Realism and time symmetry in quantum mechanics

Benedikt Bläsi, Lucién Hardy

Marie-Christine Combourieu, Jean Pierre Vigier

Charles P. Tresser

In Bohm's version of the EPR gedanken experiment, the spin of the second particle along any vector is minus the spin of the other particle along the same vector. It seems that either the choice of vector along which one projects the spin of the first particle influences at superluminal speed the state of the second particle, or naive realism holds true i.e., the projections of the spin of any EPR particle along all the vectors are determined before any measurement occurs). Naive realism is negated by Bell's theory that originated and is still most often presented as related to non-locality, a relation whose necessity has recently been proven to be false. I advocate here that the solution of the apparent paradox lies in the fact that the spin of the second particle is determined along any vector, but not along all vectors. Such an any-all distinction was already present in quantum mechanics, for instance in the fact that the spin can be measured along any vector but not at once along all vectors, as a result of the Uncertainty Principle. The time symmetry of the any-all distinction defended here is in fact reminiscent of (and I claim, due to) the time symmetry of the Uncertainty Principle described by Einstein, Tolman, and Podolsky in 1931, in a paper entitled ``Knowledge of Past and Future in Quantum Mechanics" that is enough to negate naive realism and to hint at the any-all distinction. A simple classical model is next built, which captures aspects of the any-all distinction: the goal is of course not to have a classical exact model, but to provide a caricature that might help some people.

Stanley Gudder

Kaj Börge Hansen

Brian Ellis

This chapter develops metaphysics to include quantum mechanical realism. Quantum mechanics has always been the main obstacle to acceptance of scientific realism, and hence to any metaphysics based on scientific realism. Realism about Schrdinger waves requires the belief that particles are always transmitted as quantum mechanically indeterminate waves, but always act as classically determinate particles on whatever absorbs them. Acceptance of a realistic theory of Schrdinger waves, and of the very rapid or instantaneous changes of state that quantum theory implies, commits one to the view that all transitions from indeterminate to determinate states are temporally irreversible, apparently in contradiction to the widely accepted T-symmetry thesis. Realism about Schrdinger waves already implies a kind of non-locality, because what was spatially distributed before the event of determination suddenly becomes localized in that event. Quantum realism implies that there is a common species of elementary physical causation that is probabilistic, and directed from one determinate state of affairs to another.