QUOTE (abalone @ Feb 26 2006, 10:51 AM)
Sound like a good story but how does one use this analogy to explain that a surface that emits a photon experiences a reactive pressure in the opposite direction, and a surface that reflects a photon experiences a pressure 2X that of a surface that absorbs it.
It is best to to regard a photon as having momentum which has to be consereved both during absorption, reflection and emission.
The photons energy is totally different to if moment. A photon does not exert pressure because it energy is changed into kinetic or mechanical energy
This is not a theory, but what I learned at school, about how photons are absorbed. But I don't know all the possible processes.
In the case of an emission, I guess that there is some reverse process of that of the absorption (for instance in a heated surface, a phonon is absorbed, thus removing a momentum from the surface) but I am not sure of this.
In the case of a reflection, it is a very different process, as the photon is not absorbed. But, as an electromagnetic wave, it induces a current into the reflective surface. This current in turn results in another wave, the reflected wave, that the photon follows. But in order to give an impulse to the surface, the photon must lose energy, and thus lower its wavelength, a thing which is preciselly not observed into a reflection. Are you sure of what you say about a 2X force? If it is true, I don't know exactly what happens. Perhaps simply a Lorentz force between the incoming wave and the current induced into the surface.
What I am sure on the other hand is that a photon don't have a momentum, and you are both false, abalone and Bruce. Momentum is the product of mass per speed square (Ec=1/2 mv2), and it is an energy. Mass of a photon is null, and speed is the speed of light, equivalent of an infinite speed in classical mechanics. The current notions of mechanics don't apply into these conditions, and especially zero multiplied by infinite can give either zero, a discrete value, or an infinite value. It happens that the actual energy of a photon has no relation with its speed. You can no more take the energy of a photon, and put its speed in the formula to calculate its mass.
So radiation pressure result of subtle interactions of light with matter, not of a mechanical reflection of grains of light. If it were so, a beam of sun light (1kw/m2) would slam our faces, and sun light would project the planets into space.