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I Loop Quantum Gravity and Virtual Particles...

  1. Sep 9, 2016 #1
    Hi all,

    Following up on another post - for a layman, can someone describe the status of virtual particles in Loop Quantum Gravity models? Since LQG avoid UV divergences, and has a different structure from field-based theories, are virtual particles still talked about? (in the context of closed loops in Feynman diagrams and virtual particles annihilating with real particles etc., etc.)

    The other important aspect, of course, is that virtual particles show up in perturbative QFT, but there still seems to be general disagreement regarding their 'reality' - even here on Physics Forums. A. Neumaier has written a great Insights post on the reality of virtual particles, and yet, in another thread on quantum foam and virtual particles, individuals seemed to place them on exactly the same footing as 'real' particles (i.e., not just terms that make sense in perturbation theory, but actual fluctuations occurring the vacuum, force mediators, etc.).

    I think I'm more concerned with the first answer than debating the second.

  2. jcsd
  3. Sep 9, 2016 #2

    A. Neumaier

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    Last edited: Sep 9, 2016
  4. Sep 9, 2016 #3
    My apologies to A. Neumaier - I think my choice of wording was poor, but I now have some serious confusion. From the first Insights articles, I was talking about the phrasing:

    "They cannot be said to exist in space and time, have no position, no meaningful probabilities to be created or destroyed anywhere, no life-time, cannot cause anything, interact with anything or affect anything." (Reference https://www.physicsforums.com/insights/misconceptions-virtual-particles/)

    and from the second:

    "Misconceptions about virtual particles: That virtual particles transmit the fundamental forces proves the ”existence” of virtual particles in the eyes of their afficionados. But since they lack states (multiparticle states are always composed of on-shell particles only), they lack reality in any meaningful sense." (Reference https://www.physicsforums.com/insights/physics-virtual-particles/)

    So, given the above, are virtual particles simply terms used in the perturbative method of calculating path integrals, or should I believe that (although of course unobservable) all of the interactions described by the terms are actually physically occurring? Since there are other methods for computing the required integrals that make no mention of virtual particles, what does that mean for their physicality?

    Another quick quote from Reddit (perhaps not the most reliable source, but...):

    "Virtual particles are an artifact of a particular way to calculate what happens in particle physics. Other ways to calculate, such as lattice gauge theory, explain particle physics without virtual particles. The different ways of calculating give the same results. Since you can get the same results about everything you can actually observe in a calculation scheme that does not have virtual particles, there does not seem to be a good basis for suggesting that virtual particles actually exist, only that they are a sometimes useful language for talking about quantum processes."
  5. Sep 9, 2016 #4
    I think the above issues about virtual particles have been discussed here on Physics Forums on a number of occasions. My main question was, is this type of perturbative expansion still required in Loop Quantum Gravity - I think A. Neumaier had already indicated indicated that virtual particles are effectively associated with QFT, and that if you 'mess around' with space-time, all bets are off in terms of what applies (sorry if I'm mis-quoting again or paraphrasing poorly).

    So, is there someone knowledgable about LQG here on Physics Forums that could describe whether virtual particles, or something similar, are used in LQG theory?
  6. Sep 9, 2016 #5

    A. Neumaier

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    Of course you shouldn't believe it. Interactions between the fields are physically occurring, but they are given by terms in the action, not by Feynman diagrams. The diagrams featuring virtual particles are just pictures for (individually mostly meaningless, divergent) integrals. And real particles are asymptotic objects, physically meaningful only at distances where interactions are effectively absent; they interact through the fields of which they are excitations.
  7. Sep 10, 2016 #6


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    One should add that the perturbative diagrams are completely well defined and finite after renormalization has been used to express everything in finite quantities. For each divergent diagram one needs to take into account the various counterterm contributions according to Zimmermann's forest formula to get a finite result, so you have to evaluate all diagrams contributing to a proper vertex function. Feynman diagrams shouldn't be read too much as pictures "what's going on" but ingeneous notations for mathematical formulae to evaluate the expressions for S-matrix elements in perturbation theory.
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