Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Violating of CHSH inequality

  1. Mar 18, 2012 #1
    Obviously a violation of the CHSH inequality means that local realistic theories are untenable.

    If we sent two entangled photons towards detectors (far enough away that for information to travel, you'd require it to go faster than light). One reaches a detector before the other, so collapses.

    How do we know the partner photon doesn't assume the same polaristion until it reaches its filter and detector?
  2. jcsd
  3. Mar 19, 2012 #2
    By the way, none of what you're saying is based on the CHSH inequality in particular, just Bell's theorem in general. So you might as well work with a much simpler Bell inequality for entangled photons, described in the fantastic explanation "quantumtantra.com/bell2.html" [Broken]. (The only advantage of CHSH is that it's more practical to test experimentally.)

    And incidentally, you should know that there are various fringe people who try desperately to cling to local realism by either exploiting experimental deficiencies in practical Bell tests (e.g. fair sampling), or by theoretical loopholes in Bell's theorem (e.g. superdeterminism); you can read about many if not all these attempts in Bell threads on this forum. But experimental and theoretical advances may make such positions increasingly difficult to maintain.
    That is precisely the Copenhagen view of things.
    Last edited by a moderator: May 5, 2017
  4. Mar 19, 2012 #3
    So its not correct to say that when one photon takes on a polarisation, the other does (even if it's mid-air heading towards a detector)?
  5. Mar 19, 2012 #4
    I told you, the idea of the other photon assuming the same polarization as the first photon is precisely the view of the Copenhagen interpretation.
  6. Mar 19, 2012 #5
    Yes, but I'm asking when the photon takes on that same polarisation? I thought I made that clear.
  7. Mar 19, 2012 #6
    As you said, exactly when the first photon's polarization is measured, at least according to Copenhagen.
  8. Mar 19, 2012 #7


    User Avatar
    Science Advisor
    Gold Member

    Experiments confirm that it is completely consistent with instantaneous collapse. However, the same experiments also confirm it as consistent with collapse delayed until detection.

    So I am not contradicting lugita15, but merely pointing out that to know the precise time collapse occurs is not possible. You would really need to know more about the mechanism, which we don't. For example, the "when" may not really be any specific point in time if you consider a block time structure. Then it is more like "when-where".
  9. Mar 19, 2012 #8
    i guess it has to be the moment any of the entangled photons is detected (first), else you need to assume a third (undiscovered) entity that holds/stores the (spin info) information (and then transmits) to the photon that is detected later.
  10. Mar 20, 2012 #9
    I wonder in this case:
    Two pairs of entangled photons - (VV+HH) description for both pairs.

    Measure each photon along the 45/135 basis.

    Does QM predict that 1/2 the times we get 45 for photon 1 and photon 2 (from pair 1), pair 2's photons take on 135 polarisation, when photon 3 and 4 are measured?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook