Albert Einstein made significant contributions to the field of quantum mechanics, but he also had reservations and disagreements with certain aspects of the theory. One of the most famous objections he raised was against the concept of "spooky action at a distance," which he referred to as "entanglement."
Einstein, along with Boris Podolsky and Nathan Rosen, published a paper in 1935 presenting what became known as the EPR paradox. They argued that quantum mechanics appeared to allow for instantaneous effects over large distances, violating the principle of locality. According to their analysis, if two particles were entangled, measuring one particle's properties would instantaneously determine the properties of the other particle, regardless of the distance separating them.
Einstein found this concept problematic and suggested that it violated the principle of causality, which he regarded as a fundamental aspect of physics. He famously referred to this as "spooky action at a distance" and believed that there must be some underlying hidden variables that would explain the apparent non-locality of quantum mechanics.
However, subsequent experimental investigations, particularly by John Bell in the 1960s and later by Alain Aspect in the 1980s, provided evidence that seemed to contradict Einstein's skepticism. Bell's theorem and Aspect's experiments indicated that the predictions of quantum mechanics were incompatible with the existence of local hidden variables. The experimental results strongly supported the notion of entanglement and non-local correlations.
Einstein's reservations about certain aspects of quantum mechanics highlight his quest for a more complete and deterministic theory that would explain the behavior of particles at a deeper level. However, his objections were largely refuted by experimental evidence and the development of quantum information theory, which has since confirmed the validity of entanglement and non-local correlations in quantum systems. Nonetheless, Einstein's contributions to physics, including his work on the photoelectric effect and the theory of relativity, remain highly influential and significant.