 At Caltech most of his scientific research focused on the study of "cosmic rays" (a term which he coined).In the 1930s he entered into a debate with Arthur Compton over whether cosmic rays were composed of high-energy photons (Millikan's view) or charged particles (Compton's view).Millikan thought his cosmic ray photons were the "birth cries" of new atoms continually being created to counteract entropy and prevent the heat death of the universe.