A Brief Update on The Problem of Time

The "Problem of Time" is a century old. There are various ways to state the problem, but a common version is: the general theory of relativity precludes any a priori notions of time in the laws of physics, while quantum mechanics appears to require a fixed notion of time for its construction and physical interpretation. For most of the usual physics endeavors this problem can be suppressed by the various approximations and idealizations which are in play. But the problem remains as a matter of principle, and it hampers progress in certain areas of high energy theory, quantum gravity, and quantum cosmology. After a hundred years, the myriad versions of the problem of time and the approaches to its resolution are too numerous to review and explain properly. I will try to do this anyway, of course. In particular, I will emphasize relatively recent approaches to resolving the problem. These approaches strongly suggest new features of quantum mechanics necessarily appear when the problem of time is resolved.