Fermi Teaches Independent Thinking | Generated by AI
The Story of Enrico Fermi and Tsung-Dao Lee: A Lesson in Independent Thinking with a Custom Slide Rule
Tsung-Dao (T.D.) Lee, the Chinese-American physicist who later shared the 1957 Nobel Prize in Physics for his work on parity violation, was a graduate student at the University of Chicago in the late 1940s. There, he had the rare privilege of studying under Enrico Fermi, the Italian-born Nobel laureate (1938) renowned for his work on nuclear reactions and as a master of both theory and experiment. Fermi was known for his rigorous, hands-on teaching style, which emphasized self-reliance and deep understanding over rote memorization.
One day, as recounted in Lee’s recollections and various biographical accounts, the young Lee—eager and ambitious—approached Fermi with a question: “How do you calculate the core temperature of the Sun?” This was a classic “Fermi problem,” the kind of order-of-magnitude estimation that Fermi loved, involving astrophysics, nuclear fusion, and rough calculations to arrive at a ballpark figure (the Sun’s core is about 15 million Kelvin, where hydrogen fuses into helium to power the star).
Fermi didn’t give a direct answer. Instead, he paused and replied something to the effect of: “You see, there are things that I would like to know.” This humble deflection was deliberate—it wasn’t evasion but a profound teaching moment. Fermi, at the peak of his career and already a legend for estimating everything from the yield of the first atomic bomb (using scraps of paper during the Trinity test) to the number of piano tuners in Chicago, wanted Lee to grapple with the problem himself. He believed true learning came from wrestling with unknowns, not spoon-fed solutions.
To reinforce the lesson, Fermi didn’t stop at words. He spent two full days crafting a large, custom slide rule—a mechanical analog calculator popular before electronic computers—specifically tailored for the calculations needed to estimate the Sun’s luminosity (total energy output) and core temperature. Slide rules allowed quick multiplications, divisions, logarithms, and trigonometric functions by sliding scales against each other, making complex astrophysical estimates feasible by hand.
The result was a “giant” slide rule (likely oversized for precision and demonstration), which Fermi helped Lee build and use. With it, Lee could input key parameters—like the Sun’s radius, surface temperature (about 5,500 K, observable from Earth), and assumptions about fusion efficiency—and slide through the steps to derive the core conditions. This tool turned an abstract problem into a tangible exercise, teaching Lee not just the math but the art of approximation and verification.
Lee later reflected on this as a pivotal experience. It built his confidence (Fermi even had him “lecture” back to him on topics) and exemplified Fermi’s philosophy: Physicists must question, estimate, and verify independently. Tragically, Fermi died in 1954 from stomach cancer (possibly linked to radiation exposure), before seeing Lee’s Nobel triumph in 1957.
This anecdote highlights why Fermi is still celebrated for “Fermi problems”—challenges that sharpen critical thinking. Lee’s PhD thesis under Fermi was on the hydrogen content of white dwarf stars, blending nuclear physics and astrophysics, and he credited Fermi’s mentorship for his lifelong approach to science.
For more on Lee’s life and work: Tsung-Dao Lee Obituary, Nature (2024)
For Fermi’s teaching legacy: Enrico Fermi: His Life and Work (excerpts)