Victor Hess and the Discovery of Cosmic Rays

On a summer morning in 1912, Austrian physicist Victor Franz Hess climbed into the basket of a hydrogen-filled balloon in the Bohemian town of Aussig. He carried three electrometers — delicate instruments for measuring electrical charge — and a determination to solve a mystery that had puzzled physicists for over a century. Six hours later, having ascended to 5,300 meters, he would land near Berlin with data that would change our understanding of the universe.

The Mystery of the Leaking Charge

The puzzle began in 1785, when Charles-Augustin de Coulomb noticed something strange: a charged metal sphere, left alone in air, gradually lost its charge. The air itself seemed to conduct electricity, slowly draining the sphere.

By the late 19th century, physicists understood that this "atmospheric conductivity" was caused by ionization — something was knocking electrons off air molecules, creating charged particles that could carry current. But what was doing the ionizing?

The discovery of radioactivity in 1896 seemed to provide an answer. Radioactive elements in the Earth's crust emitted penetrating radiation that could ionize air. This terrestrial radioactivity, physicists assumed, was the source of atmospheric ionization.

There was a simple prediction: if the radiation came from the ground, it should decrease with altitude. The higher you went, the more air would absorb the radiation, and the less ionization you'd measure.

Early Experiments: The Tower and the Balloon

In 1909, German physicist Theodor Wulf built a portable electroscope and carried it to the top of the Eiffel Tower — 300 meters above Paris. According to theory, ionization at the top should be far less than at ground level.

It wasn't. Wulf measured ionization about half of the ground-level value — far more than expected if the radiation came solely from the Earth. Something didn't add up.

Several physicists began making balloon measurements. In 1910-1911, Albert Gockel ascended to 4,500 meters over Switzerland. He found that ionization decreased at first, as expected, but then leveled off — it didn't continue decreasing at higher altitudes.

Victor Hess, working at the Institute for Radium Research in Vienna, decided to make systematic measurements. Between 1911 and 1912, he made ten balloon flights, carefully measuring ionization at various altitudes with improved instruments that could distinguish between different types of radiation.

August 7, 1912: The Decisive Flight

Hess's seventh flight of 1912 was the decisive one. The hydrogen balloon lifted off at 6:12 AM, carrying Hess and his three electrometers. The weather was clear — important for accurate readings.

As expected, ionization decreased slightly as the balloon rose through the first 1,000 meters. But then something remarkable happened: it began to increase. By 2,000 meters, it matched the ground-level rate. By 3,000 meters, it exceeded it. At the maximum altitude of 5,350 meters, Hess measured ionization rates more than twice the ground-level value.

The balloon drifted 200 kilometers, landing six hours later in a field near Berlin. Hess emerged with data that demanded a revolutionary conclusion.

Radiation from Above

In his 1912 paper, Hess stated his conclusion carefully but firmly:

"The results of the present observations seem most likely to be explained by assuming that a radiation of very high penetrating power enters our atmosphere from above."

This was extraordinary. Radiation from space? The idea seemed almost fantastical. Where could it come from? What was it made of?

Hess had one more piece of evidence. On April 17, 1912, he made an ascent during a near-total solar eclipse. If the radiation came from the Sun, it should decrease when the Sun was blocked. It didn't. The source wasn't the Sun — it had to be from deeper in space.

Confirmation and Controversy

Not everyone was convinced. The measurement techniques were difficult, the instruments temperamental. Some physicists argued that radioactive contamination of the balloon itself could explain the results.

But in 1913-1914, German physicist Werner Kolhörster made even more dramatic balloon flights, reaching 9,300 meters — higher than Mount Everest. At this altitude, he measured ionization six times greater than at ground level. The evidence was becoming overwhelming.

World War I interrupted research, but by the 1920s, multiple groups had confirmed the existence of radiation from space. In 1926, American physicist Robert Millikan coined the term "cosmic rays" — though he incorrectly believed they were high-energy gamma rays. The name stuck even after it became clear they were actually particles.

The Nature of Cosmic Rays

Throughout the 1920s and 1930s, physicists worked to understand what cosmic rays actually were. Were they electromagnetic radiation (gamma rays) or particles?

The key evidence came from studies of how cosmic rays behaved in Earth's magnetic field. If they were charged particles, their paths should curve; if they were gamma rays, they should travel straight.

In 1927, Jacob Clay discovered that cosmic ray intensity varied with latitude — stronger near the poles, weaker near the equator. This "latitude effect" proved that cosmic rays were charged particles deflected by Earth's magnetic field.

In 1932, Carl Anderson discovered the positron — the antiparticle of the electron — in cosmic ray tracks. This was the first evidence of antimatter, and it came from studying cosmic rays. Anderson shared the 1936 Nobel Prize with Hess.

Hess's Later Life

Victor Hess continued his research on cosmic rays and radioactivity throughout the 1920s and 1930s. He established a cosmic ray observatory on the Hafelekar mountain near Innsbruck, at 2,300 meters elevation, for continuous monitoring.

In 1936, Hess received the Nobel Prize in Physics "for his discovery of cosmic radiation." His Nobel lecture detailed the decade of careful measurements that led to his conclusion, and the subsequent work that confirmed it.

As an Austrian Jew (through his first wife), Hess was forced to flee after the Nazi annexation of Austria in 1938. He emigrated to the United States, becoming a professor at Fordham University in New York, where he continued his cosmic ray research until his retirement in 1958.

Victor Hess died on December 17, 1964, in Mount Vernon, New York. A crater on the Moon bears his name.

From Balloon to Observatory

Hess couldn't have imagined where his discovery would lead. The cosmic rays he detected with his electrometers had energies of perhaps billions of electron volts. We now know that cosmic rays span an enormous energy range — up to 10²⁰ eV, a hundred million times more energetic than the Large Hadron Collider can produce.

The simple electroscopes he carried in a balloon basket have evolved into vast observatories covering thousands of square kilometers. The Pierre Auger Observatory in Argentina would seem like science fiction to the physicist who ascended over Bohemia in 1912.

But the fundamental question Hess asked — where does this radiation come from? — remains only partially answered 113 years later. We know cosmic rays come from outside the solar system. We know the highest-energy ones come from outside our galaxy. But identifying specific sources remains one of astrophysics' greatest challenges.

Every cosmic ray detected by Auger or Telescope Array carries a faint echo of that August morning in 1912, when a determined physicist in a balloon basket proved that the universe was raining energy down upon us.