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Could you make a snowball out of neutrinos?

“How many neutrinos would you need to make a snowball? Would it feel cold and mushy if you were hit in the face or would it just go through? “

– Hugo S., Montpellier, France

You are being hit in the face by neutrinos. Trillions of neutrinos, most of which are emitted by the sun, flow 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 hardly interact with normal matter. You not only pass your face without stopping, but generally also the earth. If you are an astronomer who wants to detect neutrinos, put your detectors deep underground. The rock and dirt block cosmic rays and other noise sources, so that neutrino detectors can pick up the weak signals of ghostly neutrinos without interference.

Neutrinos are very light, so you would need many of them to make a snowball. For a long time, the researchers thought that neutrinos like photons weigh nothing at all, but it turned out that they have a very small mass. It is still not clear how much (or little) that is, but it would take approximately 300,000,000,000,000,000,000,000,000,000,000,000,000 neutrinos to weigh as much as a snowball. You could also write this in scientific notation as 3×10³⁵, but sometimes it is nice to write down all the numbers to remind you how dazzling these numbers are.

If you tried to pick up a snowball from 300 decillion neutrinos (I had to look up the word for that number), it would fall right through your hand. Slowly moving neutrinos pass matter more easily than fast moving ones, so few, if any, of these 300 decillion neutrinos would ever interact with your hand. They would just fall right through your palm and disappear into the earth.

Because neutrinos interact with matter so rarely, all of the high-energy samples discovered by scientists were moving at almost the speed of light. But the universe is also probably full of slower ones. The Big Bang created many neutrinos and most should still be floating around us. Because these “relic neutrinos” move much slower than the speed of light, astronomers sometimes refer to them as “stationary”, even though they are still traveling at a speed of several million miles per hour.

If the snowball came towards you as quickly as the solar neutrinos it contained moved, many of them would interact with your body, most likely by bouncing off an electron in one of your atoms. You would still probably not feel the impact, but it would likely provide enough energy to give you a lethal dose of radiation.

Perhaps the best thing is that there is no easy way to make a snowball from neutrinos. If you don’t have a particle accelerator in your garden, you need to stick to normal snow.

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