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About the photon exchange and the electromagnetic force

  1. Feb 19, 2015 #1
    As many of you know better than me, photons are the carriers of the electromagnetic force, so they exchange is necessary for example in order to keep an electron around a proton in a hydrogen atom. So how does this work exactly? What is this “exchange”? In the mentioned system, which one of the two particles (if any) emits the photon and which one receives it? Why does that particle emitting a photon does not decay? Is it necessary that a constant exchange of many photons is made in order to keep the electron at that “orbit” or “state of energy” or “distance”? Is there an infinite flow of photons an all directions being emitted from the nucleus to catch the electron at whatever position it might be?
    Thanks and best regards,
  2. jcsd
  3. Feb 19, 2015 #2
    This topic could actually be arbitrarily long. However, I'll try to answer even if I'm sure I'll leave many important aspects outside.
    First of all, when you talk about exchange of photon between, for example, a proton and an electron, you are talking about virtual photons. In particular, this means that there is no such thing as an "emitting" and a "receiving" particle, especially because the meaning of future and past is not well defined anymore (everything must be covariant).
    Now, from a "classical" point of view, i.e. if you want to talk about attractive/repulsive forces (which is a meaningless concept in relativistic quantum mechanics since everything is local) then you need to ask to youself: what configuration of single photons generates something that look like a classical electromagnetic field? It turns out that the configuration you are looking for is a coherent state of photons, i.e. an infinite superposition of states with different number of photons.

    This is a very vast topic. I hope this answers, at least partially, your question.
  4. Feb 20, 2015 #3


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    It's way better to think about the electromagnetic interactions in terms of fields than to think in terms of a naive particle picture of photons. The Coulomb field is not described by a single-photon exchange but of a ladder resummation over virtual-soft-photon states, with the result that you can treat, in the non-relativistic limit, the interaction between an electron and a proton (to describe a hydrogen atom) as the electron moving in the Coulomb field of the proton and use the Dirac equation to get the energy eigenstates. On top you can then evaluate radiative QED corrections, leading to phenomena like the Lamb shift of the energy levels. This program has been performed up to the 4- or 5-loop order over the past decades (Kinoshita et al), and the agreement between the QED calculation and the measured Lamb shift is one of the most accurate results in both experimental und theoretical physics.
  5. Feb 23, 2015 #4
    Ok for the moment I understand from both your answers that the electric field is not described by the exchange of photons but more like a superposition of infinite states of virtual photons. I don’t know the math but I can picture it in my head. Now, I don’t know if these questions are related to the original but these are doubts that appear in my mind after adapting to this new idea:

    a) Can you name/explain a situation where there is an exchange of real (not virtual) photons?
    b) Can the exchange of real photons cause electromagnetic attraction/repulsion? Can there be exchange of photons without electromagnetic attraction/repulsion?
    c) Do the photons of, for example, visible light carry electromagnetic force? Are there imperceptible forces of attraction/repulsion in objects that get hit by light (visible or invisible)?
  6. Feb 23, 2015 #5


    Staff: Mentor

    Hmmmm. Maybe if you have two antennas where each receives the radio emissions of the other.

    Check out:

    Note that virtual photons are simply artefacts of the mathematical methods called perturbation theory used - they don't really exist. If you do the math another way they don't even appear. Its associated with something called the Dyson series whose terms leads to what are called Feynman diagrams and are called virtual particles:

    Last edited: Feb 23, 2015
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