VOLUME 3 Nuclear Fission 117 New Elements 117 The Discovery of Fission 122 The Nuclear Chain Reaction 127 The Nuclear Bomb 131 Nuclear Reactors 141 Nuclear Fusion 147 The Energy of the Sun 147 Thermonuclear Bombs 149 Controlled Fusion 151 Beyond Fusion 159 Antimatter 159 The Unknown 164 Reading List 166 [Illustration: _A field-ion microscope view of atoms in a crystal. Each tiny white dot is a single atom, and each ring system is a crystal facet or plane. The picture is magnified 1,500,000 times._] MASS AND ENERGY In 1900 it began to dawn on physicists that there was a vast store of energy within the atom; a store no one earlier had imagined existed. The sheer size of the energy store in the atom—millions of times that known to exist in the form of chemical energy—seemed unbelievable at first. Yet that size quickly came to make sense as a result of a line of research that seemed, at the beginning, to have nothing to do with energy. Suppose a ball were thrown forward at a velocity of 20 kilometers per hour by a man on top of a flatcar that is moving forward at 20 kilometers an hour. To someone watching from the roadside the ball would appear to be travelling at 40 kilometers an hour. The velocity of the thrower is added to the velocity of the ball. If the ball were thrown forward at 20 kilometers an hour by a man on top of a flatcar that is moving backward at 20 kilometers an hour, then the ball (to someone watching from the roadside) would seem to be not moving at all after it left the hand of the thrower. It would just drop to the ground. There seemed no reason in the 19th century to suppose that light didn’t behave in the same fashion. It was known to travel at the enormous speed of just a trifle under 300,000 kilometers per second, while earth moved in its orbit about the sun at a speed of about 30 kilometers per second. Surely if a beam of light beginning at some earth-bound source shone in the direction of earth’s travel, it ought to move at a speed of 300,030 kilometers per second. If it shone in the opposite direction, against earth’s motion, it ought to move at a speed of 299,970 kilometers per second.