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- Newsgroups: sci.physics
- Path: sparky!uunet!psinntp!scylla!daryl
- From: daryl@oracorp.com (Daryl McCullough)
- Subject: Re: QM non-causal?
- Message-ID: <1992Dec13.134955.4201@oracorp.com>
- Organization: ORA Corporation
- Date: Sun, 13 Dec 1992 13:49:55 GMT
- Lines: 53
-
- In article <1992Dec9.011426.5296@smsc.sony.com> markc@smsc.sony.com
- (Mark Corscadden) writes:
-
- >So the many worlds version does not deserve serious debate as a scientific
- >theory. Why is that, exactly? Unfortunately I don't have the mathematical
- >background to fully appreciate Hugh Everett's book on the topic but my
- >gut instincts (which I trust but wouldn't offer to anyone else as evidence)
- >tell me that Everett is dead on.
-
- From what I gather, the major criticism of Everett's work concerns his
- derivation of the probabilistic predictions of quantum mechanics (that
- is, the interpretation of |Psi|^2 as a probability). What he shows (in
- an interesting derivation) is that it is possible to give a measure on
- entire histories of the universe, and that the set of histories in
- which relative frequencies of events don't match the quantum
- mechanical predictions have measure zero. This is an interesting
- result, but somewhat circular, since his measure on histories has no
- more justification than the quantum probabilities they are supposed to
- explain.
-
- The main virtue, in my mind, of Everett's work is that he shows that
- it is almost completely unnecessary to consider the wave function
- collapsing measurements that plague other interpretations. The major
- mathematical effect of the hypothesized wave function collapse is to
- change a superposition into a mixture; that is, to change, for
- instance, from a situation described by
-
- |Psi> = 3/5 |Psi_1> + 4/5 |Psi_2>
-
- before a measurement to one described by:
-
- |Psi> = |Psi_1> (with probability 9/25)
- |Psi> = |Psi_2> (with probability 16/25)
-
- after the measurement.
-
- (The difference between these two descriptions is that elements of
- a superposition may interfere, while elements of a mixture do not.)
-
- Everett showed that the use of mixtures can be derived from the
- assumption that there is a universal wave function describing both the
- system and the observer. Therefore, it is not necessary to postulate
- any special act of decoherence or wave function collapse to go from
- superpositions to mixtures.
-
- Everett's work is solid quantum mechanics, and is not dependent on any
- particular interpretation of quantum mechanics, so it is not necessary
- to "believe" in many-worlds or universal wave functions in order to
- appreciate it.
-
- Daryl McCullough
- ORA Corp.
- Ithaca, NY
-