“Anyone who is not shocked by quantum theory has not understood it.” –Niels Bohr
“If you think you understand quantum mechanics, you don’t understand quantum mechanics.” –Richard Feynman
Quantum mechanics are utterly bizarre. Quantum particles behave like spread out waves, until their position is measured, when they suddenly behave like a particle with definite position. The mathematics of what happens are well understood, but the ontology, the actual reality of what is going on, is not. There are multiple interpretations, all of which require throwing some cherished aspect of reality under the bus.
Sean Carroll participated in a debate on the different interpretations. This is a long video, but if you’re short on time, the first 20 minutes or so are worth watching for Brian Greene’s primer on quantum mechanics and the principle problem. But if you want a good understanding of each interpretation, you’ll need to watch the whole thing.
As I said above, all of the interpretations require throwing some aspect of long accepted reality under the bus. Causality, locality, counterfactual definiteness, realism, and other concepts, all end up being in the crosshairs of one or more of the interpretations.
None of them can currently be empirically tested, although I was interested to hear in the video that the instantaneous collapse interpretation makes distinct predictions from traditional QM, predictions that are in principle testable with some future technology. This is important as many physicists dislike talk of interpretations, preferring a “shut up and calculate” approach, mainly due to the fact that most of the interpretations make the exact same predictions.
Over time, I’ve favored one or another of the interpretations. The many world interpretation has always seemed pretty enticing, and I’ve always had a soft spot for the particle pilot-wave interpretation. But as I’ve gotten older and more epistemologically cautious, I’ve become increasingly agnostic on these interpretations, and probably will remain so until some way of testing becomes possible.
Toward the end of the video, Carroll’s confident assertion that one interpretation will eventually gain dominance in the physics community, reminded me of the recent evidence requirement debate. Short of actual experimental or observational testing, any such domination will be one of belief, not of knowledge. Whether or not that should ever be considered settled science goes to the heart of the debate.