What is matter? That is one of oldest questions in science. It is also among the most productive, because as we learn more about matter, we also find new questions.
For example, you probably know that every substance on Earth is made of atoms and combinations of atoms called molecules. But do you know what makes hydrogen different from nitrogen? What makes carbon different from silicon? What makes what makes any one kind of atom different from any other?
The scientific theory of atoms goes back to 1803, when John Dalton (1766—1844) adopted an idea of the ancient Greek philosophers Leucippus and Democritus. Twenty-three centuries earlier, long before modern science, they imagined cutting up a piece of matter until it was atomos, meaning indivisible.
Dalton used that notion of indivisible particles to interpret laboratory observations. In Dalton’s theory, the atoms of Leucippus and Democritus turn out to be molecules. A water molecule is made of two hydrogen atoms and one oxygen atom, and is therefore not indivisible. But it is the smallest unit of matter that can still be called water. Dalton spoke of elements, which are made of only one kind of atom, and compounds, which are made of only one kind of molecule.
Dalton’s theory still considered atoms indivisible, but as more scientists discovered more elements, it was natural to wonder if the atoms were made of even smaller particles that distinguished one kind of atom from another. By the end of the nineteenth century, physicists (scientists who study matter and energy) began to discover those subatomic particles. And in the early twentieth century they realized that one of the most important of those was the proton.
This book is the story of how protons were discovered, where our understanding of protons has led us, and what lies ahead.