(Theory colloquium)
A sharply-defined object in a theory of quantum gravity is plausibly the S-matrix. Its high-energy behavior apparently probes the realm of greatest tension between quantum mechanics and classical gravity, where a paradox has been encountered, and where we might expect the emergence of new principles. I will summarize some aspects of what proposed theories of quantum gravity like string theory tell us, and what they might be missing. Basic features of gravity give important clues about novel behavior compared to conventional local field theory. The missing elements could be important keys to unlock the mysteries of quantum gravity.
