Gravity causes all matter to be attracted to other matter, from the subatomic to the cosmic levels. The earliest people could observe gravity at work, noticing objects falling to earth, but they did not begin to theorize systematically about the reasons behind such motion until the era of Classical Greece. The discovery of how gravity works progressed in several stages, beginning with Democritus and proceeding through the work of al-Hasan ibn al-Haytham, Galileo Galilei and Sir Isaac Newton.
Aristotle, Democritus and Atomism
In the fourth century B.C., Aristotle proposed a theory that dominated physics for over a millennium, but his ideas did not, strictly speaking, constitute a theory of gravity. Aristotle believed that bodies were drawn from one place to another because they fundamentally belonged there due to their inherent nature; air belonged in the heavens, for instance, while rocks belonged to the earth. Democritus, born more than 70 years before Aristotle, proposed a theory of atomism, matching more closely what modern physicists observe about gravitation. Atomism posited that matter is made up of essential particles, and Democritus theorized these particles – atoms – moved and collided due to a force that Panagiotis Papaspirou and Xenophon Moussas, writing in the “American Journal of Space Science,” call a precursor to the theory of gravity.
Ibn al-Haytham’s Observations of the Sky
Born in the 10th century in what is now Iraq, ibn al-Haytham formulated a theory of optics that influenced Newton, proposing that light includes colors. He also reconciled – if incorrectly – the conflicting work of Ptolemy and Aristotle, retaining Ptolemy’s heliocentrism but theorizing that the sun and other celestial bodies are material objects. For his work in astronomy, he was nicknamed Ptolemy the Second, according to Joseph A. Kechichian, in a biographical profile in Dubai’s ''Gulf News Weekend Review.'' Ibn al-Haytham also insisted on the scientific method, relying on observation and experimentation, and refuted astrology, both important scientific stances. One of his chief astronomical observations was that the sun and moon were solid, material objects, a theory that underlies later work on planetary mechanics.
If ibn al-Haytham declined to refute Ptolemy’s theories entirely, Galileo had no such qualms. He was born in 1564 in Pisa, Italy and became one of the most notorious and, eventually, influential thinkers of the Renaissance. Where Democritus and ibn al-Haytham’s observations underpinned the theory of gravity, Galileo’s work directly informed it. He defied the authority of both Aristotle and Ptolemy, becoming a pariah in the eyes of the Catholic Church and the scientific establishment alike. Most relevant to gravitation, he posited that gravity works on objects regardless of their mass; differences in the speed of a drop result from air resistance due to different shapes, not weight. Galileo is famously said to have dropped balls of the same shape but different weight from the Leaning Tower of Pisa, and although the story may be apocryphal, the resulting theory is at the heart of the theory of gravity.
Another apocryphal story underlies Newton’s work; famously, the great mathematician is said to have been inspired to study gravity when an apple dropped on his head. Born in 1642, Newton was only in his forties when he published his hugely influential book, “Philosophiae Naturalis Principia Mathematica,” often known simply as the “Principia.” Testing the theories of astronomer Johannes Kepler, a contemporary of Galileo’s, Newton set out the Three Laws of Motion, which deal with inertia and mechanics, as well as his theory of gravitation; that theory states that every object in the universe attracts every other object in proportion to its mass. This principle, although revised by Albert Einstein and later physicists, still informs scientific thought, mechanical engineering and astronomy today.