By the late nineteenth century, science was one of the main pillars supporting Western society’s optimistic and rationalist worldview. To progressive minds, unchanging natural laws seemed to determine physical processes and permit useful solutions to more and more problems. All this was challenged by the new physics.

An important first step came at the end of the nineteenth century with the discovery that atoms were not like hard, permanent little billiard balls. They were actually composed of many far-smaller particles, such as electrons and protons. Polish-born physicist Marie Curie (1867–1934) and her French husband, Pierre, discovered that radium constantly emits subatomic particles and thus does not have a constant atomic weight. Building on this and other work in radiation, German physicist Max Planck (1858–1947) showed in 1900 that subatomic energy is emitted in uneven little spurts, which Planck called “quanta.” Planck’s discovery called into question the old sharp distinction between matter and energy: the implication was that matter and energy might be different forms of the same thing.

In 1905, the German-Jewish genius Albert Einstein (1879–1955) went further than the Curies and Planck in undermining Newtonian physics. His theory of special relativity postulated that time and space are relative to the viewpoint of the observer and that only the speed of light is constant for all frames of reference in the universe. In explaining his idea, Einstein used analogies involving moving trains. For example, if a woman in the middle of a moving car got up and walked forward to the door, she had gone, relative to the train, a half car length. But relative to an observer on the embankment, she had gone farther. To Einstein, this meant that time and distance were not natural universals but depended on the position and motion of the observer.

The 1920s saw breakthrough after breakthrough in physics. In 1919, Ernest Rutherford (1871–1937) showed that the atom could be split. By 1944, seven subatomic particles had been identified, the most important of which was the neutron. Physicists realized that the neutron’s capacity to shatter the nucleus of another atom could lead to chain reactions of shattered atoms that would release unbelievable force. This discovery was fundamental to the subsequent development of the nuclear bomb.

Although few nonscientists truly understood the revolution in physics, its implications, as presented by newspapers and popular writers, were disturbing to millions of men and women in the 1920s and 1930s. As unsettling as Einstein’s ideas was a notion popularized by German physicist Werner Heisenberg (VER-nuhr HIE-zuhn-buhrg) (1901–1976). In 1927, Heisenberg formulated the “uncertainty principle,” which postulates that nature itself is ultimately unknowable and unpredictable. He suggested that the universe lacked any absolute objective reality. Everything was “relative,” that is, dependent on the observer’s frame of reference. Such ideas fascinated ordinary people, who found the unstable, relativistic world described by the new physicists strange and troubling. Like modern philosophy, physics no longer provided comforting truths about natural laws or optimistic answers about humanity’s place in an understandable world.