Einstein, like Newton, expected light particles to follow one path from source to screen, so when the data favored quantum theory, that waves interfere then hit the screen at a random point, he had two options: either quantum theory was wrong or the physical causes were unknown:
“This is the fundamental problem: either quantum mechanics is incomplete and needs to be completed by a theory of hidden quantities, or it is complete and then the collapse of the wave function must be made physically plausible. This dilemma has not been solved until today, but on the contrary has become more and more critical.” (Audretsch, 2004), p73.
This problem, which Einstein raised and Bohr ignored, still haunts physics today. On the one hand, all attempts to find hidden variables that make quantum randomness physically plausible have failed. On the other hand, all attempts to show that quantum theory is incomplete have also failed, so the rules of the quantum world predict perfectly but have no physical explanation.
What then is the solution? Both theories, that quantum theory is incomplete or that it is physical, have led nowhere, so the answer must lie elsewhere. That quantum theory is either wrong or physical assumes that matter explains everything. But if materialism is wrong, quantum theory could be true without being physical. We know it is true because it always works, and we know it isn’t physical because nothing physical can do what quantum theory says it does, so perhaps physical events can’t explain quantum events because they are created by them, as quantum theory says. It follows that physical variables can’t explain quantum theory because it is outside their domain.
For example, the rules of Minecraft don’t explain how its blocks exist, nor do the rules of chess say how its pieces exist, because that is outside their domain. A created scene can’t be fully explained by its local rules, so perhaps physical rules can’t explain photons for the same reason. I can turn off a game or tip over a chess board so likewise, what generates physical events doesn’t have to follow their rules. But if quantum events are outside the physical domain, what are the rules?