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March 20, 2006

Nobel laureate James Cronin to deliver Hofstadter Lecture on cosmic rays

By Melissa Fusco

Cosmic rays are one of astronomy's great unsolved mysteries, and the solution of their origin may point the way to the origin of the universe itself.

Nobel Prize winner James Cronin will deliver this year's Hofstadter Memorial Lecture on the history and ongoing investigation of cosmic rays. Cronin, a professor of physics and astronomy at the University of Chicago, is the U.S. principal investigator of the world's largest cosmic-ray detector, the Pierre Auger Observatory. The brainchild of Cronin and Alan Watson of the University of Leeds, the observatory brings together 200 physicists from 15 countries. It was completed in Argentina in 1995.

"Jim Cronin has done an outstanding job leading the Auger project to study the very highest energy cosmic rays," said physics Professor Roger Blandford, who directs the Kavli Institute of Particle Astrophysics and Cosmology at Stanford. "We are all looking forward very much to learning the latest news on trying to understand how nature can make a subatomic particle acquire the energy of a well-hit baseball."

Cronin's public lecture, titled "Cosmic Rays: A Fascinating Scientific History," will begin at 8 p.m. Monday, April 3, in Room 201 of the Hewlett Teaching Center. Cronin will explain the adventure, false starts, rivalry and controversy surrounding the discovery of cosmic rays and the development of the theory explaining them. A more technical afternoon colloquium on the science and construction of the Pierre Auger Observatory will be held in Hewlett 201 at 4:15 p.m. Tuesday, April 4.

High-energy cosmic rays—comprised of subatomic particles including protons—were discovered in 1912 by physicist Victor Hess during a hot-air balloon ride. Hess discovered a puzzling radiation that increased in intensity at altitudes high above the Earth, indicating an extraterrestrial origin. Later that year, Hess made similar measurements during a solar eclipse and proved that cosmic rays did not come from the sun. Debate raged throughout the 1930s as to the nature and composition of the rays, and embroiled two Nobel Prize winners, Robert Millikan and Arthur Compton, in a public spat.

Cosmic rays are so powerful that they can traverse space with little deflection. In 1949, physicist Enrico Fermi proposed that the rays were caused by the acceleration of particles in enormous exploding stars, or supernovae. But in the 1990s, two "impossible" cosmic ray events were recorded by detectors in the United States and Japan. These rays had energies on the order of 10^20 electron volts—10,000 times higher than Fermi's theory allowed.

Scientists then dubbed the high-energy subatomic particles, which showered through the atmosphere and defied all explanation, "Oh-my-God particles."

"It was as if they went out to catch butterflies and caught an F-111," according to the website for the Pierre Auger Observatory.

Cosmologists now theorize that cosmic rays may be caused by exotic objects called "topological defects" left over from the Big Bang. These objects—monopoles, cosmic strings and domain walls—are predicted by cosmological theory but never have been observed in space. High-energy rays may provide clues to their existence.

The Hofstadter Memorial Lecture series is named in memory of Nobel laureate Robert Hofstadter, who was a professor of physics at Stanford from 1970 until his death in 1990.

Melissa Fusco is a science-writing intern at Stanford News Service.

Editor Note:

Science-writing intern Melissa Fusco wrote this release. A photo of Cronin is available on the web at



Dawn Levy, News Service: (650) 725-1944,

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