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Science

Uncertainty Sets Limits On Quantum Nonlocality 223

Posted by Soulskill from the your-teleporter-concept-will-need-a-redesign dept.
An anonymous reader writes "Research in today's issue of the journal Science helps explain why quantum theory is as weird as it is, but not weirder. Ex-hacker Stephanie Wehner and physicist Jonathan Oppenheim showed that the Heisenberg Uncertainty Principle sets limits on Einstein's 'spooky action at a distance.' Wired reports that the discovery was made by 'thinking of things in the way a hacker might' to uncover a fundamental link between the two defining properties of quantum physics (abstract, supplement). Oppenheim describes how uncertainty and nonlocality are like coding problems, enabling us to make a quantitative link between two of the cornerstones of quantum theory."
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Uncertainty Sets Limits On Quantum Nonlocality More

Uncertainty Sets Limits On Quantum Nonlocality

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  • for the lulz (Score:5, Funny)

    by Pojut ( 1027544 ) on Friday November 19, 2010 @11:40AM (#34282624) Homepage

    I want to believe in quantum physics, but I'm not sure.

    • I don't believe in myths, like quantum physics and octopuses.

    • Which part?

      I mean, like any theory, it has its holes
      (Like Gravity, Black Holes, Dark matter, still unexplained).

      Quantum Mechanics has enough empirical evidence behind it (We've preformed and verified quantum entanglement at least) - you should be as willing to accept it as any other scientific theory you've come to accept.

      • Actually QM is probably one of the best attested theories in the history of science. To disbelieve it at this point is no different than being a geocentrist.

        • What does it mean to believe in QM, though? I believe that it makes predictions that accurately describe what we can see and measure. Does that mean that the models it uses constitute a true understanding of how the universe behaves? These aren't things you can believe in or not believe in - either they work in a given situation or they don't. I don't "believe" in Newton's laws of motion. They fairly accurately predict what happens in certain real-life situations, but we know that they are only helpful

          • Geocentrism isn't a useful theory. It's not even a theory, it's a "disproved theory." It's an idea that's know specifically to be false.

            QM on the other hand better models much of life than anything we've come up with to this point. Doesn't mean that there isn't an alternate explanation or one which encompasses more in a more reliable way, but it would be quite unlikely that it's wrong the way that geocentrism is.

            • Geocentrism (the belief) is a known false idea that you'd have to be an idiot to continue to believe in. Geocentrism (the model) is useful insofar as its predections match reality. Taking just the earth-moon-sun trio, I can accurately model eclipses, tides, lunar cycles, seasons, etc.. It's just a model - an abstraction (although not particularly useful).

              Similarly, just because the earth is round, doesn't mean that a map of my city can't be projected onto a flat piece of paper and still be useful. Likew

        • Re: (Score:2)

          by lgw ( 121541 )

          Well, what do you mean by QM? Do electrons have quantized energy levels around a nucleus, mirrored in quantized energy levels in photons? Sure, can't get much more grounded in data than that. But that's not the contentious part. The really interesting part that's not just "interpretation" is the Bell inequalities, which seem surprisingly not well tested - is the field really content with a few experiments for something this important to our understanding?

      • Whoosh!

    • Re: (Score:2)

      by mcgrew ( 92797 ) *

      You know what Hawking says...

    • Well as soon as you believe in it, I'm going to stop believing in it.

      Sorry, it was the best I could come up with today. I'm just feeling quirky.

  • There's no link to the full article here in pDF (Score:3, Informative)

    by Anonymous Coward on Friday November 19, 2010 @11:45AM (#34282706)
    Definitely not a link to the full article here [sharepdfbooks.com] [Not-PDF warning].

  • Einstein, Heisenberg... (Score:5, Funny)

    by srussia ( 884021 ) on Friday November 19, 2010 @11:48AM (#34282736)
    Heck, they even hinted at Gödel. Why not throw in Monty Hall too... wait, they did.

    • There's a quantum version of the Monty Hall problem. Just knowing that scares the shit out of me.

      • Re: (Score:2)

        by mcgrew ( 92797 ) *

        At least it wasn't a quantum version of the Monty Python problem.

        Nobody expects the Monty Python problem.

  • Locality is the only thing stopping me from concluding the universe is entirely deterministic and free will doesn't exist.

    • Re: (Score:3, Funny)

      by adonoman ( 624929 )
      No, it's the absolute determinism of the universe that is stopping from concluding that the universe is deterministic. Neither you, nor locality had any choice in the matter.

    • Re: (Score:3, Insightful)

      by somersault ( 912633 )

      Even if there were external forces acting to control your will in this universe, how do you know they're non-deterministic themselves?

      Individuals certainly are responsible for their own choices anyway, even if you can accurately simulate 100% beforehand what they're going to choose.

      • Even if there were external forces acting to control your will in this universe, how do you know they're non-deterministic themselves?

        If these "external forces" are what define who you are as a person, are they really external?

      • I think as long as you are in the same system as the individual, then that's true. If you can sit outside of the system, I don't think it's true anymore. It would be like saying my computer is responsible for the BSOD that happened this morning.

      • If an individual has a choice, then by definition they have free will and so I would agree with you that they are responsible for their choice.

        If the universe turns out to be deterministic, then there is no choice to be made. What does "choice" mean in the context of a giant DFA?

        • What does "choice" mean in the context of a giant DFA?

          It means the outputs of any part of the system at any given time are predictable based on certain input. Give them another input and they may have a different output though. That is their "choice".

          But really at some stage there must be some determination. Whether that's outside of our ability to measure or not, I don't think it really matters. Even if some random component comes into play in each decision, how does that make it any more a real "free will" type decision than if it was entirely predictable?

      • If there are such forces we haven't discovered them yet. As in there is no unit of measure that I'm aware of which is capable of quantifying such forces.

        But, if you can accurately simulate 100% accurately what it is that they're going to do, they aren't responsible for the actions, because they're not the ones choosing to do it. You'd be the one that's responsible as you're the one that knows what's going to happen and are able to alter the outcome.

      • Re: (Score:2)

        by mcgrew ( 92797 ) *

        But it's pretty obvious that there are external forces controlling your will. For example, a lonely guy who meets the women of his dreams is going to have his life changed forever, and there's nothing whatever he can do about it. Thought and feelings are nothing more than complex chemical reactions. Hell, life itself is just complex chemistry, and it all has to follow the laws of physics. Every action has an equal and opposite reaction, whether a rocket burn or brain chemistry.

    • I misread the title of that post. I thought someone was giving away a Wii to whoever was in the right place at the right time.

    • Even with locality, the universe is still deterministic on the macroscopic level.

      • Go grok the Butterfly Effect [wikipedia.org] and you'll see how a probabilistic microscopic universe means that the macroscopic universe is also probabilistic.

        Really, you're sounding like one of those people trying to make a distinction between micro and macro evolution. It's the same thing with the same mechanics. I mean, sure, it's a pretty safe bet that every atom in the sun won't decay overnight and the sun will indeed rise tomorrow. An astronomically good bet. But the fact that it's a bet at all means that it's not
    • Really? Locality? Maybe I'm missing something, but I'd say that the uncertainty principle is a pretty solid case for the existence of free will. That whole probabilistic universe thing is kinda antithetical to a deterministic universe.
      I mean, with locality, venus isn't going to be influence my immediate actions, but if it's a deterministic universe, it doesn't really matter if it's the rest of the universe steering the boat, or if it's just my surroundings, the same things going to happen regardless. And s

  • a coding problem? (Score:5, Funny)

    by phantomfive ( 622387 ) on Friday November 19, 2010 @11:57AM (#34282850) Journal

    describes how uncertainty and nonlocality are like coding problems,

    In that case, I guarantee there is a bug.

  • Define 'observe' (Score:5, Insightful)

    by Twinbee ( 767046 ) on Friday November 19, 2010 @12:08PM (#34282970)
    Okay, rant time.

    Whenever I see a beginner's guide to quantum theory, I always invariably see a phrase similar to:
    "Stranger still, the electron doesn't even have properties like position and momentum until an observer measures them. "

    And every time, I always think "define 'observe'", because that word is incredibly fluffy, vague as well as being immensely irritating. If a bat miles away happens to look in that direction with nothing in the way, is that counted as an observation? Are there a trillion different ways to observe it, and have they all been tried out to see the phenomenon stands? None, I repeat NONE of the articles I have ever read actually even remotely begins to touch upon that subject.

    • Re: (Score:2, Informative)

      by Anonymous Coward

      You are asking a great question, the problem is that no-one knows the answer. This is the "measurement problem", one of the biggest conceptual problems in Quantum Mechanics.

      • Re: (Score:3, Insightful)

        by Sockatume ( 732728 )

        Indeed. It's phenomenologically pretty well-defined, inasmuch as we can set up systems and we know whether we're observing them or not, and what'll happen to them if we do observe them, but we haven't a clue as to the mechanistics of it all.

        • I'm afraid I don't have much advance physics, so I may be misunderstanding the problem so this question might be irrelevant.

          Does this mean that the problem is that we cannot establish a system in which we can fail to observe the electron but still have knowledge of it's behavior?

          I can grasp the oft described concept that the particle's behavior is influenced by our observation. I am failing to grasp why our observation or non observation has any effect on the particle at all.

          If anyone can explain this to

    • Re:Define 'observe' (Score:5, Informative)

      by MozeeToby ( 1163751 ) on Friday November 19, 2010 @12:17PM (#34283052)

      I think the closest plain English definition would be: has an interaction with something. More accurate, but more confusing might be: things are undefined until something happens that requires them to be defined in order for that something to happen. An electron doesn't have a position or a momentum until something occurs which require the electrons position and momentum to be known in order to determine the outcome. That might be a human being with an incredibly complex apparatus measuring the properties of an individual electron, or it might be a chemical reaction that is sweeping through the entire sample of whatever the electron is a part of.

      • An electron doesn't have a position or a momentum until something occurs which require the electrons position and momentum to be known in order to determine the outcome.

        Which kind of leads back to the idea that the Electron itself isn't there until it's been observed. And thats where Einstein was all like "Umm. no. Just because I can't see the moon doesn't mean it isn't there".

        Thats where a lot of the curfluffle is about.

        • The electron's there (with "there" being defined by a fuzzy cloud of possible positions/momentums), it's just that it doesn't have a precise position or momentum except at points in time when one of these quantities are "observed", as stated above.

        • You didn't hear? China blew up the moon while it was passing over them this last time. It's no longer there.

          (this is an example of the quantum possibility)

        • You're misinterpreting that.

          Just because I can't see the moon at present doesn't mean that it isn't there. But it also doesn't mean that it is there because I saw it yesterday and it should still be in orbit either.

          What it means is that there is a high likelihood that it is there, and not likely to have been stolen by some aliens in a plot to render our tidal tables worthless. Or whatever other possible way that the moon could go missing without crashing into the Earth.

      • Re: (Score:3, Interesting)

        by Just Some Guy ( 3352 )

        That might be a human being with an incredibly complex apparatus measuring the properties of an individual electron, or it might be a chemical reaction that is sweeping through the entire sample of whatever the electron is a part of.

        Fair enough. But does that chemical reaction require an observation to define its outcome if it depends on those quantum events? At what point do you decide that the decision must be made?

      • Re:Define 'observe' (Score:5, Funny)

        by Anonymous Coward on Friday November 19, 2010 @12:38PM (#34283240)

        So what you're saying the the universe uses "just in time" physics.

        • Re: (Score:2, Interesting)

          by Anonymous Coward

          The universe lazily loads the details when we want to inspect it. In reality the algorithm has optimized these things away. Its like forcing it to load the debugging symbols when you attach to the physics.exe process.

      • While your post adequately describes the accepted thinking, it defies rational thought. While we may lack the ability to measure something beyond a certain limit, that is not evidence that the underlying physics is indeterministic, or inherently unknowable.

        By accepting that it was, we veered right out of the realm of science, and physics continues to be mired in probabilistic nonsense. You can calculate things, but the model provides no insight into the underlying reality. Seventy years later, and we are

      • Part of the problem is the view that an electron is some physical entity that occupies a specific location, has a specific radius, etc..

        All we know of what electrons 'look like' or 'physically are' are what we can measure.. which turns out to be a force vector. Thats it. A god damned force vector.

        All those typical macro-descriptions of things just dont really apply except in a statistical sense..

        ...those macro descriptions are of physical properties, but an electron doesnt really have an analog to a

      • Re: (Score:2)

        by skids ( 119237 )

        Except that quantum superposition happens in aggregates [scientificamerican.com], not just single ... eh ... particles.

        (Which is why Q-gates are supposed to work.)

        I think the analogy to lazy evaluation is apt, just doesn't help with this particular question.

      • Re: (Score:2)

        by srussia ( 884021 )
        It boils down to this: position is time-independent (albeit with a big caveat--can't get into it right now, but you are welcome to subscribe to my newsletter), while momentum entails t.

    • Re: (Score:2, Funny)

      by guybrush3pwood ( 1579937 )

      None, I repeat NONE of the articles I have ever read actually even remotely begins to touch upon that subject.

      Perhaps they don't touch it because you read them. Don't read them, and there's a 50/50 chance they will...

    • The technical term is a "measurement", which is an interaction with the particle which requires information on a property (which is defined by an operator). If a billiard ball strikes you, it observes your momentum and position. That's my understanding. I'm more puzzled by how it's possible to interact with a particle in a manner which doesn't cause its superposition to break down...

    • Re: (Score:2, Informative)

      by Anonymous Coward

      In the quantum mechanical sense "observe" means to measure the property. That is a particle does not have momentum until someone measures it. Thus if a bat miles away happens to measure the the property it would count as an observation.

      Essentially, in order for a quantum mechanical system to be observed there must be an interaction between the system and whatever does the observing (such as a photon). Prior to observation the system is thought of to exist in a superposition of states and after observatio

    • Definition: [princeton.edu] Observation - The act of making and recording a measurement

      In the case of an electron, it is the means used to measure position or energy that necessarily precludes the ability to know both. If I remember my lay-physics right, it has to do with choosing to measure a wave or a particle. Measure one, and measurements of the other become impossible. (Someone please correct my interpretation.)

      • Again lay-physics here, but i think it was measure one, and it changes the other. So as soon as you know one, the other becomes obsolete.

    • Re: (Score:2)

      by ardle ( 523599 )
      The important word is "measure" not "observer". Your sample sentence could be:

      "Stranger still, the electron doesn't even have properties like position and momentum until an armadillo measures them. "

      Furthermore, we define the things we measure, not nature. We might not be measuring the most useful things yet.
      We know that matter isn't made up of particles but we measure it that way because we know how to do that...

    • Re:Define 'observe' (Score:5, Informative)

      by gsliepen ( 303583 ) on Friday November 19, 2010 @12:41PM (#34283262)

      The best definition I have heard is this: suppose we have an observer O in state A, and a system S which is in the superposition of the states 1 and 2. When the observer observers the system, the state of S does not collapse, rather the observer and system become one, say OS, and is in a superposition of the states A1 and A2.

      You can interpret this in various ways; one could say that this means the observer, or even the whole universe for that matter, branches all the time, and/or all possible states of the observer/universe exist simultaneously, however that again is just a description, not what might really be the case.

      Disclaimer: I am a physicist.

      • Re: (Score:3, Informative)

        by Ignatius ( 6850 )

        No, if the System would end up in state 1/sqrt2 * (|A1>+|A2>), then no observation has taken place as

        1/sqrt2 * (|A1>+|A2>) = |A> x 1/sqrt2 * (|1>+|2>)

        with "x" being the tensor product. The post measurement state would have to be an entangled state, e.g. something like

        1/sqrt2 * (|A1>+|B2>)

        with |B> being the state of the observer after having heard a click on the Geiger counter, while in |A> there has been no click.

    • I overheard one physicist refer to it not as observation or measurement, but rather *amplification*. If the information about the property (position, momentum, spin, whatever) is amplified to a larger scale than the original property (e.g. neurons firing, pencils moving, printer printing, beam of light moving in a different direction), only then does that collapses the uncertainty.

      If the information is merely transferred to another particle without amplification (say, by bumping into it) the uncertainty re

    • Re: (Score:2)

      by u19925 ( 613350 )
      You have hit the heart of the problem in QM. The observer is YOU. Yes, that is what it is. I can assume everything in the world is made out of fundamental particles and describe a quantum state of it. At this point "nothing" exist (the qm wave is just mathematical probability equation) until I observe. In Schrodinger cat analogy: what if you keep a scientist inside a cage? Well in this case, the scientist mind is in two states too (dead cat and alive cat) until you open the cat and make the observation at w

      • Re: (Score:2)

        by .sig ( 180877 )

        until you open the cat and make the observation

        Just when we thought the poor cat couldn't take any more abuse...

    • Re: (Score:2)

      by 0123456 ( 636235 )

      None, I repeat NONE of the articles I have ever read actually even remotely begins to touch upon that subject.

      How do you know where something is unless you look for it?

      It's really that simple: the object is somewhere, but you can't tell where it is unless you look. Until then it could be anywhere.

      As for the 'spooky action at a distance', that's merely a consequence of not using the relativistic version of Schrodinger's Equation which gives you two waveforms going in different directions in time. Which is a mathermatical shorthand for 'that object was somewhere but I couldn't tell where until I looked, and now I kno

    • According to the ideas in Decoherence, the measurement problem, and interpretations of quantum mechanics [arxiv.org] the term "observe" means to strongly interact with a system that is large enough and complicated enough to behave classically. I believe the majority of physicists who think about these things would agree with this definition.

      I like to think of it as the particle (or system) being measured becoming classical-like during the measurement before going on its own merry quantum way again.

    • Re: (Score:2)

      by lelitsch ( 31136 )

      Actually, if you would have read past that point, you'd have seen that the process is actually very rigorously defined. It's when whatever particle you use to observe the system interacts with the system. So if you bounce a photon of an electron, that's the observation, not when the photon comes back to hit the photo receptor.
      The problem is that this rigorous definition is way past what you want to go into in a beginners guide. A good place to start is looking up quantum decoherence. But the short version i

    • Yeah, the term "observer" confuses a lot of people, resulting in confusing it with consiousness.

      "Observer" means any event that requires the cause to happen before the effect. For example, two particles that change the energy configuration of a point in space are observers, because their collision has macroscopic consequences.

    • Re: (Score:2)

      by fermion ( 181285 )
      To back up what other people have said, an observation is a a measurement. In the act of measuring a quantity, we fix certain properties of the object being measured.

      IMHO, part of this is due to the historical language we use to communicate. We speak of massive particles and waves even though we do not know if these words apply for very small things. It is the same way we talk about electricity and magnetism as if they are separate things. There are several ambiguous words that are used in science bec

    • Re: (Score:2)

      by radtea ( 464814 )

      And every time, I always think "define 'observe'", because that word is incredibly fluffy, vague as well as being immensely irritating.

      This is the central question. Having studied QM more than a little, and spent a great deal of time on questions of interpretation, I see the fundamental question as not "why is QM so weird?" but "why does the classical world manifest at all?"

      No one has any idea why this is so, and no one is even asking the question, so far as I know.

    • I'd recommend listening to Dr. Feynman's lecture on that. He does a pretty good job of explaining Heisenberg's uncertainty principle. My recollection of it is a bit fuzzy, but it's not just seeing it or being there, it's actively trying to observe it. As in if you're measuring or recording it weird stuff happens. Heisenberg's uncertainty principle doesn't even pop into it until you try to measure either the velocity or the position. There is no problem with measure the number of electrons in a cathode ray a

    • You need to keep in mind that all of quantum mechanics is exclusively about probabilities, and no "reality" separate from the probabilities can exist. Niels Bohr repeatedly stated [google.com] that quantum mechanics forbids any ontological questions to be raised; "is it real?" is simply not a question that this theory can answer. Unfortunately, people have been attempting to do just that ever since '37 and confused the issue beyond repair, because a quantum probability really is a purely epistemological quantity, descri

  • Paywall (Score:3, Informative)

    by zrbyte ( 1666979 ) on Friday November 19, 2010 @12:24PM (#34283116)
    If anybody cares to read it, a preprint of the whole article can be found here. [cam.ac.uk]

    • Re: (Score:2)

      by malakai ( 136531 )

      I have to say, it's an interesting read. Even if you don't understand the math, three's some Alice/Bob like narrative that lets you 'kinda' figure out what they are talking about.

      For the past 20 or so years, I've felt that software reverse engineers "crackers" could really aid many different disciplines in understanding 'natural' black boxes. The black boxes are the natrual demarcation points where for lack of better technology or limits of physics, we can't look past that point. We can only monitor a set o

  • Wired article completely misleading (Score:5, Informative)

    by DrJimbo ( 594231 ) on Friday November 19, 2010 @01:14PM (#34283568)
    The Wired article "explains" entanglement by talking about Bob predicting what Alice did even though Alice is far away from Bob. This is the fundamental misunderstanding of quantum entanglement and has led to all sorts of wacky (and false) speculations and "theories".

    The actual paper [cam.ac.uk] correctly says:

    Non-locality can be exhibited when performing measurements on two or more distant quantum systems – the outcomes can be correlated in way that defies any local classical description. This is why we know that quantum theory will never by superceded by a local classical theory. Nevertheless, even quantum correlations are restricted to some extent – measurement results cannot be correlated so strongly that they would allow signalling between two distant systems.

    Quantum entanglement (QE) provides a correlation not a communication. What this means is that not only can't you use QE to pass signals (or any information) between Alice and Bob, you actually need some other form of after-the-fact communication between them to detect the correlation in order to determine if QE happened at all. If QE was a method of communication then you could verify it by sending Bob a "cheat cheat" of what Alice was going to do or transmit. Instead, you need to look at the outcome of a series of measurements taken by Alice's and the outcome of a series of measurments taken by Bob just to see if QE actually happened.

    Correlation is not communication.

    • I have never been able to find a place to answer this idea: Is it possible that quantum entanglement is actually just fixing two particles to a stable spin?

      (If I understand it correctly) If a person takes two entangled particles and take each one to a place farther than it takes the speed of light to travels, each can be measured faster than it would take for light to travel to "inform the other" of its state, yet each particle will always have the opposite of the other's state. The kicker for me is: one

  • Higgs Boson particle = Null value? (Score:2, Interesting)

    by Anonymous Coward

    Let me guess... The "God" (Higgs Boson) particle is nothing more than nature's null value to assign properties of something without it actually existing. How would that be for irony?

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