By the end of the nineteenth century, after more than two thousand years of intellectual struggle that began with the Greek philosophers, physical scientists had reason to believe that they were beginning to understand the universe. Their theories of matter and energy, of electricity and magnetism, of heat and sound and light were confirmed in laboratories throughout the world with increasing precision. Experimentation was the method, and mathematics the language, of a powerful, coherent body of knowledge called classical physics.

For a few years before and after the turn of the century, the world was taking a breather from war and rebellion. The monumental achievements of science, technology, and industry - such as the installation of a transatlantic telegraph cable - inspired hopes for a peaceful and prosperous future. But beneath the calm surface, in politics as well as in science, the roots of future turmoil were quietly gathering strength. Even the sturdy foundations of classical physics were developing alarming cracks. Some discrepancies were found when experiments clashed with theory. Perhaps the most unsettling of these was the failure to discover the ether. When their results plainly contradicted the ether hypothesis, physicists were dismayed. How could there be vibrations without something to do the vibrating?

Other puzzles cropped up by accident. On November 8, 1895 the German physicist Wilhelm Conrad Röntgen stumbled upon a way to make strange rays with the power to penetrate black paper, and even living flesh. Since x is the unknown in algebra, Röntgen called them X rays. By December, he had used them to take pictures of human bones, and within a year their practical value was well understood. The rapid spread of the use of X rays throughout the world foreshadowed the way scientists, engineers, and inventors would turn fundamental discoveries into technological applications in the coming century. But no one knew where X rays came from. The chance discovery of radioactivity finally signaled the beginning Marie Curie of a new era in physics. As the element polonium, identified by Polish-born Marie Curie (right) in 1898, emits radiation, it changes spontaneously into lead. This discovery shattered the belief inherited from the Greeks that the elements are immutable and their atoms indestructible.
What causes atoms to decay?
What are they made of?
What forces are at work inside them?
Such questions were new to physics, and were to remain at its cutting edge throughout the twentieth century. The answers would affect our lives in ways no one could imagine in the year 1900.