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American physicist who studied subatomic particles. Alvarez built a device called a hydrogen bubble chamber that made it possible to analyze the reactions occurring between atomic nuclei inside it. His observations led to the theory that protons, neutrons, and electrons are made of quarks. Alvarez won a 1968 Nobel Prize for physics for this work. With his son, geologist Walter Alvarez (born 1940), he later developed a theory that the extinction of dinosaurs was caused by climate changes resulting from a giant asteroid striking the Earth.