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B Does observation *Determine* the past?

  1. Jul 31, 2017 #21

    kimbyd

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    This description you've written depends critically upon the interpretation of quantum mechanics you're assuming. If you make the assumption that the weirdness of quantum mechanics simply doesn't apply to macroscopic objects such as you and me, then you end up with weird conclusions like this, where observations that collapse wave functions tend to lead to nonsensical statements about causality.

    It's worth noting that you don't need to understand this as affecting the past, though. But such a wavefunction collapse does impact things far away. In the case of the photon, for example, if the photon you've observed is entangled with another photon traveling in a different direction, then the observation of one member of the pair can be interpreted as having an immediate impact on the far-away photon.

    There are a few reasons to believe that this interpretation of what's going on is just not correct. Of course, it's very bizarre: it's stating that quantum mechanics is non-local, which makes nonsense of relativity. The wavefunction is changed in whose reference frame? Simultaneous events at different locations, after all, are only simultaneous for certain observers. Other observers won't see them as simultaneous. Assuming this non-local behavior, then, also assumes that relativity is all wrong and there really is a preferred reference frame. There's also the problem that when you look at the details of this apparently non-local behavior, it's possible to prove that you can't actually transmit information this way.

    The most reasonable conclusion, then, is that this apparent non-locality of quantum mechanics is just an illusion. The simplest explanation for how this could be the case is that wavefunction collapse is itself an illusion. It can be shown that the appearance of wavefunction collapse will occur in a quantum system even if there is no actual collapse. In this interpretation, the appearance of non-locality disappears entirely: instead of collapsing, the observer's wavefunction is split into two components that can't communicate. One component sees outcome A, and interacts with the entire universe as if outcome A is the only outcome. The same with the component that sees outcome B.
     
  2. Aug 1, 2017 #22
    It does depend on the interpretation. I don't care if it works for all of them. I just care if I can find a valid interpretation that allows for retrocausality and take it as far as it can go. Things are just much more interesting that way.

    Causality is a funny thing. We think of time as necessarily flowing forward. But there are some views that the universe is a geometrical object where time is just a coordinate. In those block universe models, time does not flow. I'm not saying that it has to work this way. What I am saying is that it is possible that we think of time in a different way and not run into paradoxes.

    Why is non-locality an illusion? Bizarre doesn't mean it's wrong. Things were already different across reference frames(i.e time and space). Status of the wave function differing relative to differing vantage points wouldn't be a paradox if it were true. Why would to be wrong if for me, the wave function collapsed, but for you, it didn't? If it's relative it's relative. If it's not, then it's not. If it is, It would just have to result in a change in the way we view the world. Suppose that an observer on the other side of the universe made an observation and collapsed a wave function on an electron there. Well, it is meaningless to us, because they are in a separate quantum system to us.
     
  3. Aug 1, 2017 #23

    PeterDonis

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    Then it's not a suitable topic for discussion here, since the physics itself cannot be interpretation-dependent.

    Thread closed.
     
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