Reply To: What’s wrong with the wave function?

Robert Griffiths

Speaking as a physicist, the problem with thinking of the wavefunction as representing a quantum system goes back to the days when Born proposed that Schrodinger’s wave be interpreted not as something physically real but as a device for assigning probabilities. Thus a spherical wave emerging from a scattering event can be reconciled with the fact that the particles tend to come out in well-defined directions. The consistent histories approach is in some ways an updated version of Born’s idea. An important part of the update is is being precise about what the probabilities refer to: subspaces of the Hilbert space. These, and not Bell’s classical hidden variables, are the quantum mechanical “beables”.

In your discussion of EPRB in Sec. 6 you seem unaware of the fact that competent experimentalists regularly interpret their experiments as revealing properties possessed by measured systems before the measurement took place. This is retrodiction, not retrocausation. If Alice has a good piece of apparatus, and it indicates that the z component of spin of the spin half particle was +1/2 and not -1/2, she is perfectly justified in saying that the particle had precisely this property before the measurement took place, and that fact is the cause of the measurement outcome. There is no need whatsoever to invoke retrocausation. For a detailed discussion, see my “EPR, Bell, and Quantum Locality”, Am. J. Phys. 79 (2011) 954. arXiv:1007.4281.

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