stuart@rochester.UUCP (05/29/84)
From: Stuart Friedberg <stuart>
References: <3894@utzoo.UUCP>, <937@dciem.UUCP>
I initially responded directly to dciem!ntt, and then decided this was
of general interest. (assuming anyone is interested at all)
While I agree that FTL looks like time-travel, I don't think it looks
like travel into the past. Instead it always looks like travel to and
from the future relative to any observer. Thus there is not a chance
of creating paradoxes by combining FTL and remote control, etc. At least
not the way it's normally imagined (remember, this is still fiction)
to work. You don't have to use any complicated arguments to make a
pretty strong case that while the relative future may be involved, the
relative past never is.
Imagine two star systems, 10 light years apart, and that people have
FTL (assume instantaneous) travel. Observers in system A note
"current" events in system A and ten year old events in system B.
Analogously for observers in system B. Now someone blips from A to B.
As far as the "A" viewpoint goes, the traveller has gone into the
future (not the past) since the traveller is getting "on-line" what
will take ten years to affect those still in system A. As far as the
"B" viewpoint goes, the traveller has come from the future (not the
past) since he can report events which will not impact B for ten
years.
Now, what happens when the traveller "blips" back from B to A? Well,
the "A" viewpoint is that he went into the future and came back from
the future. The "B" viewpoint is that he came from the future and
went back the future. At no point does any modification of the
(relative) past occur. Perhaps paradoxes can arise, but they aren't
of the "go back and kill your grandfather nature". I would have to
have a more detailed "explanation" of FTL and the way paradoxes can
arise before I would agree, though.
P.s.: From the standpoint of the traveler, he is *always* going to the
future. Calendar dates at *each* of his stopping points are monotonically
increasing and, of course, his local "biological" time always advances!
(substitute she for he if you prefer)
Stu Friedberg
{seismo, allegra}!rochester!stuart
stuart@rochesterguy@rlgvax.UUCP (Guy Harris) (06/01/84)
I'm not sure that FTL travel doesn't imply "time travel into the past".
In your example, star systems A and B weren't moving relative to one
another, so their reference frames differed only by choice of the origin
of the space and time coordinate systems. However, if I remember my
relativity correctly, if one has someone leaving system A at time TA and
arriving at system B at time TB (in, say, system A's reference frame), a
reference frame can be found in which time T'A is the same or earlier than
time T'B. This reference frame would be moving relative to the frames of
systems A and B. The space and time separation between the departure from
A and the arrival at B is "spacelike", which means that there exists a
reference frame in which it is purely a spatial separation (i.e., the
trip *is* instantaneous!) and also that the difference in times can be
positive *or* negative (i.e., the guy can be observed arriving at B earlier
than they leave A). It's geometrically equivalent to the spatial distance
along a particular coordinate between two events with a "timelike" separation
(i.e., connected by something moving slower than light; there exists a
reference frame in which the separation is purely a temporal separation)
being positive in one frame and negative in another - just rotate the
frame of reference. Ask a physicist about this one; I don't have my
old relativity text at hand so I can't check it out easily.
Guy Harris
{seismo,ihnp4,allegra}!rlgvax!guy