Robert McNees
Enrico Fermi, one of the foremost physicists of the 20th century, was born #OTD in 1901. While most physicists focus on either experiment or theory, Fermi excelled at both.
Now, estimate how many new physicists will be born today.
Image: New York Public Library
Robert McNees
Fermi is perhaps best known as the first person to initiate a controlled and self-sustaining fission reaction. Chicago Pile-1 went critical on December 2, 1942, under the stands of the University of Chicago’s Stagg Field.
Image: Melvin Miller/Argonne
Robert McNees
But Fermi was also famous for asking students and colleagues questions that seemed, at first, to require complex physics or information they did not have access to.
On a road trip he once asked: How thick will dust pile up on the windshield if we’re driving at 60 mph?
A precise answer wasn’t the point. His questions were exercises in approximation. You chain together a bunch of estimates to arrive at an answer that gives you an order-of-magnitude result. You’d be surprised what you know!
Schamschula
@mcnees I heard a story about his harsh grading of papers @ the University of Chicago. The university had to put notes in student transcripts to explain.
Robert McNees
Physicists refer to these sorts of questions as “Fermi Problems."
Perhaps the most famous one – it's often used as an example of the genre – is “how many piano tuners are there in Chicago?”
The answer is almost certainly different than it was back in the 40s or 50s, but you can imagine various ways of arriving at an estimate.
3x10⁶ 👤
x 1👨👩👧👧 / 5👤
x 1🎹 / 20👨👩👧👧
x 1 tuning / 300📅
x 1 tuner x 📅 / 2 tunings
~ 50 🎹 tuners in Chicago
Robert McNees
By which I mean: There are about 3 million families in Chicago, and maybe 4 people per family-sized unit, and one actual piano (not a keyboard) out of every 20 family-sized groups. Probably they need tuning one a year or so, and I’d guess a tuner can handle two tunings per day? That gets us to around 50 tuners in Chicago.
That may be off, but hopefully by no more than a factor of 10. It should be a good order-of-magnitude estimate.
Robert McNees
Physicists combine this sort of reasoning with basic physics to quickly get a feel for the contours of complex phenomena, without going through detailed calculations.
Fermi was famously good at this. He estimated the yield of the Trinity test from his vantage point 16 km away, by dropping little pieces of paper and watching how they moved as the pressure wave passed. He was running estimates as his nuclear pile in Chicago went critical.
Robert McNees
(More on this later, if I have time!)
Robert McNees
Fermi was only 21 when he completed his PhD at Pisa, and after a few years abroad working with Born and Ehrenfest he became a lecturer at Florence. It was there that he worked out the statistics of a collection of particles satisfying Pauli's exclusion principle.
Fundamental particles like leptons and quarks, and many composite particles like protons and neutrons, satisfy these "Fermi-Dirac statistics.” For this reason, we refer to this broad class of particles as Fermions.
Robert McNees
Besides deep contributions to fundamental physics, and his work establishing a controlled and self-sustaining fission reaction, Fermi also made a few observations of the late-night-conversation-in-the-dorm-room variety.
For example, he wondered why we haven't heard from any aliens. This lonely state of affairs is now known as the “Fermi Paradox.” If the universe is so big, so full of places where life might take hold, then where is everybody?
(Imo the answer is that we are in an alien zoo.)
The Geoff
@mcnees You're coming down on the side that there are probably civilisations capable of interstellar communication in our galactic-ish neighbourhood? Interesting. I'm still not convinced *because* of the lack of signal, but we're on a circular argument there. Fun experiment of our own capabilities would be to try bouncing a high-ish power signal off a nearby exoplanet and see if we can detect the "reply".
Robert McNees
@sirlaffalot “On this day"
Ivan
CapSlog
@mcnees In reference to Fermi the word “excelled” needs to be in all caps and 200 pt high.
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