WESLEY CLARK ON TIME TRAVEL: I’m with him on this. And against the critics I invoke Clarke’s (not Clark’s) Law.

On the other hand, there’s still Niven’s Law to contend with.

CLARKE’S LAW: “When a distinguished but elderly scientist states that something is possible he is almost certainly right. When he states that something is impossible, he is very probably wrong.”

NIVEN’S LAW (couldn’t find a good Web reference) says that if time travel that changes the past is possible, people will keep changing history until it reaches the one stable state — one in which nobody ever invents a time machine, anywhere, ever. Thus, in a universe in which time travel can change the past, no time machine will ever be invented.

More background here.

I hid this background in the “extended entry” area because, well, you should already know this stuff!

UPDATE: By the way, Clark was really talking about Faster Than Light travel more than time travel. The original story confused these somewhat, but they have made the point a bit clearer.