Realism or Locality: Which Should We Abandon?

Note for Raymond Y. Chiao, John C. Garrison - Realism or Locality: Which Should We Abandon.pdf

Assumptions#

Bell's inequalities are violated in observation. This means that a subset of the assumptions used in the proof of Bell's theorem are inconsistent with experiment. There are two main assumptions and a certain number of auxiliary assumptions. Here the two main assumptions are scrutinized:

Realism: there exists an objective world independent of observations.
Locality: what happens in a finite spacetime region \(A\) cannot be affected by what is done (e.g., by the choice of setting of measuring devices) in a spatially well-separated spacetime region \(B\). This is usually assumed to be equivalent to relativistic causality, the nonexistence of controllable superluminal signals.

Either one rejects locality, or realism, or both.

Rejection of Realism, if locality is assumed, is incomplete#

The concept of locality as applied to EPR experiments must implicitly carry with a realist's view of spacetime:

In the proof of Bell’s theorem, it is assumed from the start that spacetime, as embodied in special relativity and the standard notions of causality, is an element of physical reality.

This, on look of it, is not inconsistent, but as the spacetime manifold is itself dynamical, and affected by matter, in general relativity, it has physical difficulties. (Comment: it may also involve metaphysical and epistemological difficulties.)

Locality cannot be salvaged#

Quantum theory predicts, and experiments demonstrate, that certain counterintuitive superluminal effects exist. These affects arise from the tension between the local nature of all physical phenomena required by special relativity, and the global nature of superposed states required by quantum theory.

These superluminal effects do not “in fact” violate relativistic causality due to the acausal nature of the correlations imposed by the superposition principle. The effects include

EPR experiments
Tunneling: the photon transit time through a barrier was found to be smaller than the light transit time through an equal distance in the vacuum, though since the front velocity of the tunneling photon wave packet that represents the speed at which an effect is connected to its cause, according to standard theory, is still exactly \(c\).

Summing up#

Rejection of realism in the localist position doesn't go far enough, but denial of reality for spacetime makes it hard to formualte the notion of locality.

Rejection of locality in the realist position still makes it possible to formulate the existing notions of locality, but now a nonlocal theory that doesn't violate relativistic causality in the approximation that spacetime fluctuation can be neglected needs to be constructed. In particular, the explicitly nonlocal features of the theory should not permit the sending of controllable superluminal signals. And that

It is difficult to see how this condition could be satisfied without taking the unpleasant step of postulating elements of physical reality which are in principle unobservable.