B. H. Fields - Understanding Protons

To Alex, with every positive wish.

Published in 2016 by Cavendish Square Publishing, LLC
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Library of Congress Cataloging-in-Publication Data

Bortz, Fred, 1944- author.

Understanding protons / Fred Bortz and B.H. Fields.

pages cm (Exploring the subatomic world)

Includes bibliographical references and index.

ISBN 978-1-50260-546-7 (hardcover) ISBN 978-1-50260-547-4 (ebook)

1. ProtonsJuvenile literature. 2. Particles (Nuclear physics)Juvenile literature.

I. Fields, B. H., author. II. Title. III. Series: Exploring the subatomic world.

QC793.5.P72B68 2015

539.72123dc23

2015003670

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Contents

Atoms, Molecules, and Matter

Discovering the Nucleus

Particles Within the Nucleus

Protons and Nuclear Fusion

W hat 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 (17661844) 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 Daltons 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.

Daltons 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.

ATOMS,
Molecules, and Matter

T he ancient idea that matter is made up of tiny, indivisible pieces has led scientists in directions that Leucippus and Democritus could never have imagined. Todays world of human-made substances depends on our knowledge of atoms, the subatomic particles within those atoms, and the forces that hold them together or break them apart.

Those ancient Greek philosophers began with two simple questions: What is matter made of, and why do different kinds of matter behave so differently from each other? When they imagined dividing everyday substances into smaller and smaller bits until the pieces were indivisible atoms, they also decided that the atoms of each substance would have a particular shape and texture. For instance, they concluded that water atoms would be round and smooth, while rock atoms would be hard and sharp or gritty.

The presumed atoms were much too tiny for the Greeks to test their ideas. Besides, the notion of testing theories by observation, a cornerstone of modern science, was not yet part of human culture. Toward the end of Democritus life, Greek philosophy entered what is often called a golden era, where great thinkers such as Socrates, Aristotle, and Plato used their powerful minds and logic to deduce what they believed to be the truth about the natural world.

Aristotle was so brilliant in many fields that his ideas were rarely questioned. For nearly two thousand years, most people accepted his conclusion that all the worlds matter was made of four elements: earth, air, fire, and water. The idea of atoms all but disappeared. Today we know that both Democritus and Aristotle were right in general but wrong in detail.

Democritus. This sculpture depicts Democritus, the founder of an ancient Greek philosophical movement known as atomism. He proposed that matter is made of tiny indivisible particles, an idea that eventually led to the modern theory of atoms.