Counterfactual definiteness
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In some interpretations of quantum mechanics, counterfactual definiteness (CFD) is 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 outcome 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 violate either 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.
[edit] See also
[edit] References
- ^ Henry P Stapp S-matrix interpretation of quantum-theory Physical Review D Vol 3 #6 1303 (1971)
- ^ David Z Albert, Bohm's Alternative to Quantum Mechanics Scientific American (May 1994)
- ^ John G. Cramer The transactional interpretation of quantum mechanics Reviews of Modern Physics Vol 58, #3 pp.647-687 (1986)
