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Testing Quantum Behavior — From Earth to the ISS
KentuckyFC writes "Einstein famously believed that the instantaeous effect of quantum entanglement would allow 'spooky action-at-a-distance' in violation of special relativity. Every test of entanglement on Earth has so far agreed with quantum mechanics but naysayers continue to point out various loopholes that might allow the results of these experiments to be determined in advance rather than instantaneously as QM suggests. Today, an international team of scientists is proposing the mother of all entanglement experiments, to be performed in space. The plan is to send entangled photons between an observer on the ground and one on the International Space Station. By the peculiarities of special relativity, the high relative velocity between the observers means that both will always be able to claim to have carried out their measurement first, thereby ruling out the naysayers' arguments (abstract). The experiment, called Space-QUEST, would be housed aboard Europe's Columbus module and would give the much-derided ISS a stab at doing some decent science for a change."
As I understand it, a quantum entangled photon is very fragile. I don't understand how or why it's fragile, but wouldn't that make this extraordinarily difficult to do? The trip to the ISS is pretty bumpy.
One of our competitors trademarked the term "hypothesis". From now on, we will call them "boneheaded ideas".
What's already been done is to ensure that Alice and Bob decide what measurement to do, and make the measurement, so close to the same time that it's impossible for there to be any way for Bob's equipment or photon to "know" what Alice is going to do (or vice-versa) except at superluminal velocities.
Just to clarify this paragraph because I've realized it's confusing.
Alice and Bob both randomly decide to measure the H/V polarization or the +/- (45 degree) polarization.
Then they get together and compare results. Where one has measured H/V and the other +/- then they throw the results away because they don't tell them anything useful, but where they've made the same measurement they find they always get the same (or opposite) results.
It's when they make the measurement that neither they, nor their equipment or photon can "know" what the other is doing.
There's also something called Bell's inequality that basically proves that the results of all of Alice and Bob's possible cannot be "known" by the photon ahead of time. (no local hidden variables).
Tim.
God said, "div D = rho, div B = 0, curl E = -@B/@t, curl H = J + @D/@t," and there was light.
I've never quite understood why this conflicts with GR. It seems like from Alice's perspective, when they both make their measurements, Bob is the superposition of having the same result or the opposite result. It is only after they communicate (restricted by the speed of light) that his results can be compared with hers and his superposition collapses. In other words, there are two measurements done by Alice, one of her photon, one of Bob, and they don't require any faster-than-light communication.
Am I missing something here?
Classical physics tells us that if you know the angle of polarization of a photon, then its chance of passing through a polarizer is the square of the cosine of the difference in angles between the photon and the polarizer. If you have a 45 degree photon, it will always pass a 45 degree polarizer, have a 50% chance of passing a 90 degree polarizer, and will never pass a 135 degree polarizer.
QM tells us that when you have two entangled photons and measure both of their polarizations, the chance the results will correlate is the square of the cosine of the difference in angles between the two polarizers . If you measure them at the same angle, the results always correlate. If you measure them at 45 degrees apart, the measurements correlate 50% of the time. If you measure them 90 degrees apart, the measurements never correlate (the results are always opposite). No matter how you look at it, this means either the results are predetermined at the time the photons are created based on the angles the polarizers will be at the time the measurements are taken, or that one measurement somehow influences the other later so the past isn't immutable.
Either way you look at it, it means the universe doesn't work the way we expect it to. If you're a glass-half-full person you want to believe in FTL and time travel, and if you're a glass-half-empty person then you think maybe the universe is deterministic.
That's why this stuff gets everyone who understands its implications all in a tizzy.
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