A Neglected Route to Realism about Quantum Mechanics

Bell’s Theorem depends on the assumption that hidden variables are not influenced by future measurement settings, and it is widely recognised that some of the puzzling features of quantum mechanics could be explained if this assumption were invalid. The suggestion has generally been regarded as outlandish, however, even by the taxed standards of the discipline. (Bell himself thought that it led to fatalism.) This paper argues that there is surprisingly little justification for this reaction. Using an informal model of the Bell correlations, the first part of the paper shows that the arguments against advanced action in QM, where not simply invalid, are easily evaded on good physical grounds. In the absence of such objections the approach has striking theoretical advantages, especially in avoiding the apparent conflict between Bell’s Theorem and special relativity. The second part of the paper considers the broader question as to why advanced action seems so counterintuitive, by investigating the origins of our ordinary intuitions about causal asymmetry. It is argued that the view that the past does not depend on the future is largely anthropocentric, a kind of projection of our own temporal asymmetry. Many physicists have also reached this conclusion, but have thought that if causation has no objective direction, there is no objective content to an advanced action interpretation of QM. This turns out to be a mistake. From the ordinary subjective perspective, we can distinguish two sorts of objective world: one “looks as if” it contains only forward causation, whereas the other “looks as if” it involves a mix of backward and forward causation. This clarifies the objective core of an advanced action interpretation of QM, and shows that there is an independent symmetry argument in favour of the approach. 1 An early version of this paper was read at the AAHPSSS Conference in Melbourne in May, 1988. Many people have helped me with comments and discussion since then, and I am especially indebted to Jeremy Butterfield, Peter Menzies and Jack Smart. I am also grateful for research funding from the Australian Research Council. The most profound conceptual difficulties of quantum mechanics are those that stem from the work of J. S. Bell in the mid-1960s.As Bell’s Theorem became well known, its author was often asked to survey the state of the subject, particularly in the light of his own contribution. He would typically conclude such a lecture by listing what he saw as possible responses to the difficulties, indicating in each case what he took to be the physical or philosophical objections to the response concerned. His intuitions in this respect were somewhat unfashionably realist—like Einstein, he disliked the common view that quantum mechanics requires us to abandon the classical idea of a world existing independently of our observations. He therefore appreciated the irony in the fact that from this realist standpoint, his own work seemed to indicate that there are objective non-local connections in the world, in violation of the spirit of Einstein’s theory of special relativity. As he puts it in one such discussion, the cheapest resolution is something like going back to relativity as it was before Einstein, when people like Lorentz and Poincaré thought that there was an aether—a preferred frame of reference—but that our measuring instruments were distorted by motion in such a way that we could not detect motion through the aether. Now, in that way you can imagine that there is a preferred frame of reference, and in this preferred frame of reference things do go faster than light. Bell reached this conclusion with considerable regret, of course, and would often note that there is one way to save both locality and realism. Bell’s Theorem requires the assumption that the properties of a system at a time are statistically independent of the nature of any measurements that may be made on that system in the future—“hidden variables are independent of later measurement settings”, to put it in the jargon. Bell saw that in principle one might defend a local (and hence special relativity friendly) Einsteinian realism by giving up this Independence Assumption. He found this solution even less attractive than that of challenging special relativity, however, for he took it to entail that there could be no free will. As he puts it, in the analysis leading to Bell’s Theorem it is assumed that free will is genuine, and as a result of that one finds that the intervention of the experimenter at one point has to have consequences at a remote point, in a way that influences restricted by the finite velocity of light would not permit. If the experimenter is not free to make this intervention, if that also is determined in advance, the difficulty disappears. (Davies and Brown, 1986, p. 47) It is surprising that philosophers have not paid more attention to these remarks. In effect, Bell is telling us that Nature has offered us a metaphysical Bell (1964). Bell’s papers on the subject are collected in his (1987). In Davies and Brown (1986), pp. 48-9. The irony actually runs deeper than this, for Bell’s Theorem seems to undercut Einstein’s strongest argument in favour of his view that there is more to reality than quantum mechanics describes; more on this in section 1 below.