1
$\begingroup$

Imagine two charges A and B separated by some distance.

Charge A emits a photon which is absorbed by charge B.

Is the recoil momentum received by charge A always equal and opposite to the momentum gained by charge B?

Is this true both for static Coulomb fields and radiation fields from accelerating charges?

I suppose there is no momentum "left over" in the EM field after the interaction so that all the momentum lost by A is absorbed by B. Is this how it works?

Improve this question
$\endgroup$
2
  • $\begingroup$ Yes, momentum conservation. $\endgroup$
    – pfnuesel
    Commented Oct 25, 2013 at 15:01
  • $\begingroup$ So when I accelerate A perpendicular to the line A-B then classical EM says B receives some perpendicular momentum. Does A receive recoil momentum in the opposite direction? $\endgroup$
    – John Eastmond
    Commented Oct 25, 2013 at 15:43

1 Answer 1

Reset to default
2
$\begingroup$

The momentum and mass-energy of a quantum system are together conserved. So a classical notion of recoil momentum is only partly true. The electron can absorb or emit photons but photons are quantized so will not conserve momentum classically like your question implies. Nor will it behave as a classic inelastic collision where energy, Instead both are conserved together so we can observe scattering of an incoming photon often as a change of wavelength and direction. This is called Compton scattering and proved the need to quantize photons.

Improve this answer
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.