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Some discrepancies were found when experiments disagreed with theory. Perhaps the most unsettling of these was the failure to discover the ether. Classical physics seemed to require that the universe be filled with an invisible universal medium, the ether, to carry light waves the way air transmits sound waves. In July 1887, an ingenious experiment designed to detect this hypothetical fluid was performed in a basement laboratory in Cleveland, Ohio. When it 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 R"ntgen 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, R"ntgen called them X rays. By December, he had used them to take a picture of his wife's hand, 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 investors 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 of a new era in physics. As the element polonium, identified by Polish-born Marie Curie 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 20th century. The answers would affect our lives in ways no one could imagine in the year 1900.
Editor's Note: As the APS begins its year-long countdown to the Centennial Celebration in Atlanta, APS News will feature brief monthly summaries of the last 100 years in physics and its times beginning in this issue with the dawn of modern physics at the turn of the century. Look for it here.
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