Billions of ghost particles are passing through you right this second. You can’t see them, you can’t feel them–scientists need huge underground detectors like the one above, the Super-Kamiokande detector in Kamioka, Japan, to detect them–but they’re there. Neutrinos are some of the most elusive particles in existence. Although trillions of neutrinos are passing through them right this moment, the number of neutrinos actually detected each day by Super-Kamiokande or its cousin, the Sudbury Neutrino Observatory in Ontario, Canada, can be counted on two hands.
The ceiling, walls and floor of the detectors are lined with photo detectors, and the room is filled with very pure water. Neutrinos usually fly by like drive-by ghosts, but occasionally they crash into an atomic nucleus or electron. When they do, charged particles are created, and faint flashes of blue light can be detected. This is Cherenkov light, and arises when a particle travels faster than light. This doesn’t contradict General Relativity, which only states that nothing can travel faster than light in a vacuum. Underwater, light only travels at 75% of its speed in a vacuum, and the charged particles can overtake them. Cherenkov radiation is what gives underwater nuclear reactors their distinctive blue glow.
Today, Takaaki Kajita and Arthur B. McDonald, key members of the research teams behind Super-Kamiokande and Sudbury, respectively, were awarded the 2015 Nobel Prize in physics for discovering the solution to a neutrino mystery: where did the neutrinos created in the Sun go? The detectors could only pick up one third of the neutrinos expected to reach Earth after being emitted from the Sun. Here’s a rather engaging exposition about their work written for the general public from the Nobel Institute.
Neutrinos come in three flavors, each with its own charged partner: the electron neutrino, muon neutrino and tau neutrino. Supernovas blast all three kinds of neutrino towards the Earth, but only electron neutrinos are created in the Sun. So where did the remaining 2/3rds of the electron neutrinos go? Kajita and McDonald are awarded for discovering that neutrinos change identity mid-flight, and that they therefore must have mass, however tiny. When you count the electron neutrinos detected in Kamioka and Sudbury, two thirds are missing. When you tally all the neutrinos–tau, mu and electron neutrinos–the numbers match. Neutrinos aren’t just ghost particles, they’re space chameleons.
The fact that neutrinos have mass is, excuse me, massive. The Standard Model of physics is the best description we have of how the universe works at the smallest possible scale, and it has no credible challengers. But the Standard Model is built on the assumption that neutrinos are massless, a fact we now know is false. Although the mass is tiny, the neutrino is the second most common type of particle in the universe, second only to the photon. The combined weight of the neutrinos is roughly the same as the weight of all the stars in the entire universe.
How does it feel to know that, as you’ve been reading this, trillions of space chameleons have rushed through your innermost parts and out the other side undisturbed, as if you never existed? As if you were the ghost? That is what you are, to a neutrino. A ghost. And it is an enigma to us.