New developments in string theory

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Our understanding of the physical universe rests on two corner-stones: quantum mechanics and general relativity. Quantum mechanics describes subatomic particles using statistical notions to predict their behavior. In contrast, general relativity, Einstein's theory of gravity, provides a precise description of physics on astronomical scales using spacetime geometry. Unifying these two theories and their remarkably dissimilar views of nature in a single framework remains the outstanding fundamental question for theoretical physicists in the twenty-first century. String theory stands as the leading candidate in this quest. The distinguishing feature of string theory is that rather than regarding elementary particles as point-like, they are extended in one direction, i.e., "strings." A noteworthy property of this approach is that gravity emerges as a necessary ingredient of these theories. Yet the burden of a unified quantum theory of gravity is to consistently describe physics on all scales, from subatomic out to astronomical scales. At present, our knowledge is inadequate to answer whether string theory truly fulfills this goal and so a great deal of work remains to understand this remarkably rich and complex structure and its physical implications. Much of my research involves studying how these new ideas may be tested experimentally. For example, the stringy character of matter may be revealed at particle accelerator experiments or in cosmological observations. One incisive example would be provided by "cosmic superstrings": Microscopic strings created in the early universe and stretched to macroscopic sizes by cosmological expansion. Such strings could be seen with astronomical observations.