Luis Alvarez 
Luis Alvarez never met a particle he didn't like. The Nobel Prize-winning physicist built one of the world's first particle accelerators. He studied neutrons and cosmic rays, and discovered that a form of hydrogen is radioactive. He used the products of cosmic rays to probe one of the pyramids of Giza. And with his son Walter, he used an element in ancient layers of rock to infer that the dinosaurs were killed when an asteroid slammed into Earth.
Alvarez was born 100 years ago today in San Francisco. He earned his Ph.D. at the University of Chicago, then joined the faculty of the University of California.
During World War II, he helped develop several radar systems, then joined the atomic-bomb effort at Los Alamos.
After the war he returned to California, where he became one of the top researchers in particle physics. And late in his career, he spent much of his time studying cosmic rays -- particles produced by exploding stars and other powerful events.
In the 1970s, Alvarez and his son noted that there was a high concentration of an element known as iridium in layers of rock that were 65 million years old -- the time when the dinosaurs vanished. The element is rare on Earth, but common in space rocks. The Alvarezes concluded that the iridium was deposited when a giant asteroid hit Earth, causing a global catastrophe. Today, their hypothesis remains the leading explanation for the demise of the dinosaurs.
Script by Damond Benningfield, Copyright 2011