Neutrino detectors, like the one used in the BOREXINO collaboration here, generally have an enormous tank that serves as the target for the experiment, where a neutrino interaction will produce fast-moving charged particles that can then be detected by the surrounding photomultiplier tubes at the ends. However, slow-moving neutrinos cannot produce a detectable signal in this fashion. (INFN / BOREXINO COLLABORATION)

Ask Ethan: Do Neutrinos Always Travel At Nearly The Speed Of Light?

If they have mass, then why don’t we see any slow-moving ones?

For decades, the neutrino was among the most puzzling and elusive of cosmic particles. It took more than two decades from when it was first predicted to when it was finally detected, and they came along with a bunch of surprises that make them unique among all the particles that we know of. They can “change flavor” from one type (electron, mu, tau) into another. All neutrinos always have a left-handed spin; all anti-neutrinos always have a right-handed spin. And every neutrino we’ve ever observed moves at speeds indistinguishable from the speed of light. But must that be so? That’s what Patreon supporter Laird Whitehill wants to know, asking:

“I know neutrinos travel almost at the speed of light. But since they have mass, there is no reason that they couldn’t travel at any speed. But [you’ve implied] their mass dictates that they must travel almost at the speed of light.

But light travels at a constant speed. But anything with mass can travel at any speed.”

So why, then, do we only see neutrinos traveling at velocities consistent with the speed of light? It’s a fascinating question. Let’s dive on in.