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When electronic transitions(excited to ground) take place in a hydrogen atom, does the linear momentum of proton-electron system remain constant?

BioPhysicist
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Shreyansh Pathak
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    Don't forget the linear momentum of the emitted photon. – Thomas Fritsch Oct 16 '19 at 19:15
  • Recoil effect, most clearly observed in Mossbauer spectroscopy. – Sunyam Oct 16 '19 at 19:31
  • @Thomas Fritsch what is the external force on the system? – Shreyansh Pathak Oct 17 '19 at 06:49
  • @Unique there is no external force when an atom emits light. The light goes one way, the atom goes the other: net momentum = 0, before and after the event. Light has momentum. – Gilbert Oct 19 '19 at 03:09
  • @Gilbert But linear momentum conservation law predicted that whenever there is a change in linear momentum of a system there is a net external force – Shreyansh Pathak Oct 19 '19 at 03:11
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    @Unique Indeed. But the total momentum does not change. Again: light has momentum. Momentum is a vector. When the atom decays, it emits light. The momentum of the light cancels the momentum of the recoiling atom, because their momenta are in opposite directions. Zero net momentum before emission, zero net momentum after. – Gilbert Oct 19 '19 at 04:48

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You are not saying it in your question, but you are asking about electronic transitions of the atom, and I assume you are asking about excitation and relaxation of the atom/electron, and photon absorption and emission.

You can talk about two things here:

  1. photon absorption, you are basically asking whether an atom receives a recoil when it absorbs a photon, and the answer is yes, this is called radiation pressure. Radiation pressure is the pressure exerted upon a surface due to the exchange of momentum between the object and the EM field.

https://en.wikipedia.org/wiki/Radiation_pressure

This is how solar sails work.

https://en.wikipedia.org/wiki/Solar_sail

  1. photon emission, you are basically asking whether an atom receives a recoil when it emits a photon, and the answer is yes.

Does an atom recoil when photon radiate?

Árpád Szendrei
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  • Hello Sir, linear momentum of a system is not conserved when a net external force is present on the system.So what is that net external force? – Shreyansh Pathak Oct 17 '19 at 03:36