From: hinson@bohr.physics.purdue.edu (Jason W. Hinson)
Newsgroups: rec.arts.startrek.tech
Subject: Re: Einstein and Warp Speed
Date: 14 Dec 92 07:02:01 GMT
Organization: Purdue University Physics Department

william@cs.arizona.edu (Dragon (William Hertling)) writes:
>hinson@bohr.physics.purdue.edu (Jason W. Hinson) writes:
>>Be glad to:  I am still working on my post that will explain stuff like
>>this, but basically let me say that it doesn't matter how you get from point
>>A to point B.  As long as you do so faster than light does, you can create
>>a causality problem.  If warp worked this way, the big E could basically
>>use warp to go back in time just by turning around after they enter warp.
>
>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.
>


OK, let me make a couple of statements without proving that they are
true by going into the whole theory of relativity.  It is a fact that
measurements of space and time are not absolute.  Furthermore, no one
person's measurements of space and time are any less valid that any one
else's.  For example, one observer may think that some event is in their
past, while another observer thinks that the same event is in their 
present.  Both are correct from their frame of reference.  Also, if 
observer A thinks that observer B's clock is running slow because of
special relativity, then observer B will think that observer A's clock
is the one that is going slow.  These are not effect that seem to take
place just because it takes light a while to get from one place to another,
but they are effects due to the fact that the speed of light will look
the same to ALL inertial observers.  Again, I don't want to get into
how these effects come about, rather I will now show how they can lead
to the idea of FTL travel causing time travel.

Consider two observers, you and I.  Now, from my point of view, you are
traveling past me at a velocity close to that of light. As you pass me,
we wave and say "hi."  Now, a few years later, I recieve a signal from
a friend who is a few light years away.  He says that he was just hit
by a bullet and he will die soon.  Now, by knowing how far away my 
friend was when he sent me this signal (which traveled the speed of 
light), I figure out that he sent this signal a couple of hours after
you past by me and we said "hi."  

Now, at some point after you pass me, you too pick up this signal.  
However, if you calculate when the signal was sent from your point of 
view, you find that it must have been sent a few hours BEFORE we you 
and I passed and said "hi."  

So when you and I passed and said "hi," in your frame of reference, my 
friend was long since dead.  However in my frame of reference he had
not yet been shot.  This IS a consequence that can occur because of
relativity.  I went through all the trouble above to show that even 
though we may disagree as to when it happened, neither one of us can 
no about it until it is in our past because the signal cannot travel
faster than the speed of light.

Now assume that the signal CAN be sent faster than the speed of light.
From my point of view, this doesn't help; after all, I still don't think
I can get the signal until after the event.  HOWEVER, from your frame of
reference, the event has already occured, and a signal can be send at
at any speed one likes, then YOU can get the message from my friend
before you ever pass by me. Remember, your frame of reverence is just as
valid as mine, so you can think, "the event has already occured, and if
the signal can get to me fast enough, then I can know about it by the time
I get to you."  

So, as you pass by me, you tell me that you have recieved a signal from my
friend saying that he was shot.  BUT, in my (no less valid) frame of
reference, the event of his shooting has NOT yet occured.  So I can 
obviously send a signal that will get to him before he is shot (because 
we have assumed faster than light speed for the signal).  So I can warn 
him of his fate.  Basically, in my point of view, you recieved a message
sent from the future, and in your point of view I sent a signal into the
past.

This seems very strange, and it usually can't occur because the speed 
of light is the same in all reference frames and cannot be exceeded.  

I hope this helps some.  Again, when I get a chance to finnish my long
post on this subject, it should give a better explanation for relativity
and FTL travel. (Don't fret, there is an interesting way around the 
problems).

-Jay