Reality Is Not What It Seems: The Journey to Quantum Gravity

by Carlo Rovelli

For Aristotle, physics works as follows: first, it is necessary to distinguish between the heavens and Earth. In the heavens, everything is made up of a crystalline substance that moves in a circular motion and turns eternally around Earth in great concentric circles, with the spherical Earth at the center of everything. On Earth, on the other hand, it is necessary to distinguish between forced motion and natural motion. Forced motion is caused by a thrust and ends when the thrust ends. Natural motion is vertical—upward or downward—and depends both on the substance and the location. Each substance has a “natural place,” that is to say, a proper altitude to which it always returns; earth at the bottom, water a little way above it, air a little higher still, and fire even higher. When you pick up a stone and let it fall, the stone moves downward because it wants to return to its natural level. Air bubbles in water, fire in the air, or children’s flying balloons move upward instead, again seeking their natural place.

Aristotle's elemental theory