The Delft Tower Experiment

In 1654, twelve years after the death of the brilliant Italian astronomer and scientist Galileo Galilei, Prince Leopold de' Medici, brother of the Tuscan grand duke and a key patron of Tuscan science, requested that Galileo's student, Vincenzo Viviani, write a biography of his late mentor. Viviani, who had assisted Galileo in his later years, responded to Prince Leopold with an account of Galileo's achievements, which he claimed to document "with historical integrity and complete honesty." This letter remained unpublished for over sixty years before finally being printed in 1717 as the book Racconto istorico della vita di Galileo Galilei (Historical Account of the Life of Galileo Galilei).

A mid-19th century sketch of the Leaning Tower of Pisa by an unknown artist.

In this book, Viviani was the first to recount Galileo's discovery of the pendulum's principle, after observing a lamp swinging in the Pisa Cathedral, and his experimental disproval of Aristotle’s theory of gravity by dropping weights from the top of the Leaning Tower of Pisa. Due to Viviani’s own fame as a scientist and devoted disciple of Galileo, his account became the primary source for Galileo’s biographers for many years.

However, Viviani’s biography contains several inaccuracies, most notably the story of Galileo’s famous experiment at the Leaning Tower of Pisa. Many 19th-century historians doubt whether this experiment actually took place, as Galileo himself left no written record of such a demonstration. Instead, what Galileo did leave behind were several unfinished manuscripts where he refuted Aristotle's theory of falling bodies through a thought experiment.

In these writings, Galileo presented his arguments in the form of a dialogue between three characters—Simplicio, Sagredo, and Salviati—who engaged in discussions about the scientific questions he was trying to resolve. Simplicio, whose name reflected his unyielding and simplistic adherence to Aristotelian views, served as a stand-in for Galileo’s critics. Salviati, who championed the Copernican model, represented Galileo’s own perspective. Sagredo, an open-minded nobleman, acted as the neutral party, ultimately being persuaded by Salviati’s arguments.

Galileo illustrated this thought experiment through a logical dialogue between these three characters:

Salviati. If then we take two bodies whose natural speeds are different, it is clear that on uniting the two, the more rapid one will be partly retarded by the slower, and the slower will be somewhat hastened by the swifter. Do you not agree with me in this opinion?

Simplicio. You are unquestionably right.

Salviati. But if this is true, and if a large stone moves with a speed of, say, eight while a smaller moves with a speed of four, then when they are united, the system will move with a speed less than eight; but the two stones when tied together make a stone larger than that which before moved with a speed of eight. Hence the heavier body moves with less speed than the lighter; an effect which is contrary to your supposition. Thus you see how, from your assumption that the heavier body moves more rapidly than the lighter one, I infer that the heavier body moves more slowly.

Another way to visualize this thought experiment is to assume two objects of unequal weight connected by a piece of string. If the heavier object were to fall faster than the lighter one, the string would become taut as the lighter object slows the fall of the heavier object. However, the combined system is heavier than either object alone, and by Aristotle’s logic, it should fall faster. This paradox reveals the inconsistency of the assumption that heavier bodies fall more quickly, leading to the conclusion that the assumption is false.

Galileo’s alleged Leaning Tower of Pisa experiment.

Aristotle claimed that not only does the heavier body fall faster than the lighter one, but the ratio of their speeds is proportional to the ratio of their weights. Galileo challenged this idea by pointing out that if Aristotle’s theory were correct, a lead ball that is one hundred times heavier than a lighter ball would take a hundred times less time to fall from a great height. Yet, as Galileo reasoned, no such extreme difference in falling times is observed in practice.

Galileo’s intellectual brilliance allowed him to prove his point through logic rather than physical demonstration. He didn’t need to haul weights to the top of a tower to disprove Aristotle’s theory—his thought experiments were more than sufficient. However, such an experiment did take place, though not in Pisa as the famous myth suggests. In 1586, a real-world test of falling bodies was conducted in Delft, the Netherlands, by the Flemish engineer and mathematician Simon Stevin, along with the Dutch nobleman and scholar Jan Cornets de Groot.

Stevin was employed as a military adviser for the court of William the Silent, and as such resided in the city of Delft while William's government occupied the city. One of Stevin's main benefactors was Maurice, Prince of Orange, whose patronage allowed Stevin to further his scientific interests. Although Stevin’s primary role at court involved designing defensive fortifications, he was also deeply interested in fluid dynamics, which led him to develop improvements for Delft’s windmills. To gain the necessary permissions to experiment with the city's mills, Stevin enlisted the help of Jan Cornets de Groot, a local lawyer who would later become the father of the famous legal scholar Hugo Grotius (Hugo de Groot). The elder De Groot and Stevin developed a close friendship, and De Groot eventually invested in several new mills built according to Stevin's designs.

In 1586, Stevin and De Groot collaborated on an experiment that directly challenged Aristotle's theory that the speed of a falling object is proportional to its mass. To conduct their experiment, the two carried two lead balls of different weights up the bell tower of the Nieuwe Kerk in Delft. From a height of 30 feet, they dropped the balls onto a wooden platform below. Contrary to Aristotle’s predictions, both balls hit the platform at nearly the same time, demonstrating that objects of different masses fall at the same speed when other variables such as shape and air resistance are controlled.

Nieuwe Kerk in Delft, where the experiment took place. Photo credit: Sven Wildschut

Although Stevin and Galileo had successfully disproved Aristotle's theory of falling objects, their understanding of gravity itself remained limited. Like Aristotle, neither Stevin nor Galileo considered the idea that the motion of stars and planets in the universe are governed by the same forces that made leaden spheres fall back to Earth. It would take another century before Isaac Newton would publish his revolutionary theories of universal gravitation. Newton's theory unified the motions of celestial bodies and objects on Earth under a single force—gravity—finally explaining that the same force pulling an apple to the ground also governs the orbits of planets around the Sun.

Incidentally, the popular story that Newton was inspired by an apple falling on his head is also a myth, much like Galileo’s supposed experiment at the Leaning Tower of Pisa.

A version of Galileo’s thought experiment would be conducted nearly three centuries later during NASA's Apollo 15 mission in 1971. Astronaut David Scott, standing on the Moon, dropped a feather and a hammer simultaneously. Because the Moon has almost no atmosphere, there was no air resistance to slow the feather, and both objects fell at the same rate, hitting the lunar surface at the exact same time. This dramatic demonstration of Galileo's principle was captured on video and can be seen below.