American scientist Robert Millikan confirms the existence of cosmic rays

On November 9, 1925, at the National Academy of Sciences meeting at the University of Wisconsin, physicist Robert Millikan announces a stunning fact: Radiation in the Earth’s upper atmosphere originates in space. Millikan describes how penetrating rays from space, more powerful than x-rays, are constantly bombarding the planet. He calls them cosmic rays and says they are “harder and more penetrating than any which had before even been imagined.”

The monumental announcement sent shockwaves through the scientific community. “Hearing Dr. Millikan was like being present when the Curies announced the discovery of radium, Roentgen reported the first generation of X-rays or Hertz sent the first wireless waves,” reported The Science News-Letter, a weekly publication.

As significant as Millikan’s research was, it drew on the earlier work of Austrian physicist Victor Hess. In the early 1910s, Hess conducted multiple balloon experiments to measure ionization levels in the atmosphere and discovered concentrations were highest several miles above the ground. One test during a near-total solar eclipse led Hess to argue that the source of this “high altitude radiation” wasn’t the sun but elsewhere in space. Scientists didn’t accept this theory until Millikan confirmed it within his own research the following decade.

Cosmic rays are actually high-speed particles, either electrons or atomic nuclei, that travel through space, moving at nearly the speed of light. Most cosmic rays come from within the Milky Way galaxy, and others come from the sun. The strongest cosmic rays usually come from outside the Milky Way, allowing scientists to uncover some of the universe’s makeup and history. The Earth’s magnetosphere and atmosphere largely protect humans from the dangers such cosmic radiation poses, though astronauts, pilots and flight attendants face increased risks.

Beyond illuminating previously unknown facets of the universe, cosmic rays ushered in the field of modern particle physics and led to the discovery of antimatter. They also underpin the radiocarbon dating technology that allows scientists to determine the age of various artifacts and geologic specimens.

Two years before he coined the term cosmic rays, Millikan, who served as director of the Norman Bridge Laboratory of Physics at the California Institute of Technology, won a Nobel Prize for his study of the elementary electronic charge and the photoelectric effect. During World War I, Millikan played a significant role in developing meteorological and antisubmarine devices as vice chairman of the National Research Council.

Hess received a Nobel Prize for discovering cosmic radiation in 1936.