Cosmic rays are a component of interstellar matter. Their composition is similar to ordinary interstellar gas, but they are hurtling through space at very high speeds.

In 1911, physicist Victor Franz Hess made balloon ascensions with an electroscope to measure background radiation that seemed to be emanating from everywhere on the Earth. To his surprise, the higher he went, the more radiation he detected.

In an attempt to discern the source of the radiation, Robert A. Millikan and his associate George H. Cameron (1902-1977) lowered an electroscope into a lake in 1925 and detected radiation more powerful than any previously known emission. By taking measurements at different locations, they concluded that the source of the radiation was outer space, and Millikan coined the term cosmic rays. Determining what they were, however, was another matter.

Two theories were forwarded; Millikan suggested that cosmic rays were high-energy photons, created as...