Geoff asked:

I am interested in the idea of time travel, as described in H.G. Well's novel The Time Machine.

Is time travel consistent with the laws of logic?

How would a philosopher explain the time travel paradoxes?

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While I am a physicist, I am no expert on time-travel, but here is a link:

http://www.pbs.org/wgbh/nova/time/.

I am not sure that there are any paradoxes: If a system is accelerated, its clocks begin to slow down
against the non-accelerated system, since acceleration makes a real difference to constant inertial
motion and in this way is not "relative" anymore. If the accelerated system once more gets into
contact with the place where it started, the clocks of the accelerated system are behind the clocks at
the starting point. But they are not behind the starting time, since both clocks have advanced. There
are no paradoxes of causality, since you never get behind your starting time. Then all the famous
films on time-travel are physically nonsense — while attracting to movie-directors by their bizarre
logic. But I will check that.

Carl Sagan in the interview to which the second sublink points is not sure if travel "backwards in time"
is possible. There seem to be ideas along that line, but not proven still. Maybe the "superstring
theory" for which Jean asked will give an answer some day, but superstring theory is speculative too
up to this time. For superstring theory generally go to http://superstringtheory.com.

And there may be some ride "backwards in time" after tunnelling through wormholes — but in a
parallel universe not causally connected to ours. Then too all the films on going "backwards in time"
— paradoxes, which like the "grandfather paradox" (you kill your own grandfather; then why then are
you here?) — depend on causality, would become nonsense once more.

In my opinion the most important "pedagogical" value of the theories of modern physics, astrophysics
and biology is to force people to ask for strict mathematical argument from proven data. From a
philosophical point of view this is a major progress from the the old speculative theories of alchemy,
"vis vitalis", astrology, kabbala etc., which were self-generating systems without any "Popperian"
check of validity. Even if "time-travel" and "Grand Unified Theory" are not proven yet, they at least are
serious and falsifiable hypotheses to work on. But many people don't like this merciless insistence on
evidence and proof.

Hubertus Fremerey

This is really an interesting and extremely difficult question. Right now, as far as I know, there are a
few physicists who think that some form of time travel (to the past) is possible in some limited fashion
by using a "black hole" (which, remember, is not a hole, nor is it entirely black). I am have not looked
at this in the depth necessary to evaluate it (assuming I could understand the math). But these
physicists are in a minority, I believe.

Evaluating whether "time travel" is possible must wait on a good theory of "time"... and there is none.
On the face of it, time travel is not possible, because it would require movement "through" time, which
is the source of movement. What does that even mean? If you read Kant on time, you'll think that
whatever is "out there", time itself is something we have created to make some sense of "it": the
noumenon. But we don't know, really, that Kant is correct. If you look at fairly classical special
relativity, then time travel would be possible if there were other dimensions to the universe which
were inaccessible to us, so that whatever entities they wereaccessible to could employ ourtime
dimension as a spatial one. Right. But all that would get thosebeings would be entry into what they
would perceive (remember, this is all according to particular ideas in special relativity) as a sort of
solid block of the four dimensions we inhabit. To them, our past and future would be all there, all
fixed. So for them changing our "past" would entail nochanges whatsoever in any other part of that
block, just kind of chiseling out a small piece of a big brick. And we,as just a part of that brick, of
course could not travel in time... whatever we've done is all there already. But this, as I say, is just
oneof a multitude of theories, none of which, as far as I know, have any experimental evidence for
them which makes one preferable, really, to any other.

Given all that, what paradoxes are you talking about? There are none, because there is no theory
from which to construct any.

Steven Ravett Brown