For more than 20 years, string—or superstring—theory has been heralded as the path to a unified theory of all interactions, including gravity. The subject has occupied the attention of a large fraction of the theoretical high energy physics community. By its nature, it is an ambitious program, and while it has had some impressive successes, there are, as yet, many puzzles to be resolved if string theory is to be shown to be the theory of the world around us.
The two largest puzzles have been around from the beginning. First, the theory seems to describe not just universes like our own, but many, many others: universes with more than three spatial dimensions (at least as many as ten), with varying numbers of particles and fields, and with different interaction strengths. The second puzzle is related to Einstein’s famed cosmological constant, . From a modern perspective, this is...
