Robert McNees Profile picture
Sep 29, 2018 15 tweets 4 min read
Enrico Fermi, one of the foremost experimental and theoretical physicists of the 20th century, was born #OTD in 1901. Now estimate how many other physicists were born on that day.
Image: New York Public Library
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, as well as composite particles like protons and neutrons, satisfy these Fermi-Dirac statistics (Dirac obtained the same results a few months after Fermi). For this reason, we refer to this broad class of particles as Fermions.
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
(A short thread about how it all went horribly, horribly wrong.)
Okay, not really.

Anyway, Fermi famously encouraged his students and colleagues to develop the sort of practical command of physics that would allow them to quickly and accurately estimate the outcomes of experiments or physical processes.
Fermi was the sort of colleague who, in a car, would challenge another physicist to estimate how thick a layer of dust might accumulate on the windshield as they cruised down the road. Physicists refer to these exercises in estimation & basic physics fluency as “Fermi problems.”
These days, we often focus on the estimation part as a useful way of helping students sharpen their quantitative reasoning and problem-solving skills. Fermi might ask “how many piano tuners in Chicago?” Could you estimate without looking anything up.
(3x10^6 👤/🏙) x (1👨‍👩‍👧‍👧/5👤) x (1🎹/20👨‍👩‍👧‍👧) x (1 tuning/ (🎹*300📅)) x (1 tuner*📅/2 tunings)
~ 50 piano tuners / Chicago

It could be 20 or 100 or 200, but we can often use common sense and basic facts to estimate to within an order of magnitude.
Please share your favorite Fermi problems and emoji-based solutions here! #ITeachPhysics
Finally, no twitter thread can do justice to the breadth of Fermi’s accomplishments. But if we wander outside of physics for a moment, he was known to ask “Where are all the aliens?” We call this the FERMI PARADOX, and you should follow @trekonomics to learn about its resolution.
One of my favorite examples of the sort of estimation and deductive physical reasoning that Fermi adored, from a thread on the utility of dimensional analysis:
PS: When I was little, my grandfather (who worked at Oak Ridge starting after the war) had a very big empty wine bottle (Chianti, maybe? But *really* big) that he said was from a party with Fermi. I was too young to remember much more than that. Alas, the bottle is long gone.
Well, maybe not its resolution, but at least a good accounting of the many different attempts to answer the question. My favorite is that we are basically a zoo.
"We're Cookin!" Enrico Fermi's lab notebook from the day Chicago Pile-1 went critical.

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Oct 8, 2018
Good afternoon.
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The physicist Niels Bohr was born #OTD in 1885. He applied Planck’s new ideas about quanta to develop the first modern model of atomic structure in 1913, which motivated the development of quantum mechanics over the next few decades.
Inage: Atomic Heritage Foundation
Here’s a thread from last year about the historical context for his model, and how it set physicists on the path to quantum mechanics.
A story about Bohr. In 1939, Rosenfeld told the Princeton Physics Journal Club about his work with Bohr on fission, after Meitner & Frisch's discovery but before its publication. Bohr quickly wrote a letter to Nature asserting priority of Meitner & Frisch. nature.com/articles/14333…
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Oct 5, 2018
Jason Van Dyke found guilty of 2nd degree murder and multiple counts of aggravated battery with a firearm.
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Oct 4, 2018
Koba & Takeda submitted their paper "Radiation Reaction in Collision Processes, III" to the journal Progress of Theoretical Physics #OTD in 1948. It described a method for calculating certain quantities in quantum electrodynamics via "transition diagrams." academic.oup.com/ptp/article/4/…
Their paper was submitted just two days before Freeman Dyson's ground-breaking article on Feynman diagrams was submitted to Physical Review. The approaches are very similar, as was clear to all three scientists!
Upon learning of their results, Dyson amended his own paper to recognize their work. He lamented the effect of the war on the dissemination of important ideas: "The isolation of these Japanese workers has undoubtedly constituted a serious loss to theoretical physics."
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Oct 4, 2018
He took out a full page ad in the newspaper demanding the death penalty for five black kids who turned out to be innocent, and still insisted on their guilt even after the real perpetrator confessed.
Here is the ad Trump placed in all four of New York’s major newspapers, months before the end of the trial, when I’d assume presumption of innocence would still apply.
Then in 2014 he *wrote an op-ed* in the Daily News saying they were probably still guilty, even though someone else confessed to the crime.
nydailynews.com/new-york/nyc-c…
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Oct 4, 2018
Sputnik 1, the first satellite made by humans, was launched into low Earth orbit #OTD in 1957.
Image: NSSDC, NASA
The launch of Sputnik marked the beginning of the Space Age. Here, let a young Jeff Goldblum show you what the mood was like in the Pentagon.
Before young squire Goldblum was warning us about the perils of toying with dinosaur DNA (@SUEtheTrex), he was racing down the hallways of the Pentagon with the latest bit of intel on the threat from above.
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