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Why are only some photons virtual?

  1. Feb 12, 2013 #1
    Sorry it was hard to think of a good title.

    Essentially, why do only virtual photons carry the electromagnetic force? What stops 'normal' photons from carrying the EM force? Is it the fact that we can detect them? What decides whether a photon will be virtual and hence carry the EM force, or is it vice versa that a prerequisite of carrying the EM force is being virtual?
  2. jcsd
  3. Feb 12, 2013 #2


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    Hi Mukilab! :smile:

    What is your level of knowledge of virtual particles?

    The maths tells us that we can model reality by assuming that there are virtual electrons and virtual photons in "position space" which obey conservation of momentum but do not have to obey conservation of energy …

    those must have the "correct" mass of an electron or photon​

    after a coordinate transformation, and the insertion of an extra variable, the maths also tells us that there are virtual electrons and virtual photons in "momentum space" which obey conservation of momentum and conservation of energy …

    those do not have the "correct" mass of an electron or photon​

    virtual electrons and photons do not "carry" force, they (both) mediate force …

    "mediate" is a meaningless word deliberately chosen so as not to suggest anything physical :wink:
  4. Feb 12, 2013 #3
    It's not so easy to describe...here are a few of my favorite descriptions...

    Perturbation theory of the standard model of particle physics produces virtual particles....
    see here: http://en.wikipedia.org/wiki/Virtual_particle
    " the accuracy and use of virtual particles in calculations is firmly established, but their "reality" or existence is a question of philosophy rather than science."
    This covers some of the points just posted by Tiny Tim....

    [If you know Fourier transforms, the virtual portion is the imaginary part. The real number part is what we can observe/detect.]

    What is Quantum Field Theory, and What Did We Think It Is?
    Steven Weinberg-Physics Department, University of Texas at Austin


    and perhaps my favorite description, Wikipedia:

    If that isn't crazy enough, consider this: You want to create real photons from virtual photons?? You can!!!

    Just accelerate! Examples are the Unruh effect, Hawking radiation and hanging around any cosmological horizon, like a black hole horizon. Most of the 'real' early particles in the universe were created during high energy 'inflationary' era when the high vacuum energy of virtual particles were 'bound' by the cosmological horizon...and poof, out popped real particles!! If this hadn't happened, our universe would be almost empty and expanding without bound and in which any life such as we know it would be impossible.

    An intuitive way to think about this 'particle creation' is that virtual particle wave-forms [say via the Schrodinger wave equation] are spread out all over the place, ill defined, not detectable. But when a horizon appears, it acts like a bound, a confinement. This is analogous to confining a particle, if you are familiar with that, like positing an electron in an atom when it can have only certain energies, or like clamping the ends of a vibrating violin...clamping, bounding, the ends, forces it into quantized harmonic behavior...
    Last edited: Feb 12, 2013
  5. Feb 13, 2013 #4
    Nothing stops real photons from carrying EM force. They do - just as easily as virtual.

    From the classical end, we can look at 3 types of electromagnetic field:
    1) Static fields - electrostatic and magnetostatic
    2) Evanescent waves. These are time-varying fields which weaken exponentially from their origin. For example, if you look at an antenna which emits radio waves at a different direction, but are near the antenna, you will observe variable electromagnetic fields.
    3) Freely propagating electromagnetic waves
    All 3 are electromagnetic fields. For a short period observation, all 3 look alike. For a long period but local observation, 2 and 3 look alike. Yet it is only 3 which carries energy, momentum and angular momentum away from source.

    On quantum level, if the propagating waves consist of real photons, how about the first 2?
    Virtual photons are particles of static fields and evanescent waves, just like real photons are particles of propagating waves.
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