Two thousand years ago, the force of gravity did not exist. No, really. If you asked a learned person why apples fall, he wouldn’t say it was because of gravity. That is not to say that the world worked like Roadrunner cartoon where things sometimes forget to fall. Water still ran downhill and stones sank to the bottom of the pond. But not because gravity. Apples fell and wood floated not because of gravity but because of their intrinsic nature, more specifically the nature of earth, water, air and fire.
Long before they provided inspiration for various cartoons, fantasy series and even a rock band, earth, water, air and fire were believed to be the fundamental elements of which everything was composed in different combinations. An important person who accepted this was the natural philosopher Aristotle, whose efforts to systematically analyze the natural world played an important role in the eventual development of modern science.
Aristotle attributed the heaviness or lightness of objects to their natural tendency to move toward or away from the center of the cosmos. Specifically, the element of earth had the strongest tendency to move towards the center, thus forming the Earth, and fire the strongest tendency to move away. Things would intrinsically move down or up depending on the elements they contained relative to to the surrounding substances; rocks would sink in water, water fall through the air, wood (combination of earth and air) would float in water but not in air, and sparks would fly up. Greater amounts of heavier elements gave stronger downward motion, and the deciding factor was the relative concentrations.
Aristotle’s ideas had merit; his simple framework conceptually explained a wide range of motions. The modern scientific explanations are more complicated, invoking gravity, buoyant forces, convection currents and more. Aristotle's framework suggested that heavier objects fall faster and a given object falls at a constant rate; educational research has found these types of ideas are actually fairly common among people learning physics. To explain how a rock could being thrown upwards, Aristotle included a second category of motion: “violent” (non-natural) motion which is due to an agent exerting a force on an object to move it contrary to natural motion. As Aristotle only recognized contact forces, this did not include gravity.
Unlike modern scientists, Aristotle did no experiments or careful measurements. Thus, there were problems when his conceptual framework was subjected to rigorous testing. At first, people sought to modify his claims to match experimental results. Data forced them to give up claims that speed of fall is proportion to the mass, but clung to idea that heavier objects are intrinsically faster if only by a little. To explain how tossed objects continued to move up after leaving the hand it was suggested that one could somehow impress a force on an object that would continue to act on it until it was exhausted or used up disappearing as it reached the maximum height where natural motion would take over. And it was observed the stone picked up speed as it fell, unlike a stone sinking in water. Eventually, however, Isaac Newton--building on the work of Galileo and others, proposed a different, quantitative framework that provided precise numerical predictions that a system based on Aristotle’s ideas could not. And in Newton’s system there was an intrinsic force of attraction between any two objects with mass--the force of gravity.
So, while apples have been falling for a long time, the idea that it is due to gravity is much more recent. Gravity is an explanatory construct--a theoretical model--that explains the observation that unsupported objects fall. Gravity is just a theory--one that has been tested so many times with high precision that we can be extremely sure of it. The high degree of agreement suggests that we can be reasonably confident that it reflects the intrinsic nature of reality we live in, but it is and always will be an explanation, not an observation; a theory, not a scientific law.
It is also humbling to remember that Aristotle’s explanation was dominate for far longer--around two thousand years--than Newton’s. Explanatory frameworks like that laid out in Aristotle’s Physics provide us with comprehensive, generally self-consistent frameworks through which we interpret the world around us. Once someone has adopted one and interprets the world through it, it is generally hard to make the switch to another, or even conceive that there are other, possibly better conceptual frameworks that could be used to make sense of the world around us. Thus, there was great resistance as evidence accumulated by Galileo and others seemed to undermine Aristotle’s framework, and vestiges of the system of four elements persisted for several more centuries; for example the practice of bloodletting in the eighteenth century came out of medical ideas of restoring balance among the four humors, corresponding to the four elements. Conceptual frameworks are powerful forces in shaping our thinking, and may have their roots in centuries past. And for many centuries, gravity was not part of it.
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