Quantum antenna thought experiment

[jaydnul]

If you have a classical antenna absorbing an electromagnetic wave, the charged particles inside the antenna will be given momentum in the directions perpendicular to the direction of propagation of the EM wave (because the E and B fields are perpendicular to propagation).

If just a single charged particle is absorbed by a single photon, the conservation of momentum would seem to indicate that the charged particle would gain momentum in the same direction as the photon.

How does this classical phenomenon emerge from the quantum one?

 

[mfb]

An antenna gets pushed back a bit if it absorbs radiation, while the net effects on perpendicular acceleration cancel.
The overall force is in the direction of radiation in both cases. You just don't see the effect on antennas as it is tiny, and you see collective effects (back and forth of electrons) in the antenna due to the coherent radiation - you can get the same motion on a microscopic scale in intense laser fields.

 

[jaydnul]

...you see collective effects (back and forth of electrons) in the antenna due to the coherent radiation,,,.

This is what has me confused. How is the photon interacting with the electron to give it perpendicular momentum?

 

[jaydnul]

The photon has linear and angular momentum. The linear momentum is conserved along the direction of propogation when the photon is absorbed. What about the angular momentum? Does the electron gain the angular momentum?

 

[mfb]

This is what has me confused. How is the photon interacting with the electron to give it perpendicular momentum?

Well, don't consider photons for classical antennas. Consider the electromagnetic field - in particular the electric component.

Does the electron gain the angular momentum?

A single photon cannot absorb a photon without any further interaction, that would violate energy/momentum conservation. The overall reaction conserves angular momentum, of course.

 

[jaydnul]

I assume you mean a single electron cannot absorb a photon without further interaction. So does that mean a single isolated electron under acceleration couldn't emit a photon because it would violate the conservation of energy/momentum?

 

[mfb]

Under acceleration of what? If it accelerates, it interacts with something.

The process photon + electron -> electron is impossible. This is easy to see if you consider the reverse process in the electron rest frame: an electron at rest "decays" to an electron and a photon. Wait, where did the energy come from?