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A question on photons

  1. Mar 19, 2005 #1
    I'm trying to piece this all together, but I am having some trouble.

    If photons are created by virtual particle pair annihilation and virtual particle pairs are produced by photons, then how did photons come into existence in the first place to produce the particle pairs that produce them? I hope I'm not getting something, but this seems circular.

    Anyone care to give me a laymen explanation?

  2. jcsd
  3. Mar 19, 2005 #2


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    Are you under the impression that photons can ONLY be created via pair annihilation?

  4. Mar 19, 2005 #3
    Well, they are created by emissions from electrons, but doesn't the electron require high energy photons to cause the electron-positron to exist in the first place?

    It's also created by "any fluctuation in the electromagnetic field", but I don't really understand what this means. I'm really new to the concept of quantum anything, I'm just trying to make sense of this.
  5. Mar 20, 2005 #4
    Photons are the quanta of the Electromagnetic (EM) field. The light you see with at night emitted from you house's lightbulbs is not created from electron/positron annihilation. Maxwell's equations govern EM in the classical domain. Pick up an undergraduate book on EM and look at the part on EM radiation.

    As far as vacuum flucuations and the creation of virtual virtual pairs in the vacuum due to these flucuations- this subject is governed by Quantum Field Theory and requires a great deal of learning (at least for me) to begin to feel comfortable with the implications and notions. The uncertainty principle can be used to understand how the vacuum can create particles out of seemingly nothing in a very crude way.
  6. Mar 20, 2005 #5


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    As Norman has said, maybe you should consider seriously if you really believe that the light coming from your light bulb is due to some electron-positron anhilation.

    Classical E&M indicates that ANY accelerating charged particle (and not just electrons) will generate EM radiation. Synchrotron centers all over the world make use of this fact to generate their "light". You can also get light from atomic/molecular/vibrational/etc transition. This is when a system goes from one excited quantum state to another quantum state, and if the "rules" are OK, you get photons!

    Moral of the story: electron-positron anhilation is VERY rare, and I wouldn't use it as a source to light up my living room.

  7. Mar 21, 2005 #6

    I'll try.

    Photons are the quantum mechanical explanation of the way in which
    charged particles exert forces on one another. For your discussion
    there are two "classes" of photons, virtual and actual.

    Photons, both virtual and actual are created by charges, not by other
    photons. Some virtual photons can briefly be created out of nothing
    but they do not persist.

    Actual photons carry a precise energy and momentum, and they can
    impart those quantities to a(nother) charged particle in an encounter.

    Virtual photons are the QM explanation for two different things.

    First, the virtual photon is the way two charges can exert a FORCE on
    one another WITHOUT exchanging energy or momentum. Hence virtual
    photons do NOT carry energy or momentum. This is worth pondering.

    Second, the virtual photon is postulated in a more exotic setting as
    an entity which pops in and out of existence briefly. Their reality is not
    in doubt, but in their behavior they do not resemble the virtual photon
    of electrostatic charge or the actual photon of energy/momentum exchange.

    (As far as what came first, the photon or the charged particle I don't know.
    But I can tell you which came first, the chicken or the egg: The chicken
    came first. Eggs don't form out of nothing. The first chicken to lay an
    egg could have been a mutuation of one that didn't lay eggs.)

    Hope that helps.
    Last edited: Mar 21, 2005
  8. Mar 21, 2005 #7
    I don't think this is true because virtual photons have to carry a well defined momentum. If an electron absorbses a virtual photon at a vertex the momentum of this photon has no uncertainty. Thus the position of this photon can be everywhere. No the orther particle takes this photon and gets a change in momentum. So both, electron and the other electron have momentum changed.
  9. Mar 21, 2005 #8
    You make good sense Sterj, but I have a couple of problems with this.

    In particular, if a photon has a definite momentum, it also must have
    a definite energy as well as a definite frequency. The frequency of a
    virtual photon is 0 Hz. This is what makes them different animals from
    actual photons. Also, all actual quanta can be localized to something
    on the order of their wavelength (but certainly no smaller.) Virtual
    photons of the coulomb type cannot. Note that virtual photons of the
    zero-point-fields *do* have a definite wavelength, energy, and momentum.
  10. Mar 21, 2005 #9
    Suppose that we are trying to calculate the probability (or, actually, the probability amplitude) that some amount of momentum, p, gets transferred between a couple of particles that are fairly well- localized. The uncertainty principle says that definite momentum is associated with a huge uncertainty in position.

    from http://math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html

    This text is about virtual particles (exchange particles).

    Listen, An electron with momentum k can absorb a virtual photon with momentum m. (The momentum is well defined, thus an infinite uncertainty in position is aquired). Antother electron with momentum z can take this photon and has then momentum of m+z when the first electron has now momentum of k-m. These electrons just exchange momentum, else it wouldn't make sense.
  11. Mar 21, 2005 #10
    Sterj, everything you say makes sense- I think you are right.

    Upon deeper reflection, I think they must convery both energy
    and momentum. I will ponder my error further.
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