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Virtual Photons over Large Distances

  1. Nov 2, 2011 #1
    It's my understanding that virtual photons are 'allowed' because their existence does not violate the uncertainty principle (the form using ΔE and ΔT). If this is the case though, how is the electromagnetic force transmitted over large distances?
  2. jcsd
  3. Nov 3, 2011 #2
    I think one should not take the virtual particles too literally. Terms that look like intermediate particles (propagators) show up in the perturbation expansion of the interaction between two interacting external particles. However, their energies are allowed to take any value, not just those allowed by the relativistic energy/momentum relation. They are said to be "off the mass-shell".
    The perturbation expansion calls for us to integrate over all possible paths these intermediate or virtual particles can take - even paths of infinite range.
  4. Nov 3, 2011 #3


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    I take it then that you do not believe in the existence of quarks, which are always virtual.
  5. Nov 3, 2011 #4
    Bill_K: Good point.

    I try to keep myself from thinking about of what "really" exists. This allows me to sleep fairly well.
  6. Nov 4, 2011 #5
    You can scatter off virtual b quarks in a proton during high energy physics experiments (like at the LHC) which can produce b jets, which leave a unique signature in your detector. So, from an experimentalist point of view, virtual particles are real. Plus, any "real" particle is always slightly off shell anyway, so they are in essence virtual as well. But this is just slightly off topic...
  7. Nov 4, 2011 #6
    Thanks for making that so perfectly clear. Here on PF questions about virtual particles seem to come up quite often, which then are often addressed by some 'experts' as mere fictious calculation tools with no physical meaning whatsoever. Of course, notwithstanding that every particle physic or qft book and every working particle physicist would deny that, or disregarding the fact that momentum transfer of repelling charges or attraction of two charges can't simply not explained without the help of virtual processes.
    Last edited: Nov 4, 2011
  8. Nov 4, 2011 #7


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    I assume you say this because free quarks are not observed, correct?
  9. Nov 4, 2011 #8


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    Yes, and I would further argue that the same thing can be said not just about quarks, but everything. Sometimes I see the statement made that virtual particles are somehow not real. On the contrary, virtual particles are the only ones that are real. It is the concept of a free particle that is a mathematical fiction!

    Every particle in the world is ever so slightly off the mass shell, because it is en route from one interaction to another. This remains true even if those interactions happen to be millions of years apart. A free particle is a solution to the free wave equation. It's convenient to use the idealized concept of free particles when we talk about scattering problems, but they don't really exist. No free particle has ever been observed. By definition!
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