starship-design: One more try...

[wharton@physics.ucla.edu (Ken Wharton)]

Isaac writes:

>>You're right... according to what I said anyway.  But there's a subtle
>>sign issue here, and I'm pretty sure I am right that FTL is equivalent to
>>staying in the same place and travelling backwards in time.
>
>What is this nonsense?  No, FTL is not equivalent to staying in
>place and travelling backwards in time.

Yeah, I was getting pretty nonsensical there.  Too many references to time, 
space, speed, and other imprecise, frame-dependant terms that totally 
obscured my underlying point.  Here it goes from scratch.

There are four classes of journeys one can even consider making.  

A) Within your forward time cone (All STL and light-speed falls in here)
B) Out of your forward light cone, but still (from Earth's perspective) 
forward in time.
C) With your backwards time cone (Staying in the same place but travelling 
backwards in time.
D) Our of your backwards light cone, but still (from Earth's perspective) 
backwards in time.

The first point is that there is no difference between B and D.  They are 
both FTL and they are both identical.  The only distinction is made from 
Earth's perspective, so it is not a real distinction. For those of you who 
think that B-type FTL journeys are okay and D-type are not, that is an 
artifical separation that can only be true if there is a preferred 
reference frame.  You can't outlaw D without outlawing B, and therefore 
outlawing all FTL.

My point about FTL (B and D) being equivalent to travelling backwards in 
time in the same place (C) was obviously not literally correct.  What I was 
trying to say was that they are equivalent in the sense that if it is 
possible to move out of your forward light cone with FTL (i.e. if B and D 
are possible) then C becomes possible as well.  

This is obvious; A D-type trip away from Earth, followed by a D-type trip 
back to Earth, will add up to a C-type trip.  Other examples include Adding 
a B-type and a D-type trip to get a C-type.  There are lots of ways to go 
backwards in time once FTL (B and D) is allowed.

The only way to avoid C-type trips is to demand that D-type trips are 
impossible.  But Because D-type trips are equivalent to B-, if time travel 
is forbidden, so is FTL.

That was all I was trying to say.  Sorry for trying to do it in unspecific 
terms; I confused even myself.

Ken