Atoms

The turn came at the start of the nineteenth century, in the unglamorous work of chemistry. John Dalton, a self-taught English schoolteacher and Quaker, was studying how gases combine, and he kept running into a peculiar regularity. When elements reacted to form compounds, they did so in fixed, whole-number ratios of weight. Hydrogen and oxygen made water in the same proportion every time. Carbon and oxygen made two different gases, and the amounts of oxygen involved sat in a simple ratio to one another.

Dalton drew the obvious conclusion around 1803: matter comes in discrete units, each element has its own kind of atom with its own characteristic weight, and compounds form when these atoms join in fixed numbers. This was the Greek idea reborn, but with a crucial difference. Dalton's atoms carried numbers. They had weights you could measure, or at least compare, and those numbers explained the laboratory results. For the first time, the atom paid its way — it did work no other theory could do.

Almost as soon as atoms were confirmed to exist, they began to fall apart. In 1897, J.J. Thomson at Cambridge showed that a mysterious ray inside vacuum tubes was actually a stream of tiny, negatively charged particles — electrons — that were far lighter than any atom and seemed to come out of atoms themselves. The uncuttable thing had parts. Thomson pictured the atom as a diffuse positive blob with electrons scattered through it, a model affectionately remembered as plum pudding.

That picture did not last a decade. Around 1909, Ernest Rutherford's team in Manchester fired positively charged particles at a thin sheet of gold foil, expecting them to pass gently through. Most did. But a tiny fraction bounced almost straight back, which Rutherford said was as astonishing as a shell rebounding off tissue paper. The only explanation was that nearly all of an atom's mass sat in a minuscule, dense, positively charged core. The atom was mostly empty space, with a nucleus at its center and electrons somewhere out in the void around it.

By the mid-twentieth century, physicists had powerful new tools — particle accelerators that smashed matter together at enormous energies — and what came out was a mess. Dozens of new particles appeared, so many that the physicist Enrico Fermi is said to have grumbled that if he'd wanted to memorize this many names, he'd have become a botanist. The neat atom of proton, neutron, and electron had become a bewildering zoo, and it badly needed an organizing principle.

That principle arrived in the 1960s. Murray Gell-Mann and, independently, George Zweig proposed that protons and neutrons were not fundamental at all but built from still smaller things, which Gell-Mann named quarks. Over the following decades this idea grew into the Standard Model — the framework that now organizes all known matter into a small set of quarks and leptons, held together by force-carrying particles. It is, by any measure, the most precisely tested theory in the history of science, matching experiment to a staggering number of decimal places. The 2012 detection of the Higgs boson at CERN filled in its last predicted piece.

Twenty-four centuries after Democritus, we can weigh individual atoms, image them, split them, and rearrange them. We know the word he chose — uncuttable — was wrong, and gloriously so, because everything interesting about matter has turned out to live in the cutting. The atom was not the bottom. It was a doorway, and behind it were nuclei, and behind those quarks, and behind those a set of questions that the finest theory ever built cannot answer.