Cosmic rays
It is raining through you right now
Every flash on the screen marks a particle made in the air, some 15 km above you, by a cosmic ray. The flashes fall at random, but at the measured rate: about this many particles really are passing through your screen right now.
Scroll slowly. The rain keeps falling.
What is passing through your screen?
They are called muons, heavier cousins of the electron. At sea level, about one a minute crosses every square centimetre held flat.
Your screen is about 110 cm², so roughly 1.8 go through it every second. Since you opened this page: 0.
You have not felt a single one. Nobody does. They pass through glass, flesh and concrete. A typical muon gets through several metres of rock, and the most energetic reach laboratories dug a kilometre under a mountain.
Who discovered the rain?
On 7 August 1912 the Austrian physicist Victor Hess rose in a hydrogen balloon to 5,350 metres, carrying three instruments that measured how strongly the air was ionised, that is, how many ion pairs formed in it each second.
The leading explanation was that the radiation came from the rocks beneath us, so it should have faded with height. It grew: at the top of the flight his instruments read more than twice what they did on the ground.
The radiation was coming from above. Nor was sunlight the source: Hess had flown at night and during a partial solar eclipse, and found no difference. He shared the 1936 Nobel Prize in Physics with Carl Anderson.
Where does the rain come from?
Each shower starts with a single particle, most often a proton. The galaxy's magnetic fields have bent its path so often that the direction it arrives from no longer shows where it set out.
Some 15–20 kilometres above you it strikes the nucleus of a nitrogen or oxygen atom. New particles burst out of the collision, and they break up in turn, again and again. One proton becomes a cascade.
The blue branches, electrons and photons, almost all die out in the air. The gold ones are muons. Many of them reach the ground.
Why do muons make it all the way down?
There is a problem here. A muon lives for 2.2 millionths of a second on average. Even at the speed of light, that is only 660 metres. If its clock ran like ours, most muons would die within the first few kilometres.
From 15 kilometres up, almost none should arrive: about one in ten billion. The left-hand column shows what that would look like.
But they do arrive. At their speed, a muon's clock runs 38 times slower than ours. For us the trip takes 50 microseconds. For the muon, only 1.3. Of muons with typical energy falling straight down, about half reach the ground: the right-hand column. From the muon's point of view it is the other way round: the atmosphere shrinks.
This is Einstein's relativity, first measured on muons by Bruno Rossi and David Hall in 1941. That so many reach you is one of its simplest proofs.
How many particles pass through your body?
Standing up, by a simple estimate, about 17 muons a second pass through you. Lying in bed, five times as many.
Since you opened this page: 0.
Each one crosses you in a few billionths of a second and leaves through the floor, into the ground and beyond. You have lived in this rain all your life.
What did muons find inside the pyramid of Khufu?
Because they pass through stone, muons can image things far too big for X-rays.
In 2016–2017 three teams put detectors in the Queen's Chamber of the Great Pyramid and outside it, and counted the muons arriving from every direction. From one direction more came through than solid stone would have let pass.
There, above the Grand Gallery, is a void at least 30 metres long. What it was for is still unknown. Muons also found, in 2016–2017, a hidden corridor behind the north face. In 2023 it was measured precisely and filmed with a camera slipped between the stones.
And neutrinos?
Muons are the rain you can see, if you know how. There is another, far denser one, coming straight from the heart of the Sun: neutrinos.
About 65 billion a second pass through every square centimetre, roughly your thumbnail. To them the Earth is almost transparent.
At night, when the Sun is below the horizon, its neutrinos pass through the whole planet and come up through the ground.
What is the most powerful particle ever seen?
On 15 October 1991, above the Utah desert, the Fly's Eye detector caught the shower of a single particle carrying 51 joules. That is the energy of a baseball thrown at 96 km/h.
All of it in one proton, or a heavier nucleus. Physicists called it the “Oh-My-God” particle.
In 2021, also in Utah, one almost as energetic arrived. Its direction points to a huge void between galaxies, where nothing is known that could have sent it. Its discoverers named it Amaterasu, after the Japanese sun goddess. Nobody yet knows what can push a particle that far.
How can you see them with your own eyes?
Since you opened this page, 0 muons have passed through you. None of them is still here.
You can see them at home, in a cloud chamber. You need a small fish tank or a large wide-mouthed jar, isopropyl alcohol, dry ice and a torch.
- Stick a piece of sponge or felt to the bottom of the container and soak it with isopropyl alcohol of at least 90%.
- Turn the container upside down onto a metal tray painted black, resting on the dry ice.
- Turn off the lights, shine the torch in from the side and wait a few minutes.
- Thin trails of mist start to appear just above the tray. The long straight ones are muons. The short thick ones are alpha particles from radon in the air.
Dry ice burns skin: wear thick gloves. Never seal it in an airtight container, as the gas can burst it. Dry ice turns into carbon dioxide, and isopropyl alcohol is flammable: work in a ventilated room, away from flames.
That long trail above the tray belongs to the same rain that, at that very moment, is passing through you as well.