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It's impossible !.

  1. Apr 8, 2009 #1
    I'm not convinced, I'm with Einstein's side on this.
    But I want to ask a question first, regarding the function collapse :
    Where was the position of the observer detector in the double-slit experiments ?
    Also here are my thoughts about the uncertainty principle, we are also here uncertain about the position of the tire parts that are spinning faster than what our eyes angular resolution can identify ?? :
     

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  3. Apr 8, 2009 #2

    DrChinese

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    Not really sure what your question is regarding tire parts. As to the double-slit, there are a number of variations. Again, what are you asking?

    As to Einstein: had he lived longer, his views would have changed in the face of evidence collected after his death. The final conclusion of EPR is simply wrong, although the paper itself is great.
     
  4. Apr 8, 2009 #3
    Interesting, what interpretation would he chose... In his time there was only one.
     
  5. Apr 8, 2009 #4
    Hi,
    What I meant by posting the spinning tire image is that , hypothetically, If an observer (that didn't see any non-spinning tires before) saw that tire before the invention of slow motion cameras that can process images faster than his our eyes do, he would also say that there is uncertainty here regarding the spinning parts of the tire, as he can't tell the exact positions of those parts at any certain moment (Heisenberg's uncertainty principle !).
    I meant, Where was the position of the observer detector in the first double-slit experiment that proved function collapse (the disappearing of interference) ?.
     
    Last edited: Apr 8, 2009
  6. Apr 8, 2009 #5
    This could apply to Niels Bohr too ! :smile:.
     
  7. Apr 8, 2009 #6

    Vanadium 50

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    Good heavens.

    First, this is not an analogy for the Heisenberg Uncertainty principle. The HUP does not say that the world behaves as if things are moving along definite trajectories, only really really fast so it appears as if they are not. Nor is this view consistent with observation.

    Second, why start by lecturing us on QM? Wouldn't it make more sense to study it first and lecture us second?
     
  8. Apr 8, 2009 #7
    Hi, I'm not lecturing here, and I did somewhat study QM , but I'm as Einstein was, still not convinced!.
     
  9. Apr 8, 2009 #8

    DrChinese

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    Well, there are a lot of ways to address these issues about the HUP. We have covered this a lot previously on this forum, and it is covered in the PhysicsForums FAQ as well: https://www.physicsforums.com/showthread.php?t=104715

    But if your question is: maybe we just can't "see" those tiny particles too well and that is why we have the uncertainty principle... that is completely incorrect. We can home in on any particular attribute to the level of precision we want. The HUP comes into play long before we get to the limit of our observational capability. There are probably thousands of published experiments that exhibit this principle, so no shortage of references are out there. You can google Heisenberg Uncertainty Principle and see some.

    But the easiest examples are those associated with so-called Bell tests. I would recommend that you read and understand this one and then come back with some questions. The HUP can apply to entangled particles that are separated, which cannot happen in the classical world.
     
  10. Apr 8, 2009 #9

    ZapperZ

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    Before we go on any further, have you corrected your faulty understanding of the HUP? Your example has nothing to do with the HUP at all, and unless this is corrected and you have understood why this is incorrect, there's no point in continuing. Your starting premise is wrong.

    Zz.
     
  11. Apr 8, 2009 #10

    alxm

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    Funny, I'd say it was the initial premise that's wrong. (I.e. that Einstein's idea of what constitutes a 'complete' theory)
     
  12. Apr 8, 2009 #11
    One cannot help but remember the Afshar experiment...

    Something that everyone did wrong for 100 years...

    It's impossible, Ok.
     
  13. Apr 8, 2009 #12

    DrChinese

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    Huh? There is no need to be unnecessarily cryptic. I remember the experiment and have no idea of the relevance to this thread, other than a double slit was involved.

    I believe the OP is questioning some of the key principles of quantum mechanics (such as the HUP) prior to having any kind of reasonable understanding of the experimental background or the theory involved. Hopefully, the OP will spend some time learning a bit more and then ask a follow-up question, if there is one.
     
  14. Apr 9, 2009 #13
    It's not so simple. There are a lot of issues here:

    realism
    locality
    hidden variables
    completeness of the theory

    maybe something more. Can we say with certainty which is wrong?
     
  15. Apr 9, 2009 #14
    I thought what I am referring to was clear. But if that seemed irrelevant, sorry about that. What I was trying to say was:
    very basic facts about quantum mechanics will likely to remain although higher order corrections can always refine the theory. Just like Newton's laws.

    So inspired by an enlightening stroke of light, the OP thinks that the entire physics/philosophy/science community skipped that insight he has seen, and for hundred years!

    That's what Afshar tried to do, and he was insulted out of the community by concrete refutations.

    Here's what I was thinking Dr.Chinese, I was being sarcastic.
     
    Last edited: Apr 9, 2009
  16. Apr 9, 2009 #15

    DrChinese

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    Thanks for clarifying. :smile: A lot of folks might not have caught your shorthand. And I wouldn't count Afshar totally out yet, although I really didn't think he had anything special anyway. If you can outwit the HUP, now that would blow my mind. (Or at least my nose.)
     
  17. Apr 9, 2009 #16
    Hi all, I'm sorry I was having problems with my ISP (in a critical time !).
    Well, I think my analogy is true because, the spinning tire observer's interpretation of his uncertainty is analogous to Heisenberg's interpretation of his uncertainty principle, as explained by the following:
    Yes, I was going to argue using that , according to the HUP :
    Even if an electron was the observer of itself, its exact location would be still uncertain !. And it's true , but it's because :
    As space is indefinitely divisible, also the photons/electrons are. And this is why the exact location of any known particle is uncertain !. And hence:
    Einstein's hidden variable is particles indefinite divisibility.

    So, I think this is the actual and physical interpretation , and also it explains the double-slit experiments interference phenomenon.

    And as time of events is also indefinitely divisible, everything in space and time is uncertain !.
     
  18. Apr 9, 2009 #17

    f95toli

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    There are many things that are wrong in this statement, but in this context I guess the most important point is that -as Dr. Chinese and the others have tried to explain to you- the HUP does notlimit the accuracy by which we can determine a single variable such as e.g. position.
    The HUP is important when we try to measure pairs of variables such as position and momentum.
    Of course one could argue that we can never measure anything with infinite precision but that is irrelevant here; in cases where the HUP does limit what we can measure it "kicks in" well before that becomes an issue.
     
  19. Apr 9, 2009 #18

    DrChinese

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    As I have told you, and f95toli and others have indicated, multiple times, you REALLY need to read the basics on this first. It is obvious you have mostly read a couple of short summaries and have not yet gone deep enough. That is precisely why I made the analogy I did about "little" thinks... it has little or nothing to do with the smallness!

    In fact, the HUP is a specific limitation on PAIRS of measurements. But not just any pairs: the HUP applies only to so called non-commuting observables (sometimes called canonical conjugate pairs). Let's say there are 6 observables for a photon which form 3 pairs. Position and momentum form such a pair, let's call them p and q. Also there are a pair of spin observables, let's call those y and z.

    Then the rule is that knowledge of p precludes knowledge of q, and vice versa.
    Another rule is that knowledge of y precludes knowledge of z, and vice versa.
    But knowledge of p does NOT preclude knowledge of y or z, and vice versa.
    Knowledge of q does NOT preclude knowledge of y or z, and vice versa.

    1. So you can experimentally determine p and z to ANY level of precision your microscope allows (of course we have some pretty good measurement apparati these days). The reason is that they are not a non-commuting pair, and so the HUP does not apply! If your analogy made any sense at all, that wouldn't be possible for any pair of observables. There would be no differentiation between any particular 2.

    In addition, you can measure any spin observable - say y - 5 times in a row and you will get the same answer. But measure z even once and y will change. So clearly the issue is not some disturbance caused by the observation itself. So everything is NOT uncertain, as you imagine, only some things.

    2. There are additional arguments which are ironclad as well, which I won't repeat because you need to read up first.

    Now, please keep in mind that everyone who first learns the HUP pretty much has the same view and the same skepticism to start with. So no issue about that. But I might ask: you use a computer, do you trust that the computer manufacturer knows how a computer works? Or do you insist that computing is impossible in the face of the evidence otherwise? You should take a moment to learn where other have gone before you before you start dissing arguments that have been put forth before and found lacking. This particular one evaporated around 1928 or so.
     
    Last edited: Apr 9, 2009
  20. Apr 10, 2009 #19
    Dr. Chinese, I have a question about this ( could as well start another thread for this one, but let me ask it anyway)

    I am sure there are pages of discussions on the experiment Afshar made, but do we have an established explanation in the community for what he has done, or how his argument about complementarity fails?

    I think it'd be really instructive if we summarized the main findings.

    And when I first saw it, I found it amazing that people are still trying to get hold of a theory (conceptually) - that works so easily (mathematically) .
     
  21. Apr 10, 2009 #20

    DrChinese

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    Actually, I had not seen much new on this recently so I went to Wikipedia (not much on PhysicsForums that is much newer). I also found a recent version of his experiment, Violation of the principle of Complementarity, and its implications (the date is 2007 but the experiment is essentially the earlier 2001 one). I personally had never gotten very excited on this because he never claimed the HUP was violated (in case you didn't pick this up, I am a strong believer in the fundamental nature of the HUP). I always looked at Bohr's complementarity more as a "popular" way of describing the HUP rather than a principle in itself.

    So I looked at the critiques of Afshar, which I had not previously read (they are not particularly new). The current view is essentially either: a) the which-way information is not really obtained (Unruh, Kastner, Reitzner) and complementarity is not violated; or b) the visibility parameter V which he claims is close to 1 is actually closer to 0 (Motl) and the experiment is falsified.

    I don't see that it has attracted much subsequent interest, except perhaps from Cramer (who might have an iron in this fire). I don't see where anyone has bothered to question whether the results are repeatable, which implies that it is currently looked at as more of a parlor trick (not meaning to insult Afshar, just pointing out that it isn't leading to anything more meaningful in terms of our understanding at this time).

    I would hope that Afshar would follow-up with something that might drill deeper into what he is actually observing; but wish he would drop the whole "I proved Bohr wrong" thing which is a major turn-off.
     
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