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

Experiment with single photons

  1. Aug 20, 2010 #1
    I came across an article in HuffPost entitled : Does the Past Exist Yet? Evidence Suggests Your Past Isn't Set in Stone

    Could anyone comment on the following quote:
    Could you post links to this research, and comments on this experiment?
  2. jcsd
  3. Aug 20, 2010 #2
    This is a very questionable interpretation of quantum entanglement.
    For reviews, see:

    Hawking is referring to particular unmeasured quantities of particular particles. A 'history' on the other hand, refers to a set series of interactions--which, in interacting, have already been 'measured.' Which means that the results are (except for rare exceptions, e.g. at black-hole event horizons) agreed upon by any observer.
  4. Aug 20, 2010 #3


    User Avatar
    Science Advisor
    Gold Member

    There are a variety of ways to interpret the results of experiments such as this one, and the above is one:


    "A seemingly paradoxical situation arises — as suggested by Peres [4] — when Alice’s Bellstate analysis is delayed long after Bob’s measurements. This seems paradoxical, because Alice’s measurement projects photons 0 and 3 into an entangled state after they have been measured. Nevertheless, quantum mechanics predicts the same correlations. Remarkably, Alice is even free to choose the kind of measurement she wants to perform on photons 1 and 2. Instead of a Bell-state measurement she could also measure the polarizations of these photons individually. Thus depending on Alice’s later measurement, Bob’s earlier results either indicate that photons 0 and 3 were entangled or photons 0 and 1 and photons 2 and 3. This means that the physical interpretation of his results depends on Alice’s later decision.

    "Such a delayed-choice experiment was performed by including two 10 m optical fiber
    delays for both outputs of the BSA. In this case photons 1 and 2 hit the detectors delayed by about 50 ns. As shown in Fig. 3, the observed fidelity of the entanglement of photon 0 and photon 3 matches the fidelity in the non-delayed case within experimental errors. Therefore, this result indicate that the time ordering of the detection events has no influence on the results and strengthens the argument of A. Peres [4]: this paradox does not arise if the correctness of quantum mechanics is firmly believed."
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook