# It is raining through you right now Muons made in the air by cosmic rays are passing through your screen right now: about one per square centimetre per minute, some 17 a second through your body. Follow them from the proton that strikes the air above you into the pyramid of Khufu. Then the second rain: the Sun's neutrinos. And how to catch muons at home, in a jar. Cosmic rays 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. ## Method and sources The flashes at the top and in the body scene are drawn at random, but at the measured average rate: one muon per minute per square centimetre held flat, at sea level (Particle Data Group). The figure is rounded; newer measurements come in 10–15% lower. The screen area is an estimate: on a phone we start from a width of 7.1 cm, on a computer from 96 pixels per inch. A screen held upright receives fewer muons, and one on a mountain or in a plane receives more. For the body we take an area seen from above of 0.1 m² standing and 0.5 m² lying down. Slanted muons also enter through the sides, so the real number is somewhat higher. The counters count random events at that average rate, so two visits never give the same number. The muon columns use one scenario: muons made 15 km up, falling straight down at the mean ground energy of 4 GeV, losing no energy on the way. In reality they start with more energy and lose it in the air; the exact result differs, but one column still has billions of times more survivors than the other. The cascade is a drawing built on the real stages of a cosmic-ray shower, not a physics simulation. The number of branches and their angles are chosen so the drawing is easy to follow. - Particle Data Group, “Cosmic Rays” (Review of Particle Physics, 2022), sea-level muon flux, mean energy, production height. https://pdg.lbl.gov/2022/reviews/rpp2022-rev-cosmic-rays.pdf - Particle Data Group, muon properties (mean lifetime 2.1969811 μs). https://pdg.lbl.gov/2024/listings/rpp2024-list-muon.pdf - V. F. Hess, “Über Beobachtungen der durchdringenden Strahlung bei sieben Freiballonfahrten”, Physikalische Zeitschrift 13, 1084 (1912); English translation by A. De Angelis and C. Schultz. https://arxiv.org/abs/1808.02927 - B. Rossi, D. B. Hall, “Variation of the Rate of Decay of Mesotrons with Momentum”, Physical Review 59, 223 (1941). https://doi.org/10.1103/PhysRev.59.223 - K. Morishima et al., “Discovery of a big void in Khufu's Pyramid by observation of cosmic-ray muons”, Nature 552, 386 (2017). https://doi.org/10.1038/nature24647 - S. Procureur et al., “Precise characterization of a corridor-shaped structure in Khufu's Pyramid by observation of cosmic-ray muons”, Nature Communications 14, 1144 (2023). https://doi.org/10.1038/s41467-023-36351-0 - J. N. Bahcall, A. M. Serenelli, S. Basu, solar neutrino flux (standard solar model), Astrophysical Journal 621, L85 (2005). https://doi.org/10.1086/428929 - D. J. Bird et al., “Detection of a cosmic ray with measured energy well beyond the expected spectral cutoff due to cosmic microwave radiation”, Astrophysical Journal 441, 144 (1995). https://doi.org/10.1086/175344 - Telescope Array Collaboration, “An extremely energetic cosmic ray observed by a surface detector array”, Science 382, 903 (2023). https://doi.org/10.1126/science.abo5095 - CERN S'Cool LAB, guide to building a cloud chamber. https://scoollab.web.cern.ch/cloud-chamber - CERN, what tracks look like in a cloud chamber. https://visit.cern/cloud-chamber Canonical: https://mariuscomper.uk/ploaia-invizibila/en/