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B How fast is entanglement?

  1. Jul 5, 2017 #1
    I am getting mixed messages on how fast entanglement is. Sometimes I am told it just happens instantly, sometimes it will say it is 10,000 times faster than light. I cannot comprehend how it can be instant because that would imply that if I change something in a quantum particle the change in the other particle would happen "now", and in time there is no now.
  2. jcsd
  3. Jul 5, 2017 #2


    Staff: Mentor

    From where?
  4. Jul 5, 2017 #3


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    No real experimental apparatus can measure anything with infinite precision, so no measurement can ever give an exact answer. Thus, when you read real scientific papers (as opposed to to popularizations, which tend to skip over this subtlety) you won't ever see anyone saying "we measured something and found its value is ##x##"; they'll say something like "the difference between ##x## and the thing we're measuring must be less than ##y##", where ##y## is some very small number reflecting the limits of accuracy of the experiment. Of course this result is consistent with the thing being measured being equal to exactly ##x##, and if better and better experiments keep on coming up with smaller values of ##y##, we may decide that it is a pretty good bet that ##x## really is the correct value.

    So with that background.... The best experiments so far say that entanglement (actually the collapse of an entangled state) must propagate at a speed no less than 10,000 times that of light, and that's probably what you're thinking of here. This result is consistent with "instantaneous", and you should read it as saying "it's at least 10,000 times the speed of light and it might well be instantaneous" or "if the two measurements are separated by distance ##D##, the 'travel time' is less than ##D/10000c## and may well be zero".
    You're right to be a bit confused. This entire discussion is happening in the context of non-relativistic quantum mechanics, so it's not surprising that you find that it doesn't fit well with special relativity. You might want to look for some of our old threads on why you can't use entanglement to send signals faster than light, and be aware that collapse interpretations do not work well with relativity.
    Last edited: Jul 6, 2017
  5. Jul 6, 2017 #4
    Well from what I understand the whole concept of quantum theory doesn't work well with relativity.
  6. Jul 6, 2017 #5


    Staff: Mentor

    Sure it does. Google "quantum field theory".
  7. Jul 6, 2017 #6
    Still doesn't explain that black hole thing at the Planck Scale.
  8. Jul 6, 2017 #7

    Vanadium 50

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    Lunct, you made a statement that was - as Peter pointed out - not in general correct. Trying to dig yourself out of that hole by pointing out the one time it is correct is not helpful.

    You're going to have to decide what you are here for. If it's to learn, you should ask more questions and make fewer statements. Especially incorrect ones. If it's to teach, you should learn some physics first.
  9. Jul 6, 2017 #8


    Staff: Mentor

    What "black hole thing at the Planck scale?" We have no observations of anything at the Planck Scale, or even close to it, by many orders of magnitude. All we have are speculations about how quantum gravity might look when we have a good theory of it.
  10. Jul 6, 2017 #9


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    To add to the excellent comments by Vanadium 50 and PeterDonis:

    What is not in question here is that timing is not a factor in any experimental test of entanglement. It does not matter if you measure A before B, B before A, or A and B as close to simultaneous (your "now") as possible. Distance between measurements does not matter either. You can see this from the math, as neither time nor distance is a factor in the prediction. So you can see that relativity is not really an issue here.

    There are various ways to interpret this. What you choose to make of it is up to you.
  11. Jul 6, 2017 #10
    Some authors have suggested that if the non-local effects observed in Bell-type experiments propagate at any finite speed, then non-locality could be exploited for superluminal communication (e.g. 'can't stay hidden'):

    Looking Beyond Space and Time to Cope With Quantum Theory


    Quantum non-locality based on finite-speed causal influences leads to superluminal signalling

    Full article posted in arxiv:
  12. Jul 6, 2017 #11


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    From the math : P++ , P-- = (sin a-b/2)2 and P+-,P-+ = (cos a-b/2)2 that satisfy the statistical predictions of QM ( including when a = b )
    At spacelike separation measurements can be made in any order.
    Settings can be changed after emission at source of entangled particles and before arrival at detectors A and B.
    Why is collapse/non locality even necessary ?
    Due to non separability of QM the distant correlations are related to cos and sin and not to any action at distance. A to B, B to A
    For example when A = -1 at setting = 2400 and B = +1 at setting 1800 then P -+ equals ( cos 240 - 180/2)2 = .75
    In the collapse interpretation when Alice outcome is -1 at 2400
    Bobs state changes instantaneously with spin direction Φ = α = 2400
    When Bob makes measurement at β ( 1800) he will get +1 probability ( cos β-Φ/2)2
    ( cos 180-240/2)2 = .75 that equals the QM prediction without collapse.
    Last edited: Jul 6, 2017
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