Re: FTL transmission?

From: Dan McGuirk (
Date: Thu Jun 01 2000 - 02:30:40 MDT

[Non-member submission]

Brian Atkins <> writes:

> Anyway, what you are describing is not the kind of paradox people keep
> going on about. It seems like whenever talk of FTL communications comes
> up, people start spouting off about the future, the past, paradoxes, etc.
> Just seems silly to me.

The reason is that when you have FTL communication, you can reverse
the order of cause and effect.

An "event" in special relativity specifies a position and a time. The
interval between two events A and B is called "timelike" if you can
get from A to B going slower than the speed of light; "spacelike" if
you would have to go faster than the speed of light; and "lightlike"
if you would have to go at exactly the speed of light.

For timelike and lightlike intervals, absolute ordering is preserved in all
reference frames; that is, if A happens before B in one frame, A will
happen before B in all frames.

For spacelike intervals, that is not the case. One person may see A
happening before B; another person zipping by in a spaceship going
another direction might see B happen before A.

Now if you can travel from A to B, or even transmit information from A
to B, and A and B are spacelike-separated, you have a problem. Send a
signal from A to B, and some observers will see people at B receiving
the signal (and acting on it) _before_ it was sent.

The book _Spacetime Physics_ by Taylor and Wheeler is probably a
pretty good introduction to this stuff.

I think the experiment is just some kind of phase vs. group velocity
confusion. In a wave packet, individual Fourier components can travel
faster than c (phase velocities), but they don't transmit any information.
The whole packet has to travel at or below c (group velocity).

Dan McGuirk
Hang on, but I know that you're gonna lose the fight.

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