Cover: The Year of Dr. Einstein

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import for all of physics: it not only laid down the basic principle of the laser some 40 years before the first such device was made but, more broadly, also advanced quantum theory. In addition, Einstein contributed significantly to the rebirth of cosmology, the study of the origin, history and shape of the universe. The Dutch astronomer Willem de Sitter and later the Russian scientist Alexander Friedmann had concluded that Einstein's equations pointed to an unstable universe —possibly an expanding one. Because such a changing, dynamic universe was totally at odds with the popular picture of the heavens portrayed by most astronomers, Einstein had opted for a stable, unchanging universe; he had managed that feat with a mathematical sleight of hand that involved what he called the cosmological constant. A decade later, after the American astronomer Edwin Hubble had shown that the distant galaxies were all receding from one another and that the universe was indeed expanding, Einstein reversed himself and accepted the fact toward which his original equations had pointed. The cosmological constant, he allowed, was the worst mistake of his scientific career.

But he was stubborn on other scientific issues. As he admitted in his later years: "I have become an obstinate heretic in the eyes of my colleagues. In Princeton, they consider me an old fool." He had earned this new reputation by his continued objections to what had become the basic conceptual tool for studying atomic structure: quantum mechanics, a statistical way of looking at the atom that Einstein himself had helped develop by using Planck's quanta to explain the nature of light.

Nowadays physicists rank quantum mechanics alongside relativity as one of the twin pillars of their science. But at its heart is an almost philosophical aspect that deeply troubled Einstein. It is the uncertainty principle, which says, for example, that it is impossible to tell both the exact position and the momentum of a single atomic particle—an electron, say—because the very act of observing disturbs it. Only by statistical means (like those used to determine probability in dice or poker) can a scientist predict what the results of such an experiment will be.

Einstein, who had helped revolutionize 20th century physics, now was resisting the revolution's latest turn. To him, quantum mechanics was fundamentally incomplete. Nature, he was sure, operated by strict rules that scientists : could uncover. But because of the role of probability in quantum mechanics, Einstein felt that it failed to meet his crucial standard. The universe, he insisted, could not operate on chance. Causality had to exist. Again and again, he would say such things as "God does not play dice." Exasperated, the Danish physicist Niels Bohr, Einstein's friendly adversary, finally replied, "Stop telling God what to do."

Einstein, however, was determined to go his own way. Despite criticism he spent much of the second half of his life pursuing the development of what scientists call a unified field theory. In Einstein's time, this meant an all encompassing mathematical construct that would unite under a single set of equations not only gravity but also electromagnetism. Since then the task has become even more difficult, with the discovery of two other basic forces: the nuclear forces. Most