# Re: starship-design: FTL idea

```Kyle R. Mcallister writes:
> Timothy van der Linden wrote:
> >
> > How nice it would have been if you'd not left out the calculations for this
> > transform... Now I still don't see why it goes backwards in time.
>
> It doesn't really go backwards in time. Just in some reference points.
> Nothing goes backward in time. Even if you think causality violations
> prohibit FTL, no problem. FTL can be done without causality violations
> in EVERY REFERENCE POINT. See my follow up message.

This is just plain wrong.  Since I can give you an easy reference to a
very nicely worked-out example, and I don't have a lot of time at the
moment, I will refer those of you who still aren't getting this to the
example in Chapter L of _Spacetime Physics_ (if you have a copy, you'll
know it when you find it -- it involves the Enterprise watching the
Klingons firing a hypothetical FTL missile at a planet).

If you have a "photon torpedo" that travels at c or less, then all
observers will agree that if the Enterprise fires it at a Klingon ship,
it left the Enterprise and struck the Klingon ship some time later in
the observer's future, causing the Klingon ship to blow up.

If you have a "tachyon torpedo" that travels faster than c, then
observers below a certain velocity with respect to the Enterprise will
see it leave the Enterprise's launcher and travel FTL towards the
Klingon ship, striking it and detonating.  Other observers above that
certain velocity with respect to the Enterprise will see a detonation
from which the tachyon torpedo emerges, then travels FTL straight back
to the launcher on the Enterprise and neatly berths itself.  At the
critical velocity (determined by the FTL speed of the tachyon torpedo)
the events of the Klingon ship exploding and the torpedo
leaving/berthing in the Enterprise's launcher will be _simultaneous_.

Now, the question here is which was cause and which was effect.  In the
photon torpedo case, all observers can agree on the cause and the
effect; the Enterprise firing the torpedo is the cause, and the torpedo
striking the Klingon ship and detonating is the effect.  In the tachyon
torpedo case, some observers will say that the Enterprise caused the
destruction of the Klingon ship, and others can equally rightly say that
the Klingon ship exploded spontaneously and somehow threw a perfectly
functional tachyon torpedo back to the Enterprise's launcher, and an
obserer moving at just the right speed will not be able to determine any
causal relationship at all.  FTL conflicts with unambiguous causality.

My example is a bit backwards from the _Spacetime Physics_ example.
However, the _Spacetime Physics_ example is quite neatly diagrammed and
worked out mathematically, better than I could hope to do in an email
you insist again that there is no conflict betwen FTL travel and
causality.

```