Physicists just won the Nobel prize for proving that this is true. I’ve read the articles and don’t get it.
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Actual physicist here to clear a few things up. “Locally real” doesn’t actually mean anything and is a term that has been floating around today because of a pop sci article on Scientific American that is almost complete gibberish that fundamentally misunderstands the issues and is confused about a number of elementary concepts. Here’s an itemized list of what it does wrong and what it should be explaining instead :
– To start, “locally real” doesn’t mean anything. Nor is it pointing to a concept that might itself mean something. The violation of Bell inequalities don’t show that some single thing must be false. Instead they show that one of two things must be false.
– One of those things is “locality”. What this implies is that any point in a physical system is only impacted by other points whose state has had time to reach it. So for example if something is 300,000 kilometers away from me, I’m only impacted by its state one second ago. This is a consequence of special relativity.
– The other thing is “realism”. This one requires delving just a bit into quantum theory. Simply put, the fundamental physical object in the equations of quantum mechanics are not really particles, certainly not in the way laymen imagine them. You don’t compute the properties of some point mass and conclude where it will be at any given point in time. You compute the probability of that. “Realism” is the idea that this is all just theoretical artifact, and that in reality the particle really is somewhere. Our theoretical model gives probabilistic answers, but the idea of “realism” is that there is still a definite answer to the question “where is the particle” at any point in time, and not just at points of measurement.
– What exactly constitutes a measurement is a massive and still very much open debate among experts so there’s simply no way for me to get into it here.
– Bell inequalities are a set of inequalities that, if measured to be violated, show that it is not possible for both “locality” and “realism” to be simultaneously true. Understandably, in the early days of quantum mechanics, physicists had a hard time giving up on “realism”. But Bell showed that if somehow “realism”, as required by basic human intuition, and “locality”, as required by relativistic theory, were both true, then a set of inequalities should hold.
– Experiments have shown that these inequalities do not hold. At this point, you have to abandon either “realism” or “locality”. Again, “locally real” doesn’t mean anything. I would guess most physicists tend to conclude “realism” should be abandoned, but as with measurement these are fairly open ended questions. Quite simply, it’s what quantum theory has been telling you to do all along. There are some non local interpretations of quantum mechanics that preserve realism and are seducing for that reason, but I find them to be very ad-hoc and overly convoluted. There’s also a lot of wiggle room here too if you really understand these issues and play around with them at the border of what’s possible. Personally I like to think there’s some grander theory waiting for us that is globally deterministic but locally probabilistic. Some people refer to that as super-determinism, but I don’t like the term as it implies some difference between determinism and super-determinism (which there isn’t).
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