The Second Industrial Revolution 757

postulate the structure of the atom, with a positively charged nucleus and
negatively charged electrons circling around it.
“Particle theories” in physics cast doubt on contemporary assumptions
about the universe. They demonstrated the complexity of motion, light, and
matter, which appeared to consist of electrically charged particles. Since the
time of Sir Isaac Newton in the seventeenth century, scientists had believed
that any two objects, whether the sun and the earth, or a coffee cup and a
bowl of sugar, acted on each other through gravitational force. There
seemed no mechanism for transmitting action but rather only empty space
between the two. James Maxwell (1831 — 1879), a British scientist, solved the
“action at a distance” problem for the case of electromagnetic forces. His
theory of electromagnetic fields postulated that one object creates an elec­
trical field around particles, which in turn exerts forces on electrically
charged objects, and that light itself consists of electromagnetic waves. The
German scientist Max Planck (1858-1947) discovered that radiant energy is
emitted discontinuously in discrete units, or quanta. Planck’s quantum the­
ory, not finalized until 1925, challenged the fundamental scientific under­
standing of energy that had survived almost intact since Newton’s day. More
than this, it seemed to add an element of chance to the story of the universe,
suggesting that its operations were not absolutely predictable. Here, too, sci­
entists now realized that they knew considerably less about the nature of
matter and about the universe than they had long assumed.
These discoveries were only a beginning. The much more difficult
problem of gravity remained. In Switzerland, German-born Albert Einstein

Albert Einstein explaining his theories to a stunned audience.