Counterfactual definiteness

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Counterfactual definiteness (CFD) is a property of some interpretations of quantum mechanics. It refers to the ability to speak meaningfully about the definiteness of the results of measurements, even if they were not performed.[1]

For example, by the Heisenberg uncertainty principle, one cannot simultaneously know the position and momentum of a particle. Suppose one measures the position: this act destroys any information about the momentum. The question then becomes, is it possible to talk about the measurement one would have received if one did measure the momentum instead of the position? In other words, had one conducted a different experiment, is there a single alternate time line that would have resulted from it?

Counterfactual definiteness is a basic assumption, which, together with locality, leads to Bell inequalities. In their derivation it is explicitly assumed that every possible measurement — even if not performed — would have yielded a single definite result, and this result can be treated as a fixed, but unknown, number. Bell's Theorem actually proves that every quantum theory must either violate the locality or CFD.[2] [3] CFD is not a property of the Copenhagen interpretation of quantum mechanics, as the complementarity principle is directly excluding it. However, it is always present in the hidden variables interpretations. It also is not a property of the many worlds interpretation with its multiplicity of results in different worlds or elements of the universal wavefunction It is not a property of some other decoherent interpretations such as consistent histories. Abandoning CFD allows one to claim that violation of Bell's inequalities does not necessarily imply a violation of the locality principle.

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[edit] References

  1. ^ Henry P Stapp S-matrix interpretation of quantum-theory Physical Review D Vol 3 #6 1303 (1971)
  2. ^ David Z Albert, Bohm's Alternative to Quantum Mechanics Scientific American (May 1994)
  3. ^ John G. Cramer The transactional interpretation of quantum mechanics Reviews of Modern Physics Vol 58, #3 pp.647-687 (1986)