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 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 constraining them to be no larger than 10-19 meters across. Somehow, protons, neutrons, and electrons combine together to create atoms, which occupy much greater volumes of space than their components added together. It’s a mysterious fact that atoms, which must be mostly empty space in this regard, are still impenetrable to one another, leading to enormous collections of atoms that make up the solid objects we’re familiar with in our macroscopic world.
So how does this happen: that atoms, which are mostly empty space, create solid objects that cannot be penetrated by other solid objects, which are also made of atoms that are mostly empty space? It’s a remarkable fact of existence, but one that requires quantum physics to explain.
