On Thu, Jun 01, 2000 at 03:51:56PM -0700, Dan McGuirk wrote:
> > 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.
> 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?
Now - couldn't it perhaps work out that if this happened, their signal
would never be able to reach the point of the cause in time to stop it?
In fact, doesn't it definitely work out this way, given that we already
have multiple experiments demonstrating signals arriving before they
depart which arrange themselves such that this effect cannot be used to
practically signal back in time *to the same point in space*?
> 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.
I have an idea.
What if a signal were sent, by the methods used in recent experiments,
faster than light - but only a little so. Let's say the distance
involved is one metre.
Normal light will take 3 x 10^-8 seconds to travel this distance.
A signal sent by one of the methods demonstrated can be arranged to
arrive after a negative time delay.
Now, for some value of the signal velocity in between, the signal will
An instantaneous signal cannot create a paradox.
Or, taking practical considerations into account, one arranged such that
it was *almost* instantaneous.
My question: could we 'get away' with using these methods to send FTL
signals for near-instantaneous communication over distances?
-- -----[ Martin J. Ling ]-----[ http://www.nodezero.org.uk ]-----
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