The quantum reason behind the solidity of matter | by Ethan Siegel | Starts With A Bang! | Dec, 2024

If atoms are mostly empty space, then why can’t two objects made of atoms simply pass through each other? Quantum physics explains why.

Here on planet Earth, as well as in most locations in the Universe, everything we observe and interact with is made up of atoms. Atoms come in roughly 90 different naturally occurring species, where all atoms of the same species share similar physical and chemical properties, but that differ tremendously from one species to another. Once thought to be indivisible units of matter, we now know that atoms themselves have an internal structure, with a tiny, positively charged, massive nucleus consisting of protons and neutrons surrounded by negatively charged, much less massive electrons. We’ve measured the physical sizes of these subatomic constituents exquisitely well, and one fact stands out: the size of atoms, at around 10^-10 meters apiece, are much, much larger than the constituent parts that compose them.

Protons and neutrons, which compose the atom’s nucleus, are roughly a factor of 100,000 smaller in length, with a typical size of only around 10^-15 meters. Electrons are even smaller, and are assumed to be point-like particles in the sense that they exhibit no measurable size at all, with experiments…