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B Reality in Quantum physics

  1. Oct 31, 2015 #1
    According to quantum physics, things don't exist until being observed/measured. If this is the case why can we predict the same reality being generated? For example, if the moon doesn't exist when it is not looked at, then why does the moon keep appearing when we do look at it. Is "reality" like a video game where programmed scenery is generated as the player moves around?
     
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  3. Oct 31, 2015 #2

    mfb

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    It depends on the interpretation of quantum physics. There are many of them, with different descriptions what "reality" is. For predicting measurements, it does not matter, they all agree (they have to, because otherwise experiments would rule them out).
    That is wrong in all interpretations.
     
  4. Oct 31, 2015 #3

    phinds

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    This is a mis-interpretation of QM. The moon is there whether you are looking at it or not. The Copenhagen Interpretation is sometimes taken as saying that things observed do not exist, but it is my understanding that it doesn't say that and in any case, ALL interpretations of QM reduce in reality to "shut up and compute" in which things exist.

    Now there are things at the quantum level, such as the position of an electron, that have no value until they are measured but that is not at all the same as saying there is no electron.

    EDIT: I see mfb beat me to it :smile:
     
  5. Oct 31, 2015 #4
    Because measurements don't give completely random results. Observations of the Moon are strongly likely to detect the same Moon, not a giant cheese donut. For most macroscopic cases, chances of observing something "unusual" due to quantum nature of physics are unimaginably tiny.
     
  6. Oct 31, 2015 #5

    bhobba

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    Thats not what Quantum Physics says - its silent on things existing until observed or measured. We have interpretations where such is the case ie it exists at all times regardless and ones where it doesn't. Pick the one that suits you - its your choice.

    Thanks
    Bill
     
  7. Nov 1, 2015 #6

    bhobba

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    Just to expand on that point. All interpretations have the same quantum formalism and that formalism has something called decoherence. The moon at all times is constantly observed by the environment and in fact that's what gives it its classical properties of a definite position and momentum at all times - at least way way below our ability to detect quantum effects. Its not just the moon BTW - its everything around us and why we don't observe quantum effects in everyday life. Remove that interaction and some very strange things occur:
    http://physicsworld.com/cws/article/news/2010/mar/18/quantum-effect-spotted-in-a-visible-object

    To the OP you probably got that from something Einstein said. He had his issues with QM - he eventually conceded it was correct (that happened after a famous exchange with Bohr that required his equivalence principle to resolve - he tipped his hat to Bohr - literally - and never questioned its validity from that point on) but believed it was incomplete. That is not to say he didn't understand QM - he understood it very well - always keeping a copy of Dirac's famous text with him saying I will need to consult my Dirac. What the issue is, QM has moved on a lot since those times and so has our understanding. It actually turns out the joke was on Bohr and Einstein - they were both wrong:
    http://www.fisica.ufmg.br/~dsoares/cosmos/10/weinberg-einsteinsmistakes.pdf

    Thanks
    Bill
     
  8. Nov 1, 2015 #7

    atyy

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    So do bhobba and mfb disagree here? If it is wrong in all interpretations, then it is not true that quantum physics is silent on such things.
     
  9. Nov 1, 2015 #8

    bhobba

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    Of course we don't disagree. From the QM formalism alone objects like the moon are there if no one is looking because its being observed all the time by the environment. That's the exact point Weinberg was making as to why both Einstein and Bohr were wrong.

    And to forestall a thread that goes nowhere and confuses the OP, its exactly as Weinberg says, considerable progress has been made, but some issues remain, although they are not what Bohr and Einstein thought. That is a legit thread in itself but should not be sidetracked in this thread.

    To the OP - its basically a clarification on what an observation is.

    Thanks
    Bill
     
    Last edited: Nov 1, 2015
  10. Nov 1, 2015 #9
    When in doubt, stick to known facts, if available. I find the following video very illuminating esp. the end where prof. Brian Cox discusses the non-newtonian universe and his conclusion at the very end(note the GR slant, it's still relevant to your questions albeit from a different and more 'visible' angle).

    https://m.youtube.com/results?q=brian cox feather ball&sm=3

    Lots of people find it difficult to deal with a potentially unknowable post newtonian reality.
     
  11. Nov 1, 2015 #10
    This changes nothing. (The Moon + the environment) is still a physical quantum system which you, a human, then observe. It still exhibits quantum effects.

    This means that "the environment" is in superposition of seeing the Moon in all possible states, even some unexpected states ("a giant cheese donut"), with all strange state having unimaginably tiny superposition coefficients.

    And then you, the observer, "collapse" (The Moon + the environment) into one of these states. Most probable states are where "the environment" sees the Moon as a rocky ball with all familiar craters where they should be.
     
  12. Nov 1, 2015 #11

    bhobba

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    It does. But this is not the thread to discuss it. It is now well understood that observations can be defined using nothing but QM. Start a new one if you want to pursue it.

    It has been discussed many times in many meandering threads. Derailing this one will not really serve any purpose.

    Thanks
    Bill
     
  13. Nov 2, 2015 #12

    atyy

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    That is not correct because the issues that remain are not minor. Decoherence at best causes apparent collapse.
     
  14. Nov 2, 2015 #13

    Demystifier

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    Actually, there are interpretations which say that the Moon does not exist when it is not observed. This is the most explicit in the solipsistic HV interpretation
    http://lanl.arxiv.org/abs/1112.2034 [Int. J. Quantum Inf. 10 (2012) 1241016]
    but also implicit in many interpretations that insist that nature is completely local and that neither local nor non-local HV's of the Bell type do not exist.
     
  15. Nov 2, 2015 #14

    bhobba

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    Its got nothing to do with interpretations, apparent collapse, or anything of that ilk. Its simply a fully quantum definition of what an observation is.

    It interpretatively resolves nothing. All it does is say an observation occurred by the interaction on the moon by the environment - hence its being observed all the time.

    If you don't want to accept that definition you are free to do so - but if you do you need to define it in some other way. Copenhagen was rather lax in doing that - hence Wienberg's comment.

    Thanks
    Bill
     
  16. Nov 2, 2015 #15

    bhobba

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    That's not the point. The point is its interacting with the environment, gets entangled with it, and that's is the modern view of what an observation is.

    Thanks
    Bill.
     
  17. Nov 2, 2015 #16

    Werner Heisenberg said in a private letter that the moon didn't exist until we look at it, but I think he was exaggerating.

    My personal interpretation that very simple things exist in an undefined state until they whack into something that is more complicated than they are. The Moon is very complicated. The Moon has more states than do we, so it would be more correct to say that we exist in an undefined state until the Moon observes us. As this interpretation is unflattering to the ego of Humankind, it is unlikely to garner a large following.

    I have no patience with including "conscious" in all this. There is nothing in the math that has anything to do with consciousness.
     
  18. Nov 2, 2015 #17
    Well, that could be a perfectly consistent interpretation of quantum mechanics, which would mostly take the "mystery" out of it (except for "heck, what kind of computer is this implemented on?") And since pondering over interpretations can ultimately lead one to your suggestion, I have to confess that my favorite interpretation is "shut up and calculate" :biggrin:
     
  19. Nov 2, 2015 #18

    bhobba

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    Things have moved on a lot since Heisenberg's time.

    You are taking a very weird view of things if you believe looking at the moon affects anything. This whole business is from the weakness in Copenhagen - its lax in defining what an observation is:
    http://motls.blogspot.com.au/2011/05/copenhagen-interpretation-of-quantum.html

    That is rectified in modern times by fully quantum theories of what an observation is - although various interpretations have differing takes on it. For example in Consistent Histories its a theory about histories, Quantum Darwinism it comes from the emergence of stable pointer states. Its really not germane exactly how its done - the point is we now have fully quantum theories of observation. The moon is always there because its being observed constantly by the environment.

    Thanks
    Bill
     
  20. Nov 3, 2015 #19
    Is is established that there exists a classical environment made of solid ball-like atoms that can decohere quantum systems in their motion?
     
  21. Nov 3, 2015 #20

    bhobba

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    Everything is quantum - nothing is fundamentally classical. The classical world emerges from the quantum.

    Thanks
    Bill
     
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