Counterfactual definiteness (CFD) is a property of some interpretations of quantum mechanics but not others. 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, if one conducted a different experiment, is there a single alternate time line that would have resulted from it? Counterfactual definiteness has an important application to Bell's Theorem which implicitly assumed that every possible measurement — even if not performed — would have yielded a single definite result and is often claimed to show that no quantum theory can satisfy the locality principle. What Bell's Theorem really proved was that every quantum theory must either violate locality or CFD.[2] [3] CFD is a property of the Copenhagen interpretation of quantum mechanics as well as the hidden variables interpretation. It is not a property of the many worlds interpretation with its multiplicity of results in different worlds or elements of the universal wavefunction, and this allows many worlds to resolve Bell's inequalities without violating the locality principle. It is not a property of some other decoherent interpretations such as consistent histories.
See also
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)
