# Re: starship-design: FTL and time travel

```Steve VanDevender wrote:
>
> 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.

You told me it was caused by increasing the magnetic permeability of the
vacuum. There's no need to go off the deep end, I wasn't intending to
use this (not until we find a way to use something like it on a larger
scale. Wouldn't a simple casimir cavity violate causality if you passed
photon through it?

>
>  > > >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.

Then I don't see why causality violation is such a big deal. A
disagreement I can live with.

Kyle Mcallister

```