Feynman (Volume 1, 38-4) seems to explain the normal force of a floor acting on a table as quantum mechanical, arising from the Pauli Exclusion Principle and also, the Heisenberg Uncertainty Principle. (see quote below). This seems somewhat at odds with an explanation that I have heard many times, which is that the electrostatic repulsion of the opposite-facing electron shells generates the required force. Presumably if Feynman considered electrostatics to contribute to this, he would have said so. I am also surprised because I have heard of neutron star formation through electron degeneracy pressure, and had the impression that this pressure only appears manifest under extreme conditions.
The wikipedia page on electron degeneracy pressure mentions that it plays a role in the bulk modulus compressibility of metals, and the description may imply that the strength of the contribution is undersood. Is there any way to decompose the normal force contributions of the floor supporting a chair into contributions from electrostatics, uncertainty principle, and exclusion principle? If so, what provides the primary contribution?
Feynman Quote, Volume 1 Chapter 38, Section 4
"So we now understand why we do not fall through the floor. As we walk, our shoes with their masses of atoms push against the floor with its mass of atoms. In order to squash the atoms closer together, the electrons would be confined to a smaller space and, by the uncertainty principle, their momenta would have to be higher on the average, and that means high energy; the resistance to atomic compression is a quantum-mechanical effect and not a classical effect. Classically, we would expect that if we were to draw all the electrons and protons closer together, the energy would be reduced still further, and the best arrangement of positive and negative charges in classical physics is all on top of each other. This was well known in classical physics and was a puzzle because of the existence of the atom. Of course, the early scientists invented some ways out of the trouble—but never mind, we have the right way out, now! (Maybe.)
Incidentally, although we have no reason to understand it at the moment, in a situation where there are many electrons it turns out that they try to keep away from each other. If one electron is occupying a certain space, then another does not occupy the same space. More precisely, there are two spin cases, so that two can sit on top of each other, one spinning one way and one the other way. But after that we cannot put any more there. We have to put others in another place, and that is the real reason that matter has strength. If we could put all the electrons in the same place it would condense even more than it does. It is the fact that the electrons cannot all get on top of each other that makes tables and everything else solid.
Obviously, in order to understand the properties of matter, we will have to use quantum mechanics and not be satisfied with classical mechanics."
