Entanglement is a phenomenon in quantum mechanics where the properties of two or more particles become correlated in such a way that the state of one particle cannot be described independently of the state of the other(s). This correlation exists even when the particles are physically separated.
What makes entanglement strange and non-intuitive is that it violates classical notions of locality and individuality. In a classical system, you can describe the properties of each particle independently, and any changes to one particle do not instantaneously affect the other particle.
However, in an entangled system, the properties of the particles are not independent. When two particles become entangled, their states are linked, and measuring the state of one particle can instantaneously affect the state of the other, regardless of the distance between them. This instantaneous influence is known as "spooky action at a distance," a term coined by Albert Einstein.
This non-local correlation between entangled particles has been experimentally verified in numerous studies, including the famous Bell's theorem experiments. These experiments have shown that entangled particles can exhibit correlations that cannot be explained by any local classical theory.
Entanglement is a fundamental aspect of quantum mechanics and plays a crucial role in various quantum technologies, such as quantum computing and quantum cryptography. While it may seem strange and counterintuitive, entanglement is a well-established phenomenon supported by extensive experimental evidence.