2018 Workshop on Wigner’s Friend

How we avoid getting to the Wigner's Friend situation in the first place

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    Ruth Kastner

    The Wigner’s Friend scenario was created to amplify the measurement problem as illustrated by Schrodinger’s Cat. We don’t encounter a Wigner’s Friend dilemma in the first place if we have a means of delineating, in physical terms, what constitutes ‘measurement’ (where that is described by von Neumann’s Process 1 non-unitary transition) One can do so in the transactional interpretation, as discussed in these publications in IJQF: https://www.ijqf.org/archives/4398 and https://www.ijqf.org/archives/4871
    The above analysis shows that the measurement transition will occur with overwhelmingly high probability before the systems involved get to the fully macroscopic level of Geiger counters and cats (so that that the chain of superpositions ends well before the point that Wigner and his Friend could enter).
    This resolves the issue pointed to by the recent ‘no-go’ proofs, as follows. According to TI, quantum states are not assigned universally. They are assigned only to the systems that can really be in quantum superpositions (such as the initial unstable atomic nucleus). Under TI, the quantum superposition attributable to the initial unstable nucleus does not propagate to systems in the experiment that participate in the non-unitary transition (e.g. the absorbing atom(s) that receive(s) the decay products). Rather, we have real (time-dependent) physical probabilities (rather than just amplitudes) describing the likelihood of absorption of decay products by specific absorbers, and these interactions are described by weighted projection operators (essentially the mixed state arising from vN’s Process 1, where the weights are the Born Rule). Absorbers are precisely and unambiguously defined in the relativistic version of TI (see above papers).

    The community has been widely skeptical of TI, primarily because of Maudlin’s claimed refutation of it in 1996. However, that objection is fully nullified at the relativistic level (see https://arxiv.org/abs/1610.04609). Another reason for skepticism has been the stigma attached to the ‘absorber’ theory since Wheeler and Feynman turned away from it in the mid-20th century. What is little known is that Wheeler was in fact resurrecting the absorber theory in 2003 (Wesley, D. and Wheeler, J. A., “Towards an action-at-a-distance concept of spacetime,” In A. Ashtekar et al, eds. (2003). Revisiting the Foundations of Relativistic Physics: Festschrift in Honor of John Stachel, Boston Studies in the Philosophy and History of Science (Book 234), pp. 421-436. Kluwer Academic Publishers)

    I should perhaps add that TI questions the usual assumption that there is a universal ‘superposition principle’. Rather, superpositions are not universal in the direct-action (‘absorber’) theory, and that is what allows it to define ‘measurement’ and also to resolve the contradictions pointed to by the no-go proofs. (I recognize that loopholes in the proofs are being explored, but if those attempts to circumvent the proofs do not decisively succeed, then these proofs clearly point to the need to reject the idea that there is a universal ‘superposition principle’.)

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