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Quantum entanglement

  1. Mar 21, 2010 #1
    I have two simple questions concerning the entanglement and its repercussions from the EPR paradox/bell violation.

    Are we assuming this is a problem from our previous knowledge of the information or is it rather a matter of quantum states being really linked, thus letting information travel faster than speed of light (i.e. non-locality)?

    Also, is there any new theory that explains somehow non-locality properties?

    If I've done any basic, trivial mistakes feel free to bite me.
  2. jcsd
  3. Mar 21, 2010 #2


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    Welcome to PhysicsForums, Redsummers!

    No one really knows the explanantion at this point. The short story is: The formalism of Quantum Mechanics explains the relevant phenomena (i.e. makes predictions) effectively but is silent as to the underlying physical processes. There are "interpretations" of this formalism - some which are equivalent and some which have subtle differences - without adding any useful new predictions.

    So we don't *really* know what quantum non-locality is.
  4. Mar 21, 2010 #3
    I don't think I understand this question. Could you please be more elaborate?

    If you have two photons (or any other particle for that matter) in an entangled state, they "communicate" through no medium (no ether or interaction-carrier exists), and the "communication" happens instantaneously, so, yes, it is faster than light.
    Consider a small gedankenexperiment: you have a scalar particle, like a pi meson , it decays into two photons. But "scalar" means that the pi-meson has spin 0. The photon has spin 1. So, in order to conserve the overall spin, one of the photons must have spin +1 and the other spin -1.
    The state describing these two photons is known as a singlet state. It is an entangled state. The spin of one of the photons depends on the spin of the other.

    Non-locality is not explained. It has only been tested experimentally (see Aspect et. al. Orsay exp. 1984).
  5. Mar 21, 2010 #4
    Thank you DrChinese,

    It's alright, I expected something like this since I didn't find any interpretation regarding this issue elsewhere.

    According to your formalism, it does make sense to deem Quantum Entanglement as an unexplained property. However I think non-locality is too much of intriguing to stop seeking a clearer definition because QM limitations. So I guess that we may know more about it in the upcoming future.

    I was first wondering if entanglement is in fact something we cannot predict and it makes itself obvious as soon as we start observing the phenomenon.

    Yeah, that's basically what I've been reading recently about. Even though I didn't encounter this example before, it's always interesting to see new observations.

    Right, and it hasn't been until this year that Aspect and the others get recognition for the experiment (i.e. Wolf prize). http://physicsworld.com/cws/article/news/41633" [Broken]
    Last edited by a moderator: May 4, 2017
  6. Mar 21, 2010 #5
    Well, in Quantum Mechanics we are not allowed to make assumptions prior to the measurement. So, at least in this sense, we can only detect entanglement when physically interacting with the system. Regarding the ability to predict whether two (or more) particles will be in an entangled state, we can predict that. Think of the above example, we know the pi-meson is a scalar so we know the two photons will be entangled.
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