elementary particle

Think of taking apart a clock. The clock comes apart into gears, the gears into smaller springs and pins, but at some point you reach a single solid screw that has no smaller parts inside it — it is just a screw, all the way through. In nature, the equivalent of that screw is called an elementary particle: a piece of matter or force that, as far as every experiment can tell, has no smaller parts and no internal structure at all.

An elementary particle is a particle with no known substructure — it is not built out of anything smaller. The electron is the classic example: no experiment has ever found anything inside it, so we treat it as a true point with no size. By contrast, a proton looks like a single particle but is actually made of quarks held together by gluons, so the proton is composite, not elementary. The currently known elementary particles are the quarks, the leptons (such as the electron), the force-carrier bosons, and the Higgs boson — a surprisingly short list for building the entire visible universe.

Calling a particle elementary is always a statement about what we have been able to probe, not a claim of certainty for all time. The word elementary was once applied to the atom (whose name literally means uncuttable), and later to the proton, both of which turned out to have parts. So today's elementary particles might one day reveal structure if we probe them at still higher energies and shorter distances. For now, though, the electron and the quark behave exactly like indivisible points, and the theory built on that assumption works extraordinarily well.