It took string theory 23 controversial years and two revolutions to get where it is today, and the question still remains: where exactly is it? The theory has always offered the promise of a quantum theory of gravity, uniting quantum mechanics with Einstein’s general relativity, but does so by postulating 10-dimensional universes—perhaps an infinite number of them—while providing no experimental predictions and no method of distinguishing one of those universes as the one in which we happen to be living. String theorists will argue that they’ve made great progress understanding the nuances and boundaries of the theory. Critics have questioned how "progress" can be defined if string theorists are no closer than ever in making predictions that can be tested in experiment.
     Despite the continuing controversy, string theory has come to dominate the theoretical end of high-energy physics. Among those physicists who have been dominant players in the field since the earliest days is Ashoke Sen of the Harish-Chandra Research Institute in Allahabad, India. Sen, who was recently......

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