When a photon that has an energy of at least twice the rest energy of an electron passes by an atomic nucleus, it turns into an electron and a positron. But why? Nothing I know about quantum/particle physics tells me why this should happen. From my very limited and unprofessional experience in particle physics, I can only venture a guess it's because the electric field around a nucleus is so strong that it becomes more energetically favorable to split the photon into an electron-positron pair and have them fall down their potentials than to keep the photon.

Technically, this can't be quite right, since the electric field is just an approximation of QED, but I don't know the QED equivalent of what would be happening.

Edit: The answer to this question here says pair production needs to happen around nuclei to conserve momentum. It's helpful to know, but it still doesn't explain why the photon doesn't stay a photon.

  • $\begingroup$ Possible duplicate: physics.stackexchange.com/q/274322/50583 $\endgroup$
    – ACuriousMind
    Sep 4, 2021 at 22:07
  • $\begingroup$ @ACuriousMind I'm sure that question is at least somewhat related, but I was asking specifically about pair-production near atomic nuclei. One of the answers I saw seemed to touch on pair-production, and it looked like it has something to do about bound energy turning into rest mass and flux vortices and somehow that creates an electron/positron pair. Unfortunately, I don't know nearly enough to say whether they're fundamentally the same question. $\endgroup$
    – zucculent
    Sep 4, 2021 at 22:30
  • 3
    $\begingroup$ See also: Everything not forbidden is compulsory $\endgroup$
    – ACuriousMind
    Sep 4, 2021 at 22:30
  • $\begingroup$ I suggest it's a duplicate because the second photon in gamma-gamma pair production fulfills exactly the same role as the nucleus in your question - it makes it kinematically possible (i.e. energy+momentum conservation can be maintained, as opposed to a lone photon producing a pair) to get a pair. $\endgroup$
    – ACuriousMind
    Sep 4, 2021 at 22:32
  • $\begingroup$ So it sounds like it can happen without violating any conservation laws, so therefore it does happen at least sometimes. $\endgroup$
    – zucculent
    Sep 4, 2021 at 22:34


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