The 2015 Nobel prizes: Physics

SCIENTISTS are the organised sort, putting together charts and lists of the constituent parts of the natural world in a bid to make sense of it. Chemists have the periodic table, which outlines all the chemical elements. For physicists, things remain a bit more tentative: they have the Standard Model, a recipe listing all the particles and forces (except gravity, which has its own rules) from which the universe is made. On that list are three slippery characters: the neutrinos. Although few facts are known about them, perhaps the most important is that they have mass. It is such a striking finding, in fact, that Sweden's Royal Academy of Science granted this year's Nobel prize in physics to Takaaki Kajita of Japan and Arthur McDonald of Canada, for having proved it.

Neutrinos, meaning "little neutral ones" in Italian, were proposed in 1930 as a way to balance out the equations of nuclear decay. Once physicists agreed that they existed, it became clear that there must be a great many of them. They seep from the Earth's core, stream from the sun, and scoot across the galaxy from far-flung supernovae. Yet they are exceptionally disinclined to interact with any of their other particle pals—it took until 1956 to catch one in a laboratory.

Neutrino science advanced, and the existence of the three different "flavours" of neutrino—electron, tau and muon—was worked out. They were thought to be massless. But a couple of things were amiss. The first, spotted in the 1960s, was that the sun appeared to produce fewer of the electron neutrinos than theory would suggest. This "solar neutrino problem" plagued researchers. The second was spotted in the Super-Kamiokande experiment in Japan, in 1998. There, Dr Kajita and his colleagues studied atmospheric neutrinos, produced when high-energy particles from the cosmos smash into atoms in the air. But, again, the tally of muon neutrinos that the team expected came up short. Meanwhile, Dr McDonald, working at the Sudbury Neutrino Observatory in Canada in 2001, was running a slightly different experiment, looking just at neutrinos from the sun. His detectors could spot electron neutrino reactions in particular, as well as the sum of all neutrino reactions. Since the sun is supposed to produce only electron neutrinos, those two numbers should have been the same. Instead, the "all-neutrinos" count was far higher.

One explanation tied the whole mess together: that neutrinos could change their flavour en route. If they can do that, then they experience time. If they do that, then they must not be moving at the speed of light: that is, they must have some mass. At a stroke, a slew of experimental confusion was resolved and the solar neutrino problem had an explanation. Details of solar and nuclear physics, of the sort that fusion scientists worry about, were saved. But it still left questions. What mass, exactly, does each of the three types have? Why do they have any mass at all? And if they can swap identities on the fly before reaching Earthly detectors, can scientists be sure that there are only three? Answers to these questions could help resolve just how much of the universe is made of neutrinos, or even explain why the universe got its start with more matter than antimatter—in other words, why there is a universe with Nobel prizes in it at all. Drs Kajita and McDonald have done their bit to open a little crack in the Standard Model's universal recipe; more ingredients are sure to come.