Of the four known fundamental forces of nature, the weak nuclear force is the one with the least obvious purpose. Gravity holds stars together and keeps us on the ground. The electromagnetic force ensures the stability of atoms and makes chemistry happen. The strong nuclear force holds the kernels of matter, atomic nuclei, together, and powers the burning of stars.
The effects of the weak force were first discovered at the turn of the 20th century, in the place where it is most obviously at work: in radioactive beta decay. In the most common form of this decay, beta-minus decay, a neutron decays into a proton, also spitting out a negatively charged electron in order to conserve electric charge; beta-plus decay does the reverse and turns protons into neutrons.
To understand what this, and the weak force, is all about, we first need to mention the strong nuclear force. The strong force binds the fundamental particles known as quarks together to form particles such as the protons and neutrons of the atomic nucleus. Protons and neutrons are both composites of three quarks of two types, or “flavours”, up and down. Protons have the configuration up-up-down, and neutrons up-down-down. So if the strong force binds quarks together, it becomes apparent that the weak force allows them to change flavour: for example switching a down quark to an up quark or vice versa in beta decay.
It sounds quirky, but it is far from irrelevant: only the action of the weak force changing protons into neutrons within a star like the sun allows nuclear fusion to get off the ground within its core at all. The burning of stars – and so the existence of life – depends on the weak force.
Why is it so weak? Back in the 1930s, as physicists were just devising the quantum theory of the much more muscular electromagnetic force, they came up with an explanation. The photon, the quantum particle that transmits electromagnetism, has no mass, so it is easy to make photons and transmit them over large distances – infinite distances, in theory. If the weak force were transmitted by a similar particle, but one that’s very massive, it would be very difficult to make just like that according to the rules of quantum field theory, explaining the weak force’s weakness. In fact, there are three such “boson” particles that carry the weak force – the W+, W– and Z0, whose existence were confirmed by physicists at the research centre CERN near Geneva, Switzerland, in 1983.
The similarities between the weak nuclear force and electromagnetism went so far as to suggest they could be described by one “electroweak” quantum field theory. But physicists investigating this tantalising prospect of force unification persistently encountered a stumbling block: electroweak theories demanded not only that the photon, W+, W– and Z0 should all be massless, but that absolutely all particles that interacted through the new force should be too.
A way out was found by physicists Abdus Salam and Steven Weinberg in 1967. They proposed that all particles were indeed born massless in the big bang, and that there was perfect symmetry between the weak and electromagnetic forces, and between the four particles that carry them. But this symmetry was unstable. As the universe cooled, it underwent a process called spontaneous symmetry breaking, rather like a phase transition when a gas condenses to a liquid, say, by which particles acquired the different masses they have.
To realise this transformation, they made use of another mathematical trick that had been devised by Peter Higgs and others in 1964. This demanded the existence of yet another particle with a field that interacts with different particles to different degrees as they pass through it, providing them with different masses. It is this Higgs boson, when eventually discovered at CERN in 2012, that provided the crowning triumph of electroweak theory and the “standard model” of particle physics of which it is an integral part.
One other wrinkle of the weak force is worthy of mention. It goes back to beta decays. As the physicist Wolfgang Pauli noted with puzzlement in 1930, the energy sum of the decay doesn’t add up if you assume a neutron just decays to a proton and electron. He proposed that another particle must be emitted in the decay, one with no electric charge and a negligible mass. He was proved right – these “neutrinos”, among the most elusive particles we know of, interacting to any degree only through the weak force, are now an integral part of the particle physics canon, and perhaps the key to many mysteries about the universe.