Reality Doesn’t Exist Until You Measure It, Quantum Parlor Trick Confirms

upstart writes:

Two players leverage quantum rules to achieve a seemingly telepathic connection:

A quantum particle can exist in two mutually exclusive conditions at once. For example, a photon can be polarized so that the electric field in it wriggles vertically, horizontally, or both ways at the same time-at least until it's measured. [...] The polarization emerges only with the measurement.

That last bit rankled Albert Einstein, who thought something like a photon's polarization should have a value independent of whether it is measured. He suggested particles might carry "hidden variables" that determine how a two-way state will collapse. However, in 1964, British theorist John Bell found a way to prove experimentally that such hidden variables cannot exist by exploiting a phenomenon known as entanglement.

Two photons can be entangled so that each is in an uncertain both-ways state, but their polarizations are correlated so that if one is horizontal the other must be vertical and vice versa. Probing entanglement is tricky. To do so, Alice and Bob must each have a measuring apparatus. Those devices can be oriented independently, so Alice can test whether her photon is polarized horizontally or vertically, while Bob can cant his detector by an angle. The relative orientation of the detectors affects how much their measurements are correlated.

Bell envisioned Alice and Bob orienting their detectors randomly over many measurements and then comparing the results. If hidden variables determine a photon's polarization, the correlations between Alice's and Bob's measurements can be only so strong. But, he argued, quantum theory allows them to be stronger. Many experiments have seen those stronger correlations and ruled out hidden variables, albeit only statistically over many trials.

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