The fact that we can detect neutrinos at all is a testament to human ingenuity. Photograph by Volker Steger/Science Source

This week the 2015 Nobel Prize in Physics was awarded jointly to Takaaki Kajita and Arthur B. McDonald for their discovery that elementary particles called neutrinos have mass. This is, remarkably, the fourth Nobel Prize associated with the experimental measurement of neutrinos. One might wonder why we should care so much about these ghostly particles, which barely interact with normal matter.

Even though the existence of neutrinos was predicted in 1930, by Wolfgang Pauli, none were experimentally observed until 1956. That’s because neutrinos almost always pass through matter without stopping. Every second of every day, more than six trillion neutrinos stream through your body, coming directly from the fiery core of the sun—but most of them go right through our bodies, and the Earth, without interacting with the particles out of which those objects are made. In fact, on average, those neutrinos would be able to traverse more than one thousand light-years of lead before interacting with it even once.

The very fact that we can detect these ephemeral particles is a testament to human ingenuity. Because the rules of quantum mechanics are probabilistic, we know that, even though almost all neutrinos will pass right through the Earth, a few will interact with it. A big enough detector can observe such an interaction. The first detector of neutrinos from the sun was built in the nineteen-sixties, deep within a mine in South Dakota. An area of the mine was filled with a hundred thousand gallons of cleaning fluid. On average, one neutrino each day would interact with an atom of chlorine in the fluid, turning it into an atom of argon. Almost unfathomably, the physicist in charge of the detector, Raymond Davis, Jr., figured out how to detect these few atoms of argon, and, four decades later, in 2002, he was awarded the Nobel Prize in Physics for this amazing technical feat.

Because neutrinos interact so weakly, they can travel immense distances. They provide us with a window into places we would never otherwise be able to see. The neutrinos that Davis detected were emitted by nuclear reactions at the very center of the sun, escaping this incredibly dense, hot place only because they so rarely interact with other matter. We have been able to detect neutrinos emerging from the center of an exploding star more than a hundred thousand light-years away.

But neutrinos also allow us to observe the universe at its very smallest scales—far smaller than those that can be probed even at the Large Hadron Collider, in Geneva, which, three years ago, discovered the Higgs boson. It is for this reason that the Nobel Committee decided to award this year’s Nobel Prize for yet another neutrino discovery.

When Ray Davis observed solar neutrinos, he only detected about a third as many as he’d expected to find. Most physicists thought this was due to poor knowledge of the astrophysics inside the sun: perhaps models of the solar interior had overestimated the number of neutrinos produced there. However, over the years, even as solar models became better, the neutrino deficit persisted. Physicists began to contemplate the converse possibility: that the problem had to do with our understanding of neutrinos**.** According to the prevailing model of particle physics—the standard model—neutrinos are massless. But perhaps, some physicists argued, some neutrinos do in fact have infinitesimal mass, and that mass, and its consequences, accounted for the missing neutrinos in our detectors.

According to this theory—the theory of neutrino oscillations—there are three different types of neutrinos in nature, and, if a neutrino has a small mass, it can convert from one type to another as it travels through space. The three types of neutrinos are electron, muon, or tau; each type of neutrino can convert into its corresponding charged particle (electrons, muons, or tau leptons) when it interacts with ordinary matter. The “oscillation” comes in thanks to quantum mechanics. The type of any given neutrino is not fixed. Instead, it changes as time passes. A neutrino that, say, starts out as an electron neutrino can evolve into a muon neutrino, and then switch back again. In this way, an electron neutrino produced in the sun’s core can “oscillate” periodically into a muon neutrino, and back again, as it travels from the sun to the Earth. Since the Davis detector could only detect one type of neutrino—an electron neutrino, which could cause a nuclear transmutation between chlorine and argon—it seemed possible that the missing neutrinos it hadn’t detected had converted into other neutrino types on their long voyage. In the nineteen-eighties**,** my colleague Sheldon Glashow and I coined the phrase “just-so neutrino oscillations” to describe this process. (As it turns out, we now know that the neutrinos actually oscillate inside the sun, rather than on the way to the Earth.)

The only way to know for certain was to build a detector that worked for all three types of neutrinos. Beginning in the nineteen-nineties, Arthur McDonald, of Queen’s University, in Ontario, led a team that built one, in a mine, in Sudbury, Ontario. It contained tons of heavy water, provided on loan from the Canadian government. Heavy water is a rare but naturally occurring form of water in which hydrogen, containing a single proton, is replaced by its heavier cousin, deuterium, which contains a proton and a neutron. The Canadian government stockpiled heavy water as a coolant for use in their nuclear reactors. All three types of neutrinos could scatter off the deuterium in the heavy water, breaking it apart into a proton and a neutron, and the neutrons could then be counted. The detector observed roughly three times the number of neutrinos found by Davis—in other words, they found the amount predicted by the best solar models. This suggested that electron neutrinos can, in fact, oscillate into other neutrinos.

Around the same time, another remarkable experiment was unfolding, led by Takaaki Kajita, of the University of Tokyo. It centered on a detector created inside a mine in Japan, designed to detect neutrinos coming not from the sun but from the upper atmosphere. As cosmic-ray protons collide with the atmosphere, they produce showers of other particles—including muon neutrinos. In the mine, those muon neutrinos scattered off of hydrogen nuclei in the water, converting into muons. The Kajita experiment had a clever twist: its detector could observe neutrinos coming in two directions. Some travelled downward, straight from the atmosphere, and others travelled upward, having passed through the Earth. The rates for the two events were different, telling the researchers that neutrinos which had travelled different distances were arriving at the detector as different kinds of neutrinos. They were at different points in their oscillation cycles.

This is exotic and amazing stuff, but why should neutrino oscillations and neutrino masses be worthy of popular, or even scientific, interest? The reason is simple. In the standard model of particle physics, developed throughout the last fifty years of the twentieth century—the model which has correctly described every other observation that has been made in particle accelerators and other experiments, and which represents perhaps the greatest intellectual adventure that science has ever seen—neutrinos have to be massless. The discovery of a massive neutrino, therefore, tells us that something is missing. The standard model cannot be complete. There is new physics remaining to be discovered, perhaps at the Large Hadron Collider, or by means of another machine that has yet to be built.

The physicist Richard Feynman once suggested that nature is like an infinite onion. With each new experiment, we peel another layer of reality; because the onion is infinite, new layers will continue to be discovered forever. Another possibility is that we’ll get to the core. Perhaps physics will end someday, with the discovery of a “theory of everything” that describes nature on all scales, no matter how large or small. We don’t know which future we will live in. But the observation of neutrino masses tells us that the adventure of discovery in which we are currently involved will not end here. There are still fundamental mysteries to be resolved. And it is the mysteries in life that make living so exciting.