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From: MJackson.Wbst@Xerox.ARPA
Newsgroups: net.physics
Subject: Re: Faster than light.
Message-ID: <325@sri-arpa.ARPA>
Date: Thu, 27-Jun-85 09:40:17 EDT
Article-I.D.: sri-arpa.325
Posted: Thu Jun 27 09:40:17 1985
Date-Received: Mon, 1-Jul-85 07:07:20 EDT
Lines: 35

[To: Mayank Prakash ]

No, you're *still* wrong about faster-than-light information transfer
and uncertainty.

Suppose one had the situation you describe, Mary trying to send signals
by "apply[ing] an external influence to the system for a certain
duration".  

I assert that in order for Mary's "influence" to raise the probability
of a state transition *above what it would be in the absence of the
influence* (i.e. above random) either:
	- the influence must be applied over the entire system, or
	- the duration of the influence must be > L/c, where L is
	  the distance to the other side of the system.

In neither case is superluminal communication at issue.

If this were not the case, an external observer would conclude
*unambiguously* that Mary's actions were *causing* an effect (albeit
perhaps a probabilistic effect) outside of Mary's light cone.  Then some
relatively moving observer would see the order of these events (cause
and effect) reversed, a contradiction which would identify a preferred
frame contrary to relativity.

Note that in the EPR situation there is a symmetry between
detection-at-A and detection-at-B which is missing in your John-and-Mary
case.  Point A may be closer to the source (so the stationary observer
says "detection at A 'causes' the particle approaching B to change
state", but since A and B are outside each other's light cones there are
moving observers who can equally say that the particle at B arrived
first, and conclude the reverse.  It is the *lack* of causal influence,
not a 2% uncertainty in it, which prevents a contradiction.

Mark