# RE: starship-design: One more try...

```Ken,

> 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
> artificial 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.

Awww, Gee, all I really wanted for Christmas was B...

> 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.

C is a matter of slight of hand, if you can move back to the same point in
your space line and backwards to an earlier point in your time line then it
doesn't really matter how many permutations in time and space it took to
get there. Only the end result will count.

> 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.

No it isn't obvious, you've defined TWO cases A and B, C is a subset and
D is simply B- NOT the other way around. To say that B isn't possible
because D isn't is the same thing as saying the square root of one isn't
possible because the square root of minus one isn't possible.

I'm not necessarily arguing with your conclusions, just your method of
getting there.

Lee

```