Understanding how the universe works at the most fundamental scale is often likened to peeling away the layers of an onion. The outermost layer of the onion represents atoms, and we have known about these for a century or so. The next layer of structure, which was revealed by Rutherford in 1911, is the atomic nucleus – a much smaller object that contains almost all of the atomic mass. Some 20 years after that discovery, physicists realized that the nucleus is composed of more fundamental objects called protons and neutrons. However, peeling back the next layer of the onion has turned out to be much more of a challenge.
It is now universally accepted that protons and neutrons are made up of fractionally charged particles called quarks: two "up" quarks and a "down" quark in a proton, and two downs and an up in a neutron. There are six types of quarks in total, but none of them has ever been observed as a free particle. Smashing protons together at enormous energies in particle accelerators, for instance, reveals not single quarks but yet more particles made of quarks. Such particles are called hadrons, and there are hundreds of them: some are "baryons", which contain three quarks, while the rest are "mesons" made up of quark-antiquark pairs. It might therefore seem, as indeed it did to particle physicists in the 1960s, that the core of the onion is forever hidden.
The only way we can understand the properties of quarks is to compare experimental measurements of hadrons with calculations based on quantum chromodynamics or QCD: the theory of the "strong force" that binds quarks together. Despite being around for over 30 years, however, the equations of QCD have proved eye-wateringly difficult to solve. Indeed, to the immense frustration of particle physicists, it has been impossible to calculate properties of hadrons with an accuracy of better than 10%.
In the December issue of Physics World, Christine Davies shows how a the technique "lattice QCD" is allowing these equations to be solved much more accurately by representing space-time as a 4D lattice – which, she says, could reveal what the final layer of the onion looks like.