What Is Physics?

Wind, rainbows, the warmth of rubbed hands, the Sun’s light, the beating of a heart. Nature looks endlessly varied. Physics is the bet that underneath it all lie a few simple rules.

Looking for the rules#

Physics is the study of nature and its laws. Its aim is to find the basic rules from which the events around us follow. The payoff is economy: Newton’s law of gravitation together with his laws of motion explains why an apple falls, why the Moon circles the Earth and why the sea rises highest on full-moon nights. Because the rules are basic, they apply everywhere, so physics feeds into engineering, chemistry, biology and medicine.

Nature’s chess game#

Richard Feynman offered a lovely picture. Imagine watching chess games without being told the rules. Watch long enough and you start to guess them: bishops move diagonally, pawns move forward. Knowing the rules isn’t the same as understanding every game, but it is the minimum needed to make any sense of it.

Now suppose a player castles for the first time. Your guessed rules said the king moves one square, yet here it jumps two. You must revise your rules. Physics works exactly like this. Nature plays the game; we watch its moves (experiments and observations) and guess the rules (laws). When a new observation refuses to fit, the old law is declared incomplete or wrong and a better one is found.

Physics and mathematics#

Saying “the gravitational force between two masses is proportional to the product of the masses and inversely proportional to the square of their separation” is clumsy. Writing $F \propto m_1m_2/r^2$ is not. Mathematics states laws compactly, and its tools (algebra, trigonometry, calculus) let us work out consequences. From that one rule, calculus tells us the pull of a whole rod or a whole planet.

So mathematics is the language of physics. But a language is a means, not the message. Fluent grammar helps you express an idea; the idea is what matters. Keep this in mind throughout the course: an equation is a compact way of saying something about nature, and the goal is to understand what it says.

What the world is made of#

Ancient thinkers proposed that matter is built from tiny indivisible particles; the Indian philosopher Kanada (whose name comes from kana, a tiny particle) is one of them. Experiments eventually showed that everyday matter is made of atoms and molecules, and each atom is a small, dense nucleus of protons and neutrons with electrons arranged around it. The numbers of these three particles decide how a material behaves.

The laws we develop for ordinary-sized objects make up classical physics. They don’t always hold for single atoms, nuclei or elementary particles, which need quantum physics. In classical physics a “particle” just means an object small compared with everything else in the problem; a speck of dust containing billions of atoms still counts. Twentieth-century experiments revealed hundreds of elementary particles that can transform into one another: nature turns out to be deeply interconnected.