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B How do we know some particles don't exist until measured?

  1. Jun 20, 2017 #1
    I don't know if I am right, but I have read about how some particles do not even exist until measured. How would we know this?
     
  2. jcsd
  3. Jun 20, 2017 #2

    ShayanJ

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    We don't. That's part of some interpretations of quantum mechanics which means quantum mechanics itself is silent on it.
    Also, even in those interpretations its not like some particles are like this and some are not. Those interpretations consider this to be a property of any quantity related to any quantum system.
     
  4. Jun 20, 2017 #3
    thank you for clearing that up. But there must me some merit to this interpretation. What is their argument for this? Is this the Copenhagen interpretation? Because I know Niels Bohr believed in the particle disappearance thing.
     
  5. Jun 20, 2017 #4
    btw what interpretation do you believe in? I lean towards the many worlds interpretation.
     
  6. Jun 20, 2017 #5

    ShayanJ

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    This mostly comes from Bell's theorem. It says that nature is either realistic or local, it can't be both. So local theories have to have this property. But of course some interpretations can work around this, like superdeterminism, or maybe many worlds too, I'm not sure.
     
  7. Jun 20, 2017 #6

    ShayanJ

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    At first, its not a matter of belief, its a matter of preferring one over others. Its an important distinction!
    I'm still not that much specific about which interpretation I prefer, but I do know that I don't like Bohmian mechanics and many worlds.
     
  8. Jun 20, 2017 #7
    Is Bohm is kind of like saying that everything follows a deterministic law and there is no free will, along with no measurement problem?
     
  9. Jun 20, 2017 #8

    ShayanJ

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    The no free will part is not part of Bohmian mechanics(BM), its a part of superdeterminism.
    But other things you said are true. And BM doesn't have measurement problem because its a realistic theory, which by Bell's theorem, means its non-local.
     
  10. Jun 20, 2017 #9

    phinds

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    It's really irrelevant since they all conform to the same math and are equal at the level of "shut up and calculate"
     
  11. Jun 20, 2017 #10
    but the way I see it, it is also important to explain why you can get to that math.
     
  12. Jun 20, 2017 #11

    phinds

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    The interpretations happen after the math says what's happening so, yeah, they are an attempt to say WHY the math says what it says but since all interpretations come off the same math, none is preferred (except as a personal preference)
     
  13. Jun 20, 2017 #12
    true *pretends to know what he is talking about*
     
  14. Jun 20, 2017 #13

    ShayanJ

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    But even that is an assumption that is just stated a lot. There may be fringe cases of applications of QM that are still not tested and different interpretations disagree in their predictions about them. It may still be possible to be able to experimentally distinguish different interpretations and we just don't know how or don't have precise enough measurement devices for. One example is quantum non-equilibrium in Bohmian mechanics which I'm not sure in what situation, if any, happens.
     
  15. Jun 20, 2017 #14
    just took the words right out of my mouth
     
  16. Jun 20, 2017 #15

    Paul Colby

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    This is only possible if either the theory differs or the way the theory is applied differs in such a way it yields
    different experimental outcomes. I know of no case where this is the claim. Has anyone made such a claim?
     
  17. Jun 20, 2017 #16
    It's just presented this way to pop science audience to make it more interesting.
    In reality they don't have defined properties until they are measured.
     
  18. Jun 20, 2017 #17
    Yup, Davies has an essay on this topic called "Particles don't exist", talks about particle detectors, and more. I'd have to read the essay again to get his points, and if needbe, I can respond tomorrow. But IIRC, particles are only a utilization of a flat minkowski space, and if you go into curved spaces, you'll have some observers seeing particles being created when in the persons rest frame they see none.
     
  19. Jun 20, 2017 #18

    Paul Colby

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    Look, someone would first need to explain to me what "exists before measurement" even means quantitatively. To get quantitative about it implies some measurement so it would seem that an explanation would be amusing. Exist before measurement isn't even meaningful classically.
     
  20. Jun 20, 2017 #19

    ShayanJ

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    As I said, one example is the quantum non-equilibrium in Bohmian mechanics.
     
  21. Jun 22, 2017 #20

    bhobba

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    We know exactly the why of the fomalism which every interpretation has - it just an extension of probability thoery:
    http://www.scottaaronson.com/democritus/lec9.html
    https://arxiv.org/pdf/quant-ph/0101012.pdf
    https://arxiv.org/abs/1402.6562

    Basicallly QM is part of an area of math called generalized probability models or theories. Ordinary probability theory has distinguishable outcomes (also called pure states) eg we can tell the difference easily between raining and not raining. But what if we relax some of these assumed things about ordinary probability theory - well you get generalized theories and that is what QM is. It has a number of distinguishing features such as you cant tell the difference between pure states. Also in ordinary probability theory you cant continuously go from one pure state to another via other pure states. But in QM you can. If you model a physical system by some generalized probability state space it turns out QM is the simplest theory that will allow you to continuously go from one pure state to another. But physically we expect a theory to be like that ie if its in one pure state at time t1 and another at time t2 it should have gone through some other state while doing that.

    But knowing formally the why of something is entirely different to knowing what it means. We face that in even good old probability theory - the Kolmogorov axioms tell us formally what it all about - but what does it mean. We have all sorts of answers - Bayseanism, frequentest, decision theory to name just a few. In a lot of respects all interpretations of QM is this applied to QM - see the following by John Bayez:
    http://math.ucr.edu/home/baez/bayes.html

    its nothing new and just like probability no one has been able to definitively answer which is correct - maybe it will never be answered.

    Personally I adhere to the Ensemble interpretation because it's just, basically, the frequentest interpretation applied to QM. Most applied mathematicians think of frequentest ideas in probability theory, but some like Bayesianism (those into Bayesian inference) which corresponds to Copenhagen, and some like Decision Theory (ie probability is the weight a rational agent would decide if faced with a deterministic situation but for some reason doesn't have all the information - its used by Actuaries in so called Credibility Theory). The decision theory approach is favored by Many Worlds adherents and maybe even BM.

    Thanks
    Bill
     
    Last edited: Jun 22, 2017
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