Scientists at the National Institute of Standard and Technology (NIST) have proven beyond reasonable doubt that Einstein was wrong about one of the main principles of quantum mechanics and that "spooky action at a distance" is actually real.
We are now certain that entanglement, the ability of particles to affect each other regardless of distance, exists and that it's an intrinsic property of the universe. When a pair or a group of particles are entangled, they cannot be described independently from each other. Measuring a particular property, like velocity, of a single particle affects all the other entangled particles.
Einstein and many other scientists believed that this phenomenon was paradoxical, as it would allow for information to be exchanged instantaneously across vast distances. He dubbed it "spooky action at a distance" and he believed that there was a way to reproduce this phenomenon with classical physics. He claimed that there were hidden variables – quantities that we didn't or couldn't know – that would make quantum mechanics perfectly predictable.
According to classical physics, the universe can be completely understood, modeled, and predicted by simply knowing the laws of physics. This is known as a deterministic view. Quantum mechanics is probabilistic, puts limits on the amount of understanding one has of a system and is rife with peculiarities that we don’t experience in everyday life.
While trying to investigate the consequence of this alleged paradox in 1964, physicist John Stewart Bell formulated a theorem (Bell's theorem), which states that quantum mechanics cannot be explained using any deterministic theory. The validity of the theorem has been corroborated with many experiments over the years, but now scientists are certain that it’s correct.
This new research from NIST has shown that quantum mechanics cannot be explained in a deterministic way. They created pairs of entangled photons with highly correlated polarizations. They separated the photons and sent them into two distant rooms to have their polarization measured. The settings of the apparatus that measured the polarization were picked at random for every photon; this was to ensure that if unknown effects were present, they didn't come from the measurements themselves.
To be published in Physical Review Letters, the photons the scientists observed were perfectly entangled. The probability that this was due to hidden variables was estimated to be 1 in 170 million, well beyond the 5 sigma (1 in 3.5 million) limit necessary in physics to announce a discovery.
“You can’t prove quantum mechanics, but local realism, or hidden local action, is incompatible with our experiment,” Dr. Krister Shalm, lead author of the study, said in a statement. “Our results agree with what quantum mechanics predicts about the spooky actions shared by entangled particles.”