How many neutrinos are passing through us?

The number of solar neutrinos that reach us on the earth is measured by something called “flux”, which is how science geeks measure the rate of flow of material. The solar neutrino flux for us on Earth is about 65 billion neutrinos, passing through just one square centimeter of area on earth, every second. That’s a lot of neutrinos. And almost all of them pass right on through the earth and out the other side (but more on that “almost” later). That means that every second, trillions of neutrinos are passing through your body, since you’re also transparent to these guys. By my rough estimate about 100 trillion, to be more specific. That means over the course of your lifetime, about 10^23 neutrinos will stream through your body – that’s almost a trillion trillions. That also happens to be about a mole. So over the course of your lifetime, about a mole of neutrinos (almost a trillion trillions) will have coursed through your body.

Now we’ve heard that neutrinos have a tiny but nonzero mass. It was once thought that they were massless, since they fly at just about the speed of light and don’t seem to be “trappable” using gravity. But it has been proven that they must have some mass, for reasons I’d rather not get into.1

So if they have some tiny mass, that means there’s a constant flow of matter coursing through our bodies all the time. For some reason I find this even weirder to picture than if neutrinos had been massless, photon-like invisible rays. That would have been weird enough, but mass-full particles? Constantly shooting straight through us at all times? Weird. So how much mass is streaming through us at all times?

Not very much – there may be a trillion trillion of them, but they don’t add up to much mass – about 10-13 kilograms will stream through your body over the course of your life. It has been estimated that about 100 billion people have ever lived on planet earth, if you added up all humans from the dawn of our species. That means that the sum total mass of all neutrinos that have passed through every single person who ever lived, over everyone’s total lifetime, is… about 0.15 grams.

Now that we’ve figured out the astronomically large amount of neutrinos that have coursed through our bodies over our lives. Coming up next, we’ll take a diversion and consider how the skies would look if we had “neutrino-vision”…

Next Up: What if we could *see* neutrinos? >>

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