Right now, about a hundred trillion neutrinos from the Sun are pouring through your body. By the time you finish this sentence they're past the far side of the Earth and gone, having touched nothing.

The particle that barely exists

Neutrinos are the universe's great introverts. They carry almost no mass, no electric charge, and they feel only the weak nuclear force — which means they hardly ever interact with anything. A neutrino could cross a slab of lead a light-year thick with a fair chance of coming out the far side untouched.

The numbers are hard to hold in your head:

• Around 100 trillion solar neutrinos stream through you every second.
• About 65 billion cross every square centimeter of sunlit ground each second.
• In your whole life, only a handful will ever so much as bump into one of your atoms.

They are predicted, in other words, to ignore you almost completely — and they keep the promise.

A particle invented out of desperation

In 1930 Wolfgang Pauli faced a crisis: in certain radioactive decays, energy appeared to go missing. Rather than abandon the conservation of energy, he proposed an undetectable particle slipping away with the missing share — and felt awful about it.

I have done a terrible thing. I have postulated a particle that cannot be detected.

He was almost right. It took 26 years. In 1956 Clyde Cowan and Frederick Reines parked a detector beside a nuclear reactor — a flood of antineutrinos streaming out of it — and finally caught the faint flashes of the rare few that deigned to interact. To beat odds this long you need a huge target and a lot of patience: later detectors grew into swimming-pool-sized tanks buried deep underground, shielded from everything except these ghosts.

The case of the missing sunlight

Then came a genuine mystery. Starting in the 1960s, Ray Davis ran a detector in a South Dakota gold mine to count neutrinos from the Sun's core — a direct check on the fusion that makes it shine. He found only about a third of the number theory predicted. For thirty years this "solar neutrino problem" festered. Was our model of the Sun wrong? Were the detectors?

Neither. Neutrinos come in three types, or flavors, and it turns out they shape-shift in flight: the electron-neutrinos born in the Sun morph into the other two flavors on the way here. Davis's detector could only see the original flavor, so it was blind to two-thirds of the arrivals. The Sudbury and Super-Kamiokande experiments confirmed it around the turn of the millennium by catching all three.

That shape-shifting, called oscillation, carried a sting. It can only happen if neutrinos have a tiny mass — which the Standard Model had assumed was exactly zero. The most ignorable particle in physics forced one of its grandest theories to be rewritten.