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Violation of Newtons third Law and conservation of momentum

  1. Mar 15, 2015 #1
    Three High energy Photons spontaneously convert into electron positron pairs, The electron on the left marked A is created and and destroyed within a short interval, sending an electric field burst towards electron B shortly after it's created, just enough so that Electrons B's electric field does not have enough time to reach and interact with electron A's, meaning that A exerts a force on B without B exerting a force on A

  2. jcsd
  3. Mar 15, 2015 #2


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    Newton's third law does not hold even in classical electrodynamics. You are right that momentum conservation is a consequence of the third law in Newtonian physics. The correct generalization is to discard the third law and keep momentum conservation.

  4. Mar 15, 2015 #3


    Staff: Mentor

    In QED electric fields are not viewed that way - rather its described in terms of the field theory of electrons, the field theory of photons as the quanta of the em field, and their interaction which, if i remember correctly, is treated as a perturbation.

    Beyond that I will have to leave it to those more conversant in QED - but what you wrote above is not a correct analysis.

  5. Mar 17, 2015 #4
    Your Feynman diagram (and, by extension, your thought experiment) is not physically possible, even though the basic observation about Newton's third law is accurate. A photon converting into an electron-positron pair is fine as an internal vertex, but such a process cannot conserve 4-momentum and so as a complete interaction has amplitude zero. You need a minimum of two photons to produce a free electron-positron pair. Similarly, an electron-positron pair cannot annihilate into a single photon. In both cases, you need a virtual electron to mediate the interaction.

    Also, BTW, your arrows are not drawn correctly. One arrow flows into the vertex, the other flows out.
  6. Mar 17, 2015 #5
    I see, thankyou
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