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How does quantum electrodynamics actually explain HOW reflection occurs on a microscopic scale?

Note that Feynman's QED lecture series/book is not sufficient, as he only assumes that light DOES reflect ('turn around and go back') in order to expound his path integral theory. My question is why does light have the propensity to turn around in the first place.

Is it just the absorption and re-emission of photons, and if so, why does it happen so uniformly (i.e. on a shiny thing, entire scenes are reflected near-perfectly). In essence, why are flat things shiny? Are all the molecules arranged at exactly the same angle?

Qmechanic
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Meow
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2 Answers2

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the process is coherent--- the same photon is bouncing off all the atoms at once, and you only get constructive interference when the angle of reflection is equal to the angle of incidence. The condition is that the surface is smooth on the scale of the wavelength of light, so that the light can excite each atom independently, and coherently add up all their contributions. This is Feynman's explanation in QED, I think you just misunderstood it as saying that reflection is assumed--- he just assumed rescattering, and then shows you that it happens in the reflected direction preferentially.

Ron Maimon
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I think this is the right question to ask as it demonstrates a weakness in QED. Firstly, the mirror is a form of partial measurement: the sea of electrons in the mirror constitutes a conductor that eliminates any electrical field parallel to the mirror. Thus the incident photon can only support an electric field normal to the mirror, and the only way for it to do so is to reflect. Thus by measuring part of the photon properties (asserting a component is zero) the mirror imposes behaviour on the photon.

However, this is a field description of reflection. As QED proposes a microscale field solution, its description must replace field reflection, not rely on it. In other words its summation of multiple paths is meant to produce the observed field properties, it therefore cannot use those field properties as an explanation. Thus QED needs to describe how the photon state is captured by multiple electrons into an excited electron state and then subsequently decays into the reflected photon. I am not aware of any such explanation and so would be eager to hear if anybody has.

Gavin Peacock
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