When two particles are entangled, it means their quantum states have become correlated in such a way that the state of one particle is dependent on the state of the other, regardless of the distance between them. This correlation exists even if the particles are separated by vast distances, and measuring the state of one particle instantaneously determines the state of the other, regardless of the spatial separation.
It is important to note that entanglement does not imply that the particles have always been together in both space and time. Entanglement can occur through interactions that happened in the past, bringing the particles into an entangled state. However, the entangled particles can subsequently be separated and their states will remain correlated.
The concept of entanglement was initially proposed by Albert Einstein, Boris Podolsky, and Nathan Rosen in a 1935 paper called the EPR paradox. They used entanglement to illustrate what they saw as a problem in quantum mechanics, suggesting that it led to "spooky action at a distance." Later, John Bell formulated his famous Bell's theorem, which provided a framework for testing the predictions of entanglement against certain assumptions, known as Bell inequalities.
Experimental tests based on Bell's theorem have consistently shown violations of these inequalities, indicating that entanglement is a real phenomenon. These experiments, along with the theoretical framework of quantum mechanics, have established entanglement as a fundamental aspect of quantum theory.
While the precise mechanism by which entanglement occurs is still a subject of ongoing research and debate, its reality has been experimentally confirmed and is now a well-established concept in physics. The notion that entangled particles share a correlation regardless of spatial separation is a fundamental feature of quantum mechanics and has been extensively tested and verified in various experiments.