Experimental evidence for retro-causation in quantum mechanics using weak values

Abstract

Physicists and philosophers have long argued about the real possibility that future events can influence what happens in the present. A time symmetric theory for nonrelativistic quantum mechanics first proposed in 1964 by Aharonov, Bergman, and Lebowitz and elaborated more recently by Aharonov et al. in terms of weak measurements and weak values has provided a formalism that encompasses this using not only standard forward in time evolving quantum states, but also requiring in its formulation quantum states that are evolving backward in time. Although the retro-causal interpretation of weak values remains controversial, experiments have verified many of the theory’s counterintuitive predictions. Here, we report an experimentally observed change in a weak value induced by controlled phase shifts occurring in an optically dark path of a twin Mach–Zehnder interferometer. These results can be explained in terms of the effect of a retro-propagating quantum state upon a weak value. This explanation provides empirical support for Aharonov’s controversial retro-causal interpretation of quantum mechanics.