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Quantum Foam

  1. Nov 20, 2007 #1

    Art

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    Do the virtual particles theorised to exist in the vacuum of space produce a gravitational effect? If so does that mean the virtual particles are a part of the 'missing mass' / dark energy picture?
     
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  3. Nov 20, 2007 #2

    LURCH

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    Wow, that's a really interestig question! Not an expert, but I'd have to say probably not. I think that the amount of activity (number density of VPP's) is dictated by the amount of energy in a given region of space. If that's correct, then the quantum foam would just be another way of expressing the energy density, the mass of which is already accounted for.

    Hope somebody a bit more qualified will respond to either confirm or deny this supposition.
     
  4. Nov 20, 2007 #3

    Avodyne

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    Yes, virtual particles must gravitate. If they didn't, the equivalence principle would be violated. For example, the rest energy (=mc^2, where m is the "inertial" mass) of hydrogen in the 2p state differs from the 2s state by the Lamb shift, which includes the effects of virtual particles such as an e+ e- pair; if these didn't gravitate, the gravitational masses would not change in the same ratio as the inertial masses.

    Now, what about the vacuum? If you naively add up all the energy of all the virtual particles in any finite volume, the answer is infinity. Ooops! So we have to subtract a constant energy density from the hamiltonian to cancel off this infinity. What's left over, if anything, is the "cosmological constant", which is one possibility for the "dark energy".
     
  5. Nov 20, 2007 #4

    LURCH

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    I thought the cosmological constant was a repulsive force, not an attractive one; the opposite of gravity?
     
  6. Nov 20, 2007 #5

    Art

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    Isn't this similar to the blackbody radiation infinity problem which was solved by Planck by his discovery of the quantization of electromagnetic radiation? Would a similar solution to the infinity problem be applicable in this case?
     
  7. Nov 20, 2007 #6

    olgranpappy

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    No. Though it may sound similar, it's not the same. Planck's law solves the ultraviolet catastrophy for thermal radiation by quantizing the e/m field. The problem these cosmologists have is not with the thermal part, but with the zero-mode part... their fields are already quantized, it's just that they are not happy with "throwing away" the zero-mode part since it sources gravity.

    unfortunately, no.
     
  8. Nov 21, 2007 #7

    Avodyne

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    A positive cosmological constant has positive energy but negative pressure; it's the negative pressure that results in the repulsion.
     
  9. Nov 21, 2007 #8

    Art

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    What does this repulsive force repulse? If it acts on matter then would it not be akin to gravity in one aspect in that large expanses of space would push clumps of matter it envelops such as matter in a galaxy closer together whilst also acting as a kind of anti-gravity in pushing separate galaxies further apart?
     
    Last edited: Nov 21, 2007
  10. Nov 21, 2007 #9

    Avodyne

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    Well, it causes the universe to expand, but doesn't act directly on matter. It doesn't push anything closer together.

    There's probably a decent lay explanation out there somewhere, but I'm afraid I don't know where to find it ...
     
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