Re: FTL transmission?

From: Dan McGuirk (
Date: Thu Jun 01 2000 - 16:51:56 MDT

[Non-member submission]

Brian Atkins <> writes:

> 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.

In the recipient's reference frame the event happened "long ago". In
somebody else's reference frame, it hasn't happened at all yet--it's
going to happen in the future.

> 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
> paradox.

If you're on Earth, or on Alpha Centauri, you don't see a problem.
But if you're in a spaceship heading from Alpha Centauri to Earth (I
think that's the right direction) at a relativistic speed, you'll see
the effect precede the cause.

> 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!

Well, if they see the effect soon enough before the cause, they can
send some information back to the cause, and prevent the cause from
happening. But then how did they ever see the effect?

Not to take your suggestion too literally, but: you know how movies
like Back to the Future never really make any sense? The basic idea
here is not "FTL communication is forbidden because we don't like it
and it's just an arbitrary restriction"; it's "FTL communication is
forbidden because Back-to-the-Future-like stuff would start happening,
and Back-to-the-Future-like stuff doesn't make any sense."

> 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 :-)

I agree. And that is why FTL communication must not be
possible. Otherwise, you _could_ send a signal back that could
prevent an event that has already happened.

from another post:
Brian Atkins wrote:
> Eugene Leitl wrote:
> > Maybe the people who're spouting off about FTL signalling being
> > equivalent to time travel have a reason for what they're saying. As to
> > being silly, reality is ridiculously silly. The more we depart from
> > area common sense was designed for, the more ridiculous it looks.
> Maybe they do have a reason, but neither they nor anyone here has
> been able to explain it.

Well, maybe they haven't been able to explain it in a way that will
convince you :) However, people who are actually experts on these
subjects are quite convinced. If you can show that they're wrong,
there's probably a Nobel Prize in it for you.

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

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