In 1980 physicist Luis Alvarez and his son ... rock that contained unusually high levels of a rare element, iridium. Chemical dating techniques put the rock at around 65 million years old.

The idea was first proposed in 1980 by physicist Luis Alvarez; his son, the geologist Walter Alverez; and their colleagues after they noticed the high levels of iridium in this layer. The team ...

Photo: Luis (left) and Walter (right) Alvarez. Luis Alvarez was a physicist with wide ranging interests. At the University of Chicago, he took a class called Advanced Experimental Physics ...