Why not anything like spontaneous absorption?

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If by "spontaneous absorption" you mean a process where you have an atom sitting in vacuum, with no photons around, and it magically gets excited, then that process is forbidden by the conservation of energy. We also wouldn't call it "absorption", since nothing is getting absorbed.

If, instead, you mean the inverse process to spontaneous emission ─ i.e., the atom is somewhere with a nonzero photon flux, and it spontaneously absorbs one of those photons to go into an excited state ─ then yes, this process is perfectly possible, though normally we call it "absorption". If you have any materials that are not transparent at hand, then this is the process that turns them from transparent to not-transparent.

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Tushar Gopalka
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Tushar Gopalka

Updated on June 23, 2020

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  • Tushar Gopalka
    Tushar Gopalka over 3 years

    I understand that the idea of emission or absorption cannot be incorporated in the framework of non-relativistic QM. We need QFT or relativistic QM to explain the creation and annihilation of particles, which indeed is the central phenomena in an emission/absorption process.

    I have 2 questions here :

    1. How exactly is spontaneous emission of photons because of atomic transitions of electrons modelled in QFT?

    2. Why don't we have any phenomena like spontaneous absorption? Is such a transition rate zero?

    • knzhou
      knzhou almost 5 years
      "Spontaneous absorption" (as opposed to just normal absorption) would correspond to magically jumping up an energy level even though there are no photons around. That violates conservation of energy.
    • Tushar Gopalka
      Tushar Gopalka almost 5 years
      I do realize that, and that is why i asked if the transition rate is exactly zero. It would be good if you can show that from QED, or refer to someplace which proves this.
    • MsTais
      MsTais almost 5 years
      Just to add, Tushar Gopalka has right intuition... In High energy QED there is an allowed process of spontaneous annihilation and creation of photons... If such process happens next to the atomic system, it can for an instance of time get excited and de-excited by a virtual photon exchange (in the absence of real photons at that particular instance)!! And btw, energy conservation can be violated on a short time scale... So before "minusing" the question, make sure, you have sufficient training for that, imho...
    • Tushar Gopalka
      Tushar Gopalka almost 5 years
      Finally.. Somebody understood the question. I want the explanation of spontaneous emission from QED, and asked that does spontaneous absorption transition rate may have corrections due to loops.. that is vacuum polarization
    • PM 2Ring
      PM 2Ring almost 5 years
      @MsTais Locally, energy is always conserved. The apparent violation of energy conservation in QM is an artifact of the virtual particles of perturbation theory. When exact calculations are performed, the violations disappear. See physics.stackexchange.com/questions/103724/… In general relativity, energy isn't conserved globally, but even there, local energy conservation holds.
    • MsTais
      MsTais almost 5 years
      @Tushar Gopalka - One thing that is good to do is to phrase the question specifying the framework that you are working in. As you see, different theories have quite diverging axiomatics oftentimes. In QM framework the folks are right. If you are asking from the prospective of QED, its different. There you can talk about radiative corrections and interaction with quantum vacuum.
    • Ján Lalinský
      Ján Lalinský over 4 years
      > "the idea of emission or absorption cannot be incorporated in the framework of non-relativistic QM" This is not quite right. It is only the spontaneous emission that cannot be described by nonrelativistic theory. Stimulated absorption and emission can be described using so-called semi-classical theory, which is just a combination of Schr. equation and classical external field and was analyzed in some detail already by Schroedinger himself.