Foundations

The word physics comes from the Greek physis — nature. That's the subject in one word: nature, and the rules it runs on.

The work is finding those rules. You watch something happen — a ball drops, a planet circles, a room warms up — and you try to describe it precisely enough to say what happens next.

Look right. Dozens of particles, each one moving and bumping into the others. Real nature is busier than that: every object is a crowd of atoms, nudged by light and heat and gravity at the same time. Try to follow all of it at once and you'd never get anywhere.

So you don't. You pick out what matters for the question you're asking and set the rest aside. To describe these particles you might need only a position and a speed for each — and that pared-down description is a model.

It's an old move. Four hundred years ago Galileo wrote that nature "is written in the language of mathematics," and that without it we wander "about in a dark labyrinth" (Il Saggiatore, 1623). A model is how we read a line or two of it.

A model always leaves things out, and that's what makes it useful. A street map drops nearly everything about the streets, which is exactly why you can fold it into your pocket and still find your way — the map is not the territory, and it isn't trying to be. A good model works the same way: simple enough to compute, close enough to trust.

Let's build one.