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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."
I posted this next week and it's still the first post.
America, Home of the Brave.
Apparently the entangled photon link they were using to host the webpage couldn't hold up under the strain of Slashdot.
Scientist used Entangle!
It's super effective!
Photon capture device go!
You entangled a photon!
It was added to your photondex.
Knowledge is power. Knowledge shared is power lost.
would give the much-derided ISS a stab at doing some decent science for a change
That won't necessarily help with the derision, as nobody denies the fact that interesting experiments are possible in space. The main point of contention will still be if you need to keep live persons there continuously to perform them. It'd have to be shown that a satellite or a simple orbiting mission couldn't have performed the same experiments for a fraction of the total costs.
Rome taught me patience and assiduous application to detail. Virtues which temper the boldness of great, general views.
Remember that Niels Bohr denied that such a test of nonlocality was possible. Einstein had said that this phenomenon was "incredible" in his "EPR" article, thus rejecting his own prediction. And Bohr replied in effect that such things were taboo metaphysics.
Michael J. Burns
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".
Sure it would be nice to do even more, and sure the costs are high (in part due to the STS, a nice but incredibly inefficient LV), but all this group-thinking about the "white elephant" ISS is akin to saying that kernel programming is easy. It's stupid, flat wrong and insulting for the people that get a lot of good work and science done.
There's a hidden treasure in Python 3.x: __prepare__()
I'm not sure what you are trying to suggest, but you can't use entangled photons to communicate faster than light.
I've not RTFA - it's down - but basically the EPR effect allows someone to create two photons and then measure if their polarization is H or V. The result is completely random BUT, both photons will always give the same result.
Now Alice measures her photon first and lets say we get H, then Bob's photon must instantaneously turn into H (previously it was a mixture of H and V - the dead and alive cat) so that when he measures his photon he also gets H.
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.
But because Alice and Bob are in the same inertial frame there's still, at least in theory, a concept of who did the measurement first and who did it second. (Alice and Bob can have synchronized clocks and record the time they did the experiment. Then they can, using normal communication, tell each other what time they did the experiment and they'll both agree who was first and who was second.)
What this experiment will do is mean that Alice and Bob won't agree about who was first and who was second. Alice and Bob's clocks cannot remain synchronized, so that according to Alice, and people sitting next to her, she did the measurement first, but according to Bob, and people sitting next to him, he did the experiment first. And BOTH will be correct because the two measurements are space like rather than time like.
Tim.
God said, "div D = rho, div B = 0, curl E = -@B/@t, curl H = J + @D/@t," and there was light.
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Please don't break reality. It's where I keep my stuff.
He's suggesting using entanglement to communicate faster than light. I think he's forgetting that once your manipulate the photon, it will no longer be in sync with its pair.
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?
It doesn't (there are other points where GR and QM conflict, but entanglement is not one of them ). Einstein thought it did because he assumed that any such interaction would be deterministic and could hence be used for communication (THAT would break GR ). Essentially Einstein never liked the idea that the universe was based on randomness, hence the famous "god does not throw dice with the universe" quote. As a consequence he repeatedly tried to disprove quantum mechanics by inventing scenarios in which the random nature of QM would conflict with GR. The surprising, and somewhat ironic, outcome of his attempts was however new insights into quantum mechanical interactions that just seem to confirm the random nature of QM.
Also, to be pedantic about it, entanglement doesn't in principle imply that an interaction is quicker than the speed of light. You could alternatively claim that the interaction occurs with the speed of light, but the ENTIRE UNIVERSE ends up in an indeterminate state similar to that of Schrödinger's cat until you receive Alice's message. Thus you can keep interactions restricted to the speed of light ( locality ) but in order to do so you would have to throw out the notion that Alice exist when you do not hear from her ( realism ).
In practice throwing away realism would force you into a rather solipsist interpretation of reality which I think even Kant would have issues trying to accept, and thus most of the time we just stick with the much more comfortable notion of having a reality with instantaneous long distance interactions. If nothing else this is a lot easier to visualize than imagining the entire world entering a superposition of states until you receive Alice's message. From a purely physical point of view the two cases are indistinguishable however, so it doesn't really matter either way.
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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they actually are both H and V polarized at the same time. you can think of them as just being "unknown" for the sake understanding why you cant use it to communicate faster than light, but it isnt what's actually going on.
we know this because in a quantum superposition, different possibilities in the superposition can interfere with other possibilities, making certain results more or less likely. this is shown by the double-slit experiment. shooting photons at a screen through two slits produced not two stripes like you'd expect, but several stripes. this is because each photon went through 2 waves of possibilities, one through each slit, and the waves then collided with each other, making certain ares of the screen more likely to be hit than others.
That is, unless I'm missing something fundamental.
Yes, you're missing something fundamental.
Going back to one photon.
We'll have four polarization states H, V (the normal horizontal and vertical polarization) and +, - the 45 degree polarizations.
Now Alice produces a stream of H photons and sends them to Bob. Now if Bob measures to see if they're H/V then he will always get H.
But if Bob measures if they're +/- he'll get 50/50 + and -, with each individual photon being + or - at random.
After measuring +/-, if Bob then remeasures H/V he'll again get 50/50 H and V. The measuring of +/- destroys the knowledge about H/V
If Bob measures at an angle other than 45 degrees then he'll get different proportions but he'll get sin^2 theta with one polarization and cos^2 theta with the other polarization.
Now lets consider entangled photons that will always give the same result for Alice and Bob. Initially we'll assume that Alice will always measure the horizontal polarization (0 degrees) Now lets consider that the photon "knows in advance" whether it will go through a horizontal polarizer i.e. it has (an infinite number) of hidden variables. Regardless of what measurement Bob does, an ensemble of photons can distribute values amongst these hidden variables so that Bob gets the expected correlations relative to Alice and the angle of his measurement.
But now let Alice vary her angle as well. Now the correlation depends on the difference in angle between Alice and Bob. But that angle isn't known (and hasn't even been decided) at the point the photon has been created. It could have a big "look up table" saying "If Alice angle is n and Bob angle is m then do/don't go through Alice's filter and do/don't go through Bob's filter BUT the photon that arrives at Bob's detector has to know what measurement Alice will/has done and the photon that arrives at Alice's detector has to know what measurement Bob will/has done.
But because Alice and Bob independently randomly decide what angle to measure "long" after the photon was created and their independent decisions are made so close together in time that neither can know what the other has/will do when they make their measurement due to the speed of light limit then there is no way for the photon to use its "lookup table" and get the correct statistical results.
It doesn't matter how you construct that "lookup table", unless you allow some sort of faster than light communication, using the lookup table will give different results to QM.
If you want the formal maths for that bit of hand waving then lookup Bell's inequality. He actually deduced the inequality that could be tested to prove no local hidden variable theorem was consistent with the results of QM based on measuring particle spins while most of the tests that have been done have used polarization of photons but the underlying theory is the same.
These experiments have already been done, and Bell's inequality has come down on the side of QM. Because Alice and Bob make their measurements so close together in time, not all observers will agree which one is first but (perhaps unfortunately) Alice and Bob will agree who was first and who was second. What this experiment does is close even that loophole - even Alice and Bob will be unable to agree who made the first measurement and who made the second.
Tim
God said, "div D = rho, div B = 0, curl E = -@B/@t, curl H = J + @D/@t," and there was light.
And there's the rub, and the reason why people are still trying to prove QM after a hundred years. The equations appear to indicate that one of the following MUST BE FALSE:
1. Quantum Mechanics.
2. Locality.
3. Realism.
All the experiments performed so far strongly support QM, so we can't dismiss that. If locality is false then we have Einstein's spooky action at a distance and a conflict with GR. If realism is false...then nobody knows where the hell we are, or what we are.
But one of them *has* to be wrong. All these experiments are trying to prove is which one.
Genocide Man -- Life is funny. Death is funnier. Mass murder can be hilarious.
For all the naysayers of ISS science, here is the list of past and present experiments for your review:
ISS Experiments by Expedition
Please note the count of experiments currently stands at 561, and the focus ranges from virology, to fluid mechanics, to relativity, to astronomy, and even engineering validation (not simply of space station components, but also of fully independent technologies). That's nothing to sneeze at.
And while a fairly large portion of them are relatively minor or PR projects like sleep habits in 0g and the Buzz Lightyear "teaching from space" program, there is an ample number of experiments designed specifically to take advantage of the unique environment the ISS offers and with a variety of potential future applications.
And don't forget the majority of these so far were conducted prior to the installation of the two primary laboratory modules on the ISS: Columbia, launched late last year, and Kibo, which is 2/3 delivered as of last week. These have also been done mainly by 2 or 3 man crews, with occassional help from shuttle crews. Once the ISS switches to a 6 man crew rotation, the rate at which science work is completed will be greatly enhanced.
But of course, carefully planned, executed, and generally useful science isn't as fun to talk about as broken toilets, so we'll just continue ignoring the successes of the ISS and focus only about the cost overruns, deleted components, and occassional operating problems.
Cats are not both alive and dead.
Exactly. But QM says they are.
H polarized photon = Cat dead. V polarized photon = Cat alive. 45 degree polarized photon = Cat dead, 135 degree polarized photon = Cat alive.
Until Alice and Bob make their measurements we really do find that the cat is both dead and alive.
But what does "measurement" mean? If we really were using dead/alive cats to do this experiment (currently we lack the ability to keep something the size of a cat in a superposition of states) then presumably the cat knows if it's alive. But where/how does this stop. If not a cat, what about a mouse? An amoeba? A virus? A protein? an O2 molecule? An atom? An electron?
I think currently we're up to the atom or small molecule point with still no sign of QM breaking down.
There are ways around these conceptual problems, many worlds being one.
Tim.
God said, "div D = rho, div B = 0, curl E = -@B/@t, curl H = J + @D/@t," and there was light.
Speak for yourself. I can certainly lift 10 times a spiders weight.
The revolution will not be televised... but it will have a page on Wikipedia