MadSci Network: Physics |
First let us consider what a mirror does: Our standard picture of a perfect mirror is that photons incident on it cause electrons (inside it) to oscillate in phase with the incoming photons in such a way as to screen out any propagating EM field within the mirror itself. A wave is therefore reflected. This phenomenon is discussed at length in standard texts on Electromagnetism and optics. See, for example, Optics by Eugene Hecht Modern Optics by B. D. Guenther Electromagnetic Fields and Waves by Lorrain and Corson However, maybe we can think about the direct interaction with the electrons. The first thing to note is that a photon doesn't just interact with one electron. The electrons in a metal are all interacting and so the photon sees the combined effect of more than millions of billions of electrons. The electrons initially (assuming that the mirror is cold) have very little energy (compared to the energy of a photon). The photon has a well-defined energy and phase. We know that energy is never transferred to the electron for a long period of time because the mirror is 100% reflective so what happens to the phase? Well, if all of the energy is passed back to the reflected photon, this photon must remain coherent since the electrons are no longer oscillating and there have no phase. Even if the mirror is not perfect and maybe absorbs a little or scatters, the fraction of incoming photons that is reflected remain coherent since the very short-lived oscillation they caused has re-emited its light. It might be expected that absorbed photons would transfer some of their phase to the electrons. However, absorption in a metal causes heating which is not a phase-conserving process. Since none of the phase can passed onto the electrons (at least not for long periods of time) it is maybe obvious that there can be no kind of entanglement and certainly none of the sort of entanglement which occurs in quantum mechanics which is actually quite difficult to generate. A tutorial on this kind of entanglement can be found at this site.
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