Recently I went to Michigan to speak at a conference devoted totime reversal.
Time reversal? Yes, time reversal. Wow! you say. Reversing the flow of time?
Perhaps no other term in modern physics inspires as much fascination and awe in the general public. Consider the enormous popularity of such recent movies as Back to the Future I, II, and III, The Terminator, and Peggy Sue Got Married. But those people curious enough to trudge to a library and look up time reversal in an advanced physics text--for instance, chapter 4 in the classic by J. J. Sakurai, which I studied as a student--are bound to be disappointed. Physicists do not know how to arrange for you to go back to your high school days and see teen romance with the wisdom of an adult, as Kathleen Turner did. Nor can they send you or Michael J. Fox back in time to help your father beat out his competitor for the affection of your mother. Sorry to disappoint you, but we physicists haven’t the foggiest idea how to reverse the flow of time.
Physicists are focusing on a much more modest question, a question you have to ask before you can wonder whether or not you can go back in time. They are trying their damnedest to find out whether the fundamental laws of physics know about the arrow of time.
We are all aware of the psychological arrow of time, a subjective feeling that time flows mercilessly from the past to the future. Physicists also speak of a thermodynamic arrow of time along which physical systems invariably become ever more disordered. Perhaps the physiological arrow of time, measured by the aging of our bodies, is just a manifestation of this thermodynamic arrow of time. The expansion of the universe provides yet another arrow of time. The big question is whether, and how, these arrows are related to one another. In particular, we would all like to know how our psychological arrow of time comes about, and then, of course, whether we can reverse this arrow.
