Superdeterministic loophole

In her excellent Physics Today report “Physics Nobel honors foundational quantum entanglement experiments ” (December 2022, page 14), Heather Hill discusses how the laureates closed loopholes in the interpretation of entanglement. She rightly concludes that the freedom-of-choice loophole remains open, but she describes it incorrectly, writing, “Taken to an extreme, the loophole can suggest that every event in all spacetime was determined by the initial conditions at the Big Bang, an idea called superdeterminism.”

Actually, that idea—that later events can be determined by earlier ones, and vice versa—is simply called determinism. There is a centuries-old philosophical tradition called compatibilism, which holds that even in a deterministic world we are free agents if we can do as we like without constraint. In the context of the Bell experiment, a compatibilist would say that experimenters are free to choose how to set their polarizers (for example, using the birthday of their grandparents or light from distant quasars), determinism notwithstanding.

Superdeterminism is much more subtle than that (and as a result is typically misunderstood or grossly oversimplified in the media). It is based around the following question: Do the laws of physics allow us to vary the Big Bang initial conditions in such a way that we could describe a hypothetical universe where the same pair of entangled particles—that is, with the same hidden variables—are measured with differently set polarizers? Such a universe is counterfactual, and superdeterminism describes an emergent restriction on such counterfactual measurements imposed by suitably formulated putative laws of quantum physics. ^{1 , 2}

No experiment to date has closed the superdeterministic “loophole.” Indeed, we are still searching for a realistic experimental protocol that can test it. We will get there one day, hopefully in the not too distant future, but it will likely not be via a Bell experiment.