Sean Carroll

The book you hold in your hands, Space, Time and Motion, focuses on the framework of classical physics pioneered by Isaac Newton, which held sway until the quantum revolution of the twentieth century.

Aristotle separated the way things move into “natural” and “unnatural” (or “violent”) motions. He thought of the world as fundamentally teleological—oriented toward a future goal. Objects have natural places to be or conditions to be in, and they tend to move to those places. A rock will fall to the ground and sit there; fire will rise to the heavens.

Here on Earth, in Aristotle’s view, if everything were in its natural state, things would be motionless. It requires some external influence to get things moving, and even then the motion will only be temporary. You can pick up a rock and throw it; that’s an unnatural or violent motion. But eventually the rock will come back down, maybe bounce around a bit, and return to its natural state at rest on the ground.

this new picture, the reason why rocks and coffee cups come to rest is not because that’s their natural state; it’s because forces—friction, air resistance—gradually degrade the impetus from the body. In the vacuum of empty space, Ibn Sīnā suggested, there would be no air resistance, and a moving body would keep moving at a constant velocity in perpetuity.