The term "spooky action at a distance" was coined by Albert Einstein as a way to express his skepticism and unease with the implications of quantum entanglement. While entanglement does indeed equalize the quantum states of two particles, it is the instantaneous correlation between their states at a distance that seemed puzzling to Einstein and others.
In classical physics, information cannot travel faster than the speed of light. Therefore, the idea that measuring one particle's state could instantaneously affect the state of another particle located far away appears to violate this principle. This apparent non-locality, where the particles' states are correlated regardless of the distance between them, seemed "spooky" and went against our common-sense understanding of causality.
Einstein, along with collaborators Boris Podolsky and Nathan Rosen, proposed the famous Einstein-Podolsky-Rosen (EPR) paradox in 1935 as a thought experiment to challenge the completeness of quantum mechanics. They argued that the theory must be incomplete because it allowed for such non-local connections between entangled particles. They suggested that there must be "hidden variables" that determine the particles' states beforehand, and that our lack of knowledge about these variables gives the appearance of non-locality.
However, subsequent experiments and developments in quantum mechanics, particularly the work of physicist John Bell, have shown that the predictions of quantum mechanics, including entanglement, are consistent with experimental observations. Numerous experiments have been conducted that demonstrate the entanglement of particles and the instantaneous correlation of their states at a distance.
While quantum entanglement and its non-local correlations may still be considered "spooky" in the sense that they defy classical intuition, they have been experimentally verified and are a fundamental aspect of quantum mechanics. The phenomena associated with entanglement have practical applications in areas such as quantum computing, quantum cryptography, and quantum communication. Nevertheless, the underlying philosophical implications and the nature of the non-local connections in entanglement continue to be topics of scientific and philosophical exploration.