Einstein, like Newton, believed that a photon particle traveled a fixed path from its start point to hit a screen at a point. So when quantum theory declared that where the photon hit the screen was random and the data supported this, physical realism had two options: either quantum theory was incomplete as “God does not play dice with the universe” or there was a hidden physical cause:

“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

Einstein raised this problem and Bohr chose to ignore it, but it still haunts physics today. On the one hand, quantum theory always works for quantum entities so how can it be incomplete? On the other hand, every attempt to “reify” quantum states (make them physical) has failed, so there seems no way to make the collapse of quantum waves “physically plausible”. Einstein’s search for “hidden variables” to explain quantum theory is in essence an attempt to make quantum theory physically plausible. It hasn’t succeeded because what quantum theory describes is not physical, as it is what creates the physical. So physics will never solve this dilemma until it accepts that quantum theory is real. Quantum realism however resolves the dilemma by recognizing that quantum waves are processing waves.