Matter's "solidity"? Where does solidity begin? The familiar saying is: “two things can’t occupy the same space at the same time” -- true of matter but not true of lightwaves that pass through one another without obstruction like ghosts. That matter keeps other matter out is of course true of even the tiniest pieces of matter – say, a crowd of helium atoms that have only two electrons each. Explanation #1 for this phenomenon is that when the atoms bump into one another the electrons inside are moving so fast that they create an external electrical repelling barrier. Another explanation is that the bumping atoms refuse interpenetration because of the Pauli exclusion principle – that their electrons have the same sets of quantum numbers. The quantum numbers explanation is hard to argue with because who knows what it means physically to say that separate atoms’ electrons have identical quantum numbers? But a mixture of argon and helium would put atoms of different quantum numbers up against one another wouldn’t it? Explanation #1, that they carve out a blur of electrical shield is easier to question since it is a physical explanation whereas the mathematics of the charge cloud picture admits nothing about electron motion or the trajectories. Also, if the charge clouds themselves act like solids, atom-to-atom, why wouldn’t they act like solids, electron-to-electron, inside the atom? I bring this subject up because I believe it deserves being thought about by thoughtful people. The explanation for matter’s solidity that appeals to me is that the individual electron’s de Broglie waves are by their nature impenetrable to one another. That they are the threshold of solidity. Any thoughts about this matter?