Atoms are remarkable little creatures. Not only are they generally very stable, but they fit together to make molecules, in which the distances and angles between nuclei are fixed quite precisely (subject to quantum fluctuation). So you can make measuring-rods out of them, among other things. And they also work as very precise clocks, keeping time by the orbital frequencies of their electrons, etc. And they communicate with each other – mainly through electromagnetic channels, but also (rarely) through exchange of nuclear particles like neutrinos. Off hand, it seems that apart from atoms (and things made of atoms), physics describes nothing that can function at all like this. An atomic nucleus by itself may have a certain definite size, but you can’t make any kind of stable spatial configuration with a bunch of nuclei, so you can’t use them to measure distances. An electron by itself may have a certain frequency, but you can’t use it as a clock, because as soon as you interact with it you change its frequency. In general, particles interact with each other and transfer various kinds of information, like momentum and spin-orientation. But an atom can store several different kinds of information at once, and can change its state in multiple ways, while maintaining a stable identify over billions of years. The heavy nucleus of the atom allows its location to be defined very exactly, while the surrounding shell-structure of its electron cloud lets it interact very sensitively to its electromagnetic environment. I’m not clear exactly how to define the “functionality” of atoms, because it’s complex and multifaceted. But it does seem that the emergence of atoms represents a very large step in the evolution of our universe, considered as a system for defining and communicating information. Before there were atoms, I’m wondering whether space and time would have been meaningfully definable? Of course we can just posit a spacetime continuum (or whatever) as a mathematical structure, along with mathematical laws of dynamics, etc. But maybe our ability to define something logically is less significant than the ability of the physical world to define itself concretely, in actual measurements. We know the universe we live in is extremely “finely tuned” in its basic parameters. My impression is that changing almost any of these parameters, in some cases by a very small fraction, would make the existence of stable atoms impossible – or at least atoms built the way ours are. I’m not interested in “anthropic” reasoning – the idea that the world has to be the way it is, or we wouldn’t be here to observe it. The existence of conscious observers seems to depend on a lot more than just laws of physics, and it hardly seems central to the way the physical world itself operates. On the other hand, maybe there’s a way of making sense of a universe that’s structured to support the existence of atoms, i.e. entities that are capable of defining that same structure, to each other. So when it comes to foundations of physics, I’m wondering whether the key really will turn out to be defining spacetime structure at the Planck scale, so many orders of magnitude beneath the atomic level. Is it likely that a quantum gravity theory will explain why atoms exist?... i.e. why the laws of physics produce tiny, finely-tuned devices for measuring and communicating information?