Hundreds of trillions of neutrinos a second, and you feel nothing

The Sun shines on you in two ways. One of them you can feel. The other one goes straight through you, through the floor, and out the far side of the planet, and has done so since before you were born.

What a neutrino is

A neutrino is a particle with no electric charge and almost no mass. It is produced whenever a proton inside a nucleus turns into a neutron, or a neutron into a proton. The Sun does that constantly: in its core, protons fuse into helium, and every fusion releases neutrinos along with the light. The light takes a very long time to work its way out. The neutrinos leave at once, in straight lines, and eight minutes later they reach the Earth.

Why this is worth a minute of your attention

Almost nothing stops them. Neutrinos ignore the electric force that holds ordinary matter together, so to a neutrino your body, the ground, and the Earth itself are mostly empty. So the number passing through you does not depend on what you are made of. It depends on one thing only: how big a shadow you would cast towards the Sun. The stream does not stop at night, either. The Super-Kamiokande detector in Japan sees the Sun's neutrinos after sunset, arriving from below, having crossed the whole planet to get there.

How to count them

A flux is a count of particles crossing one square centimetre in one second. Every time four protons become one helium nucleus, the Sun emits two neutrinos, which carry off about 2.3 percent of the energy. Spreading those over a sphere the size of the Earth's orbit gives a flux at the Earth of about 6.5 times ten to the ten. That is sixty-five billion neutrinos through every square centimetre, every second. Most of them come from the first step of fusion, two protons combining. The flux swings about 3.4 percent above and below its average over the year as the Earth's distance from the Sun changes, and Super-Kamiokande has measured that seasonal swing.

To turn a flux into a count for your own body, multiply by the area you present to the Sun. Face the Sun and the area is roughly your height times your width. Stand under it at noon, with the Sun straight overhead, and the area is your width times your depth, roughly the top of your head and shoulders: fewer neutrinos, because the target is smaller. Either way the answer has fourteen or fifteen digits, every second, and nothing in you registers it.

The reason nothing registers is the size of the target each neutrino sees. A neutrino only interacts through the weak force, and the weak force reaches less than the width of a nucleus. An atom is almost entirely empty to it. The chance of a single neutrino hitting anything on its way through a person is so small that physicists who want to catch them fill tanks with hundreds or thousands of tonnes of water or oil, wait for years, and count the events one at a time.

In one breath

Neutrinos are neutral, nearly massless particles made by nuclear reactions, and the Sun makes them in enormous quantity. They pass through matter because the only force they feel is short-ranged and weak, so a body is transparent to them. The number crossing you per second is the solar flux multiplied by the area you present, which for an adult facing the Sun comes to a few hundred trillion. That number is the same at midnight. The Earth is as transparent as you are.