The only way to preserve the ultimate speed limit of special relativity in the EPR scenario was to say that the spin state of each individual electron must have actually existed before the measurement, even if there was no way to know what it was. In other words, there must be some “hidden variable” that determined the spin of each electron from the start. We didn’t know the spin state of either electron only because we could not observe the determining factor, the hidden variable. The 50-50 probability was just an illusion due to this incomplete knowledge.
Think of it like this: suppose you were collecting ballots at a polling station, through which a steady stream of voters was passing. As they passed, male voters placed their ballot in your right hand, while female voters placed their ballot in your left hand. If you happened to be blindfolded, you could not tell men from women and would conclude that the voters were randomly placing ballots in one hand or the other. The gender of the voter would be a hidden variable from your perspective. It is only when you could see the gender of the voter that you could figure out the underlying order to the ballot handover.
The EPR authors did not say what variable was hidden in quantum physics, but they thought it could be discovered with more study. They were convinced of this conclusion, because it was the only logical way to preserve both the absolute speed limit of special relativity and objective reality. Their conclusion was that quantum physics was simply incomplete. There must be more to the story, something more yet to be discovered. If common sense logic is to hold, it is hard to avoid this conclusion.
QUANTUM QUOTE
We are thus forced to conclude that the quantum-mechanical description of physical reality given by wave functions is not complete.
—Einstein, Podolsky, and Rosen
The proponents of the Copenhagen interpretation preferred to abandon locality rather than concede that quantum mechanics was an incomplete theory. The predictive successes of the theory indicated that it was working just fine without any hidden variables. They would say that the two electrons were not truly separate wave functions, but rather two parts of one common wave function, which could and did instantaneously collapse when any measurement was performed. Our friend Schroedinger coined the term entanglement for this intrinsic correlation between quantum states. He would later call entanglement “the characteristic trait of quantum mechanics, the one that enforces its entire departure from classical lines of thought.”
DEFINITION
Entanglement refers to the tight correlation between two quantum particles due to a shared, two-particle wave function that can stretch over vast distances.
Unlike the Schroedinger’s Cat Gedankenexperiment, it has become possible (and legal) to devise and perform real experiments that come pretty close to reproducing the conditions of the EPR Gedankenexperiment. Let’s take a look at how this can be done and what we learn from such experiments.
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