Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!watmath!clyde!burl!ulysses!gamma!epsilon!zeta!sabre!bellcore!decvax!decwrl!greipa!pesnta!hplabs!sri-unix!MJackson.Wbst@Xerox.ARPA 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