Galileo taught mathematics at the University of Pisa from 1589 to 1592, and sometime during this period he mounted a dramatic public demonstration of one of his more unorthodox notions. Clutching two lead spheres of different sizes and masses, he climbed the stairs of the campanile, the bell tower in the Piazza del Duomo, behind the cathedral. The young professor then proceeded—before an assembly of expectant onlookers, many of them faculty and students from the university—to drop the test objects simultaneously from the upper balcony. The plummeting orbs reached the ground together; with no temporal interval between their terrestrial impacts, a single resounding thump announced their coincident landing. Aristotelian physics, for ages the dominant paradigm, held that the velocities of free-falling bodies moving through the same medium vary in direct proportion to their weights. Galileo’s so-called Leaning Tower of Pisa Experiment conclusively disproved Aristotle’s doctrine of natural downward motion: heavier objects do not fall to earth faster than lighter objects, after all. In a veritable instant, the old certainties, all those dusty apriorisms of ancient and medieval inheritance, were upended. Science and knowledge had at last entered the modern era.
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