It’s Not Elementary, My Dear Watson

If you think that the discovery of the Higgs Boson solved all the mysteries of particle physics, think again

Cosmos Science

Now that physicists at the Large Hadron Collider at Cern have found a Higgs particle, are they reaching the end of the road? Their observations have finally revealed a quantum manifestation of the Higgs field, which imbues matter with inertia, and can distinguish between particles that have mass and those that don’t. The bigger the mass of a particle, the larger its inertia and the greater its interaction with the Higgs field.

Of course there are still a few loose ends. The graviton, a stable quantum realisation of the gravitational field, has not yet been seen, nor the newly proposed Wang particle, an unstable entity of similar ilk, to say nothing of other possible Higgs particles. The idea — and it’s a grand one — is that all these particles fit in with the Standard Model, which describes the three quantum forces of nature: electromagnetism, the weak nuclear force, and the strong nuclear force. Along with gravity this makes four forces that are supposed to be the basis for everything, but if this is the end of particle physics, let us examine how we got to this point.

Questions about the fundamental constituents of matter have been around for a very long time. The Greeks had four basic elements: earth, fire, air and water, and many of their ideas came from the Babylonians who had similar notions, while the ancient Chinese had five, including wood and metal, but excluding air. Why four or five? The reason could be related to the natural division into four cardinal directions, shared by all cultures. The Chinese even thought of the earth as square, surrounded on its four sides by four great oceans. They also imagined a fifth cardinal direction for heaven, and the Greeks adopted five elements when they included the aether, that “ethereal” substance that no one could see or feel, which in Latin became the quinta essentia or quintessence.

Yet despite this standard model of the cosmos, there were Greeks such as Democritus who wanted to explain things in terms of more elementary constituents called atoms. This word comes from the Greek atomos meaning uncuttable, in other words indivisible, and in the 19th century we found them, in profusion. Atoms of gold, atoms of iron, copper, hydrogen, oxygen, and so on, finally quashing the dream of the alchemists that base metals could be turned to gold. No chemical process could do this because no amount of clever alchemy could convert an atom of one type into one of another type.

Then in 1897, J.J. Thomson at Cambridge discovered negatively charged electrons, which he realised were tiny constituents of atoms. The atoms themselves had no charge, so the electrons had to be moving in a positively charged environment, which led to the “plum pudding model” of the atom with electrons like tiny pieces of fruit whirling around in a positively charged pudding. Twelve years later in 1909 one of Thomson’s students, Ernest Rutherford, showed that the positive charge lay entirely in a tiny nucleus, and then the nucleus too started to reveal constituents of its own: positively charged protons, and electrically neutral particles called neutrons discovered by James Chadwick, who at one time worked under Rutherford.

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.