Einstein, like Newton, believed that a photon particle traveled a fixed path from its start to hit a screen at a point, so when quantum theory declared that where the photon hit the screen was random, and the data agreed, he had two options: either quantum theory was incomplete or there were hidden physical causes:
“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
The problem Einstein raised still haunts physics today, as his attempt to find hidden physical variables to explain the facts failed and attempts to make quantum collapse “physically plausible” have also failed It has become clear that the rules of the quantum world defy those of the physical world.
The fact that no hidden physical variables have been found and that no attempt to make quantum theory physically plausible has worked is yet another failure of physical realism. In quantum realism, quantum theory is neither incomplete nor physically plausible. It isn’t incomplete because it always works and it isn’t physically plausible because nothing physical can do what it does. A quantum world that generates physical reality has no need to follow the rules of what it creates, so physics will never solve this dilemma until it rejects physical realism.