The completeness problem
In 1935, physicists Albert Einstein, Boris Podolsky, and Nathan Rosen, all working at the Institute for Advanced Study in Princeton, published a thought experiment that became known as the EPR paradox. Their paper, "Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?", argued that quantum mechanics was an incomplete theory. The paradox centered on "entangled" particles—pairs of particles intrinsically linked so that their properties remain correlated no matter how far apart they are.
The thought experiment goes like this: if you create an entangled pair of particles and send them in opposite directions, measuring a property of one particle (like its spin or momentum) allows you to instantly know the corresponding property of the other. For example, if you measure the spin of particle A as "up," you will know with certainty that particle B has a spin of "down." Einstein and his colleagues argued that this meant the properties were have been predetermined from the moment of their creation through some "local hidden variables." The alternative, which Einstein famously called "spooky action at a distance," was that the act of measuring one particle instantaneously influenced the other, a concept that violated the speed of light.
Testing the spookiness
For decades, the EPR paradox remained a philosophical debate. Then, in 1964, physicist John Stewart Bell developed a mathematical framework, now known as Bell's theorem, that provided a way to experimentally test the question. Bell's theorem showed that if local hidden variables were real, the correlation between measurements of the entangled particles would have a specific, measurable limit. If quantum mechanics was correct, those correlations would be stronger, violating this limit, or "Bell's inequality."
The first convincing experimental tests of Bell's theorem were conducted by Alain Aspect and his team in 1982. Their experiments used entangled photons, and the results clearly violated Bell's inequality, supporting the predictions of quantum mechanics. This meant that the strange, non-local connection was real.
Since then, numerous experiments have confirmed these results with increasing precision, closing potential loopholes. In 2017, a Chinese satellite named Micius distributed entangled photons to two ground stations separated by 1,203 kilometers, demonstrating that the connection holds over vast distances. The link efficiency of this satellite-based approach was orders of magnitude higher than anything achievable with fiber optics on the ground. For their experimental work, Alain Aspect, John Clauser, and Anton Zeilinger were awarded the 2022 Nobel Prize in Physics.