In the mid-1890s, physicists — scientists who study matter and energy — looked ahead to the 20th century with justifiable pride. The more they had studied the universe in the 19th century, the more orderly they had found it to be. Its behavior was thoroughly predictable through natural laws that they expressed in the precise language of mathematics. Though there were still a few important questions to be answered, most physicists were confident that the 20th century would be devoted to refining theories and making the critical measurements needed to complete the tapestry of their science.

They could not have been more wrong. Instead of tying up a few loose ends, physicists pulled on a few snags and watched the entire theoretical fabric of physics unravel. It would take most of the new century to reweave it. The process would redefine almost everything people thought they understood about matter and energy, space and time, and waves and particles.
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