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Loophole-free Bell test ‘Spooky action at a distance’, no cheating
- dbcooper 11y agoLink to the paper: http://www.nature.com/nature/journal/vaop/ncurrent/full/nature15759.html http://www.nature.com/nature/journal/vaop/ncurrent/full/natu...
- dsp1234 11y agoCommentary by Scott Aaronson from September on this paper. http://www.scottaaronson.com/blog/?p=2464 http://www.scottaaronson.com/blog/?p=2464
- wfunction 11y agoCan someone explain to me how photons are directed from one place to another without interference with (i.e., "observation" by) other atoms? This seems impossible, but then it pops up the question: why is this interference (which seems to qualify as "observation") not a problem?
- hugh4 11y agoThe short answer is: because glass. Glass (especially the stuff they use in optical fibres) is pretty good at not interacting with photons, which is why it looks like glass. Why doesn't glass interact with photons? Because it has a large band gap. There are no excitations to be had at optical wavelengths, so there's no way for the photon's energy to be transferred to anything else. Some fraction of the photons used in this experiment will nonetheless have been absorbed along the way, but that's not a problem, we only count ones where we have a pair.
- wfunction 11y agoAnd how exactly can a photon change direction without interacting with something?
- jacquesm 11y agoA new photon is emitted after absorption by the old one, the new one is absolutely indistinguishable from the original. The few that remain absorbed will slightly warm up the glass which then regains thermal equilibrium by throwing off a photon at a much longer wavelength in a random direction.
- wfunction 11y ago> the new one is absolutely indistinguishable from the original This is where I get stuck, because no one explains this. How do we know this is true? Is this because they match in all the parameters that we're aware of (say, momentum, spin, whatever), or is this because it has been proven (through a Bell-like test) that there cannot be any other parameters that we are potentially unaware of, and hence if they match in the known ones, then they are one and the same?
- sitharus 11y agoNo, it doesn't mean that there is no magical hidden property that actually distinguishes them, but that as far as we can tell they're indistinguishable. Given that nothing about the photons is different they must be the same right? For more confusing theory, look up the single-electron universe.
- wfunction 11y agoYup I've heard about the single-electron universe. Okay, so doesn't what you said imply there could very well be a hidden variable that we're unaware of, that affects the two entangled photons as they are separated? What's ruling this out?
- sanxiyn 11y agoNothing rules that out. What Bell tests, of which this one is the best implementation so far, rule out is local hidden variable. Hidden variable that can propagate faster than light is not ruled out by the test. (For the most people, it is ruled out by being faster than light.)
- MichaelApproved 11y agoI have a question that's slightly off-topic from what OP asked. Is it possible to know which particles are entangled? If I have a pile of particles, can I say that a select group of them were still entangled while others are not? Another question along the same lines: If I give my friend one of my entangled particles, can I tell if he has measured the spin by examining my half of the pair of entangled particles? *edit missed a word
- ars 11y agoNo to both questions. Both would allow you to violate the speed of light if it was possible. Once you measure a particle it is no longer entangled, so if you could tell if a particle was entangled or not then you can violate the speed of light by instantly knowing an action taken on the other particle. Read also: https://en.wikipedia.org/wiki/No-cloning_theorem https://en.wikipedia.org/wiki/No-cloning_theorem
- MichaelApproved 11y agoSo, then is it correct to say that an outside observer can never tell if a particle he/she is looking at is actually entangled? This outside observer would simply observe that the particle has spin "up" or "down" but would never know that it was entangled earlier by someone else. This observer would never know it was in a superposition unless they had knowledge that someone had previously done this. Only the person who entangled the particle can clue the observer in on the fact that they were entangled. I can give you a particle, tell you it's in a superposition but you'd have no way to verify this. If you observe the particle, perhaps you'd see it's in spin "up" but that would be meaningless because that observation wouldn't tell you anything about whether it was entangled or not. You only know about its current state. There is nothing in your observation that shows "hey the particle just changed from superposition to spin up" Thanks for the replies! I watch "word science festival" and similar videos and I am fascinated by the quantum world but I have wondered these questions for a while and didn't know anyone who could answer them. I saw this great Veritasium video about measuring quantum particles but it still left me with the questions above https://www.youtube.com/watch?v=ZuvK-od647c https://www.youtube.com/watch?v=ZuvK-od647c
- tim333 11y agoInteraction in general with the photons doesn't effect their polarisation and so the polarisation state remains unobserved. It's only when you check their polarisation that it is 'observed'. (It's not so much observation as a reduction in the possible states. When the photon is heading down the fiber it could be in any state and has a probability amplitude for being in various states. After it goes through a polariser and into a detector it's definitely in one particular state so you don't get the multitude of probability amplitudes).
- deleted 11y ago[deleted]
- dang 11y agoThere was a significant thread on this earlier: https://news.ycombinator.com/item?id=10427654 https://news.ycombinator.com/item?id=10427654. But since this is the experimenters' own page, let's not treat it as a dupe.
- caf 11y agoCan anyone explain how the observations are used to create an instance of the CHSH game?
- Natanael_L 11y agoThe general approach: https://en.wikipedia.org/wiki/Bell_test_experiments#A_typical_CHSH_.28two-channel.29_experiment https://en.wikipedia.org/wiki/Bell_test_experiments#A_typica... Then there's this: http://www.scottaaronson.com/blog/?p=2464#comment-829791 http://www.scottaaronson.com/blog/?p=2464#comment-829791 (also read the comment by Scott just below that one, as well as the full post) I can't provide any ELI5
- caf 11y agoThat's helpful, thanks. What I was after is how the apparatus in this experiment maps onto the CHSH description given by Scott (if indeed, it does). It seems clear that the x / y values are used to determine the basis of measurement of the respective detectors, the a / b values are output based on whether each detector fires, and it's the correlations based on difference between the bases that allows you to beat the game. The basis of measurement here is analogous to the angle of a polarising filter in a photon detector, but the detectors in this experiment are directly measuring the spin of the nitrogen vacancies (so what is the physical implementation of the different basis of measurement?).
- securityfreak 11y agoCould someone please explain, how can they know beforehand the other electron in the other diamond is the first one's "sister"? What if those two are NOT entangled and will have identical spins? What defines entangled electrons? Is it a must that once they were e.g. in the same atom? Thank you. (I only completed 2 basic University physics courses which isn't enough :D )
- caf 11y agoThey use a process called "entanglement swapping" that allows you to create entanglement between two particles that have never interacted. It's described partway through this article: http://www.scottaaronson.com/blog/?p=2464 http://www.scottaaronson.com/blog/?p=2464
- comboy 11y agoThis should make things more clear: https://www.youtube.com/watch?v=ZuvK-od647c https://www.youtube.com/watch?v=ZuvK-od647c
- securityfreak 11y agoThank you. It does explain what I was missing from the video.
- tim333 11y agoI think that if you had non entangled electrons you wouldn't get the 'spooky' result. You'd just get random electrons arriving without anything statistically significant of interest.
- dools 11y agoI don't understand why there have to be hidden variables in order for their not to be anything spooky about this. This is just how entanglement works. There's no way to understand some kind of classical "mechanism" for this. It's no more spooky than, say, magnetism. There is a state where some measurements of some properties are correlated. Big deal!
- akvadrako 11y agoThe researches don't know how to explain the results in a way that is consistent with our intuitive understanding of the world, so I'd say most people consider that spooky. One of the equally reasonable explanations is that the measurement results made by Alice and Bob are not actually determined until their compare their results. Or FTL "communication", or time-travel. Or that their sources of randomness are actually pre-determined. I'm curious what would you consider spooky if not time-travel or whole universe pre-determinism?
- dools 11y agoRight they don't now how to explain it classically. In much the same way as electromagnetic waves for example are spooky if you try to explain them as propagating mechanically in the same way sound does. Light travelling in a vacuum?? Spooky! This in't time travel, there is no effect and no action at a distance. It's just how that state works. In the same way as I can't visualise a 5 dimensional sphere, I can't create a classical model that explains entanglement, but I can make predictions mathematically, observe the effects and then know that's how things are. Once you go beyond things that make sense intuitively, all you have is maths!
- akvadrako 11y agoThe problem with your argument is that you can rationalise anything that way - "that's just the way it is". Which is why I asked what you would consider spooky. If you don't consider anything within the realm of observable phenomena spooky, then you are essentially just defining spooky differently than other people.
- JulianMorrison 11y agoWhy is it that none of these experiments seem to consider the "many worlds" understanding of "spooky action at a distance" (which is non-spooky, and action only at immediate contact)? Roughly: both the samples are in both world A (up/down) and world B(down/up), and when you interact with them, you get split and are pulled into both world A and world B, and each copy of you when you interact pulls the thing you interact with also into A and B, and so when you compare results, the A-you compares the A-result and the B-you compares the B-result. The splitting is not immediate or faster than light. But it moves at exactly the speed of information, so you can't race ahead of it and watch it happen. To interact with it is to become part of it.
- danieltillett 11y agoI think Occam's razor is the cause. Many worlds suffers from the appearance of violating Occam's razor. Of course Occam's razor is just a rule of thumb, but it seems to inhibit physicists being comfortable with the many worlds interpretation.
- DavidSJ 11y agoObjects exist that we cannot see. Those words provoke little controversy when spoken about matter beyond the visible universe. Why? Because the simplest mathematical description we have of the objects we can see also includes those which in principle we cannot. We grant them the same ontological status as the Earth we stand on -- few would claim them as mere fictions to ease our task of prediction. Occam's Razor properly applied limits the amount of information in the description, not the amount of things the description describes. For example, pi's infinite decimal expansion is spared by this information theoretic razor. If you reject the multiverse, your full description of the universe must contain one bit for every binary "choice" the universe made, every qubit that collapsed. Quite a few bits that turns out to be. The multiverse description suffers no such flaw, as it contains only the initial conditions and the rules by which those conditions change over time.
- JulianMorrison 11y agoIt isn't, though. It doesn't create worlds. There's always only exactly one fuzzy-quantum-thing. It's just that some parts of it no longer interact with other parts of it.
- gambiting 11y agoQuestion. How is this any different to writing A on one piece of paper, then B on another piece of paper, and selecting one randomly, then giving the other piece to someone else. Then the other person travels a light year away from us, and we both look at our pieces of paper simultaneously, at pre-agreed time. If mine says "A" then I know the other one says "B", and vice-versa. I instantly know the state of something that is a light year away, even though no information was actually transmitted. Isn't this how quantum entanglement works in principle?
- caf 11y agoThe whole point of the Bell experiment is that it proves that which one has the "A" and which one the "B" wasn't pre-decided when the pieces of paper were separated.
- trist 11y agoIn a sense 'maybe not'. This would violate the bell test anyways as you predetermine the outcome by writing it down. Someone already knows what is on the paper. This is thus also subluminal communication as the outcome was already determined at the start (assuming you traveled at subluminal velocities). The idea of the entanglement states that the spin of the electrons is determined at the same time which guarantees their connection, not the outcome of the spin.
- Natsu 11y agoYou might like this explanation: http://www.scottaaronson.com/blog/?p=2464 http://www.scottaaronson.com/blog/?p=2464 It discusses that 'conspiracy' and explains why it can't happen.
- ibab 11y agoThis is a good question! It's different because if you pass quantum entangled information to the other person, you will be able to cooperate more effectively with them on certain games using that information. It's explained very well in the second paragraph of this article: http://www.scottaaronson.com/blog/?p=2464 http://www.scottaaronson.com/blog/?p=2464 . In a simplified way, this is what Bell's theorem tells you: That quantum informations behaves fundamentally different than "precalculated" random information.
- coldtea 11y agoIt's amazing how many people think that Quantum Mechanic's "two states" (e.g. in the Schrödinger's cat example [1]) just mean "it's actually in one state, and we just don't know which until we look". You don't need quantum mechanics for that. That's an ordinary cat (dead or alive) put in a box by someone else without telling you, e.g. the very classical, and intuitive "I don't know yet" thing we deal with in everyday life -- which poses absolutely no problem and no questions about the nature of the universe, the kind quantum mechanics do. The "two states" in QM is a different beast. It's not "a box we don't know yet what's inside". [1] Well, actually all possible states, but let's stop at the simplified 2 state cat example.
- kolinko 11y agoELI5 how it's different?
- dhimes 11y ago"ELI5" is "Explain like I'm 5" in case anybody is wondering.
- themgt 11y agoThe actual description of "how it's different" in QM is going to depend on the interpretation you choose, but classical vs. QM is basically the difference between "I don't know yet" vs. "it isn't yet"
- hohenheim 11y agoTo elaborate a bit, Copenhagen Interpretation of QM states that the cat is both alive and dead. It will only settle down on a form, dead or alive, once we observe it. You could read farther on the subject [1]. [1] https://en.wikipedia.org/wiki/Copenhagen_interpretation#Consequences https://en.wikipedia.org/wiki/Copenhagen_interpretation#Cons...
- cpd85 11y agoSure! One of the major advantages of quantum computing is that you can act on the superposition, i.e. construct some sort of operation which goes "Map the dead cat state to x, and the live cat state to y." This becomes useful when you can generate a superposition of all possibilities, and then perform operations which make it more probable to see the states you desire. In the classical version, the final result of mapping the dead cat would be only x or y
- kingkawn 11y agowhat magic (ie applications) might come from this? Not challenging; hopefully inquiring.
- denniskane 11y agoInstead of invoking the idea of action-at-a-distance and then reflexively prepending the "spooky" tag to it, why doesn't anyone ever ask what a purely local physical description would entail, and then prepend something like "spooky" to that? A purely local description would consist of a universe filled with nothing but [zero-dimensional] mathematical points that somehow affect each other in some bizarre way. But the physics involved would be entirely unknown, because mathematical points have precisely zero extent, with the side effect being that two mathematical points either a) occupy the same location, making them physically indistinguishable, or b) occupy different locations, making them absolutely disjoint (ie, "infinitely apart"). Let's think about 2 points approaching each other in a classical Newtonian universe. Now they are a meter apart and now they are a millimeter apart and now they are a nanometer apart. In each of these cases, they are perfectly disjoint, and have no possible way of communicating to each other. But now they both occupy the same location, and all possible classical physical descriptions have broken down! The only rational solution is that the universe can only have a fundamentally nonlocal description. That is, the fundamental units of reality are spatially extended fields that do not suffer from the same requirement of pure mutual exclusivity that naive Bohr-esque classical models suffer from. Abandoning this requirement is what gave de Broglie the impetus to develop his own theoretical model that would soon thereafter inspire Schrodinger to develop a mathematical infrastructure, which, when combined with Heisenberg's matrix mechanics via Dirac's new algebraic formalism, would become the modern, canonical quantum theory that we have today. Edit: downvotes land me in negative territory for this :( ... Any explanation as why this reply is so bad? I am honestly curious! Edit #2: I guess I should have just said "Hey, Schrodinger's cat!" or "Wow, how about that Many Worlds Interpretation!", then I would have inspired the peanut gallery to jump into some pseudo-philosophical discussion that has been repeated time and again. But alas, I have resorted to a thoughtful consideration of the deeper ideas of QM, and have found myself relegated to the footnotes of the "Hackernews-iverse" :P!!!
- effie 11y agoA purely local description is generally understood to be field-mediated interaction, where field can be at the same place as the particle it acts on, without being the same thing.
- poelzi 11y agoI'm happy that BSM-SG replaced QM for me :) They are looking at huge, huge objects and think they are fundamental particles and wonder why it behaves strangely... apart from fundamental errors they have.