This is a troubling question for me. Certainly there exists in our universe sequences of events which may or may not be undone easily. For example, I can walk 31 steps forward, then walk 31 steps backward and return to where I was. However, I can not easily unmix milk from my coffee after I have poured it in, though I shouldn't think it is impossible. This isn't really what we mean when we think of the concept of "time". A simple sequence of events is something we can do or undo, but no one here thinks they can return to yesterday, or last year. When we think of time, we think of us somehow floating along in a bubble that we call the present, and behind us is the past and before us the future. We can not look forward into the future, but we can look behind us into the past and around us inside the bubble at the present. I personally suspect that this is really an illusion, and that the concepts of "past' and "future" have no physical meaning, but trying to prove it in any concrete way I think is beyond my scope for now. Any thoughts on this topic?