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That example cannot describe the 'spooky action at a distance'. Because it involves 'local variables' which have been dis-proven by the bell-inequality in actua
by rocqua 5y ago
That example cannot describe the 'spooky action at a distance'. Because it involves 'local variables' which have been dis-proven by the bell-inequality in actual experiments.
Taking the standard example of an entangled pair of photons. Those must have opposite spin. We then move the photons rather far apart measure the spin on one side. Then because of entanglement and conservation, the spin someone will measure on the other side is guaranteed to be the opposite.
A 'local variable theory' explains this by saying that the moment the entangled pair of photons is created it is already determined what the two spins are. This variable (the pre-determined spin) then travels with both photons (hence being 'local' to the photon). And the one measurement does not affect the other measurement.
The bell inequality shows this to be false. Crucial to the bell inequality is that, when measuring spin, You don't get back the 'angle'. Instead you choose a reference plane, and measurement will tell you whether the spin is pointing above or below that plane.
Now in the bell experiment. You measure in two different planes at the two different ends. If you pick two planes at a 180 degree angle. You will always find the same result on both sides (perfect correlation). If you pick two planes at a 0 degree angle, you will always get the opposite results (perfect anti-correlation). If you pick the planes at a 90 degree angle, the results will be totally independent, and this completely uncorrelated.
On these 3 angles (0, 90 and 180), quantum mechanics and a 'local variable theory' agree on the correlation coefficient between the measurements. (-1, 0 and 1 respectively). Bells inequality concerns the other angles. In those cases quantum mechanics predicts a stronger correlation (either more positive or more negative) than the local variable interpretation can predict. This discrepancy is strongest around angles of 45 degrees. Where a local variable theory means the correlation cannot be above 0.5, but quantum mechanics allows a correlation of up to ±1/√2 = ±0.7071.
These correlations were measured experimentally, and something close to the QM maximum of 0.7071 was measured. Hence disproving there are local variables.
- sdwr 5y agoThanks for the clear explanation. I'm less clear on what particle spin is though.