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Question about QFT and vacuum

  1. Aug 28, 2013 #1
    I'm reading a paper by Art Hobson called "There are no particles, there are only fields" and had a question about something in there. (http://arxiv.org/abs/1204.4616) (Page 20-21)

    He basically says that since the vacuum in QFT has energy and non-vanishing expectation values, it is ultimately inconsistent with a particle view. If particles are fundamental, than what is it that has energy in a state that has no particles? Fields!

    That's all fine, but then he mentions three things that can be measured, which has the vacuum as their source: Lamb Shift, Casimir Effect and the electron's magnetic anomalous moment.

    However, he also mentions the vacuum as the source for Dark Energy (despite our current 120 order magnitude problem between theory and observation)

    Since, apparently these 4 phenomenon are ALL related to the vacuum in QFT, I am trying to figure out how they are all related to each other.

    Is Dark Energy essentially the same thing as the Casimir effect, except on a global scale?

    Does the fact that the three above phenomenon can be experimentally/empirically verified lend support to Dark Energy?

    Dark Energy can be represented by a term of constant energy density under GR. Are these other effects the result of (local) energy density from the vacuum?

  2. jcsd
  3. Aug 28, 2013 #2
    First of all, the Casimir effect is due to spatial variations in the vacuum energy density and not the vacuum energy alone. Only when there is a region where the vacuum energy (specifically, its expectation value) is lower than in other regions can there be a casimir force (and it acts on the interface between the two regions). In order for the casimir effect to occur, there must be regions of different vacuum energies. If we are in a homogenous and isotropic universe (which is the assumption in general relativistic cosmology), then there are no changes in the vacuum energy from place to place and hence no Casimir effect occurs. So the cosmological constant/dark energy is not the Casimir effect.

    It is important to understand that the "dark energy" issue is a problem in our understanding of gravity as described by General Relativity, but, on the other hand, the issue of vacuum energy is from QFT, as you mentioned. Physicists have not yet been able to figure out conclusively how to get these two theories (QFT and GR) to work together--creating a quantum theory of gravity is still an outstanding problem in theoretical physics.

    So how the vacuum energy plays into the phenomena of the cosmological constant, dark energy, and dark matter is not yet understood. Some physicists do believe that the vacuum energy is to blame for the cosmological constant and/or dark energy. But this is mere conjecture and as yet there is no definitive answer.
  4. Aug 29, 2013 #3


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  5. Aug 29, 2013 #4

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    I like Jaffe's paper very much, but I don't see that it addresses what might be called the topological Casimir effect. Namely, that the "vacuum energy" depends on the global structure of the space on which the field lives, e.g. periodic boundary conditions.

    I don't see how in this case we are forced to understand the Casimir effect as due to some interacting currents, etc. since the above energy may be defined for free theories. Although it has mathematical import, e.g. in modular invariance in CFT, perhaps it is somehow difficult to measure in a truly free theory?
  6. Aug 29, 2013 #5
    I don't know, it still seems to me that all the phenemenon I mentioned are due to the fact that the vacuum is not "empty", or "nothing". It participates in the dynamics of the Universe. Yeah, the specifics of each phenemenon is different, but they all tie back to the fact that the vacuum plays a role in physics.
  7. Aug 29, 2013 #6

    Given that the vacuum plays a role in physics elsewhere; and that the vacuum contains energy, and that Gravity "sees everything", doesn't it sound like a pretty darn good conjecture, though?

    Of course the 120 orders of magnitude between observaton and theory doesn't bode well, but it still seems like the conjecture is heading in the right direction.

    I guess we could be wrong, though.
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