# Re: starship-design: FTL and time travel

```Kyle R. Mcallister writes:
> Actually, the speed of light is not constant in a vacuum, but can be
> sped up...its too in depth to post, but it happens in a casimir cavity.
> Ask Steve, he knows more.

Don't even try to make it look like I'll support you on this, Kyle,
because I won't.  I don't think you know enough about the Casimir effect
to explain why it changes the speed of light, and I don't think you can
justify any sort of useful FTL effect based on it.  I won't argue such a
thing, nor will I claim to know enough about the Casimir effect to try
to explain it to anyone.

> > >Unanswered question: If I travel 1600 light years in 2 years earth time,
> > >how far back in time do I travel upon return?
> >
> > It depends upon how much you accelerate away (sublight) from Earth
> > before returning.
>
> Lets say I navigate around the star system 1600 lightyears away for, say
> 5 years, and return to earth in 2 years earthtime. Is there an equation
> for this?
>
> Question: How do we know time runs backwards in FTL? Then again, how do
> we know time runs forward here...

It isn't that time runs backward in FTL, it's that observers of a
hypothetical FTL trip won't agree on the direction that it proceeded
based on their measurements of the times and locations of each end of
the trip.

```