Einstein called non-locality "spooky action at a distance". He was concerned that the early founders of quantum mechanics were attributing the effects we see in the manifested, material world to be initiated by non-local causes, meaning that they were not measurable in the physical realm. Einstein’s entire argument against non-local causes involves a condition known as entanglement.

To show that quantum theory was incomplete, he asked two fellow physicists to help devise a thought experiment which later became famously known as the EPR Paradox. In it was theorized that when a particle goes through beta decay, two new particles were produced. Because they have interacted with one another, they are considered to be entangled. Those two particles are sent away from each other in either a straight line or at right angles. When the spin of both are measured, a table of results can be created showing that they are correlated in some direct manner. Einstein suggested that the only way for this to happen was that some force traveled between particle A and B, delivering the necessary information for B to drop into the appropriate state in correlation to the state of A after A had been measured.

Here’s the catch which Einstein pointed out. A force can only travel at the speed of light and can only go so far between each particle. This means that the short range of the force’s furthermost reach can be traversed at the speed of light in what amounts to no time, or instantaneously. If the distance between the particles is increased, no force can travel that far so as to instantaneously affect the other particle.

Here’s another way to think about this concept. A large building contains a very powerful electromagnet. When it is turned on, nearby metallic objects will fly toward it. But, the magnet’s field can only affect objects within a certain distance. It doesn’t affect the cars outside the building in the parking lot. If it could affect the cars at the same time and in the same way as nearby metal objects, some influence outside hidden force or information would have to travel faster than the speed of light out to the parking lot. In quantum theory, this influence is called a "hidden variable". Einstein wanted to show that no such spooky thing existed.

Later, physicist John Bell developed a matrix, or a table, showing every combination of detection at every combination of angle alignment of the two detectors and compared it to a table showing classically measured results (those with only local causes). The table showed that the quantum theory of hidden variables produced more accurate predictions than did the table of classical measurements. This was experimentally proved in 1982 by physicist Alan Aspect.

Some physicists also interpreted this experiment to mean that faster-than-light travel might also be possible. The type of correlation referred to in Bell’s Theorem was called entanglement in a 1935 paper written by Edwin Schrödinger.

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