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"How about that! Mr. Galileo was correct in his findings." That conclusion wasn’t based on the most careful experiment you’ll ever see, but it was one of the most spectacular in its way—because it was performed on the moon.
In 1971, Apollo 15 astronaut David Scott dropped a feather and a hammer from the same height and found that they hit the lunar surface at the same time. The acceleration due to gravity doesn’t depend on a body’s mass or composition, just as Galileo asserted from his (probably apocryphal) experiment on the Leaning Tower of Pisa.
Or does it? Jump forward to the front-page headline of The New York Times in January 1986: “Hints of 5th Force in the Universe Challenge Galileo’s Findings.” The newspaper was reporting on a paper in the premier physics journal Physical Review Letters by physicist Ephraim Fischbach and his colleagues, describing evidence that the acceleration due to gravity does vary depending on the chemical composition of the object in question. Gravity, it seemed, was not quite what we thought it was: its effects are modified by what the The New York Times’ reporter John Noble Wilford christened a “fifth force,” adding to the four fundamental forces we already know.
More than 30 years later, many experiments have sought to verify this putative fifth force. Yet despite their extraordinary accuracy, none has ever found convincing evidence for it. That search shows no sign of abating, however. Even in the past year a new tantalizing hint that such a force exists has emerged from experiments in nuclear physics, provoking fresh speculation and excitement.
What hangs in the balance are some of the foundational principles of modern physics. Some physicists believe that a fifth force is permitted, even demanded, by efforts to extend and unify the current fundamental theories. Others hope such a force might shed light on the mysterious dark matter that seems to outweigh all the ordinary matter in the universe. If it exists, says physicist Jonathan Feng of the University of California, Irvine, “it would imply that our attempts to unify the known forces have been premature, as now there will be a fifth one to unify, too.”
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