Why Neutrinos are so Weird
If you held out your thumb, every second about 65 billion neutrinos will pass through it. Besides photons, neutrinos are the most abundant particle in the universe, and by far the most unique.
The existence of the neutrino was first theorized by Wolfgang Pauli, after noticing how energy didn’t seem to be conserved in beta decay. He believed that the missing energy was being carried away by some “invisible” particle. He would later say “I have done a terrible thing, I have postulated a particle that cannot be detected.”
Although elusive, neutrinos can be detected, but it requires sensitive, and often massive detectors. After finding that neutrinos came in three types: electron, muon, and tau, a problem seemed to emerge. Electron neutrinos are created all the time in the Sun, as a by-product of nuclear fusion, but they would always find only a third of the total number of electron neutrinos they were expecting. So, where did the missing neutrinos go?
It turns out, neutrinos actually oscillate back and forth between the three different types. So, by the time the neutrinos from the Sun had reached Earth, two thirds of them have turned into muon and tau neutrinos. This discovery was especially surprising, since everyone thought neutrinos had no mass, like the photon. The fact that neutrinos could change in-flight implied that they could experience time, and due to special relativity, this means they must have mass.
While that mystery has been solved, we still have plenty to learn from these strange particles. Exactly how much do they weigh? Although we know they must have mass, they are so light, we can’t tell how much. Since they have no electric charge, is a neutrino its own anti-particle? Is there more than just three types of neutrinos? Answering these could help us uncover some of the biggest mysteries in physics today.