Gravity is weird. On the one hand, gravity is a good thing, because it keeps me from floating away into space and makes possible my building stable shelters from the storm. On the other hand, gravity is a major cause of natural evil, as when buildings collapse during earthquakes or people fall through cracks in the ice and drown. The quirky American businessman, Roger Babson, was so alarmed by the evil consequences of gravity that in 1948 he founded the Gravity Research Foundation in New Boston, New Hampshire with the explicit aim of understanding gravity so as to defeat it. So how does gravity work? What keeps my feet planted firmly on the ground but then sends me tumbling painfully to the earth when I trip over a rock?

Aristotle on Free Fall

In the fourth century, BCE, Aristotle thought that he had the answer. He taught that every element had a natural place in the universe and a corresponding natural tendency to seek that place. The natural place of the element earth was a sphere at the center of the universe, and earth, by its very nature, tended always to seek that natural place unless something interfered with its natural motion. Since heavy bodies, on his analysis, consisted mainly of earth, albeit mixed with some water, air, and fire, they fall simply because that’s what earth naturally does. For its time, this was not a bad theory. It explained free fall and other noteworthy “facts,” such as why, as Aristotle and his contemporaries thought, the planet, Earth, happened to be located in the centre of the universe.

But there are problems with Aristotle’s explanation, foremost among them being that it implied that the speed with which a heavy object falls was directly proportional to the body’s weight and inversely proportional to the resistance of the medium through which it falls. Thus, heavier bodies should fall faster than lighter bodies. It took almost two thousand years for that mistaken “law” of free fall to be corrected.

Medieval and Renaissance Theories

Aristotle’s philosophy provided the conceptual framework for natural philosophy for roughly the next two thousand years, up to the end of the Renaissance. But Aristotle’s theory of free fall was not without its critics. For example, the twelfth-century Islamic philosopher, Ibn Rushd (or Averroes, as he was known in the Latin West), argued that the medium - the atmosphere though which the body falls - was in motion and, consequently, drew the body down with it. Around the same time, St. Bonaventure suggested that the fall of heavy bodies was the result of a repulsion from the celestial sphere. And his contemporary, Roger Bacon, thought that a virtus caelestis spread around the center of the universe throughout the terrestrial realm and diminished in intensity with distance from the center, this virtus thrusting heavy bodies downward. This was an early example of what, today, we might term a field-theoretic explanation of free fall.

Perhaps the most important alternative to Aristotle’s explanation was the one proposed by the late-twelfth and early-thirteenth century English philosopher, Duns Scotus. He held that free fall was not a passive, natural tendency, but a consequence of an active principle - gravitas - inherent in the falling body. Gravitas, as Scotus understood it, was not a force in the modern sense of the word, but at least Scotus thought that a property of the falling body itself played a crucial role in explaining its own free fall.

Galileo