[ExI] Pesky Neutrinos

[Adrian Tymes]

2011/10/31 john clark <jonkc at bellsouth.net>

> Yeah but..., even if that is true, with a less intense beam you could
> still SOMETIMES communicate faster than light, SOMETIMES you could still
> receive the answer before you asked the question, SOMETIMES you could still
> produce a paradox.
>

I still don't get why FTL necessarily implies time travel.

Say you have 2 points, A and B.  Both start off at time T, according to
their local clocks (previously synchronized to some third source - let's
assume it is equidistant from A and B, so any light speed lag in the signal
would have affected both synchronizations equally).

Someone at A asks a question at time T.  The question contains a
representation of T, so others can know when - according to their own
clocks - it was asked.

The question is transmitted by some means - FTL or not - and arrives at
point B at time T+X as measured by B's clock, X being the transit time.

Someone at point B transmits a reply, by some means - FTL or not, maybe or
maybe not the same means as the question was sent.  It arrives at point A
at time T+X+Y as measured by A's clock.  The answer also contains a
representation of T+X.

Now, X and Y might be measured differently at A and B (though I suspect
they would not - it is the clocks of the
stationary-with-respect-to-each-other points that we are measuring by, so
relativity does not dilate time relative to each other).  Assuming FTL
communication, it is certainly possible for the answer to arrive before A
observes B receiving the question, and for B to receive and answer the
question before observing A sending it.  However, neither X nor Y can
actually be negative, and receiving a signal before you observe its origin
(which, in truth, is nothing more than receiving another signal - in this
case, light, traveling slower than FTL by definition) is not actually time
travel.
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