The World of the Atom

Abstract

The story of radiation is the story of the atom and of subatomic particles. I should warn you that I love this story because it is one of the most fascinating and compelling stories in the history of science, it involves a cast of brilliant scientists, and it changed the world. So I get enthused and want to go into too much detail—at least that is what my students think. But to really understand the story, it will be necessary to learn some complicated and apparently nonsensical ideas in physics. I will try to keep the technical details to a minimum, but if you really want to understand what radiation is and where it comes from, stick with me as we explore the story. I hope you will be fascinated, too. The beginnings of the story go back to Indian and Greek philosophers who postulated that the universe consisted of space and indivisible particles that could combine to form more complex matter. The term átomos, meaning uncuttable or indivisible, was coined by the Greek philosopher Democritus in the fourth century B.C.E . While this was purely a philosophical speculation, it was a remarkable insight. More than a thousand years later, science and experimentation began to uncover just what this meant. John Dalton is credited with being the father of modern atomic theory in chemistry. In 1803 he developed the idea that elements consist of atoms, that different atoms have different weights, and that the atoms of a specific element are all alike but are different from those of other elements. He also proposed that atoms can combine in specific proportions to make up compounds or molecules. But are atoms really indivisible, and if not, what are they made of? Nearly a hundred years after Dalton, this question began to be answered in a burst of experiments and insights at the end of the nineteenth century and the beginning of the twentieth century, starting with the discovery of radiation.