Talking terms: What’s a Theory?

By Matthew von Hippel

We call ourselves theoretical physicists. But what do we mean by “theory”?

Talk to a biologist, and they’ll tell you that theories are the closest to certainty that science can get. Only after an idea has been tested in many ways, incorporating insight from multiple experiments, will a biologist agree to call it a theory. That’s why they reserve the word for their most solid and important ideas, like the theory of evolution, or germ theory.

But different people use words in different ways. Theoretical physicists do care about experiments, and about which ideas have been tested. But we also care about “what if” questions, imaginary worlds that don’t reflect reality. And to talk about both types of ideas, we like to use the word “theory”.

For us, a theory is a very precise “what if”. A theory isn’t just a guess or a hunch, we agree with the biologists about that. Like them, we use “theory” as a technical term, with a specific definition. But while they tie their term to experiments, we tie it to precision. What we call a theory is, at its heart, a precisely imagined world. It states the kinds of things that exist: the different types of fundamental particles, and the ways in which those particles can interact. The particles in our theories aren’t always realistic, but they have to be mathematically consistent: an inconsistent idea isn’t a theory, it’s just a mess. By using the language of mathematics, we try to be as specific as possible, to avoid ambiguous words and say exactly what we mean.

The many reasons to visit a theory

When we talk about our work, we like to say we work “in” a theory, as if it were a country we decided to visit. Working “in” a theory means we do a calculation while pretending that the theory is true, asking what would happen if our theory described all of reality. That doesn’t mean we believe the theory is actually true: we’re just trying to figure out what the consequences would be if it were true.

Sometimes, we do this with a theory that even the biologists would call a theory, one that is a well-tested description of the real world: for example, the Standard Model of particle physics. Sometimes we use a well-tested description of part of the real world, ignoring the rest. We might do a calculation in gravity, or in electromagnetism, and ignore the other forces. A calculation like this can still match with experimental data, for example, an experiment with neutral particles might ignore electricity and magnetism, while an experiment in a particle collider might only have to consider the strongest of the forces, the strong nuclear force.

Sometimes, we do calculations in a theory because we don’t know yet whether the theory is true, and we want to test it. We might perform calculations in supersymmetry to find what consequences supersymmetric particles might have, or calculations in string theory to understand what it would say about black holes, if string theory were true.

Finally, sometimes we work in an unrealistic theory as a kind of practice. We pick a theory where the calculations are cleaner, and use it to try out new calculation tricks. Once we’ve practiced enough, we can apply those tricks to other theories and find out something about the real world. Those of us who study scattering amplitudes like working in a particular theory called N=4 super Yang-Mills. It’s one of our favorite practice grounds, a theory with many unusual particles that work together in surprisingly simple ways. You’re sure to hear more about it on this site soon!

About the author: Matt von Hippel is a postdoc at the Niels Bohr International Academy, researching scattering amplitudes. He blogs at where he tries to demystify physics, as well as the people who study it.

Photo by Louis Maniquet
January 26, 2021