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Why does not the graviton cause wavefunction collapse?

  1. Jul 1, 2015 #1
    The modern view of the measurement problem is that any interaction of a particle (say an electron) will cause its wavefunction to 'collapse' in the process called decoherence. No need for conscious observers, interaction with any other particle will cause decoherence hence collapse of the wave-like electron into a definite electron at a definite location and time (definite within the uncertainty principle depending on its momentum).

    Now, forces are also believed to be mediated by 'particles', photons, gluons, W and Z bosons...

    One might believe that in carefully controlled experiments, all these force carrier particles are sufficiently isolated from the 'to be observed' electron as to let it travel as a wave until it will interact with any other particle from its environment, whether it being some other fermion or some boson.

    But gravitons, if they exist, can not be isolated from the electron, they permeate the whole space in the lab. If they exist they must continuously interact with the electron and we have no way to avoid that.

    Assuming that gravitons exist, how come the interaction with them does not collapse the electron's wavefunction?
    Should we take this as a hint that gravitons do not really exist?

    Incidentally, the same could be said about the Higgs boson, in principle we have no way to avoid the electron interacting with it. So why can the electron still display wavelike behaviour? Why do not interaction with the Higgs or with the graviton cause collapse of the electron's wavefunction?
     
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  3. Jul 1, 2015 #2

    atyy

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    Collapse still needs a conscious observer in some way in all forms of Copenhagen. Decoherence only causes apparent collapse, which is the same as when the measurement is performed but the result is not recorded by the conscious observer.

    The curvature of spacetime can be considered a coherent state of gravitons. So if we wish to include the interaction with gravitons, we would use the formalism of quantum field theory on curved spacetime. However, for most experiments that we do, the curvature of spacetime is too small to be detected, so we can use quantum field theory in flat spacetime.
     
  4. Jul 1, 2015 #3

    atyy

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    This paper by Pikovski, Zych, Costa and Brukner partly addresses the issue raised by the OP.
    http://arxiv.org/abs/1311.1095
    Universal decoherence due to gravitational time dilation
    Igor Pikovski, Magdalena Zych, Fabio Costa, Caslav Brukner


    Sabine Hossenfelder has a blog post http://backreaction.blogspot.com/2015/06/no-gravity-hasnt-killed-schrodingers-cat.html with relevant remarks from her, as well as an author of the paper.


    "Besides this, the center of mass isn’t the only quantum property of a system, because there are many ways you can bring a system in superpositions that doesn’t affect the com at all. Any rotation around the com for example would do. In fact there are many more degrees of freedom in the system that remain quantum than that decohere by the effect discussed in the paper. The system itself doesn’t decohere at all, it’s really just this particular degree of freedom that does." [Hossenfelder]

    "In the scenario that we consider, the center of mass becomes correlated with all the internal constituents. This takes place due to time dilation, which correlates any dynamics to the position in the gravitational field and results in decoherence of the center of mass of the composite system.
    For current experiments this effect is very weak." [Pikovski]

     
    Last edited: Jul 1, 2015
  5. Jul 1, 2015 #4

    atyy

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  6. Jul 2, 2015 #5
  7. Jul 2, 2015 #6
    About that beam of neutrons entering from the left : it is presumably an ensemble / mixture with a range of energy levels, and the absorber is like an upper cutoff that skims off the top few levels? Is that how it works?
     
  8. Jul 2, 2015 #7
    OP this is an interesting question. Penrose and his Neuroscientist collaborator Hameroff discuss some of this in their papers on the quantum origins of consciousness. I'm not sure that this is main stream science though but it does hint at some of the things you are asking.
     
  9. Jul 2, 2015 #8

    atyy

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    I don't think this is a good pointer because Penrose and Hameroff are not mainstream science, nor even mainstream speculation. Instead for mainstream speculative modifications to quantum mechanics involving gravity, I would point to Penrose's earlier work on what is nowadays usually called Diosi-Penrose collapse. Some recent papers on Diosi-Penrose collapse are:

    http://arxiv.org/abs/1408.6460
    Phys. Rev. A 90, 062105 (2014)
    Gravity and the Collapse of the Wave Function: a Probe into Diósi-Penrose model
    Mohammad Bahrami, Andrea Smirne, Angelo Bassi

    http://arxiv.org/abs/1410.0270
    Nature Physics 10, 271 (2014)
    Testing the limits of quantum mechanical superpositions
    Markus Arndt, Klaus Hornberger
     
  10. Jul 9, 2015 #9

    bhobba

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    That isn't quite what it says. Interactions must also be strong enough in the particular models where it happens. Gravity in general is quite weak. Its the reason we can see interference effects with photons. Normally decoherence effects prevent that, but in the double slit they interact weakly while travelling to the screen.

    Thanks
    Bill
     
  11. Jul 10, 2015 #10
    "Interactions must be strong enough for decoherence to take place". Do we have any quantitative statement for what "strong enough" is supposed to mean? Do you mean strong enough among two particles, or perhaps that many particles are required to interact together?
     
  12. Jul 10, 2015 #11

    bhobba

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    Yes we do. See:
    https://www.amazon.com/dp/3642071422/?tag=skeartcom-20

    But gravity is so weak it doubtful it would be enough.

    Thanks
    Bill
     
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