String Theory and its Discontents

Burton Richter Takes a Shot at string theory in the October Physics Today.

To me, some of what passes for the most advanced theory in particle physics these days is not really science. When I found myself on a panel recently with three distinguished theorists, I could not resist the opportunity to discuss what I see as major problems in the philosophy behind theory, which seems to have gone off into a kind of metaphysical wonderland. Simply put, much of what currently passes as the most advanced theory looks to be more theological speculation, the development of models with no testable consequences, than it is the development of practical knowledge, the development of models with testable and falsifiable consequences (Karl Popper's definition of science). You don't need to be a practicing theorist to discuss what physics means, what it has been doing, and what it should be doing. ...

The last big advance in model building came a bit more than 30 years ago with the birth of the standard model. From the very beginning it, like all its predecessors, was an approximation that was expected to be superseded by a better one that would encompass new phenomena beyond the standard model's energy range of validity. Experiment has found things that are not accounted for in it—neutrino masses and mixing and dark matter, for example. However, the back-and-forth between experiment and theory that led to the standard model ended around 1980. Although many new directions were hypothesized, none turned out to have predicted consequences in the region accessible to experiments. That brings us to where we are today, looking for something new and playing with what appear to me to be empty concepts like naturalness, the anthropic principle, and the landscape.

Theory today
I have asked many theorists to define naturalness and received many variations on a central theme that I would put as follows: A constant that is smaller than it ought to be must be kept there by some sort of symmetry. If, for example, the Higgs mass is quadratically divergent, invent supersymmetry to make it only logarithmically divergent and to keep it small. The price of this invention is 124 new constants, which I always thought was too high a price to pay. Progress in physics almost always is made by simplification. In this case a conceptual nicety was accompanied by an explosion in arbitrary parameters. However, the conceptual nicety, matching every fermion with a boson to cancel troublesome divergences in the theory, was attractive to many. Experiment has forced the expected value of the mass of the lightest supersymmetric particle ever higher. The Large Hadron Collider at CERN will start taking data in 2008 and we will know in a couple of years if there is anything supersymmetric there. If nothing is found, the "natural" theory of supersymmetry will be gone.

(Via a comment in NEW).

Burton Richter is a Nobel Prize winner in physics and former director of SLAC.