Understanding gravity

What does it mean to understand something?

Richard Feynmann once pointed out that if a child asks “Why do things fall down?” the answer is likely to be “Because of gravity”, but this isn’t actually an explanation. ‘Gravity’ is just a word, after all.

What would it mean to understand gravity?

In one sense, to understand something means to be familiar with it; to know how it behaves in various situations, to know all its little tricks and quirks.

People who do a lot of manual lifting and carrying probably understand mass and gravity very well in this way. They can guess the weight of something pretty accurately just by looking at it, they know that a long thin thing is easier to carry than a big round one, and that rigid objects are easier to move around than floppy ones. They know that the trick to carrying a heavy object on your head or shoulder or back is to balance the object so that its center of mass is directly above your own (although they may not use exactly those terms to think about it).

Of course when we talk about understanding gravity we usually mean the kind of understanding that comes, not from direct experience, but from having a theory, or a story in your head, that tells you how things work.

The Greek philosopher Aristotle came up with a theory to explain why objects move the way they do. According to Aristotle, all objects on earth were composed of four elements: fire, air, earth and water, and each element had its own characteristic form of motion; earth and water naturally moved straight down, toward the center of the earth, while fire and air naturally moved straight up. Of course it was possible to push or pull an object in a direction counter to its natural motion, but this required the application of an external force.

(As a side note, Aristotle also believed in a fifth element, aether, which made up the heavens; the sun, moon, planets, and stars. The heavens, unlike the earth, were perfect, unchanging and eternal, and the natural motion of aether was circular.)

We now know, of course, that Aristotle’s theory of motion was completely wrong, but it was nevertheless a good theory. It was simple and internally consistent, and it matched up with everyday experience: the way flames seem to dance upward and smoke rises from a fire, but a thrown ball always falls back down to the ground; the way a log thrown onto water will float but a stone will fall straight down to the bottom. Aristotle’s ideas provided a useful way of making sense of the physical world.

As paradoxical as it may seem, knowledge can be useful and valuable even if it is entirely incorrect.