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Quantum entanglement shows that reality can't be local

158 points by suprgeek 14y ago

LokiSnake 14y ago
That very adorable kitten is way too distracting, and made it very hard to focus on the article.
- deleted 14y ago
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ChuckMcM 14y ago
This is for me the most interesting part of the whole teleportation / entanglement research. When these papers started coming out we debated the notion of 'faster than light' communication. The counter argument was that you had to move the particles apart and that was constrained by the speed of light. Then the question of "when" the state was resolved was pondered. There were two thought experiments proposed at this meetup. In the first, two highly accurate clocks, separated by enough distance that that speed of light issues were unambiguously resolved by the clocks, which where triggered by the resolution of the quantum state of an entangled particle. Capturing the exact time when the state was known (detangled) on each clock, and then comparing the times on the clocks to determine whether or not it was truly 'instant'. The second experiment was to set up a 'set' of entangled particles at a distance and resolve only 'some' of them. The set representing 'bits' and the resolved set representing the 'information.' The question of course is whether or not you can move information this way. Seems like we are getting closer to answering those questions in a definitive way. I always thought it would be fun to write a short story about someone doing this and discovering that it was the galactic equivalent of 'shortwave' radio and have the alien equivalent of the FCC come out to the planet to arrest the scientists for transmitting without a license :-)
- gwillen 14y ago
  This doesn't work because there is no moment at which the state is 'known' or 'detangled'. There is no way to tell, by observing one particle, when the other particle was observed. If there was you could send a signal with it, which would be no good. The universe is a strange place.
  - cma 14y ago
    Plus the other particle could later be "unobserved" through quantum erasure.. http://en.wikipedia.org/wiki/Quantum_eraser_experiment http://en.wikipedia.org/wiki/Quantum_eraser_experiment
    - jonmrodriguez 14y ago
      Not in the way that you're imagining. Otherwise, you could easily communicate by repeatedly observing and then unobserving until finally you see the bit that you don't want to send, and then you let it be.
- danielweber 14y ago
  and have the alien equivalent of the FCC come out to the planet to arrest the scientists for transmitting without a license Or the channel is already filled completely with spam.
  - bluedanieru 14y ago
    Maybe they can tell me how to make my jagon bigger.
  - wladimir 14y ago
    Or filled with get rich quick scams. That reminds me of a Charles Stross novel, I think Accelerando.
    - ajuc 14y ago
      That should be a quest in some space exploration game. You research hiperspace radio, then you decode alien languages, then you get mysterious quest involving intergalactic wars, lost artifacts and last scions of ruling families. You duly follow the leads to learn more about these subjects, find that nothing seems to check up, only to find out at the end that it was just Nigerian scam.
- klodolph 14y ago
  > short story about someone doing this and discovering that it was the galactic equivalent of 'shortwave' radio and have the alien equivalent of the FCC come out to the planet to arrest the scientists for transmitting without a license "Fine Structure" -- bits of a soft-SF story you can read online http://qntm.org/structure http://qntm.org/structure http://everything2.com/?node=fine+structure http://everything2.com/?node=fine+structure It's of varying quality, I think people usually link to "Oul's Egg" first because it's one of the best sections.
  - CoffeeDregs 14y ago
    Thank you. I had never seen "Fine Structure" and enjoyed it. It's of varying quality It's worth a lot more than what I paid for it. ;)
    - mck- 14y ago
      Hehe, you got me curious to read Oul's Egg.. quite interesting, but kind of bothered by the ending...
- DanBC 14y ago
  Greg Egan - I can't say more because spoilers.
  - ramidarigaz 14y ago
    Diaspora?
- jrl 14y ago
  That sounds like a very cool science fiction idea :-)
- rorrr 14y ago
  The most interesting question to me is - what happens when one of the entangled particles get annihilated. If the other one does too, instantly, then we can use that to communicate faster than light.
  - shasta 14y ago
    (X -> FLT) -> ~X
    - politician 14y ago
      There's a saying, "Relativity, causality, FTL. Pick two."
  - Retric 14y ago
    Entanglement is a lot more superficial / complex than that. QM is so differnet than how we are used to dealing with things the only really useful way to understand it is the Math it's based on. But, decoherence is the hart of a lot of the spooky voodo that really trips people up so it's a much better place to start than the 'cool' vs. trying to extrapilate based on the odd stuff that goes on. http://en.wikipedia.org/wiki/Quantum_decoherence http://en.wikipedia.org/wiki/Quantum_decoherence PS: Also, avoid thinking at the large and small scales at the same time. Sensors really are just more fields and particles just like everything else. Also, decoherence is not an all or nothing event it's more a sliding scale between wave and particle.
  - gus_massa 14y ago
    No. If you annihilate one of the particles, the other particle continues "unaffected". Whatever you do to one of the particles, the other particle continues "unaffected". Quantum mechanic is a little more complex, so it is difficult to explain what "unaffected" really means. It's clearer with an example. Let's suppose that you and I have each one a particle that is a half of an entanglement particle pair: * If you don't do anything with your particle, for each measurement that I can do there are some possible results with some probabilities. * If you do any measurement to your particle, for each measurement that I can do I have exactly the same possible results with the same probabilities. * If you do annihilate your particle :( , for each measurement that I can do I have exactly the same possible results with the same probabilities. * Whatever you do with your particle, for each measurement that I can do I have exactly the same possible results with the same probabilities. In that sense my particle is "unaffected". I can't do any measurement to know what you have done with your particle. So you can't use your particle to transmit information to me. The strange effect is that if we later meet and compare notes, we will see that the results you get with your particle and the results I get with my particle are related. Some measurements always are equal, some have the opposite value and some are related in more complex ways. Each measurement (yours and mine) alone are perfectly normal. The strange effect appears only when the result are compared.
- Dn_Ab 14y ago
  The experiment you all came up with sounds great, a lot like Bell's experiment but without the detail of setting up correlations and their implications. Information cannot be transferred faster than light using entanglement. This is already proved by the no communication theorem. Essentially, because since the outcome of measurements is random/probabilistic, ambiguity is introduced on the receiver end. So since we do not allow [local] hidden variables, there is no way for person B to know whether or not they need to apply a further transformation to get to the original state without person A transferring classical bits. I am not sure what the news in this article is because I found nothing surprising in it.
fixermark 14y ago
One thing I've never understood about interpretation of quantum entanglement experiments: Based on what I've learned of these experiments, it seems to me that the least mind-bending interpretation is that the entanglement event results in two particles that have some complementary property k (i.e. one particle has k and the other has ~k) and which one of the two, k or ~k, is had by one of the particles is unknowable without collapsing the quantum state. However, both particles have the property from their inception, and so no faster-than-light or non-local interpretation should be needed. When the particles are separated to great distance and one is observed to collapse to k, the other must be observed to collapse to ~k because it was always a ~k particle. Clearly, there's some aspect of the experiments that I don't understand that invalidates this simple interpretation, but I do not know what it is.
- Osmium 14y ago
  "However, both particles have the property from their inception, and so no faster-than-light or non-local interpretation should be needed." This is incorrect, both particles have a superposition of k and ~k (to use your terminology). It is not the case that when the wavefunction collapses, we just suddenly find out the "internal state" of the particle. This "internal state" simply does not exist until the wavefunction collapse has occurred. The "no faster than light" doctrine refers specifically to the transfer of information. And no information, in the strictest sense, has been transferred. Think of it like this: try to imagine how you could communicate with someone using quantum entanglement. If you can communicate something, then information has been transferred. But you can't. All you know is that if your particle is k, then theirs is ~k. Now, you could make up a rule book that says "if my particle is k, then I'll do x, and you'll do y" but that rule book will have to have been shared between both parties before hand, at less than the speed of light. Edit: should just mention I'm not an expert here, and articles like this are always interesting. Finding loopholes in our conventional understanding. But it's important to know just /what/ our conventional understanding is first to realise why things like this are important.
  - iliis 14y ago
    Even with such a preshared rule book you wouldn't get any FTL communication. You could crate a pair of random number generators this way which will always give the same result. Certainly useful for cryptographic purposes but sadly not direct communication.
mindstab 14y ago
:/ the more I learn and read about quantum mechanics I'm starting to realize the FTL quantum communication seems to be an SF pipe dream hoped and imagined on some possible theories but they seem to be less than likely. What a PITA. Ah well, maybe we can still use singularities or something to punch holes in space/time and shoot info through singularity routers :) Though that does seem even farther away then making quantum mech doing communication for us. ah well
chiph 14y ago
Not a physicist (theoretical or otherwise), but to me the introduction of the concept of "hidden variables" reminds me of the addition of Phlogiston to the body of knowledge. Unless... they're using it as a placeholder for as-yet undiscovered ways that entangled particles can interact, and it's known that it's a placeholder.
- Osmium 14y ago
  "Hidden variables" have been proven to not be a satisfactory explanation for quantum mechanical effects for some time now. http://en.wikipedia.org/wiki/Bell%27s_theorem http://en.wikipedia.org/wiki/Bell%27s_theorem Though, I should add, what makes this paper interesting is that they try and find loophole around Bell's theorem. This would be a big deal if true.
  - Anderkent 14y ago
    Local hidden variables. Bell's theorem does not address non-local variables.
    - numbsafari 14y ago
      So Satan was the angel that made reality work by using global variables. He was cast out by god for violating the coding standard. All the evil you see in the world is the result of errors caused by the use of global variables and a lack of sufficient testing prior to release that was the result of an unreasonable schedule that was pulled out of God's ass simply because he wanted to prove he could make the angels ship something in six days. We need an update to Time Bandits.
- tomjen3 14y ago
  Hm I don't think the issue with Phlogiston was that we didn't know what it was, but that rather than try to use the scientific method to make predictions about it, they just wrote down "when a, do b, Phlogiston does x". In this case they are doing experiments where they expect the hidden variables to do a very specific job (transmit information) and are trying to test whether they do what they expect them to do or not. That is science.
ffk 14y ago
This reminds me of the Ansible, a communication device allowing instant or near instant faster-than-light communication. It is seen in many science fiction works and often justified as quantum entanglement in practice. https://en.wikipedia.org/wiki/Ansible https://en.wikipedia.org/wiki/Ansible
- jmaygarden 14y ago
  I didn't realize that Ursula Le Guin coined that term. My first recollection of it was from Orson Scott Card and later, Dan Simmons. In any event, entanglement certainly doesn't look like it will lead to an ansible yet.
  - ffk 14y ago
    Yea, my first encounter was in Ender's Game. :)
    - jmaygarden 14y ago
      I loved that book as a kid. It looks like they are finally going to release the movie version in a year from now. They've been teasing us with movie announcements for the better part of a decade. I wonder if they'll directly mention quantum entanglement? http://www.imdb.com/title/tt1731141/ http://www.imdb.com/title/tt1731141/
Osmium 14y ago
As an aside, congrats to Ars for including a link to the paper (and a DOI) in the article. They always do this, and it's really something that needs to be encouraged elsewhere, especially in newspapers. I think it's fair to say if you don't link to the original paper, what you do shouldn't be counted as "science journalism."
Anderkent 14y ago
The paper is behind a paywall, so I cannot see if they researches addressed this already, but doesn't Everett interpretation preserve locality and not need faster than light propagation? Are the results incompatible, or was it simply disregarded to make the paper look more impactful?
ecliptic 14y ago
If you were able to construct a rigid beam 25 million miles long wouldn't you be able to transmit data (push a button) on the other end faster than the speed of light?
- MBlume 14y ago
  Nope. Pushing on the end of the beam sends a pressure wave through the beam. The button gets pushed when the wave reaches the other end. Wave travels significantly slower than light.
  - ecliptic 14y ago
    If you push on the beam and the other end does not move right away doesn't that imply that the beam has compressed and is not rigid?
    - Osmium 14y ago
      What you're proposing is an effect mediated by physical forces. Specifically, on a fundamental, atomic level your beam is made up of atoms. And when you push it, you're bringing atoms closer together, that feel an electrostatic repulsion. And the beam moves. So, in the best case, this method of communication is equivalent to communicating with electromagnetic waves. And they have a speed limit: the speed of light :) So this wouldn't be faster than light communication.
    - deleted 14y ago
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    - ars 14y ago
      > doesn't that imply that the beam has compressed and is not rigid? Yes it does. And in fact that is the case: The beam has compressed. A fully rigid beam is impossible.
- nessus42 14y ago
nessus42 14y ago
This article completely ignored the Many Worlds Interpretation, which preserves all of realism, locality, and Relativity.
- MBlume 14y ago
  Yeah. It shouldn't be that surprising that when you arbitrarily assume that part of the wavefunction magically disappears sometimes, you get weird results like non-locality.
  - Osmium 14y ago
    Could you expand on this a little more? I'm not quite sure I follow.
    - Symmetry 14y ago
      Lets say you have some experiment which gives you a 1 or a 0, and you have two entangled particles which will give the same result depsite the result being random and the measurements being taken outside each other's lightcones. In Collapse QM there was some instantanious communication making that possible. In Many Worlds you'd find that for each measurement the wavefunction split into two branches, one corresponding to 1 and one to 0, in each location. From the sites of both measurements the decoherence spreads outwards as particle interactions happen, and when they finally touch the two 1 segments and the two 0 segments merge, and there is one area split along 1/0 in the wave function instead of two. That's all horribly simplified, but should get the idea across.
    - btilly 14y ago
      The many world's interpretation is based on the assumption that both the observed system and the observer is modeled by a wavefunction. In that case observation is entanglement, which seems weird to us, but which makes perfect logical sense. Other interpretations of quantum mechanics start with the assumption that the world is fundamentally like our naive experience leads us to think of it as. Therefore when we observe something, we couldn't really be thrown into a superposition of states, each of which observed a different thing and which cannot meaningfully interact due to quantum mechanical principles. If we can't be thrown into such a superposition, then there must be some sort of underlying reality, or quantum collapse, or other weirdness that is not explained by QM. All of the weirdness of interpretations other than many worlds can be understood as the conflict between how they would like to understand the world (there is a reality that happened), and how quantum mechanics described things. All of the weirdness of the many worlds interpretation can be understood as, "We can't perceive the process of quantum superposition, so it seems really, really weird to us."
jcknight 14y ago
What?
Liron 14y ago
I hate this article, because locality is an important property that we think reality does have. It's like saying "Gravity violates conservation of energy!!!" No it doesn't; quit playing fast and loose with well-understood principles.
- xanmas 14y ago
  Locality in this sense refers to the presence of "hidden variables" as first put forth by bohm and later suggested by other people. It was believed that the various CHSH, HOM, and other bell-inequality inspired experiments would have put a nail in the coffin but critics always came back and say "but you didn't control for X." This experiment is just another in the long line of experiments started by Aspect and continued by others to put bounds on this.
cs702 14y ago
I've always found classical Newtonian physics, the Special Theory of Relativity, and (to a lesser extent) the General Theory of Relativity to be understandable at an intuitive level, but quantum phenomena just baffles me, even when I think I "understand" it. PS. IvoDankolov: I find it difficult or impossible to think intuitively about quantum phenomena.
- mrose 14y ago
  This Heisenberg quote puts the general "feeling" of quantum physics into an understandable form: “What we observe is not nature itself, but nature exposed to our method of questioning.”
- IvoDankolov 14y ago
  In what terms do you think you "understand" it? What do you make of this problem with distant entangled particles? The double slit experiment and interference in general? The Heisenberg uncertainty principle? (Or as I'd like to call it, Heisenberg's horribly mislabeled-in-order-to-confuse-students principle) A shot in the dark - many of the problems with coming to terms with quantum mechanics arise from trying to impose on it that it should somehow behave like classical mechanics, or that somehow we humans stand above it and look down upon it, and heaven forbid that we're part of a qunatum system).
  - elteto 14y ago
    Exactly, we humans try to learn new things by relating to what we already know. This leads to trouble when we encounter new subjects that have no connection to previous experiences, because we have a hard time relating to it. However, the problem with people finding QM esoteric and weird is a bit more nuanced than that. In most of the other areas of knowledge we can make progress because they do resemble reality in ways that we are familiar with. For example rotation in classical mechanics or special relativity can be somewhat confusing to a beginner, but if we think a little bit deeper than what we are used to then we can see that the results make sense and match what we experience. And also this experience is consistent at different scales. Now you move to QM and the first thing that hits you is that reality "breaks" after some point in the size scale, beyond that everything is different, with uncertainties, probabilities and a bunch of other odd properties. Learning QM is an exercise in mind-stretching, even for the most capable of us. For me the problem is not that it is different, but why is different. Why do we have such a gap between the large scale and the small scale? That is what baffles me.
pjscott 14y ago
Here's the paper in question: http://arxiv.org/pdf/1110.3795.pdf http://arxiv.org/pdf/1110.3795.pdf (Man, I love arxiv.org. It's so nice.)
jere 14y ago
Can someone provide or point to a layman's definition of "realism" in quantum mechanics? Recently on a call-in podcast I listen to, a caller has claimed that the following article shows that a) realism has been disproven and b) that proves we are all in a computer simulation: http://physicsworld.com/cws/article/news/2007/apr/20/quantum-physics-says-goodbye-to-reality http://physicsworld.com/cws/article/news/2007/apr/20/quantum... >Now physicists from Austria claim to have performed an experiment that rules out a broad class of hidden-variables theories that focus on realism -- giving the uneasy consequence that reality does not exist when we are not observing it Sure. Anyway, I'm certain this guy is equivocating, but I don't even know where to read up on the relevant topics. And I read articles like these and they keep using the word realism without defining it.
- toufka 14y ago
  Philosophically (as written by Einstein, Heisenberg, etc.) there are three assumptions about the world - of which one is broken by the quantum world. We do not know which one. Depending on your interpretation of the Schrodinger equation you can interpret the breaking in any of the three ways. This article says its realism - but as far as I know there is no way to prove its realism and not one of the others. One of these cannot hold: - Realism, Locality, or Separability Realism - Einstein's idea of 'elements of reality' - the idea that things actually are material and unchanging. Their properties are constant unless acted upon (ie a Hydrogen atom at time1 and point1 has a constancy of properties at time2 and point2). You could imagine a universe that had, as a property of the universe, atoms change mass every other day - just for the hell of it. Locality - The idea that a system can only be affected by things within its lightcone - no effect can travel faster than the speed of light. And two objects must be 'local' before they can interact. Separability - any system separated by non-trivial space/time can be considered independent of each other. The idea that 1/2mv^2 does not include every other part of the universe in trying to figure out what m or v are. Not quite the same as locality - as it would still require local interactions, but that net of interactions could be so thick and pervasive as to prevent any meaningful isolation of any system. As is stated above, these concepts fall out of the EPR experiment.
  - 14y ago
powertower 14y ago
I'm starting to believe that we confuse the mathematics of quantum mechanics with being the actual underlining physical phenomena, to the point where we start to believe that "wave function collapse" are real, rather than just a way of looking at something via probabilities; or being the models, or abstractions, or the artifacts of the maths involved.
- btilly 14y ago
  Take that one step farther and you'll arrive at the conclusion that if we take QM seriously, then there is never a collapse, and the many worlds interpretation is a logical consequence.
joe_the_user 14y ago
Ok, as far as I can tell, this all just a rehash of EPR paradox and the weirdness of the Copenhagen interpretation of QM. The many worlds interpretation resolves everything with full locality. http://en.wikipedia.org/wiki/Many-worlds_interpretation http://en.wikipedia.org/wiki/Many-worlds_interpretation Just consider, if I take a black marble and a white in hand, shake them up and put them in two boxes so I don't know which marble is in which box and put the two boxes on two sides of the world. Then, when I open one box, I instantly learn the state of the marble in the other. The information is "transmitted" to me "faster than the speed of light" but all because the actual switch event happened in the past. Now, as far as I can, QM entanglement is pretty much the same thing as these boxes. The only weird thing is that the standard interpretation (not the standard theory since it is not necessary for any meaningful measurement), the standard interpretation is that in QM, the equivalent of our marble switch happens when the final measurement happens. There's nothing in any of this research that involves any concrete evidence whatsoever that faster-than-light information transmittal happens, (feel free to counter-references that), but as far as I've read, all the "transmittal" is "transmittal" the sense of above.
- kybernetikos 14y ago
  Could you explain why your example isn't just a hidden variable explanation, which as we know from measurements of Bell's Inequality ( http://en.wikipedia.org/wiki/Bell%27s_theorem http://en.wikipedia.org/wiki/Bell%27s_theorem ), cannot result in a theory compatible with both locality and the predictions of quantum mechanics.
  - Anderkent 14y ago
    The example is a hidden variable explanation, simply because you cannot do metaphors for quantum physics using classical physics. Perhaps another approach will be clearer: Imagine magical filter goggles, of two types: red and blue. People wearing red goggles can only see other people wearing red goggles, people wearing blue goggles can only see other people wearing blue goggles. You put a pair of red and blue goggles to each box, then have four people pick a pair of goggles each. The result is that each person only sees one other person, near the second box, wearing the same colour goggles. This is more or less how many-worlds works, except instead of both red and blue goggles in the box you have a single pair of goggles that is both red and blue, but not at the same time (decoherence), and thus instead of two people wearing different kind of goggles, you have a single person wearing goggles that are both blue and red.
ZenoArrow 14y ago
Usual disclaimers: I am not a scientist and my knowledge of quantum mechanics is very minimal. However... "Repeated experiments have verified that this works even when the measurements are performed more quickly than light could travel between the sites of measurement" What interests me at this stage is not 'why' entanglement works, but rather how it can be used. The article indicates that repeated experiments have verified it appears to be a real process, where actions on one particle can influence another particle (seemingly) no matter how far away, so why can't we use it for communication? To use a simple example, if I encode data as movement in the 'sender' particle, and the movement can be monitored in the 'receiver' particle, then you can have a system for sending messages. Seems to me the need to control the internal state of the particles to communicate may be unnecessary. Perhaps to explore further, can anyone here the types of measurable phenomena that have been seen to be mirrored in entangled particles (the ones you know of, anyway)? Would particle spin be one example? Thanks!
svedlin 14y ago
Bohmian mechanics [1] is a deterministic interpretation of QM that's empirically equivalent to traditional quantum theory. However, it requires non-locality (instantaneous action across vast distances). From a recent paper [2]: "Bohmian mechanics reproduces all statistical predictions of quantum mechanics, which ensures that entanglement cannot be used for superluminal signaling. However, individual Bohmian particles can experience superluminal inﬂuences." This paper [3] referenced in the Ars Technica article shows that finite superluminal velocities (c < v < inf) can be exploited to achieve superluminal signalling. Very interesting result. I assume BM is still consistent with this. The paper does mention BM: "Bohmian mechanics and the collapse theory of Ghirardi, Rimini, and Weber [...] reproduce all tested quantum predictions, [however] they violate the principle of continuity mentioned above (otherwise they would not be compatible with no-signalling as our results imply)." The principle of continuity described in the paper: "In both cases, we expect the chain of events to satisfy a principle of continuity: that is, the idea that the physical carriers of causal influences propagate continuously through space." "Clearly, one may ask whether infinite speed is a necessary ingredient to account for the correlations observed in Nature or whether a finite speed v, recovering a principle of continuity, is sufficient." Actions can happen instantaneously under BM ("infinite speed"), so BM is still consistent with QM and doesn't allow FTL communication. [1] http://en.wikipedia.org/wiki/De_Broglie%E2%80%93Bohm_theory http://en.wikipedia.org/wiki/De_Broglie%E2%80%93Bohm_theory [2] http://arxiv.org/pdf/1207.2794.pdf http://arxiv.org/pdf/1207.2794.pdf [3] http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys2460.html http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys...
Xcelerate 14y ago
If you all want a good, intuitive explanation of quantum mechanics that really captures the "deep" aspects of the subject (as opposed to the shallow, rote memorization style of McQuarrie, et al.), see: http://lesswrong.com/lw/r5/the_quantum_physics_sequence/ http://lesswrong.com/lw/r5/the_quantum_physics_sequence/ It's kind of weird and has some asides about consciousness and what not but other than that, it gives you a really good mindset for thinking about the subject.
scythe 14y ago
Here's the original paper: http://arxiv.org/abs/1110.3795 http://arxiv.org/abs/1110.3795 [pdf] http://arxiv.org/pdf/1110.3795v1 http://arxiv.org/pdf/1110.3795v1 So let's clear up some misconceptions: First, this is not a problem specific to Copenhagen or any other interpretation of QM. Second, it is not just the EPR paradox. Third, it is not the first time someone has proposed a system utilizing quantum entanglement for superluminal communication; that title goes to Karl Popper: http://en.wikipedia.org/wiki/Poppers_experiment http://en.wikipedia.org/wiki/Poppers_experiment Popper's experiment was a very clever way to get around the no-communication theorem. It was eventually performed, but no FLT communication could be observed [some suggest we need to keep looking]. I presume that this new paper is another clever way to avoid the NCT, but I don't have time to read it at the moment.
mixedbit 14y ago
Are you aware of any attempt to explain quantum superposition (and entanglement) on the information theory basis? Is it possible that superposition is an optimization that allows the Universe not to compute stuff that does not have any observable effect? Maybe classical Universe would be much more computationally expensive?
- svedlin 14y ago
  Nice observation. I believe there was a recent paper that touches on this topic: http://phys.org/news/2012-03-quantum-physics-matrix-efficient.html http://phys.org/news/2012-03-quantum-physics-matrix-efficien... http://www.nature.com/ncomms/journal/v3/n3/full/ncomms1761.html http://www.nature.com/ncomms/journal/v3/n3/full/ncomms1761.h...
Xcelerate 14y ago
The way that I best understand quantum entanglement (and maybe someone like jessriedel can correct me if I'm wrong) is thus: You may be familiar with the Schrodinger equation (HΨ = ih' dΨ/dt) or the more accurate time-dependent Dirac equation. In each of these equations is a function called the wavefunction (denoted with Ψ). This function represents the "quantum state" of your system -- in other words, all the information that exists within a system. Ψ evolves deterministically in time. You can apply an operation to this function, and when you do, you get the original function back multiplied by some value. There are different operators, each corresponding to an "observable" (the thing you measure in the lab). For example, you can measure momentum, position, energy, spin etc... and each of these observables has a different corresponding mathematical operation that you perform on Ψ to get XΨ, where X is the mean value of the observable. Now in QM textbooks, you'll frequently see Ψ written as Ψ(r, t), where r is a position vector and t is time. r denotes the position of whatever particle constitutes your system. So what if you have a two particle system like hydrogen (proton and electron) or positronium (positron and electron)? Well, a QM textbook will write the state of your quantum system as Ψ1 * Ψ2 and completely gloss over the fact that this approximation does not apply in all situations. It is physically inaccurate to say that Ψ of two particles is two individual Ψ's multiplied together. For some systems, it is a good approximation and makes things easy to calculate, but in reality, there is really only one wavefunction Ψ and it is a function of all the particles in the universe. So now you can see where entanglement comes in. If Ψ is a wavefunction of all extant particles, then surely there will be correlations between every measurement of Ψ that you take! Now most of the time, you can't find correlations -- too many particles are affecting too many other particles (decoherence). But if you prepare two of them together and keep outside particles from interfering with them, then you can observe the correlations between two particles no matter what the distance! So now for the whole "information transfer" business. I said earlier that applying an operator to Ψ gives you the value of an observable -- what you measure. The weird thing though is that what you measure isn't always exactly this value. Instead, the mean value of many measurements will be this value. You can also compute the standard deviation of these measurements using Ψ, but that's about it. Nobody knows where the random "noise" in measurements comes from. So far, it seems as though our universe just has some randomness inherent to it (and we're quickly ruling out all remaining superdeterministic theories; Gerard t'Hooft seems to be a hold-out: http://physics.stackexchange.com/questions/34217/why-do-people-categorically-dismiss-some-simple-quantum-models http://physics.stackexchange.com/questions/34217/why-do-peop...) So you can't control or predict the individual measurements to as much precision as you'd like. Sucks, huh? Anyway, you can plot and analyze this data, and what you'll notice for two entangled particles separated by thousands of miles or more is that there are statistical correlations between the two sets of data (again, data you can't control -- if you can't control it, you can't send information with it). Obviously, you need both sets of data to notice that there are correlations.
teeja 14y ago
My grandma puts a donut in one of two boxes, closes both, gives one to each of us. You leave for Mars and, at an agreed-upon instant, I open my box. Now I know what is in your box, and you know what's in mine. No faster-than-light, no mystery.
teresko 14y ago
It would be interesting to see the result of experiment where one electron in entangled pair collides with a positron. I suspect that this might give some interesting results.
tripzilch 14y ago
BTW, while I'm not entirely sure what this article is saying, here's one of the best explanations of "Quantum" I've ever seen. Unlike most, it doesn't start out from physics, but from a mathematical/probability framework ... that allows for negative probabilities. Yeah. http://www.scottaaronson.com/democritus/lec9.html http://www.scottaaronson.com/democritus/lec9.html I'll quote a bit: ----------- So, what is quantum mechanics? Even though it was discovered by physicists, it's not a physical theory in the same sense as electromagnetism or general relativity. In the usual "hierarchy of sciences" -- with biology at the top, then chemistry, then physics, then math -- quantum mechanics sits at a level between math and physics that I don't know a good name for. Basically, quantum mechanics is the operating system that other physical theories run on as application software (with the exception of general relativity, which hasn't yet been successfully ported to this particular OS). There's even a word for taking a physical theory and porting it to this OS: "to quantize." But if quantum mechanics isn't physics in the usual sense -- if it's not about matter, or energy, or waves, or particles -- then what is it about? From my perspective, it's about information and probabilities and observables, and how they relate to each other. Ray Laflamme: That's very much a computer-science point of view. Scott: Yes, it is. My contention in this lecture is the following: Quantum mechanics is what you would inevitably come up with if you started from probability theory, and then said, let's try to generalize it so that the numbers we used to call "probabilities" can be negative numbers. As such, the theory could have been invented by mathematicians in the 19th century without any input from experiment. It wasn't, but it could have been. ----------- Greatly recommended. I can't say I understand it all, but the concept of a "qubit" and why it's so different from a regular bit is a lot clearer to me.
pebb 14y ago
There is another explaination: http://discovermagazine.com/2010/apr/01-back-from-the-future http://discovermagazine.com/2010/apr/01-back-from-the-future The future causing the past.
pauldirac137 14y ago
Better not tell this to Mitt Romney.
Eliezer 14y ago
It can be local in many-worlds. http://lesswrong.com/lw/q1/bells_theorem_no_epr_reality/ http://lesswrong.com/lw/q1/bells_theorem_no_epr_reality/ http://lesswrong.com/lw/q2/spooky_action_at_a_distance_the_nocommunication/ http://lesswrong.com/lw/q2/spooky_action_at_a_distance_the_n...
- wamatt 14y ago
  Looks like you're a firm believer, so I ask, is MWI by your definition falsifiable? Do any experiments past, current or future purport to test predictions made from MWI? While MWI may make sense in a universal reading of QM, I'm not sure we can simply extrapolate this onto the macro level, despite it's pragmatic successes over the last century. IMHO QM itself, still feels like a temporary hack (shim), when compared alongside ER. Until we have a TOE, it may be too presumptuous to be making positive assertive claims on the correlation between the MWI model and reality.
  - Eliezer 14y ago
    http://lesswrong.com/lw/q4/decoherence_is_falsifiable_and_testable/ http://lesswrong.com/lw/q4/decoherence_is_falsifiable_and_te...
    - pfedor 14y ago
      If decoherence is "untestable" relative to collapse, then so too, collapse is "untestable" relative to decoherence. Of course! Nobody questions that. The two are equivalent. That means, neither is more true than the other. You just happen to be in the minority which feels that thinking about multiple worlds makes their head ache less than thinking about the state vector reduction. What if the history of physics had transpired differently—what if Hugh Everett and John Wheeler had stood in the place of Bohr and Heisenberg, and vice versa? My guess is, we wouldn't have had to wait until 1957 for someone to say: "Fuck that noise, let's just forget about the multiple worlds and pretend the wave function collapses when I make a measurement." They would be like, "All right buddy, I'm sure you're totally right that there are really multiple worlds and all that, I'll just act like it's merely a wave function collapse for a moment, just to get on with my work of actually doing something as a physicist." My guess is that would happen within the year.
tehwalrus 14y ago
Having read this very carefully, I can't see how quantum mechanics as I always understood it can ever be either non-local or break relativity. The following was probably written down by someone else a long time ago, but I don't know by who or when. When you entangle two systems, they have to actually be in the same place/with slower-than-light communication distance. Thus, they each store "secret" information in some deterministic way which is impossible to measure directly, and which we can probe only using repeated experiments and probability distributions. The idea is that while quantum systems are deterministic just like mechanical ones, they hide their determinism in black boxes whose mechanisms it is impossible to discover. The "uncollapsed waveform" has in fact been collapsed ahead of time, you just don't know what the answer is until you measure. Back to our entangled particles, when you separate them the determined decision they made when they were together is revealed. They do not need to communicate, as they already have their story straight. It seems this entire area of research is a misnomer - if you let the prisoners talk to each other just after arrest, of course their stories will be complimentary later, even if one is in Alcatraz and the other in Siberia. You just have to assume that particles so small we can only "see" them in the way they interact with other particles have some internal structure/state that we also can't see, and thus like the prisoners, can remember the story. (of course, declaring an entire area of research irrelevant is usually a sign that I've missed the point wildly - if anyone would like to correct my mistake I'd be interested to hear!)

158 points by suprgeek 14y ago

LokiSnake 14y ago

That very adorable kitten is way too distracting, and made it very hard to focus on the article.

deleted 14y ago
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ChuckMcM 14y ago

This is for me the most interesting part of the whole teleportation / entanglement research. When these papers started coming out we debated the notion of 'faster than light' communication. The counter argument was that you had to move the particles apart and that was constrained by the speed of light. Then the question of "when" the state was resolved was pondered. There were two thought experiments proposed at this meetup. In the first, two highly accurate clocks, separated by enough distance that that speed of light issues were unambiguously resolved by the clocks, which where triggered by the resolution of the quantum state of an entangled particle. Capturing the exact time when the state was known (detangled) on each clock, and then comparing the times on the clocks to determine whether or not it was truly 'instant'. The second experiment was to set up a 'set' of entangled particles at a distance and resolve only 'some' of them. The set representing 'bits' and the resolved set representing the 'information.' The question of course is whether or not you can move information this way. Seems like we are getting closer to answering those questions in a definitive way. I always thought it would be fun to write a short story about someone doing this and discovering that it was the galactic equivalent of 'shortwave' radio and have the alien equivalent of the FCC come out to the planet to arrest the scientists for transmitting without a license :-)

gwillen 14y ago
This doesn't work because there is no moment at which the state is 'known' or 'detangled'. There is no way to tell, by observing one particle, when the other particle was observed. If there was you could send a signal with it, which would be no good. The universe is a strange place.
- cma 14y ago
  Plus the other particle could later be "unobserved" through quantum erasure.. http://en.wikipedia.org/wiki/Quantum_eraser_experiment http://en.wikipedia.org/wiki/Quantum_eraser_experiment
  - jonmrodriguez 14y ago
    Not in the way that you're imagining. Otherwise, you could easily communicate by repeatedly observing and then unobserving until finally you see the bit that you don't want to send, and then you let it be.
danielweber 14y ago
and have the alien equivalent of the FCC come out to the planet to arrest the scientists for transmitting without a license Or the channel is already filled completely with spam.
- bluedanieru 14y ago
  Maybe they can tell me how to make my jagon bigger.
- wladimir 14y ago
  Or filled with get rich quick scams. That reminds me of a Charles Stross novel, I think Accelerando.
  - ajuc 14y ago
    That should be a quest in some space exploration game. You research hiperspace radio, then you decode alien languages, then you get mysterious quest involving intergalactic wars, lost artifacts and last scions of ruling families. You duly follow the leads to learn more about these subjects, find that nothing seems to check up, only to find out at the end that it was just Nigerian scam.
klodolph 14y ago
> short story about someone doing this and discovering that it was the galactic equivalent of 'shortwave' radio and have the alien equivalent of the FCC come out to the planet to arrest the scientists for transmitting without a license "Fine Structure" -- bits of a soft-SF story you can read online http://qntm.org/structure http://qntm.org/structure http://everything2.com/?node=fine+structure http://everything2.com/?node=fine+structure It's of varying quality, I think people usually link to "Oul's Egg" first because it's one of the best sections.
- CoffeeDregs 14y ago
  Thank you. I had never seen "Fine Structure" and enjoyed it. It's of varying quality It's worth a lot more than what I paid for it. ;)
  - mck- 14y ago
    Hehe, you got me curious to read Oul's Egg.. quite interesting, but kind of bothered by the ending...
DanBC 14y ago
Greg Egan - I can't say more because spoilers.
- ramidarigaz 14y ago
  Diaspora?
jrl 14y ago
That sounds like a very cool science fiction idea :-)
rorrr 14y ago
The most interesting question to me is - what happens when one of the entangled particles get annihilated. If the other one does too, instantly, then we can use that to communicate faster than light.
- shasta 14y ago
  (X -> FLT) -> ~X
  - politician 14y ago
    There's a saying, "Relativity, causality, FTL. Pick two."
- Retric 14y ago
  Entanglement is a lot more superficial / complex than that. QM is so differnet than how we are used to dealing with things the only really useful way to understand it is the Math it's based on. But, decoherence is the hart of a lot of the spooky voodo that really trips people up so it's a much better place to start than the 'cool' vs. trying to extrapilate based on the odd stuff that goes on. http://en.wikipedia.org/wiki/Quantum_decoherence http://en.wikipedia.org/wiki/Quantum_decoherence PS: Also, avoid thinking at the large and small scales at the same time. Sensors really are just more fields and particles just like everything else. Also, decoherence is not an all or nothing event it's more a sliding scale between wave and particle.
- gus_massa 14y ago
  No. If you annihilate one of the particles, the other particle continues "unaffected". Whatever you do to one of the particles, the other particle continues "unaffected". Quantum mechanic is a little more complex, so it is difficult to explain what "unaffected" really means. It's clearer with an example. Let's suppose that you and I have each one a particle that is a half of an entanglement particle pair: * If you don't do anything with your particle, for each measurement that I can do there are some possible results with some probabilities. * If you do any measurement to your particle, for each measurement that I can do I have exactly the same possible results with the same probabilities. * If you do annihilate your particle :( , for each measurement that I can do I have exactly the same possible results with the same probabilities. * Whatever you do with your particle, for each measurement that I can do I have exactly the same possible results with the same probabilities. In that sense my particle is "unaffected". I can't do any measurement to know what you have done with your particle. So you can't use your particle to transmit information to me. The strange effect is that if we later meet and compare notes, we will see that the results you get with your particle and the results I get with my particle are related. Some measurements always are equal, some have the opposite value and some are related in more complex ways. Each measurement (yours and mine) alone are perfectly normal. The strange effect appears only when the result are compared.
Dn_Ab 14y ago
The experiment you all came up with sounds great, a lot like Bell's experiment but without the detail of setting up correlations and their implications. Information cannot be transferred faster than light using entanglement. This is already proved by the no communication theorem. Essentially, because since the outcome of measurements is random/probabilistic, ambiguity is introduced on the receiver end. So since we do not allow [local] hidden variables, there is no way for person B to know whether or not they need to apply a further transformation to get to the original state without person A transferring classical bits. I am not sure what the news in this article is because I found nothing surprising in it.

fixermark 14y ago

One thing I've never understood about interpretation of quantum entanglement experiments: Based on what I've learned of these experiments, it seems to me that the least mind-bending interpretation is that the entanglement event results in two particles that have some complementary property k (i.e. one particle has k and the other has ~k) and which one of the two, k or ~k, is had by one of the particles is unknowable without collapsing the quantum state. However, both particles have the property from their inception, and so no faster-than-light or non-local interpretation should be needed. When the particles are separated to great distance and one is observed to collapse to k, the other must be observed to collapse to ~k because it was always a ~k particle. Clearly, there's some aspect of the experiments that I don't understand that invalidates this simple interpretation, but I do not know what it is.

Osmium 14y ago
"However, both particles have the property from their inception, and so no faster-than-light or non-local interpretation should be needed." This is incorrect, both particles have a superposition of k and ~k (to use your terminology). It is not the case that when the wavefunction collapses, we just suddenly find out the "internal state" of the particle. This "internal state" simply does not exist until the wavefunction collapse has occurred. The "no faster than light" doctrine refers specifically to the transfer of information. And no information, in the strictest sense, has been transferred. Think of it like this: try to imagine how you could communicate with someone using quantum entanglement. If you can communicate something, then information has been transferred. But you can't. All you know is that if your particle is k, then theirs is ~k. Now, you could make up a rule book that says "if my particle is k, then I'll do x, and you'll do y" but that rule book will have to have been shared between both parties before hand, at less than the speed of light. Edit: should just mention I'm not an expert here, and articles like this are always interesting. Finding loopholes in our conventional understanding. But it's important to know just /what/ our conventional understanding is first to realise why things like this are important.
- iliis 14y ago
  Even with such a preshared rule book you wouldn't get any FTL communication. You could crate a pair of random number generators this way which will always give the same result. Certainly useful for cryptographic purposes but sadly not direct communication.

mindstab 14y ago

:/ the more I learn and read about quantum mechanics I'm starting to realize the FTL quantum communication seems to be an SF pipe dream hoped and imagined on some possible theories but they seem to be less than likely. What a PITA. Ah well, maybe we can still use singularities or something to punch holes in space/time and shoot info through singularity routers :) Though that does seem even farther away then making quantum mech doing communication for us. ah well

chiph 14y ago

Not a physicist (theoretical or otherwise), but to me the introduction of the concept of "hidden variables" reminds me of the addition of Phlogiston to the body of knowledge. Unless... they're using it as a placeholder for as-yet undiscovered ways that entangled particles can interact, and it's known that it's a placeholder.

Osmium 14y ago
"Hidden variables" have been proven to not be a satisfactory explanation for quantum mechanical effects for some time now. http://en.wikipedia.org/wiki/Bell%27s_theorem http://en.wikipedia.org/wiki/Bell%27s_theorem Though, I should add, what makes this paper interesting is that they try and find loophole around Bell's theorem. This would be a big deal if true.
- Anderkent 14y ago
  Local hidden variables. Bell's theorem does not address non-local variables.
  - numbsafari 14y ago
    So Satan was the angel that made reality work by using global variables. He was cast out by god for violating the coding standard. All the evil you see in the world is the result of errors caused by the use of global variables and a lack of sufficient testing prior to release that was the result of an unreasonable schedule that was pulled out of God's ass simply because he wanted to prove he could make the angels ship something in six days. We need an update to Time Bandits.
tomjen3 14y ago
Hm I don't think the issue with Phlogiston was that we didn't know what it was, but that rather than try to use the scientific method to make predictions about it, they just wrote down "when a, do b, Phlogiston does x". In this case they are doing experiments where they expect the hidden variables to do a very specific job (transmit information) and are trying to test whether they do what they expect them to do or not. That is science.

ffk 14y ago

This reminds me of the Ansible, a communication device allowing instant or near instant faster-than-light communication. It is seen in many science fiction works and often justified as quantum entanglement in practice. https://en.wikipedia.org/wiki/Ansible https://en.wikipedia.org/wiki/Ansible

jmaygarden 14y ago
I didn't realize that Ursula Le Guin coined that term. My first recollection of it was from Orson Scott Card and later, Dan Simmons. In any event, entanglement certainly doesn't look like it will lead to an ansible yet.
- ffk 14y ago
  Yea, my first encounter was in Ender's Game. :)
  - jmaygarden 14y ago
    I loved that book as a kid. It looks like they are finally going to release the movie version in a year from now. They've been teasing us with movie announcements for the better part of a decade. I wonder if they'll directly mention quantum entanglement? http://www.imdb.com/title/tt1731141/ http://www.imdb.com/title/tt1731141/

Osmium 14y ago

As an aside, congrats to Ars for including a link to the paper (and a DOI) in the article. They always do this, and it's really something that needs to be encouraged elsewhere, especially in newspapers. I think it's fair to say if you don't link to the original paper, what you do shouldn't be counted as "science journalism."

Anderkent 14y ago

The paper is behind a paywall, so I cannot see if they researches addressed this already, but doesn't Everett interpretation preserve locality and not need faster than light propagation? Are the results incompatible, or was it simply disregarded to make the paper look more impactful?

ecliptic 14y ago

If you were able to construct a rigid beam 25 million miles long wouldn't you be able to transmit data (push a button) on the other end faster than the speed of light?

MBlume 14y ago
Nope. Pushing on the end of the beam sends a pressure wave through the beam. The button gets pushed when the wave reaches the other end. Wave travels significantly slower than light.
- ecliptic 14y ago
  If you push on the beam and the other end does not move right away doesn't that imply that the beam has compressed and is not rigid?
  - Osmium 14y ago
    What you're proposing is an effect mediated by physical forces. Specifically, on a fundamental, atomic level your beam is made up of atoms. And when you push it, you're bringing atoms closer together, that feel an electrostatic repulsion. And the beam moves. So, in the best case, this method of communication is equivalent to communicating with electromagnetic waves. And they have a speed limit: the speed of light :) So this wouldn't be faster than light communication.
  - deleted 14y ago
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  - ars 14y ago
    > doesn't that imply that the beam has compressed and is not rigid? Yes it does. And in fact that is the case: The beam has compressed. A fully rigid beam is impossible.
nessus42 14y ago

nessus42 14y ago

This article completely ignored the Many Worlds Interpretation, which preserves all of realism, locality, and Relativity.

MBlume 14y ago
Yeah. It shouldn't be that surprising that when you arbitrarily assume that part of the wavefunction magically disappears sometimes, you get weird results like non-locality.
- Osmium 14y ago
  Could you expand on this a little more? I'm not quite sure I follow.
  - Symmetry 14y ago
    Lets say you have some experiment which gives you a 1 or a 0, and you have two entangled particles which will give the same result depsite the result being random and the measurements being taken outside each other's lightcones. In Collapse QM there was some instantanious communication making that possible. In Many Worlds you'd find that for each measurement the wavefunction split into two branches, one corresponding to 1 and one to 0, in each location. From the sites of both measurements the decoherence spreads outwards as particle interactions happen, and when they finally touch the two 1 segments and the two 0 segments merge, and there is one area split along 1/0 in the wave function instead of two. That's all horribly simplified, but should get the idea across.
  - btilly 14y ago
    The many world's interpretation is based on the assumption that both the observed system and the observer is modeled by a wavefunction. In that case observation is entanglement, which seems weird to us, but which makes perfect logical sense. Other interpretations of quantum mechanics start with the assumption that the world is fundamentally like our naive experience leads us to think of it as. Therefore when we observe something, we couldn't really be thrown into a superposition of states, each of which observed a different thing and which cannot meaningfully interact due to quantum mechanical principles. If we can't be thrown into such a superposition, then there must be some sort of underlying reality, or quantum collapse, or other weirdness that is not explained by QM. All of the weirdness of interpretations other than many worlds can be understood as the conflict between how they would like to understand the world (there is a reality that happened), and how quantum mechanics described things. All of the weirdness of the many worlds interpretation can be understood as, "We can't perceive the process of quantum superposition, so it seems really, really weird to us."

jcknight 14y ago

What?

Liron 14y ago

I hate this article, because locality is an important property that we think reality does have. It's like saying "Gravity violates conservation of energy!!!" No it doesn't; quit playing fast and loose with well-understood principles.

xanmas 14y ago
Locality in this sense refers to the presence of "hidden variables" as first put forth by bohm and later suggested by other people. It was believed that the various CHSH, HOM, and other bell-inequality inspired experiments would have put a nail in the coffin but critics always came back and say "but you didn't control for X." This experiment is just another in the long line of experiments started by Aspect and continued by others to put bounds on this.

cs702 14y ago

I've always found classical Newtonian physics, the Special Theory of Relativity, and (to a lesser extent) the General Theory of Relativity to be understandable at an intuitive level, but quantum phenomena just baffles me, even when I think I "understand" it. PS. IvoDankolov: I find it difficult or impossible to think intuitively about quantum phenomena.

mrose 14y ago
This Heisenberg quote puts the general "feeling" of quantum physics into an understandable form: “What we observe is not nature itself, but nature exposed to our method of questioning.”
IvoDankolov 14y ago
In what terms do you think you "understand" it? What do you make of this problem with distant entangled particles? The double slit experiment and interference in general? The Heisenberg uncertainty principle? (Or as I'd like to call it, Heisenberg's horribly mislabeled-in-order-to-confuse-students principle) A shot in the dark - many of the problems with coming to terms with quantum mechanics arise from trying to impose on it that it should somehow behave like classical mechanics, or that somehow we humans stand above it and look down upon it, and heaven forbid that we're part of a qunatum system).
- elteto 14y ago
  Exactly, we humans try to learn new things by relating to what we already know. This leads to trouble when we encounter new subjects that have no connection to previous experiences, because we have a hard time relating to it. However, the problem with people finding QM esoteric and weird is a bit more nuanced than that. In most of the other areas of knowledge we can make progress because they do resemble reality in ways that we are familiar with. For example rotation in classical mechanics or special relativity can be somewhat confusing to a beginner, but if we think a little bit deeper than what we are used to then we can see that the results make sense and match what we experience. And also this experience is consistent at different scales. Now you move to QM and the first thing that hits you is that reality "breaks" after some point in the size scale, beyond that everything is different, with uncertainties, probabilities and a bunch of other odd properties. Learning QM is an exercise in mind-stretching, even for the most capable of us. For me the problem is not that it is different, but why is different. Why do we have such a gap between the large scale and the small scale? That is what baffles me.

pjscott 14y ago

Here's the paper in question: http://arxiv.org/pdf/1110.3795.pdf http://arxiv.org/pdf/1110.3795.pdf (Man, I love arxiv.org. It's so nice.)

jere 14y ago

Can someone provide or point to a layman's definition of "realism" in quantum mechanics? Recently on a call-in podcast I listen to, a caller has claimed that the following article shows that a) realism has been disproven and b) that proves we are all in a computer simulation: http://physicsworld.com/cws/article/news/2007/apr/20/quantum-physics-says-goodbye-to-reality http://physicsworld.com/cws/article/news/2007/apr/20/quantum... >Now physicists from Austria claim to have performed an experiment that rules out a broad class of hidden-variables theories that focus on realism -- giving the uneasy consequence that reality does not exist when we are not observing it Sure. Anyway, I'm certain this guy is equivocating, but I don't even know where to read up on the relevant topics. And I read articles like these and they keep using the word realism without defining it.

toufka 14y ago
Philosophically (as written by Einstein, Heisenberg, etc.) there are three assumptions about the world - of which one is broken by the quantum world. We do not know which one. Depending on your interpretation of the Schrodinger equation you can interpret the breaking in any of the three ways. This article says its realism - but as far as I know there is no way to prove its realism and not one of the others. One of these cannot hold: - Realism, Locality, or Separability Realism - Einstein's idea of 'elements of reality' - the idea that things actually are material and unchanging. Their properties are constant unless acted upon (ie a Hydrogen atom at time1 and point1 has a constancy of properties at time2 and point2). You could imagine a universe that had, as a property of the universe, atoms change mass every other day - just for the hell of it. Locality - The idea that a system can only be affected by things within its lightcone - no effect can travel faster than the speed of light. And two objects must be 'local' before they can interact. Separability - any system separated by non-trivial space/time can be considered independent of each other. The idea that 1/2mv^2 does not include every other part of the universe in trying to figure out what m or v are. Not quite the same as locality - as it would still require local interactions, but that net of interactions could be so thick and pervasive as to prevent any meaningful isolation of any system. As is stated above, these concepts fall out of the EPR experiment.
- 14y ago

powertower 14y ago

I'm starting to believe that we confuse the mathematics of quantum mechanics with being the actual underlining physical phenomena, to the point where we start to believe that "wave function collapse" are real, rather than just a way of looking at something via probabilities; or being the models, or abstractions, or the artifacts of the maths involved.

btilly 14y ago
Take that one step farther and you'll arrive at the conclusion that if we take QM seriously, then there is never a collapse, and the many worlds interpretation is a logical consequence.

joe_the_user 14y ago

Ok, as far as I can tell, this all just a rehash of EPR paradox and the weirdness of the Copenhagen interpretation of QM. The many worlds interpretation resolves everything with full locality. http://en.wikipedia.org/wiki/Many-worlds_interpretation http://en.wikipedia.org/wiki/Many-worlds_interpretation Just consider, if I take a black marble and a white in hand, shake them up and put them in two boxes so I don't know which marble is in which box and put the two boxes on two sides of the world. Then, when I open one box, I instantly learn the state of the marble in the other. The information is "transmitted" to me "faster than the speed of light" but all because the actual switch event happened in the past. Now, as far as I can, QM entanglement is pretty much the same thing as these boxes. The only weird thing is that the standard interpretation (not the standard theory since it is not necessary for any meaningful measurement), the standard interpretation is that in QM, the equivalent of our marble switch happens when the final measurement happens. There's nothing in any of this research that involves any concrete evidence whatsoever that faster-than-light information transmittal happens, (feel free to counter-references that), but as far as I've read, all the "transmittal" is "transmittal" the sense of above.

kybernetikos 14y ago
Could you explain why your example isn't just a hidden variable explanation, which as we know from measurements of Bell's Inequality ( http://en.wikipedia.org/wiki/Bell%27s_theorem http://en.wikipedia.org/wiki/Bell%27s_theorem ), cannot result in a theory compatible with both locality and the predictions of quantum mechanics.
- Anderkent 14y ago
  The example is a hidden variable explanation, simply because you cannot do metaphors for quantum physics using classical physics. Perhaps another approach will be clearer: Imagine magical filter goggles, of two types: red and blue. People wearing red goggles can only see other people wearing red goggles, people wearing blue goggles can only see other people wearing blue goggles. You put a pair of red and blue goggles to each box, then have four people pick a pair of goggles each. The result is that each person only sees one other person, near the second box, wearing the same colour goggles. This is more or less how many-worlds works, except instead of both red and blue goggles in the box you have a single pair of goggles that is both red and blue, but not at the same time (decoherence), and thus instead of two people wearing different kind of goggles, you have a single person wearing goggles that are both blue and red.

ZenoArrow 14y ago

Usual disclaimers: I am not a scientist and my knowledge of quantum mechanics is very minimal. However... "Repeated experiments have verified that this works even when the measurements are performed more quickly than light could travel between the sites of measurement" What interests me at this stage is not 'why' entanglement works, but rather how it can be used. The article indicates that repeated experiments have verified it appears to be a real process, where actions on one particle can influence another particle (seemingly) no matter how far away, so why can't we use it for communication? To use a simple example, if I encode data as movement in the 'sender' particle, and the movement can be monitored in the 'receiver' particle, then you can have a system for sending messages. Seems to me the need to control the internal state of the particles to communicate may be unnecessary. Perhaps to explore further, can anyone here the types of measurable phenomena that have been seen to be mirrored in entangled particles (the ones you know of, anyway)? Would particle spin be one example? Thanks!

svedlin 14y ago

Bohmian mechanics [1] is a deterministic interpretation of QM that's empirically equivalent to traditional quantum theory. However, it requires non-locality (instantaneous action across vast distances). From a recent paper [2]: "Bohmian mechanics reproduces all statistical predictions of quantum mechanics, which ensures that entanglement cannot be used for superluminal signaling. However, individual Bohmian particles can experience superluminal inﬂuences." This paper [3] referenced in the Ars Technica article shows that finite superluminal velocities (c < v < inf) can be exploited to achieve superluminal signalling. Very interesting result. I assume BM is still consistent with this. The paper does mention BM: "Bohmian mechanics and the collapse theory of Ghirardi, Rimini, and Weber [...] reproduce all tested quantum predictions, [however] they violate the principle of continuity mentioned above (otherwise they would not be compatible with no-signalling as our results imply)." The principle of continuity described in the paper: "In both cases, we expect the chain of events to satisfy a principle of continuity: that is, the idea that the physical carriers of causal influences propagate continuously through space." "Clearly, one may ask whether infinite speed is a necessary ingredient to account for the correlations observed in Nature or whether a finite speed v, recovering a principle of continuity, is sufficient." Actions can happen instantaneously under BM ("infinite speed"), so BM is still consistent with QM and doesn't allow FTL communication. [1] http://en.wikipedia.org/wiki/De_Broglie%E2%80%93Bohm_theory http://en.wikipedia.org/wiki/De_Broglie%E2%80%93Bohm_theory [2] http://arxiv.org/pdf/1207.2794.pdf http://arxiv.org/pdf/1207.2794.pdf [3] http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys2460.html http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys...

Xcelerate 14y ago

If you all want a good, intuitive explanation of quantum mechanics that really captures the "deep" aspects of the subject (as opposed to the shallow, rote memorization style of McQuarrie, et al.), see: http://lesswrong.com/lw/r5/the_quantum_physics_sequence/ http://lesswrong.com/lw/r5/the_quantum_physics_sequence/ It's kind of weird and has some asides about consciousness and what not but other than that, it gives you a really good mindset for thinking about the subject.

scythe 14y ago

Here's the original paper: http://arxiv.org/abs/1110.3795 http://arxiv.org/abs/1110.3795 [pdf] http://arxiv.org/pdf/1110.3795v1 http://arxiv.org/pdf/1110.3795v1 So let's clear up some misconceptions: First, this is not a problem specific to Copenhagen or any other interpretation of QM. Second, it is not just the EPR paradox. Third, it is not the first time someone has proposed a system utilizing quantum entanglement for superluminal communication; that title goes to Karl Popper: http://en.wikipedia.org/wiki/Poppers_experiment http://en.wikipedia.org/wiki/Poppers_experiment Popper's experiment was a very clever way to get around the no-communication theorem. It was eventually performed, but no FLT communication could be observed [some suggest we need to keep looking]. I presume that this new paper is another clever way to avoid the NCT, but I don't have time to read it at the moment.

mixedbit 14y ago

Are you aware of any attempt to explain quantum superposition (and entanglement) on the information theory basis? Is it possible that superposition is an optimization that allows the Universe not to compute stuff that does not have any observable effect? Maybe classical Universe would be much more computationally expensive?

svedlin 14y ago
Nice observation. I believe there was a recent paper that touches on this topic: http://phys.org/news/2012-03-quantum-physics-matrix-efficient.html http://phys.org/news/2012-03-quantum-physics-matrix-efficien... http://www.nature.com/ncomms/journal/v3/n3/full/ncomms1761.html http://www.nature.com/ncomms/journal/v3/n3/full/ncomms1761.h...

Xcelerate 14y ago

The way that I best understand quantum entanglement (and maybe someone like jessriedel can correct me if I'm wrong) is thus: You may be familiar with the Schrodinger equation (HΨ = ih' dΨ/dt) or the more accurate time-dependent Dirac equation. In each of these equations is a function called the wavefunction (denoted with Ψ). This function represents the "quantum state" of your system -- in other words, all the information that exists within a system. Ψ evolves deterministically in time. You can apply an operation to this function, and when you do, you get the original function back multiplied by some value. There are different operators, each corresponding to an "observable" (the thing you measure in the lab). For example, you can measure momentum, position, energy, spin etc... and each of these observables has a different corresponding mathematical operation that you perform on Ψ to get XΨ, where X is the mean value of the observable. Now in QM textbooks, you'll frequently see Ψ written as Ψ(r, t), where r is a position vector and t is time. r denotes the position of whatever particle constitutes your system. So what if you have a two particle system like hydrogen (proton and electron) or positronium (positron and electron)? Well, a QM textbook will write the state of your quantum system as Ψ1 * Ψ2 and completely gloss over the fact that this approximation does not apply in all situations. It is physically inaccurate to say that Ψ of two particles is two individual Ψ's multiplied together. For some systems, it is a good approximation and makes things easy to calculate, but in reality, there is really only one wavefunction Ψ and it is a function of all the particles in the universe. So now you can see where entanglement comes in. If Ψ is a wavefunction of all extant particles, then surely there will be correlations between every measurement of Ψ that you take! Now most of the time, you can't find correlations -- too many particles are affecting too many other particles (decoherence). But if you prepare two of them together and keep outside particles from interfering with them, then you can observe the correlations between two particles no matter what the distance! So now for the whole "information transfer" business. I said earlier that applying an operator to Ψ gives you the value of an observable -- what you measure. The weird thing though is that what you measure isn't always exactly this value. Instead, the mean value of many measurements will be this value. You can also compute the standard deviation of these measurements using Ψ, but that's about it. Nobody knows where the random "noise" in measurements comes from. So far, it seems as though our universe just has some randomness inherent to it (and we're quickly ruling out all remaining superdeterministic theories; Gerard t'Hooft seems to be a hold-out: http://physics.stackexchange.com/questions/34217/why-do-people-categorically-dismiss-some-simple-quantum-models http://physics.stackexchange.com/questions/34217/why-do-peop...) So you can't control or predict the individual measurements to as much precision as you'd like. Sucks, huh? Anyway, you can plot and analyze this data, and what you'll notice for two entangled particles separated by thousands of miles or more is that there are statistical correlations between the two sets of data (again, data you can't control -- if you can't control it, you can't send information with it). Obviously, you need both sets of data to notice that there are correlations.

teeja 14y ago

My grandma puts a donut in one of two boxes, closes both, gives one to each of us. You leave for Mars and, at an agreed-upon instant, I open my box. Now I know what is in your box, and you know what's in mine. No faster-than-light, no mystery.

teresko 14y ago

It would be interesting to see the result of experiment where one electron in entangled pair collides with a positron. I suspect that this might give some interesting results.

tripzilch 14y ago

BTW, while I'm not entirely sure what this article is saying, here's one of the best explanations of "Quantum" I've ever seen. Unlike most, it doesn't start out from physics, but from a mathematical/probability framework ... that allows for negative probabilities. Yeah. http://www.scottaaronson.com/democritus/lec9.html http://www.scottaaronson.com/democritus/lec9.html I'll quote a bit: ----------- So, what is quantum mechanics? Even though it was discovered by physicists, it's not a physical theory in the same sense as electromagnetism or general relativity. In the usual "hierarchy of sciences" -- with biology at the top, then chemistry, then physics, then math -- quantum mechanics sits at a level between math and physics that I don't know a good name for. Basically, quantum mechanics is the operating system that other physical theories run on as application software (with the exception of general relativity, which hasn't yet been successfully ported to this particular OS). There's even a word for taking a physical theory and porting it to this OS: "to quantize." But if quantum mechanics isn't physics in the usual sense -- if it's not about matter, or energy, or waves, or particles -- then what is it about? From my perspective, it's about information and probabilities and observables, and how they relate to each other. Ray Laflamme: That's very much a computer-science point of view. Scott: Yes, it is. My contention in this lecture is the following: Quantum mechanics is what you would inevitably come up with if you started from probability theory, and then said, let's try to generalize it so that the numbers we used to call "probabilities" can be negative numbers. As such, the theory could have been invented by mathematicians in the 19th century without any input from experiment. It wasn't, but it could have been. ----------- Greatly recommended. I can't say I understand it all, but the concept of a "qubit" and why it's so different from a regular bit is a lot clearer to me.

pebb 14y ago

There is another explaination: http://discovermagazine.com/2010/apr/01-back-from-the-future http://discovermagazine.com/2010/apr/01-back-from-the-future The future causing the past.

pauldirac137 14y ago

Better not tell this to Mitt Romney.

Eliezer 14y ago

It can be local in many-worlds. http://lesswrong.com/lw/q1/bells_theorem_no_epr_reality/ http://lesswrong.com/lw/q1/bells_theorem_no_epr_reality/ http://lesswrong.com/lw/q2/spooky_action_at_a_distance_the_nocommunication/ http://lesswrong.com/lw/q2/spooky_action_at_a_distance_the_n...

wamatt 14y ago
Looks like you're a firm believer, so I ask, is MWI by your definition falsifiable? Do any experiments past, current or future purport to test predictions made from MWI? While MWI may make sense in a universal reading of QM, I'm not sure we can simply extrapolate this onto the macro level, despite it's pragmatic successes over the last century. IMHO QM itself, still feels like a temporary hack (shim), when compared alongside ER. Until we have a TOE, it may be too presumptuous to be making positive assertive claims on the correlation between the MWI model and reality.
- Eliezer 14y ago
  http://lesswrong.com/lw/q4/decoherence_is_falsifiable_and_testable/ http://lesswrong.com/lw/q4/decoherence_is_falsifiable_and_te...
  - pfedor 14y ago
    If decoherence is "untestable" relative to collapse, then so too, collapse is "untestable" relative to decoherence. Of course! Nobody questions that. The two are equivalent. That means, neither is more true than the other. You just happen to be in the minority which feels that thinking about multiple worlds makes their head ache less than thinking about the state vector reduction. What if the history of physics had transpired differently—what if Hugh Everett and John Wheeler had stood in the place of Bohr and Heisenberg, and vice versa? My guess is, we wouldn't have had to wait until 1957 for someone to say: "Fuck that noise, let's just forget about the multiple worlds and pretend the wave function collapses when I make a measurement." They would be like, "All right buddy, I'm sure you're totally right that there are really multiple worlds and all that, I'll just act like it's merely a wave function collapse for a moment, just to get on with my work of actually doing something as a physicist." My guess is that would happen within the year.

tehwalrus 14y ago

Having read this very carefully, I can't see how quantum mechanics as I always understood it can ever be either non-local or break relativity. The following was probably written down by someone else a long time ago, but I don't know by who or when. When you entangle two systems, they have to actually be in the same place/with slower-than-light communication distance. Thus, they each store "secret" information in some deterministic way which is impossible to measure directly, and which we can probe only using repeated experiments and probability distributions. The idea is that while quantum systems are deterministic just like mechanical ones, they hide their determinism in black boxes whose mechanisms it is impossible to discover. The "uncollapsed waveform" has in fact been collapsed ahead of time, you just don't know what the answer is until you measure. Back to our entangled particles, when you separate them the determined decision they made when they were together is revealed. They do not need to communicate, as they already have their story straight. It seems this entire area of research is a misnomer - if you let the prisoners talk to each other just after arrest, of course their stories will be complimentary later, even if one is in Alcatraz and the other in Siberia. You just have to assume that particles so small we can only "see" them in the way they interact with other particles have some internal structure/state that we also can't see, and thus like the prisoners, can remember the story. (of course, declaring an entire area of research irrelevant is usually a sign that I've missed the point wildly - if anyone would like to correct my mistake I'd be interested to hear!)