Einstein's objections to quantum mechanics can be attributed to several key factors, despite his significant contributions to the development of quantum theory. Here are a few reasons for his reservations:
Determinism and Causality: Einstein held a strong belief in determinism and causality, the idea that every event has a cause and that the behavior of physical systems can be predicted with certainty. Quantum mechanics, on the other hand, introduced inherent probabilistic aspects to the description of particles and their interactions. Einstein famously stated, "God does not play dice with the universe," expressing his dissatisfaction with the indeterministic nature of quantum mechanics.
Incompleteness and Hidden Variables: Einstein sought a complete and deterministic theory that could account for all aspects of reality. He was troubled by the probabilistic interpretation of quantum mechanics and felt that the theory might be incomplete. He proposed the concept of hidden variables, underlying unknown properties that would determine the outcomes of quantum events, and suggested that quantum mechanics was a statistical approximation of a more fundamental theory. However, subsequent experiments and theoretical work, such as John Bell's theorem, have posed challenges to the concept of hidden variables.
Wave-Particle Duality: While Einstein contributed to the understanding of wave-particle duality, he struggled with the concept on a philosophical level. Quantum mechanics describes particles as having both wave-like and particle-like properties, depending on how they are observed or measured. Einstein found this concept difficult to reconcile with his intuitive understanding of physical reality, preferring a more deterministic and tangible description of particles.
Nonlocality and "Spooky Action at a Distance": Einstein was troubled by the phenomenon of entanglement, in which the properties of two or more particles become correlated in a way that their states are dependent on each other, even when separated by large distances. He referred to this phenomenon as "spooky action at a distance" and questioned the completeness and locality of quantum mechanics. Einstein's skepticism towards nonlocality was captured in his collaboration with Boris Podolsky and Nathan Rosen in the famous EPR paradox thought experiment.
It is important to note that Einstein's objections did not negate his acknowledgment of the mathematical and predictive successes of quantum mechanics. He recognized its utility and made significant contributions to quantum theory, especially in explaining the photoelectric effect and the concept of the photon. However, his philosophical reservations about the fundamental interpretation and completeness of quantum mechanics persisted throughout his life, leading to his famous quote, "Quantum mechanics is certainly imposing. But an inner voice tells me that it is not yet the real thing."