From: [d--m--y] at [cats.ucsc.edu] (David Empey)
Newsgroups: rec.games.frp.advocacy
Subject: Realistic FTL?
Date: 4 Oct 1993 19:23:57 GMT

I haven't seen any very clear explanation of why FTL implies time travel
on this thread yet.  I'm not a physicist, nor do I play one on TV, but
I _think_ I finally understand this point, after reading Forward's
_Timemaster_.  Real physicists are invited to correct me if necessary,
but here's how I think it works, really short and sweet.

Lets imagine a spaceship travelling STL, but close to c, away from
Earth.  Due to 'time dilation' the observers on Earth say time has
'slowed down' for the travellers; say when 2 years have passed on
earth since launch, only one year has passed on the spaceship.
  
For the travellers, the situation is exactly reversed; _they_ see 
_Earth_ as having 'slowed' time, in exactly the same ratio.  

Now suppose the Earthlings fire up their ansible and send a message to 
the ship after two years have passed.  This sends messages 
simultaneously, with respect to Earth.  Due to time slowing down, the 
ship receives the message after only one ship-year has passed, since 
one ship-year equals two Earth-years, from Earth's point of view.
  
Now the ship decides to reply, and fires up _their_ ansible, which 
sends messages simultaneously from the ship's point of view.  
According to the ship, one ship-year has passed, but only 1/2 Earth-year 
has passed, from the ships point of view.  So their simultaneous reply 
gets to Earth after only six months have passed on Earth.  The reply
is received 18 months before the message was sent.  _That_ is a true
violation of causality, and will appear to be one to _all_ observers.

How about it?  Am I close?  Am I clear?

-Dave