# Re: starship-design: FTL idea

```Kyle R. Mcallister writes:
> Steve VanDevender wrote:
> > One observer fires a projectile that travels at 3 c, and strikes a
> > target 3 light-years away.  Consider the firing of the projectile as
> > event A and the projectile striking the target as event B.  What is the
> > spacetime location of event B relative to event A as seen by an observer
> > travelling at 0.1 c relative to the first observer?  At 0.5 c?
> > observer?  At 0.9 c?  At what velocity does a moving observer have to
> > travel to measure those events as simultaneous in his reference frame?
> >
> > Now repeat this exercise for the case where the projectile travels at
> > 1/3 c and strikes a target 1 light-year away, and determine the measured
> > spacetime locations as seen by the same three moving observers.
>
> Can you please explain why the backwards in time travel occurs?

Do the math.  It will explain more to you than I can in words.

If you go through enough contortions you can avoid talking about
"backward in time travel".  The important point is that observers won't
agree on which thing came first in time, and hence don't agree that
event A caused event B (after all, if your observations show event B
happening before event A, how could it?).  You get improbable things
like fully-formed torpedos shooting out of explosions unscathed, but the
observers who see this can construct a physically possible explanation
(wildly improbable, but physically possible; i.e. all these atoms
somehow come out of the explosion and arrange themselves into a tachyon
torpedo which then shoots away).

Causality, as we're talking about it in this context, is the notion that
there is an unambiguous time order to events that allows you to
determine that one event caused another, because that event preceded the
other in time and is linked to the other (say by a particle traveling
from the first event to the second).  In relativity, as long as nothing
exceeds the speed of light, all observers agree on the time order of
connected events and hence can trace a chain of causality through them.
If you admit faster-than-light travel, then you have to abandon
causality, because if you do then the time order of events linked by FTL
interactions cannot be the same for all observers.

> If the Causality violation has to do with the light arrival from the object,

Nope, not even close.  It's that under Lorentz transformation there is
no unambigous time order for the events connected by FTL travel, unlike
events connected by slower-than-light travel which have the same time
order under all possible Lorentz transformations.

> I have a solution you might want to consider.

Since your premise is faulty your solution is likely to be faulty too.

> Time travel in FTL might not
> occur- see my earlier mention of Stephen Hawking's time protection
> theory. Causality violations do not accur if you do not travel LOCALLY
> FTL, which is not what I'm suggesting doing. If you construct the
> spacetime metric correctly, proper time will be EXACTLY THE SAME as
> apparent time.

Huh?  You can't just construct any spacetime metric you want; you're
limited to what's available in general relativity.  Proper time is
apparent time only for the object that follows the worldline you're
talking about.  All other observers will disagree on the apparent time,
but can construct the proper time by measuring the worldline of the
object.

> Causality violations would not occur either since the
> respective light cone would be carried along with the object travelling
> FTL. Not local FTL, GLOBAL FTL. The object would actually not be moving
> FTL, but be stationary. This is real stuff, and I know it. I'm not just
> making this up.

In slower-than-light travel, there's always a frame in which an object
is (instantaneously) stationary (the qualification covers accelerating
objects).  There's no slower-than-light frame in which a
faster-than-light object can be instantaneously stationary.  That's
partly why faster-than-light travel must involve time-ordering
discrepancies.

> P.S.: Tipler's infinite rotating cylinder has to have no ends?? How is
> that possible? (sudden thought comes to my mind...)

Well, it's not, really, which is why Tipler's infinite rotating
cylinder, as well as all the other known methods of constructing what
Nick Herbert calls "closed timelike loops", don't appear to be
constructible in our universe.

```