Albert Einstein was indeed skeptical of certain aspects of quantum mechanics, including the phenomenon of quantum entanglement. Although he didn't know about the experimental proof of entanglement at the time, his disagreements with the prevailing interpretations of quantum mechanics were based on his philosophical objections to certain aspects of the theory.
Einstein's main objection to quantum entanglement stemmed from what he called "spooky action at a distance." He found it difficult to accept that two entangled particles, once separated, could instantaneously influence each other's states, violating the principles of locality and causality.
Einstein famously expressed his objections in a series of letters and debates with Niels Bohr, who was one of the key architects of quantum mechanics. The discussions between Einstein and Bohr are often referred to as the Einstein-Bohr debates or the Bohr-Einstein debates. These debates took place in the 1920s and 1930s and focused on the fundamental nature of quantum mechanics, including topics like entanglement and the completeness of the theory.
It's important to note that while Einstein questioned the interpretation and implications of quantum entanglement, he didn't dispute the mathematical formalism of quantum mechanics itself. Instead, he sought a more complete physical explanation that would reconcile quantum mechanics with his intuitions about determinism and causality. However, subsequent experiments and developments in quantum physics, such as the famous Bell's theorem and the Aspect experiments in the 1960s and 1980s, confirmed the existence of quantum entanglement and its non-local nature.
In summary, Einstein's objections to quantum entanglement were based on philosophical grounds rather than experimental knowledge of its existence. He disagreed with Bohr and other quantum physicists about the interpretation and implications of quantum mechanics.