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Does time exist at Quantum level?

  1. Jun 22, 2015 #1
    There seems to be some talk of Entanglement "defeating" the speed of light. But in these discussions people talk about c in terms of time frames - it takes Light X to travel Y ... etc

    If the Quantum universe simply doesn't facilitate such measurements then does that go someway to explaining what we perceive to be instantaneous.

    I'm interested in what time really means at a Quantum level and how it can be meaningfully measured.
     
  2. jcsd
  3. Jun 22, 2015 #2

    Nugatory

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    Not really.... It just wraps the same problem in different words....

    I send two particles out in opposite directions. They're entangled so that if we measure one of them to be spin-up then a measurement of the other one will necessarily be spin-down. I let them fly away from me for an entire year, and then someone a full light-year away to my left measures the spin of the left-moving particle and finds it to be spin-up. A naive picture says that this measurement causes the wave function of the entire system of two entangled particles to collapse so that the right-moving particle two light-years away immediately becomes spin-down. Experiments confirm that the universe really does behave in a way that is consistent with this naive picture:
    1) The two particles' spin will always be opposite, no matter how far apart they are when the first measurement is made.
    2) The spin of the two particles is determined by the first measurement; it is not possible that the two particles were just launched with opposite spins from the start. (The experiments that show this have spawned many threads in their own right - google for "Bell's Theorem").

    These experiments haven't been done at distances of light-years (hard to set up a lab on alpha centauri) but they have been done at distances of meters and kilometers.
     
  4. Jun 22, 2015 #3

    bhobba

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    Time is no different in QM - its something we parameterise our theories with and it's measured the same way.

    What you are talking about is the entanglement thing.
    http://www.johnboccio.com/research/quantum/notes/paper.pdf

    As you can see if it is instantaneous is open to question.

    Suppose I put a red slip of paper in an envelope and a green slip in another then mix them up, keep one, and send another to the other side of the universe. I open the envelope and see green. Immediately I know the other is red. No communication took place - we simply have a correlation. Entanglement may simply be like that, its a bit different as the above on Bells Theorem shows, in that its not the same as classical correlations, but it's still a correlation.

    In fact in what's called Quantum Field Theory (QFT) we have something called the cluster decomposition property which basically says for uncorrelated systems sufficiently separated regions behave independently. This is the notion locality in QFT. Note the key word - uncorrelated. Entangled systems are correlated. This leaves up in the air if locality is even applicable for entangled systems. Personally I don't think it is, but mine is very much a minority view.

    Thanks
    Bill
     
  5. Jun 22, 2015 #4

    atyy

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    Time in quantum mechanics is classical, just like space. Quantum field theory assumes a classical spacetime. So time is just what your classical clock on the wall reads.

    There is a second answer in which time is position - you treat the clock as a quantum system, and the time the clock reads is the position of its hands when you look at the clock.

    But even if you have a quantum clock in the box with Schroedinger's cat, since spacetime and your own measurement apparatus is classical, you can still have a classical clock outside of the box.
     
  6. Jun 23, 2015 #5
    "Suppose I put a red slip of paper in an envelope and a green slip in another then mix them up, keep one, and send another to the other side of the universe. I open the envelope and see green. Immediately I know the other is red. No communication took place - we simply have a correlation. Entanglement may simply be like that, its a bit different as the above on Bells Theorem shows, in that its not the same as classical correlations, but it's still a correlation."

    Could you expand on this a bit?

    Do you have any doubts about Bell's inequality?


    Thanks
     
  7. Jun 23, 2015 #6

    bhobba

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    There isn't anything to expand. Its simply a correlation - but of a different type to classical ones like the red and green slips - as Bells Theorem shows.

    I have no doubts at all about Bells Theorem or its implications. Under my view as simply a correlation it violates naive reality and is ambivalent to if its local or not. In fact from the cluster decomposition property, which is the careful statement of locality in QFT, correlated systems are excluded. Hence its even up for grabs if locality is a concept that's even applicable. But if you reject naive reality its of no consequence.

    Thanks
    Bill
     
  8. Jun 23, 2015 #7
    Good question.
    I'd love to know how wrong-er-ish these question-like non-answers are?

    Enforcement of Super Selection Rules occurs FTL
    Enforcement of Super Selection Rules is non-local
    Enforcement of Super Selection Rules is non-temporal (in our space-time at least) - this is the same thing as saying it is "non-local"
    QM symmetries are non-local, non-temporal structures (in our space-time at least)

    I keep thinking the answer is somehow that there is an aspect of the QM reality we inhabit, that is not subject to time, as all other aspects of that reality are. Whether that means the aspect is utterly non-temporal (which defies definition I think), or just more like alt-temporal seems open, but it does contain information that matters in our space-time.

    I was under the impression this was just what the ADS/CFT Correspondence and "Holographic Universe" models are trying to capture.
     
    Last edited: Jun 23, 2015
  9. Jun 23, 2015 #8
    How do you confirm the absolute simultaneity at distances of meters and kilometers by experiment when we know that in the framework of relativity the simultaneity is relative ?

    Regards
    Patrick
     
  10. Jun 23, 2015 #9
    On one hand if I had to guess, I'd say the experimenters who painstakingly set these tests of quantum mechanics up can calculate the possible effects of Einstein's theory of General Relativity (and the "Relativity of Simultaneity") over the distances the experiments are performed, accounting for the relative velocities of the pieces of the experiment, and accounting for the curvature of space-time due to the presence of mass, and rule those out as factors in the experimental outcomes.

    On the other hand, as I understand it, the idea of exactly what Einstein's space-time looks like and what "simultaneity" and "distance" mean to a photon (and other quanta) which are the subject of these experiments is not exactly clear - It can be predicted very accurately to a point, but there is not yet a widely accepted theory that formally unifies Quantum Mechanics and General Relativity. This seems to me consistent with your objection, and sort of exactly the mystery.
     
    Last edited: Jun 23, 2015
  11. Jun 23, 2015 #10

    bhobba

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    Because its usually tests involving ordinary QM which is based on the Galilean transformations.

    Thanks
    Bill
     
  12. Jun 23, 2015 #11

    morrobay

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    Consider A and B detectors separated by one km with photon entangled source on AB axis
    Detector A is .4 km from source and detector B is .6 km from source.
    Measurement at detector A at t0 and measurement at detector B at t1
    And also .2 km/c < 1 km/c
     
  13. Jun 24, 2015 #12

    morrobay

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    Δt for t1 - t0 = 6.6 * 10-7 sec with both observers stationary. In order for both measurements to be
    simultaneous for an observer in a moving frame S' 1 km from detector A overhead detector B. That is, Δt' = 0
    That observer would have velocity 5.94 * 107 m/sec. From Δt' = γ( Δt.- vΔx/c2)
    Then solve for v with x = 1000 m
     
  14. Jun 24, 2015 #13

    Nugatory

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    You're right, and that's why I used the word "naive". I'm describing the experiment from the point of view of an experimenter who is at rest with respect to both the source and the detectors, and the detections are only simultaneous using that frame. If we consider the experiment as described by an observer who is moving relative to the parts of the experiment, then the two detections are not simultaneous.

    Depending on their speeds, some observers will find that detector A triggered before detector B and others will find that B triggered before A. That creates something of a problem for the naive explanation that says that the "first" detection determines the result of the "second".
     
  15. Jun 27, 2015 #14
    Bhobba, you write "In fact in what's called Quantum Field Theory (QFT) we have something called the cluster decomposition property which basically says for uncorrelated systems sufficiently separated regions behave independently. This is the notion locality in QFT. Note the key word - uncorrelated. Entangled systems are correlated. This leaves up in the air if locality is even applicable for entangled systems. Personally I don't think it is, but mine is very much a minority view."

    The mystery to me is that probability of the 'spin' to be either up or down (50/50 probability) with the opposite 'photon' assumingly knowing it instantaneously. to me it's not the correlation that confuse me, but the way it is assumed to always 'know' a proper choice, of two. Time definitely exist, I can guarantee that one personally :) but how it correlate is interesting. Naively I'm thinking of it as a static space, when trying to see it. One where every probability is counted in, with time becoming what makes a universe (always 'Locally defined') so to speak..

    Better expressed, in such a universe it's the limits and constants that's interesting, because they should form the logic we measure on. And that is definitely interesting to me.
    ==

    This is a simplified description of a 'spin' naturally, then again, it's perfectly enough for me :)
     
    Last edited: Jun 27, 2015
  16. Jun 27, 2015 #15
    In other words an incomplete and approximate theory of gravity. Not Einstein's?
     
  17. Jun 27, 2015 #16
    I think that those two things — the meaning of time and the way in which it is measured — are always the same thing in the context of all branches of physics.
     
  18. Jun 27, 2015 #17

    bhobba

    Staff: Mentor

    That's your problem right there. You are using your classical intuition to read things into it the theory doesn't say.

    QM is a theory about observation - you cant 'assumingly' anything beyond that.

    Think back to the red and green slips - assumingly they they know it instantaneously as well - but of course there is nothing of the sort going on - you simply have correlated them. Same in QM with entanglement - but its different to classical correlations.

    Thanks
    Bill
     
  19. Jun 27, 2015 #18

    bhobba

    Staff: Mentor

    No. Standard QM is not relativistic just like classical mechanics is not relativistic. Because of that they obey the Galilean Transformations:
    https://en.wikipedia.org/wiki/Galilean_transformation

    Physically it is the Lorentz Transformations with the maximum speed C taken to infinity ie there is no maximum speed ie its not local. In fact, if you look at Chapter 3 of Balrentine you will see in QM all the dynamics follows from that alone. Right at its very foundations its non local. For locality to be an issue in QM you need to go to relativistic QM - which is called Quantum Field Theory (QFT). In QFT locality is defined by the Cluster Decomposition Property which more or less forces you to exclude correlated systems like EPR. Of course you can play around with definitions to get a different view. It needs to be pointed out the definition of locality in discussions of Bell is pretty similar to a loose statement of the Cluster Decomposition property, so its not really anything earth shattering.

    Thanks
    Bill
     
  20. Jun 28, 2015 #19
    No, its different. In classical mechanics, Newton has distinguished true time and apparent time (as measured by humans) simply out of priniciple, because they are philosophically different. In QM, they are really different. There is no operator for time measurement, and every clock goes, with some non-zero probability, even backward in time.

    I would like to object against phrases like naive reality. This suggests a solution of the problem consists of some sort of sophisticated notion of reality or so. But what is used in Bell's theorem is an extremely weak notion of reality - the EPR criterion of reality - which does not have a meaningful possibility to be weakened. Thus, to save Einstein causality from falsification, you simply have to give up realism. And causality too - because you have to give up Reichenbach's principle of common cause.

    Essentially, you preserve nothing in this way. Giving up realism means giving up any attempt to find a realistic model, a realistic explanation of the world. Giving up causality means to give up the search for causal explanations of observed correlations. The "weaker form" of realism is simply solipcism, the "weaker form" of causality would be not to care anymore about causal explanations of correlations.

    So, nor the notion of realism used in Bell's proof is "naive", nor the notion of causality based on Reichenbach's common cause.
     
  21. Jun 28, 2015 #20

    bhobba

    Staff: Mentor

    I don't agree. In both cases its a parameter. Time is what a clock measures. Newton had many many misconceptions. But that is only to be expected - in the intervening centuries much has been learned.

    Yes - giving up that things exist independent of observation means - well you give up things exist independent of observation. Its how you react to it that's the issue - you obviously react quite alarmingly to it. I, and many others, don't.

    Thanks
    Bill
     
    Last edited: Jun 28, 2015
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