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I Quantum experiments and entropy question

  1. May 22, 2016 #1
    Layman question here. I have often read that the known laws of physics, QM and Relativity, do not make a difference between past and future, they are perfectly reversible in time. The only fact which gives time its preferred arrow is the 2nd Law, the increase of entropy, which manifests in the evolution of macroscopic systems (possibly as a result of simple statistical probabilities).

    So my question is, in purely quantum experiments, such as say a simple double-slit or a delayed choice quantum eraser, I understand that entropy increase does not show up, it is not applicable to this scale of event. Is that correct?

    Because if so, the next question, this would mean that in such an experiments, the arrow of time is not present. They are dictated only by the "pure" relevant law(s) of nature, "unadulterated" by the statistical evolution effects of the 2nd Law, that is, the time-reversible law(s). I ask this because particularly in the delayed choice experiment there is always the argument that "the particles seem to behave in one or another way at the slits depending on the configuration of the apparatus they will find after the slits". That is, the "mystery argument" comes by invoking that the experiment follows the presumed arrow of time, i.e. the slits lie in the past of the eraser device and vice-versa.

    Isn't this a bit of a contradiction? Shouldn't we remove any concern about a time arrow when describing such experiments? Can we say that such an experiment can be equally described in time-reverse by the same laws of physics? Or is it that even in such quantum-scale experiments we must account for an entropy increase and therefore consider them in a single time direction past -> future?

  2. jcsd
  3. May 22, 2016 #2

    Simon Bridge

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  4. May 24, 2016 #3
    Thanks, I have read the article and I think I more or less understand it (even if I would appreciate it if it was more thorough and explicit, my fault that I didn't search for the actual paper), but I still don't quite get the connection to my OP.

    I'l try to put it more clear: in a delayed choice quantum eraser experiment, is it sensible that we insist that what happens at the slits happens before what happens at the eraser? or at such a scale of experiment, the temporal sequence before and after looses meaning?

    Thanks anyway...
  5. May 25, 2016 #4

    Simon Bridge

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    Excuse me: I thought what you were asking about was founded in your understanding of entropy, time reversal, and it's relation to quantum mechanics.
    That is what the article covers.

    QM is "mired in time" ... the order of actions is important.
    "before" and "after" are meaningful.
  6. May 26, 2016 #5
    Thanks for the answer.

    Then I must have misinterpreted some of my readings. They often say that the actual laws of QM (or GR for what matters) do not imply any time direction, that they are just as well in reverse. And that the actual time direction we observe in the universe comes from the 2nd law. I must have got something wrong. If anyone would be so kind to clear this a bit for me I would be grateful.

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