Three Merry Roads to T-violation

This paper is a tour of how the laws of nature can distinguish between the past and the future, or be T -violating. I argue that, in terms of basic analytic arguments, there are really just three approaches currently being explored. I show how each is characterized by a symmetry principle, which provides a template for detecting T -violating laws even without knowing the laws of physics themselves. Each approach is illustrated with an example, and the prospects of each is considered in extensions of particle physics beyond the standard model.