At 03:36 AM 10/23/98 -0400, firstname.lastname@example.org wrote:
>Actually, AFAICS, *all* of the possibilities have this
>feature. In general, time machines which make use of
>relativity require large twists in spacetime (like
>Tipler's cylinder), and so you can't travel to a time
>when the artifact didn't yet exist.
>I am not aware of anyone proposing time machines that
>*don't* use this sort of method...
Just so. This is a good reason why we shouldn't expect to see time travellers swarming the streets. On the other hand, it's plausible that any number of *natural* time loops exist (cosmic strings, etc) and are the strict and ferociously protected property of whoever reaches them first. These might allow (non-paradoxical) passage almost back to the Big Bang. It means that early time travellers are first of all space travellers. Since interstellar travel takes a gruesome amount of energy unless you're a nano-upload, one reason we don't see the time travellers is that we're inhaling the sneaky little buggers. The other reason is that they are three feet tall and have big black wrap-around eyes.