An experimental test of non-local realism
Most working scientists hold fast to the concept of ‘realism‘—a viewpoint according to which an external reality exists independent of observation. But quantum physics has shattered some of our cornerstone beliefs. According to Bell’s theorem, any theory that is based on the joint assumption of realism and locality (meaning that local events cannot be affected by actions in space-like separated regions) is at variance with certain quantum predictions. Experiments with entangled pairs of particles have amply confirmed these quantum predictions, thus rendering local realistic theories untenable. Maintaining realism as a fundamental concept would therefore necessitate the introduction of ‘spooky’ actions that defy locality. Here we show by both theory and experiment that a broad and rather reasonable class of such non-local realistic theories is incompatible with experimentally observable quantum correlations. In the experiment, we measure previously untested correlations between two entangled photons, and show that these correlations violate an inequality proposed by Leggett for non-local realistic theories. Our result suggests that giving up the concept of locality is not sufficient to be consistent with quantum experiments, unless certain intuitive features of realism are abandoned.
I’ve only ever read philosophers (like Tim Maudlin) on the Bell inequalities, and I don’t think I’ve ever read about the Leggett inequalities. So I shouldn’t be too snarky. But really, these judgments about comparative spookiness can’t be left to stand. They find non-local relations, of the kind we need to posit to give a realistic explanation of Bell inequalities, “spooky”. (I wonder if they find spatiotemporal relations spooky too.) But the thought that there isn’t really a world out there to perceive, and that our impressions of the world are to be explained in some way other than as the perception of a mind independent reality. That’s apparently not spooky at all. This doesn’t strike me as particularly plausible.
Comments from anyone who knows more about Leggett’s inequalities, or who wants to mock my fear of anti-realism, more than welcome.