1921 in science
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- Winkel tripel projection proposed.
- John Maynard Keynes publishes A Treatise on Probability.
- Marston Morse applies the Thue–Morse sequence to differential geometry.
- Emmy Noether publishes Idealtheorie in Ringbereichen, developing ideal ring theory, an important text in the field of abstract algebra.
- First publication of Ludwig Wittgenstein's Tractatus Logico-Philosophicus, as "Logisch-Philosophische Abhandlung" in Annalen der Naturphilosophie.
- July 18 – The first BCG vaccination against tuberculosis.
- July 27 – Researchers at the University of Toronto led by biochemist Frederick Banting announce the discovery of the hormone insulin.
- American biochemist Elmer McCollum identifies the presence of a component in cod liver oil which cures rickets, which he calls vitamin D.
- Fidel Pagés pioneers epidural anesthesia.
- July – Wolfgang Pauli is awarded his Doctor of Philosophy at Ludwig Maximilians University of Munich for his thesis Über das Modell des Wasserstoff-Molekülions ("About the Hydrogen Molecular Ion Model").
- T. Kaluza demonstrates that a five-dimensional version of Albert Einstein's equations unifies gravitation and electromagnetism.
- January 18 – Yoichiro Nambu (died 2015), Japanese-American nuclear physicist, recipient of the Nobel Prize in Physics.
- March 11 – Léopold Reichling (died 2009), Luxembourg biologist and naturalist.
- April 21 – John R. Huizenga (died 2014), American nuclear physicist.
- April 30 – Ralph A. Lewin (died 2008), Anglo-American biologist, "the father of green algae genetics".
- May 18 – Olgierd Zienkiewicz (died 2009), British civil engineer.
- June 1 – Giuliana Tesoro (died 2002), Italian-American organic chemist.
- June 9 – Forrest Bird (died 2015), American biomedical engineer.
- June 14 – George Rédei (died 2008), Hungarian biologist.
- June 15 – Gavriil Ilizarov (died 1992), Polish-born orthopedic surgeon.
- June 26 – Anne Beloff-Chain (died 1991), British biochemist.
- July 4 – Aron Arthur Moscona (died 2009), American developmental biologist.
- July 18 – John Glenn (died 2016), American astronaut.
- August 16 – Rudolf Trümpy (died 2009), Swiss geologist.
- October 5 – Mahlon Hoagland (died 2009), American biochemist, discoverer of transfer RNA (tRNA).
- October 18 – Beatrice Helen Worsley (died 1972), Mexican-born Canadian computer scientist.
- October 21 – Victor A. McKusick (died 2008), American "father of genetic medicine".
- December 2 – Isabella Karle, American physical chemist.
- "The Collisions of Particles with Hydrogen Nuclei" (PDF). Philosophical Magazine. 42 (252): 923–940. 1921.
- Crilly, Tony (2007). 50 Mathematical Ideas You Really Need to Know. London: Quercus. p. 57. ISBN 978-1-84724-008-8.
- Weyl, Hermann (1935). "Emmy Noether". Scripta Mathematica. 3: 201–220.
- Carere, Suzanne (2007-07-25). "Age-old children's disease back in force". thestar.com. Toronto. Archived from the original on 2008-05-17. Retrieved 2012-01-26.
- Conis, Elena (2006-07-24). "Fortified foods took out rickets". Los Angeles Times. Retrieved 2012-01-26.
- McClean, Franklin C.; Budy, Ann M. (1963). "Vitamin A, Vitamin D, Cartilage, Bones, and Teeth". In Harris, Robert S. (ed.). Vitamins and Hormones. 21. New York; London: Academic Press. pp. 51–52.
- United States Patent #1,394,450.
- United States Patent #1,394,519.