Re: FTL transmission?

From: Brian Atkins (
Date: Thu Jun 01 2000 - 14:09:32 MDT

Dan McGuirk wrote:
> [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.

Well this is what I am getting at: in these "paradoxes" that I keep
hearing about, and that you describe above, all that is really going
on is that someone may have quicker knowledge (i.e. see an event happen)
before someone else who is relying on normal light (travelling at normal
lightspeed) to see the event. But the event already happened long ago.

What I mean is the difference between someone watching a moon landing
in 1969 on TV vs. someone watching it on Alpha Centauri many years later.
In either case there is a delay, and even if you had instant (FTL) TV
signals from the moon to your TV in 1969 there still is no way that I
see that you actually are going "back in time" or causing any kind of

Perhaps if you explained it in terms of TV I would understand ? :-)

So what if someone sees me react to something that for them hasn't
even been transmitted yet? They just need a faster TV connection!
There is no reversal of cause and effect though. Once an event has
happened (say a car wreck), there is no way to make it not happen.
In my uneducated opinion :-)

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