# Re: starship-design: FTL

```Kyle R. Mcallister wrote:

>So you're saying that if I dissapear an object completely, light,
>energy, mass, everything, and then reappear it somewhere else faster
>than light can get there from the sender, it violates causality and
>travels back in time? Why?

Steve is confused.  Time travel doesn't occur--necessarily--in this
example.  He has a definition of causality which isn't terribly
useful, but I won't get into that.

Basically, the problem comes when you travel back.  If you decide
to thrust a little bit at sublight speeds before going back,
you can meet yourself before you left!  Now _that_ is a causality
violation (you can have an effect on your own past).

In order to understand why, you have to at _least_ understand
special relativity.

>If I travel 1600 light years in 2 years (FTL) of earths reference time,
>how much time passes aboard the ship? How far back in time do I go? (in
>earth's reference time)?

See that's the thing.  Earth's reference frame is no more or less valid
than any other reference frame.  Thus, what you can do after you
travel 1600 light years away is thrust your ship (sublight) away
a bit, and then use the same drive to return to Earth.  Due to the
way relativity works, you will return to Earth _before_ you left.

It simply doesn't matter how you specifically got there and what
happenned to you in the meantime.  The fact that the endpoints
are outside each other's light cones is enough, along with
general relativity in a flat or convex space (special relativity
is a special case of general relativity on a flat space).

>If there was a way to generate an alcubierre field around my ship, and
>send it FTL, would this eleiminate the need for 10^33Mgalaxy of exotic
>matter?

Huh?  The exotic matter is what's used to _create_ the Alcubierre
metric in the first place.
--
_____     Isaac Kuo kuo@bit.csc.lsu.edu http://www.csc.lsu.edu/~kuo
__|_)o(_|__
/___________\ "Mari-san...  Yokatta...
\=\)-----(/=/  ...Yokatta go-buji de..." - Karigari Hiroshi

```