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Is something wrong with statistical interpretation of QM?

  1. Feb 19, 2014 #1
    The probabilistic nature of QM seems apparent(in theoretical formulation as well as experimentally), then why do most of the physicists give the credit of in-determinism to the nature itself and look for many worlds, abruptly collapsing observer conscious experiment setups, and the likes.

    Whereas it seems that in-determinism lies with the fact that there are far too many degrees of freedom involved in QM. That is, why does that cat has to be dead and alive at the same time or one cat dead and other alive in two different worlds, when it is perfectly understandable that we don't know the state of the cat because there are more degrees of freedom involved for which we can solve this particular setup.

    Similarly, why do we make the Quantum Entanglement superior than the classical case of a pair of shoes at different places? Is it because, the in-determinism of QM is currently considered fundamental in nature and therefore the states of the two annihilation photons must be entangled somehow so as that when we measure the spin of one photon the spin of the other photon is known simultaneously?

    Thanks
     
    Last edited: Feb 19, 2014
  2. jcsd
  3. Feb 19, 2014 #2

    Nugatory

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    Google for "Bell's Theorem". Entanglement is indeed fundamentally different from the classical case of a pair of shoes at different places.
     
  4. Feb 19, 2014 #3

    bhobba

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    Who told you that? I don't. For me its simply the most reasonable probability model that allows entanglement.
    http://arxiv.org/abs/quant-ph/0101012
    http://arxiv.org/abs/0911.0695

    Actually many physicists don't care that much about the interpretational stuff - they simply use it.

    You have been reading too many popularizations. According to the standard Copenhagen interpretation the cat is alive or dead - period. Schrodingers cat is not about a cat thats supposed to be alive and dead - no one seriously doubted that - its about the real issue with QM - namely how a theory that's a probability model about things that occur here in the common sense classical world explain that world which it assumes from the start.

    A lot of work, particularly in the area of decoherence, has been done on that issue with a lot of progress made - but some issues still remain.

    Entanglement is totally inexplicable by classical probability theory. In fact as the links I gave above show its what distinguishes it from standard probability theory in modelling physical systems.

    If you REALLY want to understand what QM is ACTUALLY about start here:
    http://www.scottaaronson.com/democritus/lec9.html

    Its not about wave particle dualities, wavefunction collapse, Schrodinger's Cat, consciousness causes collapse, and the usual stuff in popularizations - it's really simply a generalization of probability theory with features more suitable to model continuous changes, or, equivalently, allows entanglement.

    Thanks
    Bill
     
    Last edited: Feb 19, 2014
  5. Feb 19, 2014 #4

    jtbell

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    Or a pair of Bertlmann's socks. :wink:
     
  6. Feb 19, 2014 #5

    stevendaryl

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    It's the combination of two features of quantum mechanics that make it difficult to interpret in terms of classical probability. One is entanglement, and the second is incompatible observables--the fact that perfect knowledge about the value of one observable can preclude knowledge about other, noncommuting observables.

    Entanglement by itself is not particularly quantum-mechanical; as you point out, it has an analogue in joint probability distributions in classical probability theory. Incompatible observables by themselves don't force us to a new type of probability. Early on, the uncertainty principle was given a heuristic interpretation in terms of disturbances caused by measurement: The attempt to measure precisely a property such as position inevitably disturbs the object being measured, so that other incompatible properties are made uncertain. This disturbance interpretation of the uncertainty principle is different from what we would expect from classical physics, where it is assumed that all properties can be measured as precisely as we like without disturbing the object being measured. But it's not that difficult to understand in classical terms.

    But the combination of entanglement and the uncertainty principle both together makes quantum mechanics impossible to interpret in terms of classical probability. That's what experiments such as EPR show. In the classical case of entanglement, we can always assume that an entangled probability distribution simply reflects a lack of information: If we knew more precisely what the situation was, the entanglement would disappear. In the quantum case, because of the uncertainty principle, there is no more precise knowledge possible in some cases. And because entanglement can affect objects that are arbitrarily far apart, it's not plausible to think of the uncertainty principle in terms of disturbances due to the measurement process: Measuring the spin of one particle in an EPR twin-pair experiment might be disturbing that particle, but certainly it isn't disturbing the other particle far, far away (unless disturbances travel instantaneously, which is the interpretation given by the Bohm-DeBroglie model of quantum mechanics.)
     
  7. Feb 20, 2014 #6
    Does incapability of current classical science to explain the probabilistic outcome of Entangled particles at different detectors, makes the Quantum Entanglement fundamentally different from classical case ?

    If so, then how does Entangled states are aware of eachother's measurement, according to Quantum theory? Or am I asking the wrong question, because there is NO intuitive explanation for it, except ofcourse Quantum formulation.
     
  8. Feb 20, 2014 #7
    Is it because of present understanding that there can be NO reasonable interpretation for Quantum formulation? (Entanglement)
    Well, Does this interpretation falls apart for bell's theorem, is it not ?
    What does the above mean?

    Seems like Quantum Entanglement which is very much similar to the classical pair case, cannot be explained by generalized classical probability theory, and therefore there is NO intuitive interpretation for quantum formulation.
     
  9. Feb 20, 2014 #8
    I think principle of causality is more "sacred/superior" than any interpretation for Quantum Entanglement, therefore on must consider Quantum formulation as incomplete, falling-short. But wait that was exactly the point of EPR, and it was Bell's theorem which showed that classical hidden variable cannot explain the Entangled particles detection probabilities. Well, my point is why should classical physics be able to explain Quantum Entanglement when it was not able to explain even hydrogen atom, black body radiation, photoelectric effect, etc. The bigger question is, Does quantum physics respect the principle of causality or not, and this question lies entirely with the quantum physics and it has nothing to do with classical physics.
     
  10. Feb 20, 2014 #9

    DevilsAvocado

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    Yes, very much so.

    Welcome to the club buddy! :biggrin:

    No one knows how entanglement works, except for the mathematical description. Not even the smartest professors that work with this stuff every day. At current state of affairs, we can't even tell if it's locality or realism (or both) that has to be abandoned – so how could you expect an intuitive explanation in this situation...

    Most certainly there's a Nobel Prize awaiting the genius(es) cracking this nut.
     
  11. Feb 20, 2014 #10

    DevilsAvocado

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    The key point is that QM entanglement + Bell's theorem hit quite a drastic blow on classical local realism. It's gone forever, period.

    Macroscopic "cause" and "effect", yes. Mathematically/microscopically, there are some interpretations that use retrocausality, however no one knows which interpretation is the right one (yet).

    (However, if you chose to preserve realism in the EPR-Bell case, you will run into trouble with relativity of simultaneity)
     
  12. Feb 20, 2014 #11

    bhobba

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    Why do you think we don't know why QM is probabilistic?

    Check out Gleason's Theorem:
    http://en.wikipedia.org/wiki/Gleason's_theorem

    The reason its fundamentally different is, as the papers I linked to proved, if you want continuous transformations between pure states, which is impossible in standard probability theory, you inevitably get entanglement which is impossible classically.

    But before going any further can you tell me what you think entanglement is?

    Thanks
    Bill
     
  13. Feb 20, 2014 #12

    bhobba

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    No - its because they believe in shut up and calculate.

    And why do you believe there is no reasonable interpretation? Whats reasonable and not reasonable is in the eye of the beholder.

    No interpretation falls apart for Bells Theorem or it wouldn't be an interpretation

    It means the formalism of QM is a theory about observations (or more generally 'marks') here in an assumed classical world. But that classical world is in fact quantum - so how does a theory that assumes a classical world explain it.

    What is intuitive depends on the intuition of the person concerned. No one would have come up with QM without experiments forcing you, but when you understand its rock bottom essence you start to get a feel for whats going on.

    Again I urge you to read:
    http://www.scottaaronson.com/democritus/lec9.html

    Thanks
    Bill
     
  14. Feb 21, 2014 #13
    Why? is it because nature itself is inherently probabilistic since no classical hidden variable can explain Quantum entanglement, or is it because there are far too many degrees of freedom involved in any particular quantum measurement.
    Are they the Unitary transformations which can change pure states to other pure states yet keep the probabilistic outcome(or say expectation values) same. Whereas, the argument goes similarly for quantum physics, that since quantum entanglement requires that local-realism must be abandoned, the quantum formulation must be falling-short, for there are no experiments which denies the local-realism. (ofcourse other than the quantum formulation requirement)
    OK, let's see, Quantum entanglement according to quantum physics says the state of the two particles are said to be entangled, if they produce opposite (or similar depending on the experiment) results every time there is similar measurement done on the two particles state. But since the two states are not co-related according to QM(i.e they are independent of each other) therefore the only solution according to QM is that the two states somehow change the state of each other depending on what is measured on the other, to get the opposite/similar final results.

    Classically, there is no need for one particle to change the state of the other particle, because they can be understood to be co-related to each other, just like a pair of shoes, but classically we cannot produce the probabilistic outcome.
     
  15. Feb 21, 2014 #14
    Well, if we don't know how does it work, how can we be sure that QM is not wrong or incomplete. Or does producing the statistical probabilistic outcome same as experiments, is more fundamental than let's say, what is the physics behind it?

    Since a theory can only be complete when we know/understand how does it work, right?
     
  16. Feb 21, 2014 #15

    ZapperZ

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    Let's go back a bit, because by saying that, you are implying that we DO know something that we can be SURE that "is not wrong or incomplete". Can you point to me one? Do you think classical mechanics is "not wrong and not incomplete"? How about Relativity?

    There is a very strange criteria put on physics here by claiming something it never claimed. Science, and certainly physics, NEVER makes any claim on any theory or description to never be wrong and to be complete. That is a COMPLETE fallacy. Only politicians and talking heads on TV make such definitive claims.

    What we CAN make is a claim on the validity of something over a certain range that we have encounter. And until we discover the limits on such a range, we will continue to uphold the validity of it! It is why we still use Newtonian physics to build your house! It is why we continue to use classical E&M when we design all those antennas to pick up your cell phone signals. And this why we continue to use the Drude model that give you Ohm's Law. We don't use them because they are "never wrong and complete", but rather they are VALID and correct within the range of parameters that they are applied to!

    Please note that in these things are you questioning about, it was NEVER based on either a logical inconsistency (i.e. mathematical error) or experimental evidence. You questioned it based on TASTES or personal preference! You need to be very clear on this, because a lot of people seem to think that their feelings, or uneasiness, are somehow sufficient to offer a valid evidence against something in science! This is another fallacy!

    The statistical interpretation of QM can only be shown to be wrong IF there are evidence that can separate out all the different interpretations. That's it! Experimental evidence trumps everything else, even one's "feelings" and personal tastes.

    Until that happens, this is nothing more than a discussion on one's favorite color. And unless there is a high physics content in this discussion rather than just a discussion on one's personal preferences, this discussion is about philosophy and is subject to being closed.

    Zz.
     
  17. Feb 21, 2014 #16

    TheOldDog

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    Isn't it amazing how many people in the world don't understand this? :-(
     
  18. Feb 21, 2014 #17

    Nugatory

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    We can't.

    That's always been true of all scientific theories. And, because the answer to any "why?" question can always be met with another deeper "why?", I expect that it will be true of all scientific theories in the future as well.

    It's also worth noting that QM has "improved" a lot since the early days. There's a better understanding of the limits of the formalism and the point where science stops and interpretation begins; a clearer understanding of the limits of hidden variables; the statistical interpretation cleans up the axiomatic structure no end; the discovery of decoherence has helped enormously with the macro/micro split; and more. There's every reason to expect that it will continue to improve over time.
     
  19. Feb 21, 2014 #18

    WannabeNewton

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    I would say that's not an accurate description of entanglement. There is no necessity for "opposite" or "similar" results and there are correlations. Entanglement occurs when you cannot factorize the state of a two-component system into a product of states of the subsystems. A quantum mechanically described measuring apparatus can get entangled with the system it is attempting a measurement of if the system was in a superposition of eigenstates of the observable(s) being measured which your description does not encompass.
     
  20. Feb 21, 2014 #19

    atyy

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    There are interpretations in which QM is incomplete (eg. de Broglie-Bohm theory for non-relativistic QM), as well as approaches in which QM is complete (many-worlds).
     
  21. Feb 21, 2014 #20

    stevendaryl

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    That's the way I always thought of entanglement, in terms of the impossibility of factoring the state describing distant objects. To me, this is a sense in which quantum mechanics is nonlocal in a way that classical physics is not. In classical physics, it's always possible to do the following:

    1. Partition the universe up into neighborhoods.
    2. Describe the state of each neighborhood.
    3. Then the state of the universe is completely described by those "local" states, together with geographic information about which neighborhoods border which other ones.

    You can't do that in quantum mechanics, precisely because of entanglement. The state of the universe is not completely described by giving the states of all the neighborhoods that make up the universe.

    The exact same thing happens in classical probability theory, though. If you put a pair of shoes into identical boxes, and send one box to Alice and another box to Bob, then before opening the box, both Alice and Bob would describe the state of each box as "probability 1/2 of being a left shoe, probability 1/2 of being a right shoe". But the state of the universe is not completely captured by those local descriptions, because we know that if Alice has a left shoe, then Bob has a right shoe.

    But in the classical case, you can interpret the "entanglement" as being due to lack of information about the true state of the universe, which is not entangled. The entanglement of quantum mechanics cannot (or at least not easily) be interpreted as due to lack information.
     
    Last edited: Feb 21, 2014
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