# Re: starship-design: FTL and time travel

Isaac Kuo wrote:
>
> Kyle R. Mcallister wrote:
>
> >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.
>
> Actually, the speed of light is constant in a vacuum, its just that
> what we normally think of as vacuum isn't really completely empty.
> The casimir effect is a means by which one can "suck out" from a
> normal vacuum, making it a more perfect vacuum.  The speed of
> light within this cavity is thus made slightly faster than that
> outside, closer to its "true" speed (in a "true" vacuum).  It's
> potential to "increase" the speed of light is an imperceptibly
> small percentage, however.  It's still less than 300,000km/s.

Thats what I mean. Can you tell me the difference between negative mass
and imaginary mass? I couldn't find it in Spacetime Physics

>
> >> >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?
>
> Sorry, you just don't understand.  It depends upon how much you
> accelerate away from Earth before returning.

Ah, I see now. Is this in Spacetime Physics?

> I won't go through
> all the calculations, because you won't understand them.  Not
> without drawings of space-time diagrams, at least, which really
> can't be drawn in ASCII.  Jay Hinson's FAQ on relativity and FTL
> does a noble attempt at it (he regularly posts it on rec.arts.sf.
> startrek.tech and rec.arts.sf.science less regularly).

How can I get on one of these newsgroups?

>
> >Question: How do we know time runs backwards in FTL? Then again, how do
> >we know time runs forward here...
>
> The problems with FTL causing causality problems have nothing at all
> to do with time "running backwards" in FTL.  It only has to do with
> stuff in STL frames of reference.  A naive calculation of time
> dilation with FTL frames of reference would show time running in
> an _imaginary_ direction.  Strange?  Yes.  But it's because this
> naive use of the formula for time dilation isn't valid for FTL
> frames of reference.
>
> Like I said before, you'd have real difficulty getting an atom,
> much less an entire clock, to exist in an FTL frame of reference.