Did Galileo really drop two balls from the Tower of Pisa?

Probably not, at least according to most historians. The famous scene, Galileo dropping two weights of different mass from the Leaning Tower and watching them land together, disproving Aristotle's claim that heavier objects fall faster, comes from a biography by his student Vincenzo Viviani, composed in 1654, decades after it supposedly happened. Viviani wrote of repeated experiments in front of professors and students, but Galileo himself left no account of any such test. Most historians think it was a thought experiment later retold as a real event, although one historian, Stillman Drake, argued it happened more or less as Viviani described.

Galileo didn't even need the tower to make his case. In his 1638 book Two New Sciences, he laid out a purely logical argument: if a heavier object really did fall faster than a lighter one, tying the two together with a string should produce a contradiction, since the slower object would hold back the faster one, yet the combined, heavier system should fall faster than either object alone. The assumption collapses under its own logic. Others had already tested the idea physically: Simon Stevin and Jan Cornets de Groot, in a test described in Stevin's 1586 book, dropped two lead balls, one ten times heavier than the other, from 30 feet in Delft and saw them land together, years before Galileo supposedly did it at Pisa.

The idea got its most dramatic confirmation centuries later, far from any tower. In 1971, Apollo 15 astronaut David Scott dropped a hammer and a feather together on the airless Moon, where there was no atmosphere to slow the feather down, and the two objects hit the lunar surface at exactly the same moment. The underlying principle, now called the weak equivalence principle, turned out to be far more important than a single dramatic demonstration: Einstein built on it to help develop general relativity, and modern experiments have confirmed it to a precision better than one part in a trillion.