Quantum entanglement is a phenomenon in quantum mechanics where two or more particles become interconnected in such a way that the state of one particle is instantaneously correlated with the state of another particle, regardless of the distance between them. This instantaneous correlation appears to violate the notion of locality, which is the idea that information cannot travel faster than the speed of light.
Einstein referred to quantum entanglement as "spooky action at a distance" because it seemed to challenge his theory of special relativity, which posits that information cannot be transmitted faster than the speed of light. He found the concept of instantaneous correlation between distant particles puzzling and counterintuitive.
According to quantum mechanics, the entangled particles exist in a superposition, meaning they can be in multiple states simultaneously until they are measured or observed. When a measurement is made on one entangled particle, its state becomes determined, and the state of the other entangled particle is immediately known, regardless of the distance between them.
This apparent faster-than-light correlation does not allow for the transfer of information or communication. It is important to note that quantum entanglement cannot be used to send signals or messages faster than light. The correlation between the entangled particles only becomes evident when their states are compared after the fact.
While the phenomenon of quantum entanglement has been extensively observed and verified experimentally, its underlying mechanism and how it fits into our understanding of physics are still subjects of ongoing research and debate.