What Physics Can Teach the Fields That Are Not Physics

People who move from the exact sciences into anything else tend to make one of two mistakes. The first is to assume that the subject matter transfers: that you can write down equations for a society the way you write them for a gas. It does not, and I have written about why. The second mistake is the opposite one: having noticed that the equations do not transfer, to conclude that nothing does, and to leave all the discipline behind at the door.

The second mistake is the worse one. The equations do not transfer. The habits do, and the habits are most of what made physics work in the first place.

The First Lesson Is Humility About Understanding

Start with the lesson that is least expected from a field famous for its confidence.

Physics is where humanity discovered, in the most precise setting available, that understanding has hard limits. The three-body problem has no general closed-form solution: three masses under gravity, the simplest imaginable system after two, and you cannot write down where they will be. Chaotic systems make prediction impossible in principle past a horizon, not because we lack data but because the error in the data grows faster than any instrument can shrink it. Quantum mechanics put a limit on what can be known simultaneously about a single particle.

These are not gaps that will be filled. They are proofs about the shape of knowledge, and they were obtained in the one field where everything else is nailed down.

Now transfer that. If three masses under one force law are beyond exact prediction, what is the standing of a confident prediction about an economy, a war, or an election? The honest transfer of physics into the social sciences is not more confidence. It is the recognition that if the easy case is this hard, the hard case is not going to be solved by a clever essay.

Define the Term or Do Not Use It

The second habit is operational definition.

In physics a word means one thing, and it means a procedure. Mass is what a balance measures. Temperature is what a thermometer reads. Force is what deflects a spring by a known amount. When two physicists disagree, they can never be disagreeing about what the words mean, because the words are anchored to instruments.

Almost every argument outside the exact sciences is, on inspection, an argument about what the words mean, conducted by people who believe they are arguing about the world. Freedom, justice, inequality, democracy, intelligence, culture. The habit physics offers is brutal and simple: before you use the word, say how you would measure it. If you cannot, you may still talk, but you should know that you are not yet saying anything that could be wrong.

Carry the Error Bars

The third habit is that no number travels alone.

A measurement in physics is a number and an uncertainty, and a result stated without the uncertainty is not a result. Students lose marks for it. The uncertainty is not an admission of weakness; it is the part that tells you whether the number can be used.

Outside the exact sciences numbers routinely travel naked. Unemployment is 5 percent. The policy will save a billion. Turnout was 62 percent. Each of these has an uncertainty, sometimes a large one, and the omission is not accidental: a naked number is more persuasive. The habit to import is to ask, every time, plus or minus what? And to treat a source that cannot answer as a source that has not measured anything.

State the Null Model

The fourth habit is the null hypothesis, and it is the one whose absence does the most damage.

Before a physicist claims an effect, she states what the data would look like if there were no effect, and shows that the data do not look like that. Without the null, every pattern is evidence for whatever you already believed, because human beings see patterns in everything, including noise.

In history, in economics, in politics, the null is almost never stated. A policy is followed by growth, so the policy caused the growth. A leader takes office and a war ends, so the leader ended it. The first question physics trains you to ask is: what would this have looked like anyway? Sometimes the answer is that it would have looked exactly the same, and then the story has no content.

Prefer the Thing You Can Measure

The last habit is a preference, not a rule, and it is the one that the great transitions in science have all followed.

Given a choice between a rich concept you cannot measure and a thin one you can, take the thin one. Chemistry became a science when it stopped talking about essences and started weighing things. Medicine became one when it stopped explaining and started counting outcomes. In every case the rich vocabulary felt like understanding and produced none, and the thin vocabulary felt like impoverishment and produced everything.

The social sciences are full of rich concepts, and the richness is treated as depth. The transfer from physics says: the richness is the problem. Find the thing you can count, count it, and let the rich concepts either earn their place by predicting something or go.

Why This Is Not Physics Envy

None of the above requires a single equation from physics to apply to people, and none of it assumes that societies are mechanical. It asks for a set of manners: know what you cannot know, define before you argue, never state a number without its uncertainty, say what the world would look like if you were wrong, and prefer the measurable.

Physics did not succeed because its subject was simple. It succeeded because it adopted these habits before anyone else did, and it adopted them because it was the first field in which the world pushed back hard enough to make them unavoidable. The other fields get pushed back more gently, which is exactly why they need the habits more and have them less.