In 1947, eight years before his death, Einstein wrote to a friend that he could not seriously believe in quantum mechanics because “physics should represent a reality in time and space, free from spooky actions at a distance.”Albert Einstein came up with the phrase “spooky action at a distance” because he had a problem with the completeness of quantum mechanics, particularly as it conflicted with his own special theory of relativity.
This is most famously seen in the “EPR paradox” of 1935 named after its inventors Einstein, Boris Podolsky, and Nathan Rosen which concerns how a pair of particles are “strangely” linked.
The newer term for this is “entanglement,” which is when two particles are “so deeply linked that they share the same existence.”
That’s more physics than I can really grasp , and it involves mathematical relationships such as a wavefunction.
We think of space – be it ten feet between two people or thousands of miles – as the medium that separates things. Quantum entanglement makes you question that apparent truth. Entanglement means that quantum connections between two particles can persist even if the two particles are on opposite sides of the universe.
What Einstein found “spooky” was that when these entangled particles become widely separated in space, action upon one of them (such as measurement) immediately influences the other. Distance is irrelevant.
And Einstein found this impossible according to special relativity, so, quantum mechanics must be wrong, or at least incomplete, was his conclusion.
The paradox had a solution, but it came in the decade after Einstein’s death. A physicist at CERN, John Bell, in 1964 described entanglement as an entirely new kind of phenomenon, which he termed “nonlocal.”
What I find interesting about this, as it applies more to my non-physics world, is that Bell’s theory is more concerned with the transfer of information. Physicists are still experimenting on this theory, but it fits this information age we live in.
As Brain Greene said on a NOVA episode, quantum entanglement brings to mind voodoo, but the scientific evidence that it exists is overwhelming.
Einstein was trying at the end of his life to find a single theory that wrapped all of this and energy and time into a beautiful package. He didn’t find it. He would not be pleased that modern physicists reject the notion that quantum mechanics requires general relativity to be consistent.