|Born||February 14, 1898|
|Died||February 8, 1974 (aged 75)|
|Alma mater||Swiss Federal Polytechnic|
|Known for||Dark matter, supernovae, galaxies as gravitational lenses, neutron stars|
|Awards||President's Medal of Freedom (1949)|
Gold Medal of the Royal Astronomical Society (1972)
|Institutions||California Institute of Technology|
|Doctoral advisor||Peter Debye and Paul Scherrer|
Fritz Zwicky (//; German: [ˈtsvɪki]; February 14, 1898 – February 8, 1974) was a Swiss astronomer. He worked most of his life at the California Institute of Technology in the United States of America, where he made many important contributions in theoretical and observational astronomy. In 1933, Zwicky was the first to use the virial theorem to infer the existence of unseen dark matter, describing it as "dunkle (kalt) Materie".
Fritz Zwicky was born in Varna, Bulgaria, to a Swiss father and Czech mother. His father, Fridolin (b. 1868), was a prominent industrialist in the Bulgarian city and also served as ambassador of Norway in Varna (1908–1933). The father Fridolin designed and built his family's Zwicky House in Varna. Fritz's mother, Franziska Vrček (b. 1871), was an ethnic Czech of the Austro-Hungarian Empire. Fritz was the oldest of the Zwicky family's three children: he had a younger brother named Rudolf and a sister, Leonie. Fritz's mother died in Varna in 1927. His father Fridolin lived and worked in Bulgaria until 1945, after World War II, when he returned to Switzerland. Fritz's sister Leonie married a Bulgarian from Varna and spent her entire life in the city.
In 1904, at the age of six, Fritz was sent to his paternal grandparents in the family's ancestral canton of Glarus, Switzerland, to study commerce. His interests shifted to math and physics. He received an advanced education in mathematics and experimental physics at the Swiss Federal Polytechnic (today known as ETH Zurich) in Zurich.
In 1925, Zwicky immigrated to the United States to work with Robert Millikan at California Institute of Technology (Caltech) after receiving the "international fellowship from the Rockefeller Foundation." He had an office down the hall from Robert Oppenheimer.
While there, Zwicky grew a reputation for his rash and unpredictable character, renowned for his highly hostile dispositon and feats of physical strength. German colleague Walter Baade was falsely accused by Zwicky of being a Nazi, and received several threats on his life from him. He refused that they be allowed alone together.
Zwicky developed numerous cosmological theories that have had a profound influence on the understanding of our universe in the early 21st century. He coined the term "supernova" while fostering the concept of neutron stars. Five years passed before Oppenheimer published his landmark paper announcing "neutron stars".
Zwicky was appointed Professor of Astronomy at Caltech in 1942. He also worked as a research director/consultant for Aerojet Engineering Corporation (1943–1961), and as a staff member of Mount Wilson Observatory and Palomar Observatory for most of his career. He developed some of the earliest jet engines and holds more than 50 patents, many in jet propulsion. He invented the Underwater Jet.
In April 1932, Fritz Zwicky married Dorothy Vernon Gates (1904-1991), a member of a prominent local family and a daughter of California State Senator Egbert Gates. Her money was instrumental in the funding of the Palomar Observatory during the Great Depression. Nicholas Roosevelt, cousin of President Theodore Roosevelt, was his brother-in-law by marriage to Tirzah Gates. Zwicky and Dorothy divorced amicably in 1941.
In 1947 Zwicky married in Switzerland to Anna Margaritha Zurcher. They had three daughters together, Margrit, Franziska, and Barbarina. The Zwicky Museum at the Landesbibliothek, Glarus, houses many of his papers and scientific works. Zwicky died in Pasadena, California on February 8, 1974, and was buried in Mollis, Switzerland.
He is remembered as both a genius and a curmudgeon. One of his favorite insults was to refer to people he did not approve of as "spherical bastards", because, he explained, they were bastards no matter which way one looked at them.
The Fritz Zwicky Stiftung (Foundation) was established in Switzerland to carry on his ideas relating to "Morphological analysis". The foundation published a biography of Zwicky in English: Alfred Stöckli & Roland Müller: Fritz Zwicky – An Extraordinary Astrophysicist. Cambridge: Cambridge Scientific Publishers, 2011. A review of the book is available from Acta Morphologica Generalis.
Fritz Zwicky was a prolific scientist and made important contributions in many areas of astronomy.
Ionic crystals and electrolytes
Supernovae and neutron stars
Together with colleague Walter Baade, Zwicky pioneered and promoted the use of the first Schmidt telescopes used in a mountain-top observatory in 1935. In 1934 he and Baade coined the term "supernova" and hypothesized that supernovae were the transition of normal stars into neutron stars, as well as the origin of cosmic rays. This was an opinion which contributed to determining the size and age of the universe subsequently.
In support of this hypothesis, Zwicky started looking for supernovae, and found a total of 120 by himself (and one more, SN 1963J, in concert with Paul Wild) over 52 years (SN 1921B through SN 1973K), a record which stood until 2009 when passed by Tom Boles. Zwicky did his laborious work, comparing photographic plates with the human eye, which is far more challenging and difficult than Boles accomplished using modern technology for his record.
In 1937, Zwicky posited that galaxies could act as gravitational lenses by the previously discovered Einstein effect. It was not until 1979 that this effect was confirmed by observation of the so-called "Twin Quasar" Q0957+561.
While examining the Coma galaxy cluster in 1933, Zwicky was the first to use the virial theorem to discover the existence of a gravitational anomaly, which he termed dunkle (kalt) Materie 'dark matter'. The gravitational anomaly surfaced due to the excessive rotational velocity of luminous matter compared to the calculated gravitational attraction within the cluster. He calculated the gravitational mass of the galaxies within the cluster from the observed rotational velocities and obtained a value at least 400 times greater than expected from their luminosity. The same calculation today shows a smaller factor, based on greater values for the mass of luminous material; but it is still clear that the great majority of matter was correctly inferred to be dark.
When Edwin Hubble discovered a somewhat linear relationship between the distance to a galaxy and its redshift expressed as a velocity, Zwicky immediately pointed out that the correlation between the calculated distances of galaxies and their redshifts had a discrepancy too large to fit in the distance's error margins. He proposed that the reddening effect was not due to motions of the galaxy, but to an unknown phenomenon that caused photons to lose energy as they traveled through space. He considered the most likely candidate process to be a drag effect in which photons transfer momentum to surrounding masses through gravitational interactions; and proposed that an attempt be made to put this effect on a sound theoretical footing with general relativity. He also considered and rejected explanations involving interactions with free electrons, or the expansion of space.
Zwicky was skeptical of the expansion of space in 1929, because the rates measured at that time seemed too large. It was not until 1956 that Walter Baade corrected the distance scale based on Cepheid variable stars, and ushered in the first accurate measures of the expansion rate. Cosmological redshift is now conventionally understood to be a consequence of the expansion of space; a feature of Big Bang cosmology.
Zwicky developed a generalised form of morphological analysis, which is a method for systematically structuring and investigating the total set of relationships contained in multi-dimensional, usually non-quantifiable, problem complexes. He wrote a book on the subject in 1969, and claimed that he made many of his discoveries using this method.
Catalog of Galaxies and Clusters
Zwicky devoted considerable time to the search for galaxies and the production of catalogs. From 1961 to 1968 he and his colleagues published a comprehensive six volume Catalogue of galaxies and of clusters of galaxies. They were all published in Pasadena, by the California Institute of Technology.
- Zwicky, F.; Herzog, E.; Wild, P. (1961), Catalogue of Galaxies and of Clusters of Galaxies, 1, California Institute of Technology, Bibcode:1961cgcg.book.....Z
- Zwicky, F.; Herzog, E.; Wild, P. (1963), Catalogue of Galaxies and of Clusters of Galaxies, 2, California Institute of Technology, Bibcode:1963cgcg.book.....Z
- Zwicky, F.; Herzog, E.; Wild, P. (1966), Catalogue of Galaxies and of Clusters of Galaxies, 3, California Institute of Technology, Bibcode:1966cgcg.book.....Z
- Zwicky, F.; Herzog, E., Catalogue of Galaxies and of Clusters of Galaxies, 4, California Institute of Technology
- Zwicky, F.; Karpowicz, M.; Kowal, C.T. (1965), Catalogue of Galaxies and of Clusters of Galaxies, 5, California Institute of Technology, Bibcode:1965cgcg.book.....Z
- Zwicky, F.; Kowal, C.T. (1968), Catalogue of Galaxies and of Clusters of Galaxies, 6, California Institute of Technology, Bibcode:1968cgcg.bookR....Z
Galaxies in the original catalog are called Zwicky galaxies, and the catalog is still maintained and updated today. Zwicky with his wife Margaritha also produced an important catalog of compact galaxies, sometimes called simply The Red Book.
- Zwicky, F.; Zwicky, M.A. (1971), "Catalogue of selected compact galaxies and of post-eruptive galaxies", Guemligen: Zwicky, Bibcode:1971cscg.book.....Z
Zwicky was an original thinker, and his contemporaries frequently had no way of knowing which of his ideas would work out and which would not. In a retrospective look at Zwicky's life and work, Stephen Maurer said:
When researchers talk about neutron stars, dark matter, and gravitational lenses, they all start the same way: "Zwicky noticed this problem in the 1930s. Back then, nobody listened..."
He is celebrated for the discovery of neutron stars. He also went on to consider nuclear goblins, which he proposed as "a body of nuclear density ... only stable under sufficient external pressure within a massive and dense star". He considered that goblins could move within a star, and explode violently as they reach less dense regions towards the star's surface, and serve to explain eruptive phenomena, such as flare stars. This idea has never caught on.
An anecdote often told of Zwicky concerns an informal experiment to see if he could reduce problems with turbulence hindering an observation session one night at Mount Wilson observatory. He told his assistant to fire a gun out through the telescope slit, in the hope it would help to smooth out the turbulence. No effect was noticed, but the event shows the kind of lateral thinking for which Zwicky was famous.
In a talk to a Caltech PhD student Frank Malina, who experienced some difficulties working on a dissertation regarding characteristics of oxygen-gasoline rocket engine, Fritz Zwicky claimed the engineer "must realize that a rocket could not operate in space as it required the atmosphere to push against to provide thrust". Zwicky later admitted that he had been mistaken.
He was also very proud of his work in producing the first artificial meteors. He placed explosive charges in the nose cone of a V2 rocket, to be detonated at high altitude and fire high velocity pellets of metal through the atmosphere. The first attempts appeared to be failures, and Zwicky sought to try again with the Aerobee rocket. His requests were denied, until the Soviet Union launched Sputnik 1. Twelve days later, on October 16, 1957, Zwicky launched his experiment on the Aerobee, and successfully fired pellets visible from the Mount Palomar observatory. It is thought that one of these pellets may have escaped the gravitational pull of the Earth and become the first object launched into a solar orbit.
Zwicky also considered the possibility of rearranging the universe to our own liking. In a lecture in 1948 he spoke of changing planets, or relocating them within the solar system. In the 1960s he even considered how the whole solar system might be moved like a giant spaceship to travel to other stars. He considered this might be achieved by firing pellets into the Sun to produce asymmetrical fusion explosions, and by this means he thought that the star Alpha Centauri might be reached within 2500 years.
Zwicky was a generous humanitarian with a great concern for wider society. These two sides of his nature came together in the aftermath of the Second World War, when Zwicky worked hard to collect tons of books on astronomy and other topics, and shipped them to war-ravaged scientific libraries in Europe and Asia.
He also had a longstanding involvement with the charitable Pestalozzi Foundation of America, supporting orphanages. Zwicky received their gold medal in 1955, in recognition of his services.
Zwicky loved the mountains, and was an accomplished alpine climber.
He was critical of political posturing by all sides in the Middle East, and of the use of nuclear weapons in World War II. He considered that hope for the world lay with free people of good will who work together as needed, without institutions or permanent organizations.
- In 1949, Truman awarded Zwicky the Medal of Freedom, for work on rocket propulsion during World War II. In 1968, Zwicky was made professor emeritus at California Institute of Technology.
- In 1972, Zwicky was awarded the Gold Medal of the Royal Astronomical Society, their most prestigious award, for "distinguished contributions to astronomy and cosmology". This award noted in particular his work on neutron stars, dark matter, and cataloging of galaxies.
- The asteroid 1803 Zwicky and the lunar crater Zwicky are both named in his honor.
- The Zwicky Transient Facility is named in his honor.
Zwicky produced hundreds of publications over a long career, covering a great breadth of topics. This brief selection, with comments, gives a taste of his work.
- Zwicky, F. (October 1929), "On the Red Shift of Spectral Lines through Interstellar Space", Proceedings of the National Academy of Sciences, 15 (10): 773–779, Bibcode:1929PNAS...15..773Z, doi:10.1073/pnas.15.10.773, PMC 522555, PMID 16577237. This is the article that proposes a tired light model to explain Hubble's law. (full article)
- Baade, W.; Zwicky, F. (1934), "On Super-novae", Proceedings of the National Academy of Sciences, 20 (5): 254–259, Bibcode:1934PNAS...20..254B, doi:10.1073/pnas.20.5.254, PMC 1076395, PMID 16587881, and Baade, W.; Zwicky, F. (1934), "Cosmic Rays from Super-novae", Proceedings of the National Academy of Sciences, 20 (5): 259–263, Bibcode:1934PNAS...20..259B, doi:10.1073/pnas.20.5.259, PMC 1076396, PMID 16587882. These consecutive articles introduce the notion of a supernova and a neutron star respectively.
- Zwicky, F. (November 1938), "On Collapsed Neutron Stars", Astrophysical Journal, 88: 522–525, Bibcode:1938ApJ....88..522Z, doi:10.1086/144003. The idea of a neutron star, previously introduced in the supernova paper, is explained along with the idea of critical stellar mass and black holes.
- Zwicky, F. (December 1939), "On the Formation of Clusters of Nebulae and the Cosmological Time Scale", Proceedings of the National Academy of Sciences, 25 (12): 604–609, Bibcode:1939PNAS...25..604Z, doi:10.1073/pnas.25.12.604, PMC 1077981, PMID 16588306. Zwicky argues that the shape of nebulae indicate a universe far older than can be accounted for by an expanding universe model.
- Zwicky, F. (August 1941), "A Mosaic Objective Grating for the 18-inch Schmidt Telescope on Palomar Mountain", Publications of the Astronomical Society of the Pacific, 53 (314): 242–244, Bibcode:1941PASP...53..242Z, doi:10.1086/125331. Zwicky was a great advocate for the use of the wide angle Schmidt telescope, which he used to great effect to make many discoveries.
- Zwicky, F. (1945), Report on certain phases of war research in Germany, Aerojet Engineering Corp. Zwicky did work on jet propulsion and other matters with Aerojet corporation during and after the war.
- Zwicky, F. (1957), Morphological astronomy, Springer-Verlag. In this book Zwicky gives free rein to his ideas on morphological research as a tool for making discoveries in astronomy.
- Zwicky, F. (October 1958), "Nuclear Goblins and Flare Stars", Publications of the Astronomical Society of the Pacific, 70: 506–508, Bibcode:1958PASP...70..506Z, doi:10.1086/127284. As well as proposing neutron stars, Zwicky also proposed unstable aggregations of neutron density matter within larger stars.
- Zwicky, F. (1966), Entdecken, Erfinden, Forschen im Morphologischen Weltbild, Droemer Knaur, Bibcode:1966eefi.book.....Z
- Zwicky, F. (1969), Discovery, invention, research through the morphological approach, MacMillan. Zwicky also proposed that the morphological approach could be applied to all kinds of issues in disciplines going far beyond basic science.
- "Zwicky". Random House Webster's Unabridged Dictionary.
- Arp, Halton (June 1974). "Fritz Zwicky". Physics Today. 27 (6): 70–71. Bibcode:1974PhT....27f..70A. doi:10.1063/1.3128662. Archived from the original on 2013-09-27.
- Zwicky, F. (1933), "Die Rotverschiebung von extragalaktischen Nebeln", Helvetica Physica Acta, 6: 110–127, Bibcode:1933AcHPh...6..110Z See also Zwicky, F. (1937), "On the Masses of Nebulae and of Clusters of Nebulae", Astrophysical Journal, 86: 217, Bibcode:1937ApJ....86..217Z, doi:10.1086/143864
- de Swart, J. G.; Bertone, G.; van Dongen, J. (2017). "How dark matter came to matter". Nature Astronomy. 1 (59): 0059. arXiv:1703.00013. Bibcode:2017NatAs...1E..59D. doi:10.1038/s41550-017-0059. S2CID 119092226.
- "Организират конференция, посветена на родения във Варна астроном Фриц Цвики" (in Bulgarian). Днес+. 2008-02-13. Archived from the original on 1 February 2010. Retrieved 18 March 2010.
- Ivanova, Natasha (2008), "110th anniversary of the astrophysicist Fritz Zwicky", Bulgarian Astronomical Journal (in Bulgarian), 10: 135, Bibcode:2008BlgAJ..10..135I
- Richard Panek, The Father of Dark Matter. Discover. 2009. pp. 81-87.
- "The Two Piece Jet Thrust Motor and Inverted Hydro Pulse", TIME, March 14, 1949).
- U.S. Patent # 3044252
- Muller, R. (1986), Fritz Zwicky: Leben und Werk des grossen Schweizer Astrophysikers, Raketenforschers und Morphologen (1898-1974) (in German), Verlag Baeschlin
- Swiss-American Historical Society (2006). Newsletter, Volumes 42-43. The Society. p. 17.
Zwicky has dealt critically with religion during his whole life. A 1971 diary entry states: "To base the inexplainabilty and the immense wonder of nature upon another miracle, God, is unnecessary and not acceptable for any serious thinker." According to one story, Zwicky once discussed the beginning of the universe with a priest. The priest, quoting Scriptures, stated that the universe had started with "and there is light." Zwicky replied that he would buy this, if instead God had said "and there is electromagnetism".
- "The Father of Dark Matter Still Gets No Respect - Discover Magazine.com".
- Ken Freeman, Geoff Mcnamara, In Search of Dark Matter, p22-23 ISBN 978-0-387-27616-8
- Osterbrock, D. E. (2001). "Who Really Coined the Word Supernova? Who First Predicted Neutron Stars?". Bulletin of the American Astronomical Society. 33: 1330. Bibcode:2001AAS...199.1501O.
- Baade, W.; Zwicky, F. (1934), "On Super-Novae", Proceedings of the National Academy of Sciences, 20 (5): 254–259, Bibcode:1934PNAS...20..254B, doi:10.1073/pnas.20.5.254, PMC 1076395, PMID 16587881
- Baade, W.; Zwicky, F. (1934), "Cosmic Rays from Super-novae", Proceedings of the National Academy of Sciences, 20 (5): 259–263, Bibcode:1934PNAS...20..259B, doi:10.1073/pnas.20.5.259, PMC 1076396, PMID 16587882
- List of Supernovae, retrieved 2007-07-10 (provided by CBAT)
- Zwicky, F. (February 1937), "Nebulae as Gravitational Lenses", Physical Review, 51 (4): 290, Bibcode:1937PhRv...51..290Z, doi:10.1103/PhysRev.51.290
- Walsh, D.; Carswell, R.F.; Weymann, R.J. (May 31, 1979), "0957 + 561 A, B - Twin quasistellar objects or gravitational lens", Nature, 279 (5712): 381–384, Bibcode:1979Natur.279..381W, doi:10.1038/279381a0, PMID 16068158, S2CID 2142707
- Some details of Zwicky's calculation and of more modern values are given in Richmond, M., Using the virial theorem: the mass of a cluster of galaxies, retrieved 2007-07-10.
- Hubble, E. (1929), "A Relation between Distance and Radial Velocity among Extra-Galactic Nebulae", Proceedings of the National Academy of Sciences, 15 (3): 168–173, Bibcode:1929PNAS...15..168H, doi:10.1073/pnas.15.3.168, PMC 522427, PMID 16577160
- Zwicky, F. (1929), "On the Red Shift of Spectral Lines through Interstellar Space", Proceedings of the National Academy of Sciences, 15 (10): 773–779, Bibcode:1929PNAS...15..773Z, doi:10.1073/pnas.15.10.773, PMC 522555, PMID 16577237 (full article)
- Baade, W. (1956), "The Period-Luminosity Relation of the Cepheids", Publications of the Astronomical Society of the Pacific, 68 (400): 5–16, Bibcode:1956PASP...68....5B, doi:10.1086/126870
- Singh, S. (2004), Big Bang, Fourth Estate, archived from the original on 2007-06-30
- Ritchey, T. (2002), General Morphological Analysis: A General Method for Non-Quantified Modelling (PDF), retrieved 2007-07-10
- Zwicky, F. (1969), Discovery, Invention, Research Through the Morphological Approach, Toronto: The Macmillan Company
- The Updated Zwicky Catalog of Galaxies (UZC), retrieved 2007-07-10 at the Harvard-Smithsonian Center for Astrophysics.
- Maurer, S.M. (2001), "Idea Man" (PDF), Beamline, 31 (1), retrieved 2007-07-10
- Zwicky, F. (October 1958), "Nuclear Goblins and Flare Stars", Publications of the Astronomical Society of the Pacific (journal), 70 (416): 506–508, Bibcode:1958PASP...70..506Z, doi:10.1086/127284
- Knill, O. (1998), Supernovae, an alpine climb and space travel (biographical notes), retrieved 2007-07-10
- "Pionniers & Précurseurs - FRANK J. MALINA - AERONAUTIQUE - MEMOIRES SCIENTIFIQUES - 2E TOME". olats.org.
- Zwicky, F. (August 1946), "On the Possibility of Earth-Launched Meteors", Publications of the Astronomical Society of the Pacific (journal), 58 (343): 260–261, Bibcode:1946PASP...58..260Z, doi:10.1086/125840
- Zwicky, F. (August 1948), "Morphological astronomy", The Observatory (journal), 68: 121–143, Bibcode:1948Obs....68..121Z
- Zwicky, F. (1966), "Entdecken, Erfinden, Forschen im morphologischen Weltbild", Muenchen: Droemer (book), Muenchen, Bibcode:1966eefi.book.....Z (page 237). This reference was identified from a footnote provided in an online essay: Knill, Oliver (November 1997), Moving the Solar System, retrieved 2007-07-17.
- Greenstein, J.L. (March–April 1974), "Fritz Zwicky – Scientific Eagle (obituary)" (PDF), Engineering and Science: 15–19, retrieved 2007-07-14
- Fritz Zwicky's Extraordinary Vision, American Museum of Natural History, archived from the original on 2007-07-14, retrieved 2007-07-16, an extract from Soter, S.; Tyson, N.D. (2000), Cosmic Horizons: Astronomy at the Cutting Edge, New Press, ISBN 978-1565846029
- Zwicky, F. (November 1949), "Free World Agents of Democracy" (PDF), Engineering and Science, 13 (2)
- Wilson, A. (1975), "Fritz Zwicky (obituary)", Quarterly Journal of the Royal Astronomical Society, 16: 106–108, Bibcode:1975QJRAS..16..106.
- "Dr. Fritz Zwicky, Astronomer, Jet Propulsion Expert, 74, Dies". The New York Times. February 11, 1974.
- Meeting of the Royal Astronomical Society (PDF), February 1972, retrieved 2007-07-14
- "Zwicky Transient Facility Opens Its Eyes to the Volatile cosmos". Zwicky Transient Facility. November 14, 2017.
- Johnson Jr., John (2019). Zwicky: Going Supernova, Harvard. Description & arrow-searchable preview. Book review at Marcia Bartusiak "'Zwicky' Review: Going Supernova," Wall Street Journal. Sept. 13, 2019 (Sept. 14–15, 2019, p. C9 in print ed.), retrieved 2019-09-14.
- Freeman Dyson, "The Power of Morphological Thinking" (review of John Johnson Jr., Zwicky: The Outcast Genius Who Unmasked the Universe, Harvard University Press, 2019, 352 pp.), The New York Review of Books, vol. LXVII, no. 1 (16 January 2020), pp. 42, 44. Freeman Dyson writes (p. 42): "The change [around 1935] from a peaceful to a violent view of the universe was the result of many discoveries by many scientists using a variety of instruments, but one man and one instrument made a major contribution to it. The man was Fritz Zwicky... The instrument was a little eighteen-inch telescope that he installed near the summit of Mount Palomar in California in 1935... Zwicky's small, cheap telescope was the second one built with a revolutionary design by Bernhard Schmidt... in Germany.... The Schmidt telescope had an enormous advantage over other telescopes at that time: it focused light accurately over a wide field of view...."
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- Ritchey, T., Fritz Zwicky, retrieved 2007-07-10
- Maurer, S.M. (2001), "Idea Man" (PDF), Beamline, 31 (1), retrieved 2007-07-10
- Knill, O. (1998), Supernovae, an alpine climb and space travel (biographical notes), retrieved 2007-07-10
- Fritz Zwicky foundation (in German)
- Proc Natl Acad Sci
- Initial Evidence of Dark Matter — Annenberg Learner
- O'Connor, John J.; Robertson, Edmund F., "Fritz Zwicky", MacTutor History of Mathematics archive, University of St Andrews.
- Fritz Zwicky at Find a Grave