Finding some of nature’s weirdest particles has won two experimenters the 2015 Nobel Prize in Physics. Japanese scientist Takaaki Kajita and Canadian researcher Arthur B. McDonald will share this year’s award for the discovery that neutrinos—fundamental particles that come in three types, or flavors—can actually swap identities and change flavors as they fly through space. Their turn-of-the-millennium discovery not only revealed the strangeness of these shape-shifting particles—it also contradicted the Standard Model of particle physics. At the time, physicists predicted that neutrinos would weigh nothing at all. For neutrinos to change flavors, however, they must have mass. Kajita and McDonald demonstrated that neutrinos must therefore have a very small but nonzero mass. Exactly how much mass each neutrino flavor has remains one of the most important unanswered questions in physics today.

“Neutrinos are puzzles,” Nobel physics committee member Olga Botner said during a press conference this morning. “This year’s Nobel Prize in Physics honors a fundamental step toward unveiling the nature of the neutrino.” The committee reached both Kajita and McDonald early this morning to deliver the news of their award. “As it turns out, I did not mind,” McDonald, a physicist at Queen's University in Ontario, said about his predawn wake-up call. “It’s a very daunting experience, needless to say. Fortunately I have many colleagues as well who share this prize with me in the tremendous amount of work they have done to accomplish this measurement. … We, of course, are very satisfied that we have been able to add to the world’s knowledge at a very fundamental level.” Kajita, who was not able to call into the press conference, said via a recorded message that the news of his prize was “still kind of unbelievable.”

A professor at the University of Tokyo, Kajita led the Super-Kamiokande experiment in Japan, which in 1998 was the first to detect neutrinos changing flavor—a process called oscillation. Super-Kamiokande uses a vast tank of extremely pure water to detect neutrinos that emerge from interactions between cosmic rays (high-energy charged particles from space) and atoms in Earth’s atmosphere. These interactions tend to produce large quantities of neutrinos of the muon flavor (the other two flavors are electron neutrinos and tau neutrinos). When scientists counted the number of muon neutrinos that arrived at Super-Kamiokande, however, they found far fewer than expected. The best explanation was that some fraction of incoming muon neutrinos had switched to other flavors en route.

The finding was bolstered in 2001 when the team behind the Sudbury Neutrino Observatory (SNO) in Ontario, led by McDonald, announced that they had seen a similar effect in neutrinos coming from the sun, which produces electron neutrinos in fusion reactions in its core. SNO uses “heavy water” (in which the isotope deuterium replaces “normal” hydrogen) to detect all three flavors of neutrinos. Here, too, the experiment detected a different mix of neutrinos than expected—in this case, fewer electron neutrinos and more taus and muons. And here, too, it stood to reason that some of electron neutrinos had oscillated. There certainly was a eureka moment at this experiment when we were able to see that neutrinos appeared to change from one type to another in traveling from the sun to the Earth,” McDonald said.

Both Super-Kamiokande and SNO researchers had to overcome many hurdles to make their discoveries, particularly the difficulty of observing neutrinos that—because they have very little mass and no electric charge—interact only very rarely with atoms in the detectors. “These were epic experiments, combining vast scale with delicate sensitivity,” Nobel laureate Frank Wilczek of Massachusetts Institute of Technology told Scientific American. “Neutrinos are tough prey! The history of the field is strewn with inconclusive and incorrect results. But the Super-K and Sudbury groups produced convincing work that has stood the test of time.”

Although they are hard to detect, neutrinos are extremely common, second only to photons (particles of light) as the most abundant species in space. Trillions fly through our bodies every moment but only once or twice in a lifetime (if ever) will they hit an atom inside. Even then you would not feel a thing. In addition to being produced by cosmic rays as well as the sun and other stars, many neutrinos were created in the big bang. Understanding their nature is key to understanding why the universe turned out the way it is.

According to quantum mechanics, particles, including neutrinos, can also be thought of as waves, and the frequency of these waves depends on the particles’ masses. For the particles to change flavor, they must change frequency, which requires the masses of the three flavors to be different, and therefore nonzero.

No one expected neutrinos to have mass, however, because the mechanism that explains the mass of most other particles—the Higgs field, which is associated with the Higgs boson particle discovered in 2012—does not apply to neutrinos. The revelation that neutrinos have heft implies that some mysterious mechanism beyond known physics is out there waiting to be discovered. Once scientists find out what it is, they may discover that it has had other effects in shaping the evolution of the cosmos. “The discovery that neutrinos have mass has profound consequences—not only for particle physics, pointing at physics beyond the Standard Model, but also for astrophysics and cosmology,” says Botner, who is spokesperson for the IceCube neutrino experiment at the South Pole. “The discovery of neutrino observations has opened a new, exciting and challenging field of physics.”

Kajita wrote a 1999 feature for Scientific American about his neutrino discovery, along with two co-authors: “Detecting Massive Neutrinos

McDonald, along with co-authors, wrote a 2003 feature for Scientific American: “Solving the Solar Neutrino Problem