Observation of neutrinos registered by the IceCube experiment

As you read this, 10m neutrinos are passing through your eyes every second, at almost the speed of light, unseen. These will o’ the wisps are the most enigmatic particles in the universe. They are like electrons except that they have no electrical charge and almost no mass. They are impervious to electric and magnetic forces, and interact with matter through the “weak” force—the feeblest of the natural forces other than gravity. So their faint affinity for matter makes neutrinos very hard to detect.

Neutrinos are produced in nuclear transmutations, such as the decay of a radioactive element in a rock. They are also produced during the production of power in a nuclear reactor, or in the fusion processes that fuel the Sun and stars. The chance of capturing a neutrino is so small that when Wolfgang Pauli, the Austrian theoretical physicist, proposed its existence in 1930, in order to explain some properties of radioactivity, he wagered a crate of champagne that no one would ever be able to detect the phantom. The chance of observing a neutrino is small, but not zero—and if the source of neutrinos is intense enough, occasionally you can be lucky and capture one. In 1956, two Americans, Fred Reines and Clyde Cowan, did just that at a nuclear reactor in an experiment that was aptly named “Project Poltergeist.” Pauli paid up.

Some neutrinos are coming up from the ground beneath our feet, emitted by natural radioactivity in rocks. But most of the neutrinos that are presently all around you were born in the heart of the Sun less than 10 minutes ago—just a short while before you decided to read this article. In just a few seconds the Sun emits more neutrinos than there are grains of sand in the Sahara, a number greater even than the number of atoms in all the humans who have ever lived.

If we could see neutrinos, our nights would be as bright as day: solar neutrinos shine down on our heads by day and up through our beds by night—the earth is almost transparent to them. It is not just the Sun which acts as an emitter; all stars fill the void with neutrinos.

But the neutrinos born in the Sun and stars, numerous though they are, are relative…

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