Fermi questions are named after the nuclear physicist Enrico Fermi, who won the Nobel prize in 1938 and who worked at Los Alamos on the first nuclear bomb. He was famous for doing physics out loud – devising problems and, through a process of reasoning and guessing, making very plausible estimates. Philip Morrison, one of his colleagues, describes an example:
We were walking through the wooden barracks-like structure of the Theoretical Physics Building at Los Alamos and, as we walked, the sounds of our footsteps reflected off the high surface and seemed to bounce throughout. And he said, ‘How far do you think our footsteps can be heard in this building?’ And then he began to tell me what the yield of sound would be from the impulse, how far that would go, how you have to worry about the wood conduction and the air passage. And pretty soon, by the end of the hall, he had an answer. It was a fast calculation. Sounded very reasonable. And when I tried to recalculate it, I got something like the same result – slowly and looking at the numbers over and over again.
He added: ‘this was my idea of a Fermi question – turn every experience into a question. Can you analyse it? If not, you’ll learn something. If you can, you’ll learn something.’
When he taught at the University of Chicago, Fermi set his students questions that seemed outrageous and seemingly impossible, before demonstrating that they already had (or had easy access to) the necessary knowledge and tools to answer them. Such questions came to be called Fermi questions.
Fermi questions have become popular with some science teachers and maths teachers as a way to stimulate critical thinking and estimation skills – to develop a feel for whether an answer is reasonable or not. And, as an additional benefit, students develop logical tools and experience the habit of mind to seek out interesting questions in their everyday experience. This is demonstrated if we think through some steps of a classic Fermi problem in Maths – one that Enrico Fermi asked his own students: ‘How many piano tuners are in New York City?’