[ExI] Faster than light??

[Adrian Tymes]

On Sat, Sep 24, 2011 at 2:28 AM, Eugen Leitl <eugen at leitl.org> wrote:
> I'd call retrograde signalling in time (which can be
> indefinite via a chain of routers) and causality violations
> pretty Earth-shattering.

I do not see why FTL automatically means time travel.
Aside from the paper scerir posted, let's take a simple
example:

Points A and B are 1 kilometer apart.  They have a
mechanism for sending, back and forth, signals in 1
microsecond - a bit over 3 times the speed of light.

Point A sends a FTL signal to point B.

Point B then returns a FTL signal to point A.

>From point A's point of view, the second signal was
received 2 microseconds after the first was sent.

Perhaps this could allow point B to act on something
before the physical consequences could ordinarily
reach it.  (Say, if point A were at the epicenter of an
earthquake, point B could be prepared for it.  Indeed,
because earthquakes propagate much more slowly
than light over fiber optics, this effect - without FTL -
is being investigated as part of an earthquake warning
system in California.)

The only "time travel" comes about if one believes
that time and causality propagate at the speed of
light.  If, instead, one believes that all points in the
universe have their own time, which may pass at
different rates (thanks to relativity) but each point's
rate is consistent with itself, then there is no time
travel.

Though, even if this did allow for information to be
sent to the past, it appears that you'd need
specialized equipment for those relays.  Not all of
said equipment has been constructed yet, and what
there is, is not set up in a relay.  Therefore, no
information can travel from the future into today.
The earliest point in history that information could
time travel to, is when said equipment first becomes
operational.  There would likely also be bandwidth
limitations; at any point where those are used up,
that point in history becomes inaccessible to further
future information, and likely all points before it too.