NetNews Usenet Archive 1992 #30/NN_1992_30.iso/spool/sci/physics/21130

Internet Message Format | 1992-12-14 | 1.7 KB

Path: sparky!uunet!mtnmath!paul
From: [e-mail] (Paul Budnik)
Newsgroups: sci.physics
Subject: Re: QM non-causal?
Message-ID: <[e-mail]>
Date: 14 Dec 92 17:30:33 GMT
References: <[e-mail]>
Organization: Mountain Math Software, P. O. Box 2124, Saratoga. CA 95070
Lines: 30

In article <[e-mail]>, [e-mail] (Daryl McCullough) writes:
] [e-mail]@kih (larsj.johansen) writes:
]
] >>Quantum mechanics predicts that, in the EPR
] >>experiment, if one experimenter measures spin-up for his particle,
] >>then the other experimenter will measure spin-down with probability
] >>cos^2(theta/2). Where does the assumption of collapse come in?
] >>It is definitely not necessary for experimenter A to assume that the
] >>measurement performed by experimenter B collapses the wave function
] >>in order to derive this probability.
] >
] >Well, there is a collapse in the sense that the wave function CHANGES
] >INSTANTLY (in fact, in the whole universe!) because of the first
] >measurement. Namely, the wavefunction |psi> collapses to one of the
] >eigenstates
] >
] >    |psi> = |+>|+> + |->|->  ----->    |psi> = |+>|+>
]
] I disagree. There is no reason for experimenter A to assume that B's
] measurement has this effect. Experimenter A can treat B as any other
] quantum system.

I am not sure what you are saying. There are certainly many ways to analyze
EPR experiments. However it is necessary to assume that the wave function
you use to compute probabilities changes instantaneously as the result of
a distant measurement to derive the probabilities for a joint detection
in tests of Bell's inequality. All laws of physics except this assumption
are local.

Paul Budnik