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B Do particles not only exist everywhere but also "everytime"?

  1. Jun 13, 2016 #1
    Hi, I read quite a few popular science books and try to wrap my head around quantum physics. I am reading Stephen Hawking's "Theory of everything" again in which he explains the double slit experiment and how particles take every possible path in the universe (with some paths being more likely than others). He says that in theory a photon that was sent from a distant galaxy billions of years ago would also have the same result in a double slit experiment. We could show the wavelike pattern but we could also measure through which slit the photon would go, basically forcing it to choose one of the two slits (I hope my understanding is right so far).
    This made me wonder something: do particles not only take every imaginable path in spacetime but are they also "present" simultaneously in every possible time, not being restricted by the laws that seem to govern what we call time in the macroscopic world? Interacting with the particle would somehow force it into a fixed path in the complete "history" of the particle?
     
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  3. Jun 13, 2016 #2

    Simon Bridge

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    ... be careful about how you extrapolate from pop-sci books: they contain many errors and omissions due to having to make the descriptions accessible to people with little patience (like TV producers). The books are entertainment not science.
    No - it isn't.
    The theory is silent on which path is taken - if any. All we know is that to predict the likelyhood of what will happen at the detector we have to do a calculaton based on every possible path between source and detector. It's only maths.

    It's like how you may do long division to find out how big a slice of pie each person will get if the pie gets divided evenly between them ... this will invove several steps, maybe dividing in half, then in half again etc. But that does not predict how someone would actually cut up the pie.

    No. This is not how physics is understood.

    I'd suggest you view:
    http://www.vega.org.uk/video/subseries/8
    ... which handles the subject more carefully.
     
  4. Jun 14, 2016 #3
    Hello, thank you for your answer. I will make sure to view all these videos, they look very interesting.
    When you say that the theory does not mention which path is taken, is it not so that we can force the particle through a slit by detecting it? Do we not know which path it took when this happens?

    Is there something in quantum mechanics that prohibits particles to travel back and forth in time?
     
  5. Jun 15, 2016 #4

    Simon Bridge

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    No. detection does not "cause" a particular path to be chosen ... the particle either gets detected or it does not ... there is a calculation for figuring out the probability it will be detected. To do the calculation, you work out all the possible paths the particle could take from the source to the detector. If the detector is at one of the slits, that changes the calculation.

    That's a yes and no ... but it does not invalidate previous statement because the experiment is different. If you know the path, you don't get interference. You still don't know exactly how the particle got from the source to the detector.

    Kind-of: QM is "mired in time" ... the time axis is not a special dimension of space in QM like it is in GR.
    In QM, time travel amounts to a reverse causality, where a particle is detected before it is emitted.
    There has been some speculation about what a self-consistent time travel in QM would look like.
     
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