Neutrino

A neutrino is a tiny particle that weighs almost nothing. It has no electric charge and barely reacts with other matter. Neutrinos are produced in the sun and in radioactive decay, among other places, and fly in enormous numbers through everything, including you, without you noticing.

How does it work?

Neutrinos feel almost only the weak nuclear force, a force that acts over a very short distance. As a result, a neutrino usually passes straight through a planet as if it were not there. That makes them hard to measure. Physicists build enormous detectors for this, often deep underground or in ice, which now and then see a flash when a neutrino does collide with something.

There are three kinds of neutrinos, and they can change from one kind to another on the way. That shows that they do have a very small mass.

Why it matters

Neutrinos help to answer a big puzzle: why matter was left over after the Big Bang, when almost as much antimatter was created. Many physicists think that every ordinary neutrino has an extremely heavy partner, which has never been seen. Physicists at the University of Southampton and the Indian Institute of Technology Kanpur calculated that such a heavy particle, which almost nobody had paid attention to, may have made the young universe expand differently (link). That changes the sums behind the puzzle, because it determines how large the surplus of matter ends up being. New gravitational-wave detectors could partly test that idea.