Protons, neutrons and electrons, along with photons — the tiny particles of light — became the main story, with additional roles played by things called mesons that mediated between the main characters, along with a few other odds and ends. These were the elementary particles, and by the 1960s there was a zoo-full of them, barely understood.
Slowly but surely a new theory took shape, with mathematical symmetry playing a role in understanding the nuclear forces between particles. This led to the idea that protons and neutrons themselves had structure, comprising three quarks glued together by the strong force, with mesons consisting of two quarks, and a clear scheme began to emerge. Heavy particles like the proton are called baryons, from the Greek word baros meaning heavy, and light elementary particles such as the electron are called leptons, from the Greek word leptos referring to something fine and delicate. They all now fit into the Standard Model.
But is this really the end of the story? Cosmologists working on the Big Bang theory worry that there is not enough gravitational matter in the universe, so they invent non-baryonic dark matter way out there in the wilds of interstellar space. Perhaps like the Greek fifth element, the aether, it requires a fifth force. Yet remember that the Greeks were not entirely happy with their standard model of "elements", with or without the aether. They wanted something truly indivisible that they called atoms.
Physicists today have a similar yearning. They want to see the four known fundamental forces as manifestations of something more basic, present at the Big Bang. Its glorious symmetry was broken soon afterwards, leaving the universe with the shards of the three known quantum forces plus gravity. Perhaps they are right, and there really is a fundamental force, as yet undiscovered, which like the Greek concept of an atom will yield the final end of particle physics and explain all the matter in the universe.
But a final answer in physics is a rather fin de siècle idea that reared its head toward the end of the 19th century before Relativity and Quantum Theory struck it down in the 20th. It then rose again towards the end of last century with the Standard Model. Will this finally be the end?
No, but it will serve until inexplicable contradictions create a paradigm shift. That is how Einstein's Relativity came along: the speed of light turned out to be strangely constant so that no matter how fast you were travelling you couldn't begin to catch up with it — it was as if it were infinity. Quantum mechanics too was inspired by a contradiction. As electrons orbited the nucleus of an atom they should spiral inwards, because when electric charges change their direction of motion they emit radiation and therefore lose energy. Yet atoms were stable — how so? The answer was that at its lowest level energy was quantised, so while light and other electromagnetic radiation behaved as it if it were a wave, it came in discrete units called photons, and the mathematical foundations for this new mechanics was developed by physicists such as Schrödinger and Heisenberg.
The contradictions were unexpected, but that is my point. Expect the unexpected . . . which is exciting because we don't yet know how the big new discoveries will emerge, whether from interstellar space or much closer to home as we grapple with understanding the human brain. But if you think the particle physicists are almost there bar a few more Higgs bosons and a graviton or two, then I've got a bridge over the River Thames I'd like to sell you.
