I want you to take a moment to leave your computer (or mobile device) screen and go outside. Stand under the sun, away from any trees, and hold your hand out. Congratulations! Billions of neutrinos have traveled through your hand, directly from the sun.
You didn’t have to put your hand out, stand out in the sun, or even go outside. If you’re sitting in your car, or at the dinner table, in bed or anywhere else, uncountable neutrinos pass through you and everything else on the surface of the planet. So why do they exist? And how do we know about them?
It was after Einstein published his iconic equation, E=mc2. We discovered how the sun produces energy: nuclear fusion.
As with any star, the sun produces energy because its gravitation forces the gas comprising it to undergo fusion reactions. A main sequence star (of which our sun is one) fuses its hydrogen into helium. It takes four hydrogen nuclei to make one helium nucleus. But the mass of one helium nucleus is less than the sum of the masses of four hydrogen nuclei. And that’s where our energy comes in. The difference in mass between the single helium nucleus and the four hydrogen nuclei is the m in E=mc2. And the energy we get from the sun is the result of that mass difference.
But in measurements of the sun’s luminosity, there was energy (E) missing as predicted by Einstein’s equation. Why? We know that light is made of photons, and it was those photons that emerged as energy as per the same equation. Where could those photons have gone? Why couldn’t we see them? Was Einstein’s equation wrong?
No. There was already a theoretical particle in physics at the time, and that theoretical particle was called the neutrino. In subsequent work in particle physics, it was determined that the missing energy from the sun was in the form of neutrinos, which don’t interact strongly with normal matter (that is, they don’t collide with the particles in normal matter in a way that releases other particles and energy). So then it was time to find a way to detect these neutrinos.
In 2004 I met a man by the name of Ray Davis outside of David Rittenhouse Laboratories at the University of Pennsylvania. At the time I didn’t know who he was; he was sort of a vacuous presence and didn’t do much talking. Think of the old man at the bingo hall who can never hear the letters being called. Well, Mr. Davis invented the world’s very first working neutrino detector in the late 1960s, and when the results came in, it was a genuine eureka moment.
The detector was essentially a gigantic tank of cleaning fluid deep down in a decommissioned mine in South Dakota called Homestake. Why in a mine? Because mines are deep underground, and deep underground is a place where photons from the surface and beyond don’t quite make it to. The detector itself detects neutrinos caught by the cleaning fluid because atoms that have captured neutrinos decay from one element to another (in this case, chlorine becoming argon). Part of that decay is the release of a photon, which is picked up by photodetectors lining the inside of the tank. (The process is much more detailed than this, and you can read about it here.)
Ray Davis won the Nobel prize in 2002 for his contributions to astrophysics. He died in 2006 from Alzheimer’s disease.
Neutrinos are among the most numerous particles in the universe, and they are there everywhere, all the time. You may have heard the term used in science fiction, maybe even in a way that implies that neutrinos are useful in tools or weapons. While neutrinos serve an important role in quantum physics (I encourage you to read about them), they do not interact with normal matter, so there’s no way for you to rapidly harness their energy. So, don’t fear the elusive neutrino–think of it as the neighbor you never see or hear from.