by Randall Munroe, cartoonist, author, engineer, XKCD/NY Times, firstname.lastname@example.org
“How many neutrinos would you need to make a snowball? Would it feel cold and squishy if you were hit in the face, or would it just pass through?”
— Hugo S., Montpellier, France
You are getting hit in the face by neutrinos right now. Trillions of neutrinos, most of them emitted by the sun, are streaming through your face as you read this. Every second, 100 billion neutrinos pass through each of your eyeballs.
You don’t notice this constant bombardment because neutrinos barely interact with normal matter. Not only do they pass through your face without stopping, they generally pass right through Earth. In fact, if you’re an astronomer who wants to detect neutrinos, you put your detectors deep underground. The rock and dirt block out cosmic rays and other sources of noise, allowing neutrino detectors to pick up the faint signals of ghostly neutrinos without interference.
Neutrinos are very light, so you would need a lot of them to make a snowball. For a long time, researchers thought that neutrinos, like photons, didn’t weigh anything at all, but it turned out they do have a very tiny mass. It’s still not clear how much (or little) that is, exactly, but it would take about 300,000,000,000,000,000,000,000,000,000,000,000 neutrinos to weigh as much as a snowball. You could also write that in scientific notation as 3×10³⁵, but sometimes it’s nice to write out all the digits as a reminder of how mind-bogglingly large these numbers are.
If you tried to pick up a snowball made from 300 decillion neutrinos (I had to look up the word for that number) it would fall right through your hand. Slow-moving neutrinos pass through matter more easily than fast-moving ones, so only a few, if any, of those 300 decillion neutrinos would interact with your hand at all. They would just drop right through your palm and disappear into the earth.
Because neutrinos so rarely interact with matter, all the ones that scientists have detected have been high-energy specimens moving at nearly the speed of light. But the universe is probably full of slower ones, too. The Big Bang created lots of neutrinos, and most should still be drifting around us. Because these “relic neutrinos” are moving much slower than the speed of light, astronomers sometimes refer to them as “stationary,” even though they’re still traveling at several million miles per hour.
If the snowball were coming at you as fast as the solar neutrinos in it were moving, a lot of them would interact with your body, most likely by bouncing off an electron in one of your atoms. You probably still wouldn’t feel the impact, but it would likely deliver enough energy to give you a lethal dose of radiation.
Maybe it’s for the best that there is no easy way to produce a snowball made of neutrinos. Unless you have a particle accelerator lying around your yard, you’ll have to stick to regular snow.