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The Borderworld between Quantum and Classical Physics

  1. Sep 18, 2008 #1

    mea

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    The "Borderworld" between Quantum and Classical Physics

    A perpetual problem in physics has been that there is a discrepancy between where quantum mechanics holds, such as in microscopic phenomena, and where it falls apart at the macroscopic level. For example, photons and electron, under quantum theory, are able to be in two places at once yet there has been no evidence of this happening in the visual world i.e. people are not bi-locating.

    Thinking about this today in Physics class made me ponder the possibilities that perhaps the borderland between the quantum and classical world is due to gravity. It is my understanding that gravity is a curvature in spacetime and thus would it possible that particles such as electrons and photons are affected by the curve and fall into place in a single spot? Or perhaps gravitons (or some other such particle) create an attractive field and as photons etc. pass through them they stick together into a single entity due to the attractiveness of gravity? Kind of like going through a "security checkpoint" of sorts for classical mechanics.

    I was wondering if you good guys were able to help me prove / refute such a notion. Along with this, I'm also toying around with multi-dimensional gravitons and the like but for now I'd like to keep thing rather simple
     
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  3. Sep 18, 2008 #2

    mea

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    Re: The "Borderworld" between Quantum and Classical Physics

    Anything at all?
     
  4. Sep 18, 2008 #3

    atyy

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    Re: The "Borderworld" between Quantum and Classical Physics

    Penrose (The Road to Reality, Knopf 2005) and Smolin, among others, have both proposed that a quantum theory of gravity will resolve the "borderworld" problem. There are also other approaches, which are more well-defined at the moment, such as those mentioned in the articles by Zurek. It is not clear (to me :confused:) whether a solution to the general problem is known, and what the relationship between the different approaches are.

    The case for background independence
    Lee Smolin
    http://arxiv.org/abs/hep-th/0507235

    "One can also mention a fifth unsolved problem, that of resolving the controversies concerning the foundations of quantum mechanics. ... We know from the experimental disproof of the Bell inequalities that any viable hidden variables theory must be non-local. This suggests the possibility that the hidden variables are relational. ... In our theory the non-local hidden variables are coded in a graph on N nodes, which is argued to arise from the low energy limit of a relational theory like loop quantum gravity. ... It is too soon to see if these theories will be successful."

    Decoherence and the transition from quantum to classical -- REVISITED
    Wojciech H. Zurek
    http://arXiv.org/abs/quant-ph/0306072

    Decoherence, einselection, and the quantum origins of the classical
    Wojciech H. Zurek
    http://arxiv.org/abs/quant-ph/0105127
     
    Last edited: Sep 18, 2008
  5. Sep 19, 2008 #4

    mea

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    Re: The "Borderworld" between Quantum and Classical Physics

    Thanks for those papers. They look quite helpful to my research. I'm still in high school so I don't have a lot of the math background to actually propose anything but I am very very interested in the subject from a qualitative point of view.

    Speaking of which, are there any books that would help me ease my way into quantum mechanics / theory with explanations of the math and how to work with it? I have Feynman's Lectures so I might peruse through those but is there anything more comprehensive and textbook-ish?
     
  6. Sep 19, 2008 #5

    olgranpappy

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    Re: The "Borderworld" between Quantum and Classical Physics

    This is not true. Although what you are saying certainly depends on what exactly you mean by "be in two places at once", one electron is certainly never found (measured) to be in two places at once...

    I'm glad to see that you have the Feynman lectures. I'm sure he discusses the double slit experiment with electrons, which is exactly what you are interested in. The main point that you should grasp is that the electron is never *measured* to be in two places at once; each electron is fired at the slits and hits the observing screen in one place. It is only after a large number of electrons are shot at the slits--and subsequently observed on the screen--that the interference pattern develops.

    Good luck with your studies. As far as other textbooks go, have a look at Griffiths "Introduction to Quantum Mechanics".
     
  7. Sep 19, 2008 #6
    Re: The "Borderworld" between Quantum and Classical Physics

    But doesnt an electron pass through both slits, then settles on a final location? If after a while, this wave pattern shows up, this must be true, that an electron photon whatever has to go through both slits at the same time to interfere. So maybe what mea (im pretty sure its you mike, am i right?) is proposing is that gravity could be the reason why it settles in the final location? I could be totally wrong about all of this however.
     
  8. Sep 19, 2008 #7
    Re: The "Borderworld" between Quantum and Classical Physics

    Do either of those have any experimental support? How about the decoherence program?
     
  9. Sep 19, 2008 #8

    olgranpappy

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    Re: The "Borderworld" between Quantum and Classical Physics

    What do you mean by "pass through both slits"?

    If you try and look you will always find that each electron either goes through slit number one or goes through slit number two. Never both.

    Again, I'm pretty sure that this very topic is covered in the Feynman Lectures. Also, his lectures are on tape (cd too now probably) and it is fun to hear him talk about all this. Cheers.
     
  10. Sep 19, 2008 #9

    atyy

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    Re: The "Borderworld" between Quantum and Classical Physics

    I don't know about gravity-induced wavefunction collapse, but the decoherence programme is addressed by (with references to earlier work):

    Quantum jumps of light recording the birth and death of a photon in a cavity
    Sébastien Gleyzes et al
    http://arxiv.org/abs/quant-ph/0612031

    Progressive field-state collapse and quantum non-demolition photon counting
    Christine Guerlin et al
    http://arxiv.org/abs/0707.3880
     
  11. Sep 19, 2008 #10
    Re: The "Borderworld" between Quantum and Classical Physics

    I'm afraid that, prima facie, this particular idea looks to me to have a few difficulties. The whole point of a statement such as "the electron is in two places at once" is that a single particle seems to be subject to dynamics such that there are contributions from different spatial locations. To tie the different branches together spatially would remove the whole reason for our supposing that the particle exists in a superposition of states in the first place! Although I can see where you might be coming from if you were thinking in terms of, say, energy levels instead of some variable that isn't oviously dependent on position, you'd still have to explain why we observe the energy associated with a single state rather than a sum of states. Additionally, you might like to consider the relative strengths of the gravitational (attractive) and electrostatic (repulsive) forces that two electrons exert on each other...
    Still, it has been suggested that gravity might have something to do with it. The tricky part in general for me seems to be how you would tie in the gravitational effects with our act of measurement.
     
  12. Sep 20, 2008 #11
    Re: The "Borderworld" between Quantum and Classical Physics

    The dual slit experiment proves that electrons produce interference patterns with each other. If it only went through one slit, then it wouldnt produce an interference pattern.
     
  13. Sep 20, 2008 #12
    Re: The "Borderworld" between Quantum and Classical Physics

    quantum mechanics is silent on the issue.
     
  14. Sep 20, 2008 #13
    Re: The "Borderworld" between Quantum and Classical Physics

    What do you mean by "went"? :wink:
     
  15. Sep 25, 2008 #14
    Re: The "Borderworld" between Quantum and Classical Physics

    The interference pattern is produced (say a simple photon interference on a double slit experiment) by the photons 'landing' at given positions. Quantum theory is probabilistic, if you look at the very instance where a photon passes through the slits, then it will travel through only one slit. However, gradually, the probability seemingly 'conspires' to create the interference pattern.

    Back to the original question- there's not really a 'borderworld' problem if you think about it. Certainly, the Copenhagen interpretation forces it on us, but quantum mechanics can be thought to be a complete theory (as complete as it can get atleast- Hawking, et al. suggest incompleteness theorems indicate all encompassing theories may be impossible), it doesn't have a certain threshold where it stops applying.
    The reason uncertainties and collapsing wavefunctions aren't 'around the corner' in the mundaneness of everyday is because observations are continually happening on the macro scale. The real question is whether this is a statistical effect, and gravity may play a role in explaining that.

    Consider for instance that even if some apparatus had the potential to 'observe', so long as it too is not observed it entangles with the system it tries to observe! Thus the problem exists on all scales.
     
  16. Sep 26, 2008 #15
    Re: The "Borderworld" between Quantum and Classical Physics

    The only point I would clear up here is that if you try and detect the slit through which the electron passes, then you destroy the interference patten because you collapse the wavefunction into a position eigenstate "prematurely".
     
  17. Sep 26, 2008 #16
    Re: The "Borderworld" between Quantum and Classical Physics

    Oh sorry- I was trying to say that if the slits were observed, the photons would travel through either randomly, on average half of them through one slit and half through another; and then, given they took both paths if the slits weren't observed, the fact that there'd then be an interference pattern at all indicated the curious probability distribution.
     
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