Something I heard shook me. There are 60 billion neutrinos from the Sun passing right through your thumbnail every second.

Neutrinos are one of my favorite topics. They say something about science and what we know. Here’s the story.

Alpha decay and beta decay are both types of radioactivity. Other than that, they’re about as different as two processes can be. Alpha decay involves the ejection of an alpha particle, really a helium nucleus. Alpha particles are two protons and two neutrons, and throwing them around is the equivalent of tossing bowling balls around in the subatomic world.

Beta decay involves the ejection of an electron (or a positron in special circumstances). Compared to an alpha particle, a beta particle – an electr0n – is just a bit of fluff. Though they usually penetrate deeper than alpha particles, they don’t carry nearly the oomph.

That’s the obvious difference between the two. But there’s a more subtle difference, beautiful and deep. Alpha decay is, for any particular decay of an isotope, always exactly the same. If you get an alpha from U-238, the energy of that alpha is always the same. There are some isotopes that can decay in different modes; yet even so, the emission spectrum is always discrete, like the light from a laser, rather than continuous, like a light bulb. If you know where an alpha particle came from, you know its energy.

Beta decay isn’t like that. The spectrum of electron energies from beta decay is more like the spectrum from an incandescent bulb. There’s an upper limit, but between that limit and zero, the electrons take on all sorts of values.

This bothered scientists when they discovered it. Why should beta decay have this strange property? Where did the extra energy go (or, alternatively, where did it come from)? Given this evidence, some scientists were prepared to give up the conservation of energy.

A desperate alternative was tried instead. Perhaps, it was suggested, an invisible, electrically neutral, non-reactive particle came flying out of beta decay along with the electron. This invisible particle could carry that extra energy. Voila! The conservation of energy was saved!

Enrico Fermi named the particle “neutrino”, Italian for “little neutral one.” Cute.

It seemed like a trick, fixing the books. We can’t find the extra energy? Fine, we’ll just make up a story. Here it is, in a place we can never find it. Quite convenient, and also not very scientific.

So things stood for decades. Physicists had a neat solution, but also a big problem. Where was the evidence for the neutrino? There was none. The calculated properties of the neutrino meant that a single particle would, on average, travel through several light years of solid material before interacting. Hence the 60 billion through your thumb every second. Even if you’re exceptionally odd, your thumb is nowhere near as thick as a light year, so you have little chance of stopping any particular neutrino.

Ah, but the power of large numbers means that with lots and lots of neutrinos, occasionally a few will do something unusual. Just as the average U-238 atom will last 4.5 billion years, but some are always defying the odds and decaying anyway, a few neutrinos will also defy the odds and get absorbed, even in relatively small amounts of matter.

But how do you know that you’ve caught a neutrino?


In 1955, two scientists named Fred Reines and Clyde Cowan decided to try it. They filled a tank with cadmium chloride and surrounded the tank with detectors. They were looking for two gamma rays flying away in opposite directions. This would indicate that an electron and positron had been annihilated. And that would mean that a proton had just absorbed a neutrino, in the process shooting off a positron and turning itself into a neutron.

The details aren’t that important, though they are fascinating and maybe I’ll write about them later. The important point is that Reines and Cowan found the neutrino. It was really there!

This is astonishing. First, scientists noticed something that they didn’t expect – the continuous spectrum of beta decay. The observation violated one of the physicists’ most cherished ideas, the conservation of energy. To fix it, they created out of wholecloth this invisible particle with virtually no properties. But then, their patch turned out to be a real thing! This, to me, shows that physics has in some sense found out something true about the universe.

Science isn’t just observation and classification. As this example shows, science can actually touch on something we might call truth. The neutrinos point the way. The truth is out there. Think about that next time you gnaw on your thumbnail.