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Discover and read the best of Twitter Threads about #OTD

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The mathematician and astronomer August Ferdinand Möbius was born #OTD in 1790. He is best known for devising a surface that not even the @nytimes could find a way to both-sides.
This is by far the best joke about lazy journalism and 19th century geometry.
As best I can tell, this is the source of the gif-ified “Möbius strip with ants” by Escher.…
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Good morning from a UK with slightly less dull weather! I want to discuss two topics which are often confused & misunderstood, but first I wanted to clarify a few things from yesterday's threads #earthquake #Stress #faults 1/n
Referring back to the block diagram for dip-slip faults from yesterday, @CriticalStress_ noted that the stress arrows at the top were wrong. All faults have a confining (inward) compressional stress, but stresses from other directions can be larger 2/n
This diagram from @SEELeeds shows that in the example block diagram I gave the smallest stress (sigma-3, min) is inward perpendicular to the fault. The primary stress - the largest amount of force acting on the rock to produce a normal fault - is vertical: GRAVITY! 3/n
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David Hilbert delivered his "Grundgleichungen der Physik" lecture at Göttingen #OTD in 1915. He was in a race with Einstein to develop a relativistic theory of gravitation.
Hilbert invited Einstein to attend. But Einstein, who wanted to stay home and work on his own theory, pretended to be sick and sent a postcard declining the invitation.
(From "David Hilbert and the Axiomatization of Physics" by L. Corry)
Hilbert and Einstein are in a race for priority on one of the great intellectual accomplishments of the 20th century. Hilbert invites Einstein to come hear about his recent progress. And Einstein says "Nah. I mean, no thanks, I've uh... I've got a cold." Classic scientist move.
Read 3 tweets
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For many years, it was not clear whether Einstein or Hilbert first obtained the field equations of general relativity. Hilbert's paper appeared in 1916, after Einstein's paper. But it had been submitted on November 20, 1915 – five days before Einstein's paper was submitted.
A common narrative among physicists was that Hilbert and Einstein obtained more or less the same result in parallel, with Hilbert submitting first but Einstein being responsible for the development of general relativity as a whole.
But it turns out the submission dates don't tell the whole story!
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Einstein proposed three "classical" tests of general relativity: the precession of Mercury's perihelion, deflection of light by the sun, and gravitational redshift. Irwin Shapiro proposed a fourth test #OTD in 1964: the gravitational time delay of light.…
It's a simple idea: The round-trip time of a radio signal bounced off a distant object increases a little if it passes through the gravitational field of a massive object along the way.
Shapiro estimated that signals bounced off Venus or Mercury at superior conjunction, when they are on the opposite side of the sun from Earth, would experience a delay of about 200 microseconds due to the Sun's gravitational field.
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Abraham Flexner, who reformed medical and science education in the U.S., founded and served as the first director of the Institute for Advanced Study, and helped many scientists — including Einstein — leave Nazi Germany, was born #OTD in 1866.…
Parts of Abraham Flexner's 1939 essay "The Usefulness Of Useless Knowledge" feel very timely, as disingenuous attacks on science gain currency as a political tactic.…
The language and assumptions in Flexner’s essay are dated in parts — it was written in 1939 — but its arguments for curiosity-driven research and defense of science resonate right now.
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John William Strutt, Lord Rayleigh, was born #OTD in 1842. He discovered argon, worked out the wavelength-dependence of elastic scattering of light by small particles that explains blue skies and red sunsets, and described the spectral radiance of a blackbody at long wavelengths. Lord Rayleigh at work in his laboratory. Image: Wellcome Collection
It’s great that I am just seeing this three year old tweet today, since Lord Rayleigh discovered argon.
Sorry, I was going to tweet more about Rayleigh scattering and the Rayleigh-Jeans law, and sort of got derailed by other things. Next year.
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The physicist Theodor Kaluza, who proposed a compact fifth dimension of spacetime in order to unify Einstein's general relativity and Maxwell's electrodynamics, was born #OTD in 1885.
With great encouragement from Einstein, Kaluza published "Zum Unitätsproblem der Physik" in 1921.…
But despite the attention and support of Einstein, Kaluza's work (refined by physicist Oskar Klein) received comparatively little attention until the advent of string theory many years later.
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Carl Sagan was born #OTD in 1934. He was a planetary scientist, a dedicated and gifted science communicator, and a tireless advocate for the humanizing power of science.
Image: NASA
Carl Sagan was prescient about many things, not just our impact on the climate but also how ignorance might be weaponized. Here, in an excerpt from “The Demon Haunted World,” is what he feared we might become.
Sagan passed away in 1996. Towards the end he emphasized two messages which are, unfortunately, still relevant today. I’m glad that we seem to have made at least a little progress on part of his first point with this last election.
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William Wirt, born #OTD 11/8/1772, was the longest-serving Attorney General; argued some of the most important early Supreme Court cases; prosecuted Aaron Burr; ran for President as a 3rd party; and had his skull stolen from his grave in the 1970s...
Wirt was born in Bladensburg, Maryland— if that city sounds familiar, it might be because it's the location of a famous cross-shaped memorial that is the subject of a constitutional battle at the Supreme Court this Term...
After his wife died, he relocated to Virginia, where he became an integral member of the legal elite. Once Jefferson's personal lawyer, in 1807, President Jefferson tapped him to prosecute Aaron Burr for treason (no, not for killing Hamilton!)
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Wilhelm Röntgen discovered X-rays #OTD in 1895 when he noticed that emanations from a Crookes tube caused a platino-barium screen to fluoresce. The rays exposed photographic plates but were blocked by bone & metal. One early plate shows his wife's left hand.
Image: Wellcome Trust
Röntgen presented his results in December, gave a public talk in January of 1896, and by February some clinics were beginning to use X-rays as a diagnostic tool.
Image: New York Academy of Medicine (Boston Medical and Surgical Journal 134(8), February 20, 1896)
Röntgen refused to patent his discovery so that there would be no barriers to medical applications and other beneficial uses. He donated all his Nobel Prize money to his university, and then died in poverty due to mass inflation after WWI.
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The physicist Lise Meitner was born #OTD in 1878. She discovered fission in uranium and was the first person to understand both its mechanics and implications.
Image: Atomic Heritage Foundation (photographer unknown)
Meitner’s PhD advisor at the University of Vienna was Ludwig Boltzmann -- that's the Boltzmann of S = k ln W fame. n 1907, she went to Berlin to work with Max Planck and Otto Hahn. Meitner's collaboration with Hahn would last for 30 years
Meitner handled the physics and Hahn did the chemistry. They had complementary approaches to science -- it was a partnership made for discovery. Together, they made several advances related to radioactivity. She also discovered what is commonly called the “Auger Effect” in 1922.
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The measured deflection of starlight by the gravitational field of the sun, confirming one of the main predictions of Einstein's general relativity, was announced at a joint meeting of the Royal Society and the Royal Astronomical Society #OTD in 1919.
The measurements were made during the total solar eclipse on May 29 of that year. Eddington led an expedition to Principe, while Crommelin led an expedition to Sobral.
Image: Arthur Eddington
The data from the Sobral expedition suggested a deflection of around 2 seconds of arc; the Principe data gave about 1.6 seconds of arc.…
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Freeman Dyson submitted a lovely little two-page paper to Physical Review #OTD in 1951, demonstrating that perturbation theory in quantum electrodynamics produces a divergent series. It's an absolute classic of the field, worth taking the time to read.…
I wrote a bit about this last December, but it's embedded inside a much longer thread with lots of other stuff so let me repeat the relevant parts here.
In QED we calculate physical quantities perturbatively, as a series in a small number α (about 1/137). So if we want to know, for instance, the anomalous magnetic moment of the electron (classically it should be g=2) we get a series like:
g = 2 + (1/π) α + (0.656/π²) α² + …
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The X-10 graphite reactor at Oak Ridge went critical #OTD in 1943. Built as part of the Manhattan Project, it was the second artificial nuclear reactor. (Fermi’s Chicago Pile 1 was the first.)
Image: Ed Westcott
Here's a thread from earlier this year when I visited X-10 (alas, no pictures inside the reactor) and nearby facilities.
If you, like me, enjoy a well-designed 1960s control panel, please enjoy this beauty from the High Flux Isotope Reactor (HFIR) just down the road from the graphite reactor.
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The astronomer Henrietta Hill Swope was born #OTD in 1902. She discovered thousands of variable stars, made precise measurements of the Cepheid period-luminosity relation, and gave the most accurate result for the distance to M31.
Image: Charles Reyne, via Harvard College Obs.
When she retired, Henrietta Hill Swopes gave a large gift to @CarnegieAstro, and the 40-inch telescope at Las Campanas Observatory in Chile was named in her honor.
Image: Carnegie Astronomy
The AIP has an oral history with Swopes, from an interview by David DeVorkin.…
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The physicist David Schramm, who did as much as anyone to bring about the fusion of particle physics and cosmology that informs so much of our present understanding of the Universe, was born #OTD in 1945.
Image: Clemson University
Schramm's 1977 collaboration with Steigman and Gunn used ⁴He production during Big Bang Nucleosynthesis to place limits on how many types of neutrinos exist. Two were known at the time. They initially gave a limit of at most five types; it was eventually narrowed down to three.
Schramm died in a plane crash in 1997, but particle cosmology owes much of its present day success to his pioneering work in the 70s and 80s.
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#otd 23rd October 1956, an uprising in Hungary broke out. Twelve days of street fighting ended with the arrival of Soviet tanks, but the workers councils continued for two months.…
Hungary '56 was the first outbreak of workers councils in the USSR. The official communist parties lined up behind the tanks, and their members left in droves. Peter Fryer, a correspondent for the Daily Worker, resigned so he could publish this account.…
This outbreak of workers councils, without any communist party to lead them (because the Communist Party was the state and employer) profoundly influenced people like CLR James and Grace Lee Boggs, who wrote Facing Reality in response.…
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The astronomer Emma Williams (Vyssotsky) was born #OTD in 1894. After completing her astronomy PhD at Harvard, she studied stellar motions and the kinematics of the Milky Way at UVA's McCormick Observatory. In 1946 she was awarded the Annie Jump Cannon Award by the @AAS_Office.
Her career as a researcher was cut short by a debilitating illness, but not before she compiled (with her co-author and husband, the astronomer Alexander Vyssotsky) an exhaustive catalog of stellar motions.…
I tried to track down a photo of Emma T.R. Williams (Vyssotsky), but couldn't find anything.
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The physicist Karl Jansky was born #OTD in 1905. While searching for the source of radio hiss that interfered with transatlantic phone calls for Bell Labs, he discovered that astrophysical objects emit radio waves. It was the birth of radio astronomy.
Jansky built a directional antenna to pinpoint the source of radio static that was interfering with phone calls. He referred to the antenna as his "merry-go-round" – it rotated on a set of wheel from a Model-T Ford.
Image: Bell Telephone Laboratories
Here's a thread from a few months ago, describing Jansky's discovery of radio waves "apparently of extraterrestrial origin."
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THREAD: #OTD 1966: Aberfan Disaster.

The Aberfan Disaster killed 116 children and 28 adults.

On Friday morning, the 21st October 1966, a wave of coal waste slid down the Aberfan Valley.

#Wales #History
At approximately 9.15am this coal waste hit Pantglas Junior School, Aberfan and destroyed a number of houses nearby. In less than five minutes 116 children and 28 adults were killed in the disaster- the majority within the primary school walls.

#Wales #History
The coal waste hit the classes at the back of the school building killing the headmistress, four teachers and almost all their pupils.
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1/29 #OTD in 1813, on the last day of the Battle of Leipzig, died drowned in the river Elster the commander of the forces of the Duchy of Warsaw, Marshal of the Empire, Prince of Bohemia and Poland and Poland's national hero, Prince Józef Poniatowski.
2 Poniatowski was half Czech and half-Polish. His mother, a Czech aristocrat, was one of Maria Theresa's court ladies.His father was a brother of Poland's last king Stanisław August Poniatowski.Young Józef joined the Austrian imperial army and took part in a war against the Turks
3 In 1789 when new Polish army was being formed, he was asked by his uncle,the King of Poland, to switch to Polish service in the rank of general.He did it after asking Emperor Francis II to release him from his oath, which the latter did. Poniatowski would spend much of his life
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Steven Weinberg submitted "A Model of Leptons" to Physical Review Letters #OTD in 1967. It is a key element of the Standard Model, and the most cited particle physics paper in the inSPIRE database.…
Weinberg’s model describes a unified electroweak interaction at high energies. At low energies, a spontaneously broken gauge symmetry gives mass to the W & Z bosons, while the photon remains massless. So you get a short range weak force and long range electromagnetic interaction.
“It’s what keeps you going as a theoretical physicist to hope that one of your squiggles will turn out to describe reality.”…
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The scientist Henry Cavendish was born #OTD in 1731. Among his many accomplishments, he used a torsion balance designed by John Michell to give the first accurate measurement of the average density of the Earth.
Image: Wellcome Collection
Here is his paper describing the experiment. Note that Cavendish, who is often described as the inventor of the torsion balance technique, credits Michell in the first line.…
Michell, of course, was the first person to propose what one could call, given the understanding of gravity at the time, a "black hole." He described in a letter to his pal Cavendish.
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