptually interesting area in all of physics (perhaps all of science). I think I am not exaggerating here, since questions such as Why is there one time direction and three spatial dimensions? Can our universe be multiply-connected on short distances? or What is the endpoint of black hole evaporation? all involve deep and fundamental ideas.

But I also told them, half joking, that I didn't want to work on quantum gravity (at least not all the time) until someone builds a desktop accelerator that can collide particles at Planck energies or at least make small black holes. What I meant by this comment is that physics generally cannot advance by theoretical ideas or mathematics alone. There is no evidence that a single, unique mathematical structure describes our universe. Consequently, we will likely be confronted with more than one theoretical possibility, and only experimental tests can distinguish between them.

We are barely on the threshold of detailed tests of classical general relativity (e.g., using large interferomenters such as LIGO to detect gravity waves). There are no experiments on the drawing board which will test whether these waves are indeed quantized into individual gravitons, and the current generation of particle accelerators are 16 orders of magnitude away from testing the Planck energy. So, I think quantum gravity will not, in a strict sense, be a scientific endeavor for some years to come.