What’s a neutrino?

Neutrinos are... poster

Credit: Symmetry Magazine / Sandbox Studio, Chicago


A neutrino is a particle! It’s one of the so-called fundamental particles, which means it isn’t made of any smaller pieces, at least that we know of. Neutrinos are members of the same group as the most famous fundamental particle, the electron (which is powering the device you’re reading this on right now). But while electrons have a negative charge, neutrinos have no charge at all.

Neutrinos are also incredibly small and light. They have some mass, but not much. They are the lightest of all the subatomic particles that have mass. They’re also extremely common—in fact, they’re the most abundant massive particle in the universe. Neutrinos come from all kinds of different sources and are often the product of heavy particles turning into lighter ones, a process called “decay.”

These little particles have an interesting history. First predicted in 1930, they weren’t discovered in experiments until 1956, and scientists thought they were massless until even later. While we keep learning more about neutrinos, with new answers come new mysteries.

Neutrinos are also tricky to study. The only ways they interact is through gravity and the weak force, which is, well, weak. This weak force is important only at very short distances, which means tiny neutrinos can skirt through the atoms of massive objects without interacting. Most neutrinos will pass through Earth without interacting at all. To increase the odds of seeing them, scientists build huge detectors and create intense sources of neutrinos.

Physicist Enrico Fermi named neutrinos, which is Italian for “little neutral ones.” They are denoted by the Greek symbol ν, or nu (pronounced “new”). But not all neutrinos are the same. They come in different types and can be thought of in terms of flavors, masses, and energies. Some are antimatter versions. There may even be some yet undiscovered kinds of neutrinos!

Ten quick facts about neutrinos:

    1. Trillions of the harmless particle stream through you every second, night or day.
    2. They are the second most abundant particle in the universe (after particles of light called photons).
    3. Neutrinos rarely interact with anything—a lightyear of lead would stop only about half of the neutrinos coming from the sun.
    4. About 15 billion neutrinos from the Big Bang are in the average room.
    5. Neutrinos interact only through two of the four known forces: the weak force and gravity.
    6. So far, scientists have discovered three flavors of neutrinos: electron (νe), muon (νμ), and tau (ντ).
    7. They oscillate, or change flavor, as they travel.
    8. Their masses are very tiny, but not yet known.
    9. Their speed is very close to the speed of light, but also not known exactly.
    10. They could be the reason that matter exists in the universe.