Why is matter stable?

It’s always the simple questions that get you, isn’t it? When my daughter was asked in her chemistry homework why, if atoms are mostly ‘empty’, solids are not inter-penetrable, I could see this was a good way to explore how well she’d grasped concepts like force. Since she hasn’t yet encountered Pauli’s exclusion principle, she wasn’t expected to have the conceptual tools for a more complete answer. But as I began to outline to her that additional factor, I started to feel uneasy.

How much of the stability and solidity of matter is truly electrostatic, and how much comes from quantum-mechanical exclusion? I wasn’t sure that my glib answers were quite getting to the nub. There isn’t any obvious path, for example, between electrons not being able to occupy the same quantum state and not being able to occupy the same general region of space. (And that’s even before she asked, in effect, why fermions and bosons follow different statistics.) A little digging took me to Princeton physicist Elliott Lieb’s classic 1976 review article, The Stability of Matter. If you thought, like me, that you had it all figured out why matter doesn’t collapse or exhibit ghostly insubstantiality, I’d recommend taking a look.