Cloud Chambers and Cosmic Rays: The Quest to Unravel One of the Most Dazzling Mysteries of the Universe


Silk, vapor and the substance that is life.


The Austrian physicist died in his final year of his twenties. Victor Franz Hess (June 24, 1883–December 17, 1964) climbed into the basket of a balloon, carefully stationed the exquisitely precise new electroscopes he had built himself, and ascended into the sky to probe a mystery that had long puzzled scientists: the presence of ionizing radiation and electricity in the air. Hess flew day and night into the moody skies, through icy sunshine and black storms — a naked ape afloat five kilometers above his habitat on the wings of hydrogen and silk, risking his life for this one fragment of truth the way Caroline Herschel had risked hers for another a century-some earlier.

Hess at one of his balloon ascents 1911.

Ever since the discovery of radioactivity in 1896, it was believed that any radiation in the atmosphere was emanating from radioactive elements in Earth’s rocky body, and should therefore decrease as distance from the surface increases. Hesse was able to discover something remarkable as he rose, taking meticulous measurements at every step. Radiation decreased for the first km, but then started increasing steadily at the highest elevation.

His movements seemed closer to the source of his mysterious energy than he did away from it.

This had to have come from space.

Hesse received the Nobel Prize in 1936. This was a quarter-century after his balloon ascent. Albert Einstein bowed to him in his 1939 World’s Fair speech. Cosmic rays went on to revolutionize nuclear physics and the wonderland of subatomic particles, leading to the discovery of the muon — the electron’s heavy-set cousin — and the positron, the electron’s antimatter twin.

The Crab Nebula. Hubble Space Telescope / ESA. Available as both a printed copy and stationery cards.

Today, cosmic rays still carry with them a particulate cloud of mystery — a million cosmic rays go through your body each night while you sleep, but we don’t yet know where they come from. Most likely supernovae but also possible quasars or active galactic nuclear nuclei and gamma ray bursts. These rays are thought to have originated from the Crab Nebula, a remnant of a supernova that was thousands of lightyears far. They could be from Centaurus A radio galaxy, which is close to Earth. Cosmic Rays detected at the International Space Station may provide clues about the cosmic mysteries of dark matter.

Hesse was a Swiss meteorologist and physicist who made his first ascent from a balloon several months later. Charles Thomson Rees Wilson (February 14, 1869–November 15, 1959) built a wondrously imaginative device for studying cloud formation and optical illusions in humid air, which would find an unexpected application in the study of cosmic rays and for which Wilson too would receive the Nobel Prize.

In his twenties, standing atop a mountain with his back to the Sun, Wilson had gasped at the enormous haloed shadow his body cast upon the distant clouds — an atmospheric phenomenon known as Brocken bow or mountain specter, produced when the tiny near-identical water droplets in clouds refract and backscatter sunlight.

Brocken is a spectre of glory. (Creative Commons image by Brocken Ataglory.

To recreate the effect, he built chambers and soon discovered that the kernels of the ions were water molecules.

Wilson perfected the first prototype in 1911, as Hess was soaring into the sky in his balloon, and called it a “cloud chamber.”

In the decades that followed, other scientists built on Wilson’s inventive particle detector. The year Hesse won his Nobel Prize for the discovery of cosmic rays, the American physicist Alexander Langsdorf Jr. — who had worked on the atomic bomb and became a vocal critic of nuclear weapons — reimagined the cloud chamber not with water but with alcohol, coolable to much lower temperatures before freezing, making it much more sensitive to ionization tracks.

Cloud chamber photo of the original positron observed. The positron entered from the lower right and curves toward the top left.

When the trays of alcohol are heated, the vapor sinks because alcohol molecules are heavier than air, supersaturating the chamber with vapor so that any littlest particle will kernel the condensation of droplets as subatomic particles collide with air molecules and fracture them into charged ions around which cloudlets condense — a fractal miniature of what happens when cosmic rays pass through Earth’s atmosphere, breaking air molecules apart into high-energy subatomic particles that then break more molecules apart and make more particles.

A century after Wilson’s birth, in the Summer of Love, NASA Ames Research Center donated one of their cloud chambers to the first exhibit at The Exploratorium — San Francisco’s magical museum of science and wonder, founded that year by Frank Oppenheimer.

Inside the cloud chamber, as cosmic rays drag subatomic particles through matter, they paint a constellation of wispy white lines left behind by muons traipsing through the liquid, dappled with some shorter, curlier electrons tracks and a handful of thicker scratches made by alpha particles — the nuclei of helium atoms. This dazzling, far-off mystery of the cosmic rays suddenly becomes intimate and visible. It reminds us that our main focus is restlessness and empty space.

Complete the story about how Lise Meitner, physicist, discovered nuclear fission, was exempted from the Nobel Prize, but she went on to open up the door for women scientists, enjoy this BBC In Our Time program on cosmic rays, and then revisit Alan Lightman, poetic physicist, on why our atomic lives are worth living.


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