# RE: starship-design: FTL

```Steve,

> Sure they did.  If you reappear at Tau Ceti in less time than it would
> take for light to go from Earth to Tau Ceti, then observers won't
> agree on whether you disappeared from Earth and reappeared at Tau Ceti,
> or disappeared from Tau Ceti and reappeared at Earth.

Huh? So what? It doesn't matter if you get there before the light does
as long as you can't get BACK before you left. Assuming that this mythical
drive is ONLY instantaneous or nearly so, you can't get back to the
original (rest) frame prior to leaving it.

If:

t0 = start time
a = some finite interval of time close to zero

Then:

The trip requires 2a intervals and return time equals t0+2a=>0

No causality violation.

Lee

As for Timothy's question about whether someone can affect their own
past using FTL, I believe you need a conspirator to do so, but if FTL is
possible then so is affecting your own past via the conspirator.  All
the conspirator has to do is move fast enough (but still less than c) so
that in his frame your FTL message appears to go backward in time, then
send another FTL message to you in your past.  For sufficiently fast FTL
and with enough distance between you and your conspirator the relative
velocity required can be very low.

I believe Ken and Isaac have both posted accurate worked-out examples of
this before, but I'll see if I can come up with my own later.

What's important to understand is that if _anyone_ can observe FTL
motion, then you can construct these paradoxes.  It doesn't matter how
you do it, whether the object is visible for all of its trip or not, or
even if the FTL object itself believes it never exceeded c.

> I wonder if it would be possible to generate a baby universe around your
> ship, and move it FTL with you stationary in it. Is there a lower mass
> limit to baby universes? From what I've heard this would require alot of
> energy. Is this true?

What's a baby universe?  How do you make it?  How do you get in and get
out?  Yes, it probably would take a lot of energy.

```