I did not originally intend to jump into this thread, but I think at this point, I should. There are several issues that need to be addressed or corrected here.
1. If you think about it carefully, it is more UNLIKELY that anything is "stopped", meaning it is actually unusual for anything in the universe to be in the same inertial frame as we are. In spite of our terrestial experience, most things and objects are NOT in our inertial frame. Therefore, what should be asked, really, is why is that object stopped, and not why an objected is not stopped, because the former is the more unusual, highly specific event.
2. Based on the rudimentary Newton's Law, to stop anything, one needs to make that object interacts with some external force or field. And object with mass m and charge q can interact with either a gravitational field or an electric field. Thus, to "stop" it, one apply either a gravitational force or an electric force in the opposite direction to its direction of motion. You can't do this with photons (no charge and no rest mass). While they do follow the spacetime curvature of gravitational fields, they don't really interact with gravity the same way as we know classically. So you have no mechanism to stop them!
3. One needs to define the word "stop". If I shoot an object into a blackbox, and the object never leaves that blackbox over a period of time, can I then say that I've "stopped" that object? If so, then one can say that one has stopped light when it never makes it out of an object. So anything that absorbes this light can be thought of as having stopped light.
4. However, in physics, #3 isn't usually defined as stopping light. This is because photon numbers are usually not conserved. Photons can be converted easily into other forms of energy, so they are really not stopped, but rather destroyed. Physics makes a distinction between these two.
5. So can light be stopped in the "physics" sense? Yes! What it means here is that the energy and phase coherence of light is preserved somewhere and then, at a later time, retransmitted without any loss. Lene Hau at Harvard has achived this several years ago.[1] For physicists, this is what is meant by stopping light.
6. In a typical photoelectric effect, the photon is NOT absorbed by an atom or an electron, resulting in the emission of that electron. Remember that a photoelectric experiment is done on SOLIDS, or more specficially, on metals as cathodes. In a metal, the conduction electrons are the ones being emitted in this effect. Conduction electrons DO NOT belong to a particular atom of the metal. The overlapping of the valence shells in a metal causes the formation of "bands" in which the valence electrons loses it's confinment to individual atoms and are now part of the whole bulk solid. So it is incorrect to think that the individual atom's atomic structure are still valid within such energy scale, especially in the usual photoelectric effect experiments.
Zz.
[1] C. Liu et al., Nature v.409, p.490 (2001).
