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Everybody knows that excited electrons can emit photons upon relaxation. A nucleus too (which is not an elementary particle), can be in an excited state and emit gamma rays upon relaxation:

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So is it a law of nature that elementary particles (and very small systems such as a nucleus) can have radiative transitions?

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Sparkler
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    Any physical system with a mode at frequency $\omega$ and where that mode couples to electromagnetism (i.e. there's charge) can emit photons. This is not limited to particles. Electrical circuits do this too. That's how you listen to radio or how the data in this comment is transmitted from my computer to yours. – DanielSank Feb 24 '16 at 23:01
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    "Everybody knows that electrons can emit photons upon relaxation." Er ... bound electrons in an excited state. It's really a system doing that, not a fundamental particle. – dmckee --- ex-moderator kitten Feb 24 '16 at 23:04
  • @dmckee "excited" added. – Sparkler Feb 24 '16 at 23:05
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    The electron doesn't get excited. It's the atoms that get excited. It's not clear that electrons even have any internal states... they are, after all, quanta of a field rather than independent objects. Electromagnetic emission can only occur if the system interacts electromagnetically, i.e. when it is charged or has, at least, a dipole etc. moment. In general these kinds of "do all" questions assume that these things are simple and that the world somehow factors into simple things... they aren't and the world doesn't factor like that. – CuriousOne Feb 24 '16 at 23:28
  • @DanielSank thanks, that's exactly what I missed out. – Sparkler Feb 24 '16 at 23:36
  • @CuriousOne thanks, that's exactly what I missed out. – Sparkler Feb 24 '16 at 23:36

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