Albert Einstein had a complicated relationship with quantum mechanics and, specifically, the concept of quantum entanglement and non-locality. Einstein, along with Boris Podolsky and Nathan Rosen, formulated the famous Einstein-Podolsky-Rosen (EPR) paradox in 1935 to challenge certain aspects of quantum mechanics.
Einstein's objection to quantum entanglement and non-locality stemmed from his philosophical stance on the completeness and determinism of physical theories. He famously expressed his discomfort with what he called "spooky action at a distance," referring to the non-local correlations predicted by quantum entanglement.
Einstein believed in a deterministic worldview, where physical properties of objects have definite values independent of observation. He favored local hidden variable theories, which postulate that there are underlying, yet unknown, deterministic variables that govern the behavior of quantum systems. In his view, these hidden variables could potentially explain the seemingly non-local effects observed in quantum entanglement.
However, in the late 1960s and early 1970s, physicist John Bell formulated a series of inequalities, known as Bell's inequalities, that could be tested experimentally to distinguish between local hidden variable theories and quantum mechanics. Subsequent experiments, such as the Bell tests conducted by Alain Aspect in the 1980s, consistently showed violations of Bell's inequalities, indicating that quantum mechanics and entanglement cannot be explained by local hidden variables.
Despite these experimental results, Einstein maintained his skepticism towards the non-local nature of entanglement. He famously referred to entanglement as "spukhafte Fernwirkung," which translates to "spooky action at a distance." Einstein argued that quantum mechanics was an incomplete theory and that there must be underlying principles yet to be discovered that would restore determinism and locality.
It's worth noting that Einstein's views on quantum entanglement and non-locality were controversial and did not align with the prevailing consensus among most physicists. Quantum entanglement and non-locality have been extensively confirmed by experiments and are considered fundamental aspects of quantum mechanics. However, Einstein's skepticism and contributions to the EPR paradox stimulated further research and discussions on the foundations of quantum theory and the nature of reality.