To be or not to be local

Abstract

The experimental violation of mathematical relations known as Bell's inequalities sounded the death-knell of Einstein's idea of 'local realism' in quantum mechanics. But which concept, locality or realism, is the problem? Most of us, most of the time, can get along in life and in science with a sturdy concept of what is 'real'. Generally speaking, an external reality exists independent of observation. But in the world of quantum physics, where individual events are predicted only in terms of probabilities, this will not do. According to Bell's theorem, any theory 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. Previous work with entangled pairs of particles has confirmed these quantum predictions, rendering local realistic theories untenable. Maintaining physical realism as a fundamental concept would therefore require the introduction of 'spooky' locality-defying actions. A new study combing experiment and theory now shows that a broad and rather reasonable class of such non-local realistic theories is incompatible with observed quantum correlations. This suggests that any future extension of quantum theory, if it is to agree with the experiments, must abandon certain features of realistic descriptions. Giving up the concept of locality is not sufficiently 'unreal'.