Credit: spl
Even the ancient Greeks knew that different objects were composed of compounds of more fundamental substances. This begged the question: if something can be broken down into its component parts, and they in turn can be broken down into their component parts, where does it stop?
Is there a point where we arrive at the fundamental ‘elements’ of matter? For the Greeks, these elements were air, water, fire and earth. And for physicists in the late 19th and early 20th centuries, they were atoms.
Some might argue the story of the Standard Model really began in 1897, when British scientist, J.J. Thomson, demonstrated he could pull identical particles – electrons – out of many different materials.
Soon thereafter, in 1909, New Zealander Ernest Rutherford discovered that most of an atom’s mass was concentrated in a tiny area, the nucleus.
Rutherford concluded that an atom – derived from the Greek atomon meaning ‘cannot be divided’ – can, in fact, be divided. Atoms have protons and neutrons in the nucleus, with electrons orbiting.
But the sub-divisions didn’t stop there.
“Who ordered THAT?!” Nobel laureate I.I. Rabi famously asked after yet another particle, called the muon, was discovered in 1936.
The muon was the first particle discovered that did not occur in an atom – its mere existence understandably caused consternation in the physics community. No one had thought of matter that didn’t occur somewhere around us on Earth.
The muon has similar properties to an electron, except that it is much heavier. And particle physicists now know of an even heavier version, called the tau. The electron, muon and tau are called the three ‘flavours’ of a family of fundamental particles (which cannot be divided) called leptons. Each flavour has a nearly massless companion, called a neutrino.
In 1964, Americans Murray Gell-Mann and George Zweig proposed the existence of three other fundamental particles, which together could make up hundreds of the known particles at the time.
Gell-Mann named them ‘quarks’ (pronounced ‘kworks’), after the nonsense word fabricated by James Joyce in his novel Finnegans Wake: “Three quarks for Muster Mark!”
Experimentalists found each of the three quarks as predicted … and they were on such a roll that they continued until they found three more. The most elusive quark, the top quark, was finally discovered in 1995, after its existence was first predicted in 1977. Quarks bunch together, in pairs or trios, forming the familiar particles, such as neutrons and protons.
The Standard Model, then, consists of six quarks (up, down, strange, charm, top and bottom) and six leptons (electron, electron neutrino, muon, muon neutrino, tau and tau neutrino).
There is one more twist to the physics of fundamental particles: every quark and lepton has its own antimatter twin. An ‘antielectron’ was hypothesised in 1928, but the idea seemed preposterous to most. That is, until 1932, when Carl Anderson found it, and dubbed it the positron.
Antimatter particles have the same properties as their matter analogues, except with opposite electrical charges.
In addition to the 12 fundamental particles, and their antimatter twins, the Standard Model contains four fundamental forces: electromagnetism, strong and weak, and gravity.
The strong and weak forces only operate over incredibly short distances within the nucleus of an atom, whereas gravity and electromagnetism are visible at macroscopic scales. Most of what we see springs from electromagnetism, whether it’s pressure, friction, heat or magnetic attraction.
Physicists originally struggled with how to describe these forces. They were reluctant to posit strange energies that mysteriously pulled or pushed on distant objects without anything going on in between the objects.
Then occurred the revelation that the forces were mediated by their own ‘force-carrying particles’ that were exchanged between other fundamental particles. For example, the force-carrier particle for electromagnetism is the photon. So, as two electrons get closer to each other, one emits a photon that the other absorbs, producing repulsion.
The force-carrying particle for the strong force is the gluon and for the week force it’s W or Z particle.
All this may sound increasingly like science fiction, but thus far the Standard Model has proved uncannily successful, and has yet to fail in a single experimental test. Even so, the theory is incomplete. The story of the Standard Model, like so many other stories in science, has no predictable ending.
Although gravity seems to be one of the four fundamental interactions, physicists are having trouble incorporating it into the Standard Model. Gravity’s predicted force carrier particle, the graviton, has yet to be seen.
Perhaps, though, the biggest failing of the Standard Model is that it cannot explain why each particle has a particular mass. In 1964, British physicist Peter Higgs and his team proposed the theory of the so-called Higgs field. In theory, the field would interact with other particles to give them mass. However, the existence of the Higgs field requires a particle, the Higgs boson, which has been affectionately dubbed the God Particle because of its monumental importance to particle physics. Yet, like the graviton, this fundamental piece of the puzzle has yet to be seen. The hunt continues.
Jacqui Hayes is the deputy editor of Cosmos Magazine and the editor of Cosmos Online.
