Fritz Zwicky

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14 February 1898
Varna, Bulgaria
8 February 1974
Pasadena, California, USA

Fritz Zwicky was a Bulgarian-born Swiss astronomer who worked most of his life in the USA. He made important contributions in theoretical and observational astronomy.


Fritz Zwicky was the son of Fridolin Zwicky (1868-1944), a prominent industrialist in Varna who was born in Switzerland, and Franziska Vrček (1871-1927) who was a Czech born in the Austro-Hungarian Empire. Fritz was the eldest of his parents' three children, having a younger brother Rudolf Zwicky (1900-1952) and sister Leonie (1905-1980). Fritz only spent the first six years of his life in Varna for, in 1904, he was sent to Mollis, Glarus canton, Switzerland, the home of his paternal grandparents, to be educated. Fritz's mother Franziska remained in Varna for the rest of her life. Fritz's father served as Norwegian Consul in Varna from 1906 to 1933 and remained in Varna until 1944 when he returned to Switzerland, the country of his birth, and died soon after. Fritz's sister Leonie married a Bulgarian from Varna and remained in that city for the rest of her life. Rudolf became Norwegian Consul in Varna after his father left the post in 1933 and held this position until 1940. By 1946 he was living in Switzerland.

Zwicky's father wanted his son to follow an education which would lead to a career in commerce but Zwicky found mathematics and science so fascinating that he persuaded his father to let him study engineering at university. Zwicky attended primary school in Mollis and from there went to the Höhere Stadtschule in Glarus. In 1914 he went to Zürich where he completed his school education at the Industry School (today the Mathematisch-Naturwissenschatliches Gymnasium).

After graduating from the Industry School in Zürich in 1916, Zwicky entered the Swiss Federal Institute of Technology (the Eidgenössische Technische Hochschule or simply ETH) where he switched from engineering to study mathematics and physics. He obtained his first degree in 1920 having specialised in mathematics with Hermann Weyl as the supervisor of his dissertation. He then continued to study at ETH for his doctorate advised by Peter Debye (1884-1966) who, after being Professor of Theoretical Physics at Zurich University (1911-12), then Utrecht University (1912-14), followed by the University of Göttingen (1914-20), had been appointed Professor of Physics and Principal of the Eidgenössische Technische Hochschule in 1920. Zwicky was also advised by Paul Scherrer (1890-1969), a student of Debye at Göttingen, who had been appointed Professor of Experimental Physics at ETH in 1920. Zwicky studied the applications of quantum mechanics to crystals for his Ph.D., a topic involving both physics and chemistry. During his study for his Ph.D. he met several leading scientists such as Albert Einstein and Wolfgang Pauli. After being awarded his Ph.D. in 1922 for his thesis On the theory of ionic crystals, Zwicky worked at ETH as a scientific assistant from 1922 to 1925.

Zwicky is known today for his remarkable contributions to theoretical astronomy but there is no evidence of him being involved in any work related to astronomy during his studies in Switzerland. It was, surprisingly, the Swiss mountains that influenced his move to astronomy in an unexpected way. The Rockefeller Foundation realised the importance of quantum mechanics and decided to put up fellowships to enable experts in that subject to come to the United States. In 1925 Zwicky was offered such a fellowship and, since he was an avid skier and mountaineer, he asked to be posted to a position in the United States where there are mountains. The Rockefeller Foundation sent him to the California Institute of Technology in Pasadena, California. Leaving Cherbourg, France, on the S S Lapland on 18 September 1925, Zwicky arrived in New York on 26 September en route to Pasadena. Once he had arrived he found that there are certainly mountains nearby, with the San Rafael Hills and the San Gabriel Mountains, but he was unimpressed with them saying that Pasadena only had "foothills." The San Gabriel Mountains, however, was the location of the Mount Wilson Observatory where Edwin Hubble was working and Zwicky began to become interested in astronomy.

Arriving with a Rockefeller fellowship from the International Education Board to support 2 years' work at the California Institute of Technology, Zwicky was supposed to work under Robert Millikan on quantum mechanics of atoms and metals [9]:-
Millikan had expected Zwicky to work on quantum theory of solids and liquids, and he indeed published in these areas, but his primary interests gradually turned to astrophysics, beginning with cosmic rays and ideas for how they might arise.
When his fellowship ended in 1929, the California Institute of Technology appointed Zwicky as an associate professor of physics, a post he was to hold until 1942. On 25 March 1932 he married Dorothy Vernon Gates (1904-1991), the daughter of the California state senator Egbert James Gates and his wife Emma Vernon. They had no children and were divorced in 1941. He was appointed professor of astrophysics at the California Institute of Technology in 1942, the first person to hold this chair, and he continued in this position until he retired at the age of seventy in 1968. He had married Anna Margarita Zürcher on 15 October 1947. Anna was the daughter of an hotel owner in Bern, Switzerland, who was working as a cashier when they met. Fritz and Anna Zwicky had three daughters, Margarit (born 1948), Franziska (born 1950), and Barbarina (born 1952).

Let us look now at some of Zwicky's outstanding contributions. In 1931 he began a collaboration with Walter Baade who had noticed that novae, stars which suddenly increase dramatically in brightness and are relatively close to the solar system, also were sometimes observed in galaxies which had been shown to be at vast distances from the Milky Way. Although mechanisms for relatively nearby novae were at least comprehensible, the immense energy need to power these distant objects was beyond understanding. Baade and Zwicky also noticed that cosmic rays appeared to be associated with these objects which they called supernovae. Zwicky tried to explain how cosmic rays could be produced by these supernovae but failed to find anything to satisfy himself. The other major problem which they tried to explain was how a star could produce such incredible amounts of energy. In 1932 James Chadwick had discovered the neutron and soon Zwicky and Baade saw how this could solve the problem. They wrote in March 1934:-
... we advance the view that a supernova represents the transition of an ordinary star into a neutron star, consisting mainly of neutrons. Such a star may possess a very small radius and an extremely high density. As neutrons can be packed much more closely than ordinary nuclei and electrons, the 'gravitational packing' energy in a cold neutron star may become very large, and, under certain circumstances, may far exceed the ordinary nuclear packing fractions. A neutron star would therefore represent the most stable configuration of matter as such.
Although this first prediction of a neutron star has proved correct, it was ignored at the time and even when it was independently put forward later, no mention was made of their 1934 prediction.

Zwicky, realising that supernovae occurred very infrequently, proposed building a telescope which could photograph large numbers of galaxies in a wide-angled photograph. He persuaded the California Institute of Technology to build an 18 inch Schmidt camera which was completed in 1936 and used by him for many years. Koenig writes [10]:-
In 1935 Zwicky traveled to Hamburg to visit Schmidt. Upon his return to Pasadena, he and Millikan convinced Hale to allocate $25,000 of the Rockefeller grant for an 18-inch Schmidt, "on the pretext to the Rockefeller Foundation that it was needed as a scout instrument for the 200-inch telescope". It was constructed under the direction of John A Anderson and put in operation in September 1936. Baade stated that this was "sooner than expected!" Perhaps this was a result of Zwicky's immediate visit to Hamburg and solicitation for funding.
It was Zwicky's father-in-law, the wealthy Egbert James Gates, who gave a considerable amount towards the 18-inch Schmidt telescope. Using this instrument, throughout his career Zwicky discovered 129 supernovae in distant galaxies. His belief that supernovae could be used to map out the universe was correct but again it took many years before astronomers used this technique.

Another remarkable prediction was made by Zwicky in 1933 after studying the COMA cluster of galaxies. He was able to estimate the velocities of the galaxies within the cluster and realised that the observed mass of the galaxies was not nearly enough to allow gravity to keep them bound together. He wrote in an article in Helvetica Physica Acta:-
In order to receive an average Doppler effect of 1000 km/s or more, which is what we have observed, the average density in the COMA system would have to be at least 400 times greater than that of visible matter. If this can be shown to be the case, then it would have the surprising result that dark matter is present in the Universe in far greater density than visible matter.
Although much of the data that Zwicky was using, such as the best estimate for the Hubble constant, was later modified considerably, nevertheless his prediction based on relatively limited statistics is now universally accepted. With this work he has earned himself the title of "Father of dark matter."

Another of Zwicky's brilliant ideas was that galaxies could be used as gravitational lenses. Einstein had mentioned the fact that stars could act as gravitational lenses but noted that "there is no great chance of observing this phenomenon." In 1937 Zwicky published two letters in The Physical Review. In the first, Nebulae as gravitational lenses, he wrote:-
... I made some calculations which show that extragalactic nebulae offer a much better chance than stars for the observation of gravitational lens effects.
In the second letter of 1937 to The Physical review, On the probability of detecting nebulae which act as gravitational lenses, Zwicky wrote:-
Provided that our present estimates of the masses of cluster nebulae are correct, the probability that nebulae which act as gravitational lenses will be found becomes practically a certainty. ... [For] around one in about one hundred nebulae the ring-like image of a distant nebula should be expected, provided that the chosen nebula has an apparent angular radius smaller than the angles through which light is deflected on grazing the surface of this nebula.
He also wrote that this method of gravitational lensing could be used to investigate dark matter. As with many of Zwicky's ideas, this was not followed up by astronomers for many years. In fact it was not until 1979, over 40 years later, that the first gravitational lens was discovered. This was five years after Zwicky's death.

Of course anyone who was prepared to put forward ideas as freely as Zwicky will get some wrong and, indeed, this is true of him. Let us give one interesting example. The redshift of the galaxies was taken by most astronomers to be a Doppler Effect arising from an expanding universe. Zwicky was not satisfied with this explanation and noted, correctly, that there could be other causes such as a gravitational drag, light slowly losing energy over billions of years, or even that the laws of physics could change over time. In 1939 he claimed to have a "proof" that the universe was older than the time from the "big bang" when the expansion began. His "proof" was based on the spherical nature of most clusters of galaxies which, he "proved" could only happen over a much longer timescale than that predicted by the expanding universe. There is much evidence today to show that Zwicky was wrong on this topic and the expansion of the universe is real.

Let us now look at some work that Zwicky undertook on developing rockets. Theodore von Kármán set up Aerojet in 1936, becoming its director. He was joined by a number of scientists from the California Institute of Technology including Zwicky. At this stage the basic interest of Aerojet was to produce rockets which could be used for spaceflight which would greatly advance astronomy and, in addition, other sciences. At first no practical work was undertaken and the group met occasionally to discuss their objectives. A first test rocket was launched in August 1941 and the United States Air Force became interested. Aerojet Engineering Corporation was formally incorporated in March 1942. Von Kármán wrote:-
Thus began ... the world's largest manufacturer of rockets and propellants. In only twenty years it was to grow from six people with a capitalisation of $1200 into a 700-million-dollar a year business, a staff of nearly 34,000, and a key role in the modern defence picture of the United States.
By 1943, the US Air Corps had ordered 3000 rockets from Aerojet for jet-assisted take-off technology for their aircraft. In the same year Zwicky was made head of Aerojet's research department [11]:-
At War's end, Zwicky toured secret weapons programs in Germany and Japan. Later, he helped Aerojet develop many of the high energy fuels used in today's solid rocket boosters.
Returning to the United States from Paris in August 1946 he completed an Information Sheet which gives details such as the trip was paid for by the American Air Force and personal details such as: height 6 ft 1/2 inch, complexion fair, colour of hair grey-brown, colour of eyes hazel.

For this contribution to the war effort, Zwicky was awarded the United States Medal of Freedom in 1949. Zwicky, although born in Bulgaria, had Swiss citizenship through his father. He did not become an American citizen so during World War II he had to be given a special clearance by the US government to allow him to undertake war work. This clearance was removed in 1955 and he had the option of taking American citizenship or leaving Aerojet. He refused to give up his Swiss citizenship and therefore left Aerojet.

Zwicky believed that equally important to his scientific discoveries was his method of making them which he called morphological outlook and method. He explained:-
After pursuing a dozen or so various activities ranging from mountain climbing and professional shorthand to physics, astronomy, engineering, languages, higher education, national and international politics, and mutual aid with fair success, I still did not feel satisfied. ... It was difficult to account for the lack of satisfaction until it occurred to me ... that no stereotype activity in the books of the past corresponds to my personal genius. Its nature is such that it could become fully alive only through the creation of a new profession - the morphologist.
In 1948 Zwicky delivered the Halley lecture for 1948, Morphological astronomy, at the University of Oxford. We give the beginnings of that lecture at THIS LINK.

In 1957 Fritz Zwicky published the book Morphological astronomy. We give a version of the Foreword of this book at THIS LINK.

Extracts from some reviews of two books written by Fritz Zwicky on morphology and one book of which he was both co-editor and a contributor on morphology are given at THIS LINK.

Ritchey writes [17]:-
Zwicky has been described as a notorious maverick in science, both brilliant and insufferable. On the brilliant side, Zwicky was truly a "high flyer", an "outlier", who went his own way and formulated ideas and hypotheses that were ahead of their time. Some of them were wrong - many right. His triple hypothesis about the nature of supernovae, neutron stars and cosmic rays in the 1930's was scoffed at by many, but turned out to be completely correct - and, under better circumstances, might have earned him a Nobel Prize. He was also a great humanist, engaged in a number of charitable activities, including years of work to help rebuild scientific libraries destroyed during the Second World War and participating in the Pestalozzi Foundation's program to establish war orphan villages. On the other side, the authors do not try to cover up the fact that Zwicky was, all too often, what we today would call a "difficult person". There are scores of anecdotes about his deeds and manners, his "salty" attitude and abusive statements (no doubt embellished over time). ... His sometimes overbearing and belligerent manner irritated a lot of people, and this may well have contributed to his present day lack of recognition.
It must be said that he "irritated a lot of people" because he said what he thought of them, criticism that was usually entirely justified. Perhaps the most prestigious scientific award he received was the Royal Astronomical Society's Gold Medal in 1972. Today Zwicky has received more recognition as many of his remarkable predictions are verified. Asteroid 1803 Zwicky is named for him as is lunar crater Zwicky.

Jesse Greenstein writes [7]:-
It is difficult to write a brief, conventional memoir about so unconventional a man. Fritz classified scientists into two categories, eagles and low-fliers; a low-flier like myself recognised clearly that Fritz was the high-flier. He pursued an extraordinary range of personal interests: international charities, city-planning, mountain climbing, new explosives, exploding stars, crystals and dying stars, and, especially galaxies. He always saw the Universe in his own original way; he loved the extraordinary objects it contained, and he explained them in his own fashion, sometimes wrong but never dull.
As a final quote let us record Zwicky's feelings for both his homeland and for the world:-
We hope to have served the world and our homeland by simultaneously playing the local patriots and world citizens.

References (show)

  1. R Müller, Fritz Zwicky. Leben und Werk des grossen Schweizer Astrophysikers, Raketenforschers und Morphologen (Baeschlin, Glarus, 1986).
  2. A Stöckly and R Müller, Fritz Zwick: Astrophysiker, Genie mit Ecken und Kanten (Neue Zürcher Zeitung Verlag, Zürich, 2008).
  3. A Stöckly and R Müller, Fritz Zwicky: An Extraordinary Astrophysicist (Cambridge Scientific Publishers, Cambridge, 2011).
  4. C Cluster, Fritz Zwicky: Developed General Morphological Analysis, Swedish Morphological Society.
  5. M Egdall, The Remarkable Discoveries of Fritz Zwicky, Decoded Science (2 July 2012).
  6. T Flowerdew, Review: New Methods of Thought and Procedure, (1967) by F Zwicky and A G Wilson (eds.), OR 19 (4) (1968), 482-484.
  7. J L Greenstein, Remembering Zwicky, Engineering and Science (California Institute of Technology, March-April 1974), 15-19
  8. H S Hogg, Review: Morphological Astronomy, by F Zwicky, Science, New Series 127 (3294) (1958), 343-344.
  9. O Knill, Zwicky, Fritz, in T Hockey, V Trimble and T R Williams (eds.), Biographical Encyclopedia of Astronomers (Springer Publishing, New York, 2014).
  10. T Koenig, Fritz Zwicky: Novae Become Supernovae, in M Turatto, S Benetti, L Zampieri and W Shea (eds.), 1604-2004: Supernovae as Cosmological Lighthouses, ASP Conference Series 342, Proceedings of the conference held 15-19 June, 2004 in Padua, Italy (Astronomical Society of the Pacific, San Francisco, 2005), 53-60.
  11. S M Maurer, Idea man, National Accelerator Laboratory, Stanford University.

Additional Resources (show)

Honours (show)

Honours awarded to Fritz Zwicky

  1. Lunar features Crater Zwicky

Written by J J O'Connor and E F Robertson
Last Update November 2018