In our experience, time is not symmetric. From cradle to grave things happen that cannot be undone. We remember the past, predict the future. These arrows, while not always—or even now—deemed suitable for scientific investigation, have been recognized since the dawn of thought. Technology and statistical mechanics give us a precise characterization of the thermodynamic arrow. That's what the previous chapter was about. But the biological arrow (memory, etc.) is elusive. Then we come to arrows that only recently have been recognized. The greatest of these is the fact that the universe is expanding, not contracting. This is the cosmological arrow. Related, perhaps a consequence, is the radiative arrow. Roughly, this is the fact that one uses outgoing wave boundary conditions for electromagnetic radiation, that retarded Green's functions should be used for ordinary calculations, that radiation reaction has a certain sign, that more radiation escapes to the cosmos than comes in. Yet more recently, the phenomenon of CP violation was discovered in the decay of K mesons. As a consequence of CPT invariance, and some say by independent deduction, there is violation of T, time reversal invariance. This CP arrow could be called the strange arrow of time, not only because it was discovered by means of ‘strange’ particles, but because its rationale and consequences remain obscure. There is another phenomenon often associated with an arrow, the change in a quantum system resulting from a measurement.