This piece is prompted by two articles in the April Phactum. (a) I heartily endorse Tom Napier's plea that "as skeptics . . . we should give some thought as to what evidence we would find acceptable" for extraordinary claims. (b) I don't know what point, if any, Bob Glickman is trying to make about time travel. But if he's saying that there has been no evidence for time travel, I would certainly agree with him. Put these two articles together, and you get this question: "What experiences could count as evidence for or against time travel?"

If no experiences could count either way, I'd be very tempted to say that the notion is vacuous. Some authors of a skeptical bent have claimed that there can be no evidence for time travel, since time travel is impossible on conceptual or logical grounds. For example, William D. Gray, in his Thinking Critically about New Age Ideas, says that the idea of time travel is "logically impossible." It is not merely unsupported by evidence, and not merely physically impossible because it is contrary to the laws of nature. The very concept is self-contradictory (84-85).

The "arguments" for this contention are a series of bald assertions with absolutely no justification offered. Here are three such tacit or explicit assertions: "If I went back to a point in time before I was born (already a contradiction [#1]), I could arrange it so my parents would never meet [#2]! I (an existing being) could bring it about that I never existed [#3]."

Sheer bluster substituting for reason! Rather than explain what's wrong with Gray's view -- that will become clear -- I'll establish the possibility of time travel by recounting an observable event that a thoroughly rational person could well count as evidence for time travel. (One wouldn't have to count it as evidence for time travel; but one never has to count any observation as evidence for anything in particular.) I'll invite you to imagine a series of experiences, and then ask you how you would explain what you've seen. (This story is not wholly original; but I forget where I read something like it many years ago.)

At a time T1 you see me strolling down the road at point P1 -- nothing unusual going on. Then some seconds later at time T2 a person who looks just like me pops into existence at point P6, perhaps fifty feet ahead of me, and immediately separates into two people looking like me -- one walking forward, and the other walking backward toward the original me at point P2. At time T3, there are three of me: the original at point P3 walking forward toward the second at point P5, who is walking backward toward the first, and the third at point P7 strolling in the same direction as the first, some fifty feet ahead. Then a few seconds later at time T4, the two walking toward each other (one backward) meet at point P4, merge, and disappear. So at a still later time T5, there is only one of me strolling down the road at point P8: the one that we identified as the third me at time T3.

Now I ask two questions. First, is that an observable event? I propose that it obviously is. Improbable, no doubt; but observable. (Remember: the issue is not whether time travel has ever occurred or is likely ever to occur, but whether it is strictly possible or impossible.) The second question is: how would you account for what you just saw? Of course there are indefinitely many different ways in which you might try to explain it, limited only by your imagination. So ask yourself what the simplest explanation would be. I suggest that the following is the most economical way to account for the event described: I simply walked down the road. Well, not quite "simply." For at time T4 I turned around in time and went "backward" (though still "forward" in space) to time T2, at which time I turned around again (in time) and continued "forward" with the rest of the world.

Think about it. If that short trip backward in time had actually occurred, then you would have observed exactly the event described. And that's all it means to say that the observation of a phenomenon is evidence for the occurrence of an otherwise unobservable event.

Conclusion: because straightforward evidence for time travel is possible, the notion of time travel is not self-contradictory, and time travel itself is possible.

This chart should help you visualize what's going on. It graphs the time travel proposed above. The solid line represents my movement through space and time. The x axis is spatial (to simplify the story, I imagine myself walking in a straight line: forward to the right, backward to the left), and the t axis represents the passage of time (up is "forward" in time, from earlier to later, and down is "backward", from later to earlier). If the charted event occurred, I propose that you would see exactly what I sketched here. Notice that in going "back" from time T4 to time T2, I'm not going to a time and place where I had never been before. At time T4, it is true that at time T2 I had indeed been at point P6. There would be contradiction if I claimed that I'd gone back to a time and place where I had not in fact been. But time travel does not require this, as the story shows. Nor does time travel require that I be able to do at an earlier time something that I did not in fact do when I was "there." To review: observation of the very strange episode recounted earlier would count as evidence for time travel, since the most economical way to account for that observation is by saying that things happened as on the chart. Therefore there is (logically) possible evidence for time travel, and the concept is thus a coherent, consistent one. If time travel has not happened, it's not because it's logically impossible.