Luis Alvarez, a physics Nobel laureate, had a long and brilliant career as a physicist. In 1980, he proposed a now famous thesis on the extinction of dinosaurs, suggesting it had been caused by an asteroid colliding with the Earth some 160 million years ago. The resulting cloud of dust would have blocked out the Sun for months. With the help of his geologist son, Luis Alvarez was able to find traces of this cloud responsible for the dinosaurs' starvation; a cloud that had covered the Earth's surface with a thin layer of dust containing radioactive iridium from outer space, an isotope rarely found on Earth.