Since the 1980s, string theory has emerged as the leading candidate for achieving every physicist’s dream: reconciling general relativity with quantum mechanics and thereby finding a tidy explanation ...

String theory has so far failed to live up to its promise as a way to unite gravity and quantum mechanics. At the same time, it has blossomed into one of the most useful sets of tools in science.

String theory strutted onto the scene some 30 years ago as perfection itself, a promise of elegant simplicity that would solve knotty problems in fundamental physics—including the notoriously ...