h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

From what I've seen, most of the suggested methods of time travel wouldn't allow for travelers to go to before the time machine was invented. Worm holes, spinning black holes, the Tippler method...all would only allow travelers to go to the machine's own past. If you think about it, it makes more sense that way. Imagine that you had a time-traveling car, and you tried to drive it one day into the past. Should you return to the same spot on the Earth's surface, one day in the past? Or should you be floating in space, where the Earth happened to have been located yesterday? Or should you have shifted out of the solar system, as the whole galaxy rotated in a day's time? These questions cannot be answered if time machines can travel to before they were invented. But, if they can only travel to their own past, it's easy: you'll end up wherever the worm hole, black hole, etc., was yesterday. I envision time machines more as doors than as cars: they only serve as a gateway between two places, and must be first set up in both places before they will work.

Well, appropriate worm holes and the like might occur naturally in space, in which case we could travel back before the *invention* of time travel, albeit not before the *creation* of the worm hole.

You make a good point about location, but that just tells me that any point to point time travel thingumy is likely to be able to move you in space as well as time - and be quite tricky to 'steer'...

Actually, I think that the wormhole-based time travel device could only take you back to when one end was accelerated to near the speed of light, not back to when the wormhole was originally created. Of course, if we're lucky and a naturally-existing wormhole got one end accelerated for some reason many years ago...

And it won't be diffucult to stear. You wouldn't have any choice about where it spat you out (unless you count when you originally moved around the end of the wormhole, which may or may not be what you meant).

I think this whole discussion misses the most obvious and scientifically accepted possability. If you accept the Copenhagen Interpretation of Quantum Physics. (Which is the most widely accepted!) Under that set of rules, when someone travels back in time they cause a new universe to come into existance, as any choice does.

So there can be only one choice event in any universe.

The last point is one they choose to ignore in the interpretation.
But I believe it is the most important point. The one which invalidates the entire Copenhagen Interpretation IMO.

So there can be only one time travel event per universe. That universe then becomes unmaleable, any new choice is the bifurcation of a new universe. Oh truly the travelers experience is one of many events, but you can never go home again!