Galileo, Pisa and Those Two Famous Balls

Justus Susterman's  Portrait of Galileo Galilei, 1636

Justus Susterman’s Portrait of Galileo Galilei, 1636

Galileo Galilei famously faced the Inquisition for steadfastly asserting the Earth revolved around the Sun, rather than the Sun around the Earth. But before that, he is credited with the equally famous experiment at Pisa, where he dropped two heavy metal balls, of different sizes, from Pisa’s Leaning Tower. Aristotle had written the heavier metal ball would fall faster; Galileo proved him wrong. Within the limits of detection they fell at the same rate, striking the ground at the same time. Aristotle was proven wrong.

It was revolutionary. Philosophy, which included what we now call physics, had been governed – trapped, really – by Aristotlean thinking, officially sanctioned by the Roman Catholic Church. If Aristotle could be wrong about gravity – and he demonstrably was – what else had he gotten wrong. The problem with infallibility is that when it fails, the whole system comes down with it. It was the disproof of Aristotle’s theory of gravity that made possible the other logical jumps Galileo made, leading to heliocentrism, Galileo’s Dialogue of the Great World Systems and Letters on the Solar Spots and his censure and house arrest.

That’s what they taught WC in high school.

Except it probably didn’t happen.

Galileo, in his copious writings, never once mentions the event. Galileo’s biographer and pupil Vincenzo Viviani, described it in 1654 and published it in 1717. Except for Vivani’s description in his biography of Galileo, there is no evidence at all this classic of science history actually happened.

What indisputably did happen is that Galileo, in an earlier book, On Motion, described a thought experiment: Imagine two objects, one light and one heavier than the other one, that are connected to each other by a string. Drop this system of objects from the top of a tower. If we assume heavier objects do indeed fall faster than lighter ones (and conversely, lighter objects fall slower), the string will soon pull taut as the lighter object retards the fall of the heavier object. But the system considered as a whole is heavier than the heavy object alone, and therefore should fall faster. This contradiction leads one to conclude the assumption is false.

A simple, elegant thought experiment. It was that thought experiment, not an exercise for the general public, that opened his mind to allow him to understand what he was seeing when he observed the Galilean moons circling Jupiter.

WC mentions all this because another famous natural philosopher, Albert Einstein, had his General Theory of Relativity confirmed (again) by observations of a triple star some 4,200 light years away. Two stars, a white dwarf and a rapidly spinning pulsar, of wildly different masses, are falling towards another star. At exactly the same speed. Cool.


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