Neutrinos are among the strangest particles in the Standard Model. They carry no electric charge, have an almost immeasurably tiny mass, and interact so weakly with other matter that trillions of them pass through your body every second without you ever noticing.
They come in three types, tied to the electron, muon, and tau, and they are produced in huge numbers by the sun, nuclear reactors, and violent cosmic events like supernovae. Because they interact so rarely, detecting them requires enormous, carefully shielded detectors, often buried deep underground to block out other particles.
One of the biggest surprises in neutrino physics is that they can change from one type to another as they travel, a process called oscillation. That discovery proved neutrinos have mass at all, something the original Standard Model did not predict, and it earned the 2015 Nobel Prize in Physics.
The discovery that neutrinos oscillate helped resolve a decades-old mystery known as the solar neutrino problem. Starting in the 1960s, experiments detecting neutrinos streaming from the sun consistently found only about a third as many as theoretical models of solar fusion predicted, a discrepancy that puzzled physicists for over thirty years. The answer turned out to be that neutrinos were changing type on their journey from the sun’s core to Earth, and earlier detectors were simply blind to the other two flavors. Modern neutrino observatories have grown correspondingly ambitious, IceCube, buried in a cubic kilometer of Antarctic ice, uses thousands of sensors to catch the faint flashes of light produced on the rare occasions a high energy neutrino from deep space actually collides with something.
Exactly how much mass each neutrino type carries, and in what order, remains unresolved, a puzzle physicists call the neutrino mass hierarchy. Solving it requires experiments sensitive enough to detect mass differences a million times smaller than an electron’s, and several major detectors, including Japan’s Hyper-Kamiokande, are being built specifically to help settle the question.