Subject: Re: Einstein and Warp Speed
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Organization: Keristor Systems
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Date: Sun, 13 Dec 1992 12:00:36 +0000

In article <27937@optima.cs.arizona.edu> william@cs.arizona.edu writes:

>I hate to sound like a newbie, since I've been reading this group for quite
>some time, and discussed a few issues, but I must have had to 'catchup'
>on all the days when this was discussed previously. Can someone please
>explain how being able to travel faster than light implies time travel.
>I just don't see it. Yes, I understand that because of the speed of
>light, if I looked at something 4 light years away, I'm looking at it as
>it was 4 years ago. But that doesn't mean that it is 4 years earlier
>there. It is still the same "time" there as it is everywhere. (Time
>in quotes since it is such an iffy thing.) If the big E leaves
>point A for point B, and travels in a straight line at warp 3 for
>1 year, it has travels 39 light years. If it then turns around, and
>heads back for 1 year, it will have traveled back to it's original
>starting point, and have gotten there 2 years after it left, and not
>a second sooner. On the other hand, if it simply moves away from point A
>at warp 3 for 1 year, the big E can observe what point A looked like
>39 years earlier, but that doesn't change the actual time.
>
>So how then does FTL speed travel imply time travel, and thus causality
>problems? I just don't see it.

William, you're not the only one!  People have been trying to convince
me of this for *months*, and I still don't get it.

One poster on this thread, though, did say that "in some frame of
reference, it will appear that you're going backwards in time".  Now
*that* is what you've said, and I can agree with it.

For instance, you have two stars and one spaceship (I'm going to use a
1 dimensional model here - just assume all the stars and the spaceship
are in a straight line, OK?).

The stars are A and B (the observer).  s is the spaceship (it's a little
s because spaceships are small compared to stars!).  a is the light fom
the spaceship at star A, and 1-5 are the light during the journey.
We'll see how the light (signals) reach B.  Assume that the spaceship is
travelling at twice the speed of light.

Time            A                       B
  0             s                       .
  1               a s                   .
  2                 a 1 s               .
  3                   a 1 2 s           .
  4                     a 1 2 3 s       .
  5                       a 1 2 3 4 s   .
  6                         a 1 2 3 4 5 s
  7                           a 1 2 3 4 5
  8                             a 1 2 3 4
  9                               a 1 2 3
 10                                 a 1 2
 11                                   a 1
 12                                     a

So to the observer at B, who's limited to light-speed communications,
the spaceship arrives, and then B sees it going away back to A and
getting 'younger'.  But that is *only* because he's using a slow
information medium.  Like using sound to track a super-sonic jet...

Well, that's how it seems to me, anyway - perhaps someone else's
diagrams will eventually explain to me what they're talking about with
causality violations, but until then I maintain (as you seem to) that
there will only be *apparent* violations, which don't matter (any more
than it matters that the light I see from a star may have come from
something that no longer exists).

Chris

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