Time is a huge problem. Of course, there is our social and biological sentiment about it: we all want more of it and/or are worried about its passage. As I come up against the deadline for turning in my next book, I’m feeling that strongly. But there is also a deeper physical issue, which is that we don’t know why it always moves forwards. This is true even in the context of relativity, where space and time are unified.

You would think that might give the phenomenon some flexibility.…