A look at the discovery of cosmic rays

Cosmic rays are extremely high-energy particles, most of which were originally emitted by sources far outside of Earth’s solar system. Like many breakthroughs in science, the existence of cosmic rays was discovered largely by accident in the early 20th century by scientists studying radioactive decay.

According to the University of Utah, the discovery of cosmic rays grew out of the discovery of radiation by Henri Becquerel in 1896. Radioactivity is the process by which the atomic nuclei of some unstable chemical isotopes blow apart. Over the next several years, fellow chemists and physicists began devising primitive devices to detect more radiation.

Initially, Becquerel believed that radiation could only be caused by heavy elements in the ground. Although it was soon discovered that trace amounts of ionization could be detected in the air, it was assumed that this must be caused by radiation emitted from underground or by radioactive gases escaping up into the atmosphere. In 1909, however, German Jesuit priest and scientific researcher Theodor Wulf reported something unexpected: His radiation detector actually reported increasing levels of background radiation as he climbed up the Eiffel Tower. It was a simple “experiment,” but the results were the opposite of what was expected. As Wulf ascended the Tower, radiation from ground sources should have gone down, not up.

Wulf’s finding was anomalous but no more. The actual discovery of the existence of cosmic rays, according to R.A. Mewaldt of the California Institute of Technology, is normally credited to an Austrian researcher, Victor Hess. (Hess, a Jew, later fled to the United States in the late 1930s to escape Nazi persecution, and became an accomplished professor at Fordham University in New York.) In 1912, seeking to verify and explain Wulf’s strange results, Hess took several electroscopes and ascended several miles into the atmosphere by means of a hot-air balloon. Once again, radiation levels climbed. Hess timed his flight to occur during an eclipse to rule out the possibility that the devices were simply measuring something related to sunlight.

The results were clear: Whatever the new detection devices were finding, it was coming from somewhere off the Earth and decreasing in intensity as they travelled toward the surface. Exactly what those results meant still wasn’t certain, but according to the Pierre Auger Observatory, the following decade their existence was finally confirmed by Russian scientist Dimitri Skobelzyn, who photographed the eerie, ghost-like trails left through a specially designed cloud chamber by cosmic rays as they bombarded the Earth.

Hess received the Nobel Prize for this discovery shortly before he fled to the U.S., but it would actually take years before other scientists could piece together the nature of cosmic rays. The term itself was coined by an American physicist, Robert Millikan, in the 1920s. Mewaldt says that they were called “rays” because “for some time it was believed that the radiation was electromagnetic in nature” – like gamma rays or X-rays. However, he says, by the 1930s scientists realized that so-called “cosmic rays” were actually high-energy particles, not electromagnetic rays. Science has moved on, but it was too late: The name “cosmic ray” stuck.

The first part of Millikan’s term, “cosmic,” though, has turned out to be even more accurate than he suspected. The source for the majority of cosmic rays striking the Earth is still not known for certain, but NASA says that recent research using its Fermi Space Telescope suggests that the largest emitters of cosmic rays are supernovae – exploding giant stars. Light and high-energy particles travel thousands or even millions of light-years through deep space before striking Earth.