> This is a side issue from the FTL "teleportation" story, but does anybody
> know if travelling backwards in time is theoretically excluded by General
> Relativity or any other physical theory.
CTCs (closed timelike curves) are possible in general relativity, but
it is not clear if they can occur or be created in physically
reasonable worlds (the chronology protection conjecture says they are
impossible, but it is just a conjecture). Quantum mechanics seems to
be time reversible, on the quantum level there is no real distinction
between past and future (as far as we know). One interesting
possibility is that quantum mechanics rules out arbitrary time travel
because (gross oversimplification) virtual particles could follow the
CTCs around and around, causing a buildup of vacuum energy that
destoys the time machine (see the papers on wormhole physics by
Visser). On the other hand, Novikov has shown that quantum mechanics
may allow time travel as long as it remains consistent: no paradoxes
are possible because actions that would cause them are prevented by
quantum effects, they simply do not happen.
The conclusion seems to be that it may be possible, but likely very
constrained. But the universe is a weird place, and weird things may
be possible.
> If reverse time-travel is possible, then I guess (should we fail to
> achieve biological immortality in our lifetimes) we have a chance of being
> "rescued" by future humans once time travel is invented. In this way,
> anybody who has ever lived is potentially immortal if they can be snatched
> from the past.
This is untestable, so we cannot rely on it.
-- ----------------------------------------------------------------------- Anders Sandberg Towards Ascension! asa@nada.kth.se http://www.nada.kth.se/~asa/ GCS/M/S/O d++ -p+ c++++ !l u+ e++ m++ s+/+ n--- h+/* f+ g+ w++ t+ r+ !y