Post Azvitdi5blv00udb2e by johncarlosbaez@mathstodon.xyz
(DIR) More posts by johncarlosbaez@mathstodon.xyz
(DIR) Post #Azvitdi5blv00udb2e by johncarlosbaez@mathstodon.xyz
2025-11-04T09:12:30Z
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Lise Meitner, who discovered nuclear fission, is one of my heroes. She had to work twice as hard for everything, and did so much.The Austrian government did not open the universities to women until 1901, when she was 23. They had only opened high schools to women in 1899, but luckily her father had hired a tutor to prepare her for the university before it opened, so she was ready to enter as soon as they let her in. She decided to work on physics thanks in part to the enthralling lectures and friendly encouragement of Ludwig Boltzmann. After getting her doctorate in 1906, she went to Berlin to work with Max Planck. At first she found his lectures dry and a bit disappointing compared to Boltzmann's, but she soon saw his ideas were every bit as exciting, and came to respect him immensely. In Berlin she also began collaborating with Otto Hahn, a young chemist who was working on radioactivity. Since women were not allowed in the chemistry institute - supposedly because their hair might catch fire - she had to perform her experiments in the basement for two years until this policy was ended. Even then, she did not receive any pay at all until 1911! But gradually her official status improved, and by 1926 she became the first woman physics professor in Germany.Meitner was one of those rare physicists gifted both in theory and experiment; her physics expertise meshed well with the analytical chemistry skills of Hahn, and as a team they identified at least nine new radioisotopes. The most famous of these was the element protactinium, which they discovered and named in 1918. This was the long-sought "mother of actinium".(1/n)
(DIR) Post #Azvitmc2VdQVcksOMS by johncarlosbaez@mathstodon.xyz
2025-11-04T09:28:13Z
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Here you can see what Meitner was studying: the decay chain starting with uranium-235 that produces protactinium, then actinium, and eventually lead. All the nuclei in this decay chain have atomic mass 4n+3. The reason: • In "α decay" a nucleus emits a helium nucleus or "α particle" - 2 protons and 2 neutrons - so its atomic number goes down by 2 and its atomic mass goes down by 4. • In "β decay" a neutron decays into a proton and emits a neutrino and an electron, or "β particle", so its atomic number goes up by 1 and its atomic mass stays the same.But to understand Meitner's work in context, you have to realize that these facts only became clear through painstaking work and brilliant leaps of intuition! Much of the work was done by her team in Berlin, Marie and Pierre Curie in France, Ernest Rutherford's group in Manchester and later Cambridge, and eventually Enrico Fermi's group in Rome.At first people thought electrons were bound in a nondescript jelly of positive charge - Thomson's "plum pudding" atom. Even when Rutherford, Geiger and Marsden shot α particles at atoms in 1909 and learned from how they bounced back that the positive charge was concentrated in a small "nucleus", there remained the puzzle of what this nucleus was! (2/n)
(DIR) Post #AzvitvEyQlK8Np9b9s by johncarlosbaez@mathstodon.xyz
2025-11-04T09:31:35Z
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In 1914 Rutherford referred to the hydrogen nucleus as a "positive electron". In 1920 he coined the term "proton". But the real problem was that nobody knew about neutrons! Instead, people guessed that the nucleus consisted of protons and "nuclear electrons", which made its charge differ from the atomic mass. But it was completely mysterious why these nuclear electrons should act different from the others: as Bohr put it, they showed a "remarkable passivity". They didn't even have any spin angular momentum! But on the other hand, they certainly seemed to exist - since sometimes they would shoot out in the form of β radiation! To solve this puzzle one needed to postulate a neutral particle as heavy as a proton and invent a theory of β decay in which this particle could decay into a proton while emitting an electron. But there was an additional complication: unlike α radiation, which had a definite energy, β radiation had a continuous spectrum of energies. Meitner didn't believe this at first, but eventually her careful experiments forced her and everyone else to admit it was true. The energy bookkeeping just didn't add up properly!(3/n)
(DIR) Post #Azviu4CT7RfSAf3E0G by johncarlosbaez@mathstodon.xyz
2025-11-04T09:37:46Z
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Meitner's discovery - that the electrons produced when neutrons decay into protons don't have a single specific energy but rather a range of energies - led to a crisis in nuclear physics around 1929. Bohr decided that the only way out was a failure of conservation of energy! Maybe it was only conserved on average. Pauli thought of a slightly less radical way out: maybe some of the energy is carried off by yet another neutral particle, this time one of low mass. Two mysterious unseen neutral particles was a lot to stomach! In 1931 Fermi called the big one the "neutron" and the little one the "neutrino". In 1932 Chadwick realized that you could create beams of neutrons by hitting beryllium with α particles. The neutrino was only seen much later, in the 1950s. (I hope people remember this story when they scoff at the notion that "dark matter" makes up most of the universe: even if something is hard to see, it might still exist.) Back to Meitner:When Hitler gained power over Germany in 1933, her life became increasingly tough, especially because she was a Jew. In May of that year, Nazi students at her university set fire to books by undesirable writers such as Mann, Kafka, and Einstein. By September, Meitner received a letter saying she was dismissed from her professorship. Nonetheless, she continued to do research.(4/n)
(DIR) Post #AzviuCuiinonCDog5Y by johncarlosbaez@mathstodon.xyz
2025-11-04T09:42:42Z
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In 1934, Fermi started trying to produce "transuranics" - elements above uranium - by firing neutron beams at uranium. Meitner got excited about this and began doing the same with Hahn and another chemist, Fritz Strassman. They seemed to be succeeding, but the results were bizarre: the new elements seemed to decay in many different ways! Their chemical properties were curiously variable as well. And the more experiments the team did, the stranger their results got.No doubt this is part of why Meitner took so long to flee Germany. Another reason was her difficulty in finding a job. For a while she was protected somewhat by her Austrian citizenship, but that ended in 1938 when Hitler annexed Austria. After many difficulties, she found an academic position in Stockholm and managed to sneak out of Germany using a no-longer-valid Austria passport.She was now 60. She had been the head of a laboratory in Berlin, constantly discussing physics with all the top scientists. Now she was in a country where she couldn't speak the language. She was given a small room to use a lab, but essentially no equipment, and no assistants. She started making her own equipment. Hahn continued work with Strassman in Berlin, and Meitner attempted to collaborate from afar, but Hahn stopped citing her contributions, for fear of the Nazis and their hatred of "decadent Jewish scence". Meitner complained about this to him. He accused her of being unsympathetic to *his* plight.(5/n)
(DIR) Post #AzviuKsX6op1tkr2n2 by johncarlosbaez@mathstodon.xyz
2025-11-04T09:46:23Z
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Given Lise Meitner's very tough situation, and Hahn's complaints to her, it's no surprise that she wrote to him: "Perhaps you cannot fully appreciate how unhappy it makes me to realize that you always think I am unfair and embittered, and that you also say so to other people. If you think it over, it cannot be difficult to understand what it means to me that I have none of my scientific equipment. For me that is much harder than everything else. But I am really not embittered - it is just that I see no real purpose in my life at the moment and I am very lonely...." What *is* a surprise is that this is when she made her greatest discovery. She couldn't bear spending the Christmas of 1938 alone, so she visited a friend in a small seaside village, and so did her nephew Otto Frisch, who was also an excellent physicist. They began talking about physics. According to letters from Hahn and Strassman, one of the "transuranics" was acting a lot like barium. Talking over the problem, Meitner and Frisch realized what was going on: the neutrons were making uranium nuclei split into a variety of much lighter elements!In short: fission. (6/n)
(DIR) Post #AzviuTLBeAUo8wK2Yy by johncarlosbaez@mathstodon.xyz
2025-11-04T09:52:28Z
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I won't bother telling the story of all that happened next: the calculations and experiments confirming that uranium really was splitting into lighter nuclei, the development of the atomic bomb, which Meitner refused to participate in, how she was nonetheless hailed as the "Jewish mother of the bomb" when she came to America in 1946 - and how Hahn alone got the Nobel prize for fission, also in 1946.It's particularly irksome how Hahn seemed to claim all the credit for himself in his later years. But history has dealt him a bit of poetic justice. Element 105 was tentatively called "hahnium" by a team of scientists at Berkeley who produced it, but later, the International Union of Pure and Applied Chemistry decreed that it be dubbed "dubnium" - after Dubna, where a Russian team also made this element. To prevent confusion, no other element can now be called "hahnium". But element 109 is called "meitnerium". For more, I highly recommend this biography:• Ruth Sime, Lise Meitner: A Life in Physics, University of California Press, 1997. (n/7, n = 7)
(DIR) Post #AzwJx0DA6PkgyMddY0 by BashStKid@mastodon.online
2025-11-05T15:01:21Z
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@johncarlosbaez The only thing I can add is that it’s available on Kobo for £17, which is the cheapest I can find it that’s not on Amazon.
(DIR) Post #AzwJx1SRT5ZKq2nM0G by johncarlosbaez@mathstodon.xyz
2025-11-05T15:33:46Z
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@BashStKid - thanks, I don't know about Kobo but I would like UK-friendly alternatives to Amazon for books.
(DIR) Post #AzwJx2UFdqbu1weJSi by Suiseiseki@freesoftwareextremist.com
2025-11-05T15:37:41.410499Z
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@johncarlosbaez @BashStKid Certain Kobo readers have BusyBox/Linux ports available for them as Kobo partially compiles with the GPLv2 for Linux.For example; https://wiki.postmarketos.org/wiki/Kobo_Clara_HD_(kobo-clara) can be used with AIR and free ebook software like KOReader with some fiddling - too bad postmarketOS is a proprietary distro and therefore contains proprietary software (full GNU/Linux-libre freedom is possible provided proprietary Wi-Fi software isn't included).