Fritz Zwicky

Major contributions

• Applied the virial theorem to the Coma Cluster in 1933 and found that galaxy velocities implied a mass far larger than that of the visible matter, an early and influential argument for dark matter (see Galaxy Clusters and Dark Matter)
• With Walter Baade (1934), identified supernovae as a class of stellar explosion far more energetic than novae, and suggested that they originate in the transition of an ordinary star to a neutron star and are a source of cosmic rays (see Supernovae and Neutron Stars); neutron stars were not observed until the discovery of pulsars in 1967
• Proposed in 1937 that galaxies and clusters could act as gravitational lenses, and estimated that lensing events should be observable (see Gravitational Lensing)
• Ran a long-running supernova search at Palomar and compiled the Catalogue of Galaxies and of Clusters of Galaxies, and developed the “morphological method” for creative problem solving

Named concepts

Career

Zwicky was born in Varna, Bulgaria, to a Swiss father and a Czech mother, and was raised largely in Switzerland. He studied at ETH Zurich, where he earned his doctorate in 1922 under Peter Debye and Paul Scherrer. In 1925 he moved to the California Institute of Technology on a Rockefeller fellowship and remained associated with Caltech and the Mount Wilson and Palomar observatories for the rest of his career. He kept Swiss citizenship throughout his life.

Zwicky's 1933 analysis of the Coma Cluster used a velocity dispersion of roughly 1,000 km/s and found that the cluster's mass-to-light ratio was several hundred times that of the Sun, so the bulk of the mass had to be unseen. His numbers were larger than modern values because of the distance scale and Hubble constant of the time, but the qualitative conclusion has stood. His ideas were largely set aside until Vera Rubin's galaxy rotation curves in the 1970s brought the problem back (see Vera Rubin). Zwicky is known to have had a combative personality and difficult relations with colleagues, a matter documented in biographies, and some of his ideas were ahead of their observational testability.

Key publications

• Zwicky, F. (1933). “Die Rotverschiebung von extragalaktischen Nebeln.” Helvetica Physica Acta, 6, 110–127.
• Baade, W., Zwicky, F. (1934). “On Super-novae.” Proceedings of the National Academy of Sciences, 20, 254–259.
• Zwicky, F. (1937). “Nebulae as Gravitational Lenses.” Physical Review, 51, 290.

Nobel Prize

Fritz Zwicky never received a Nobel Prize. The 2011 prize (accelerating expansion, which relies on supernovae as distance indicators), the 1974 prize for pulsars, and the 2019 prize in physical cosmology each concern fields that his work anticipated, but none was awarded to him; he died in 1974. He did receive the Gold Medal of the Royal Astronomical Society in 1949.

Historical context

Zwicky's work shows how a result can be correct yet unconvincing in its time: his dark-matter inference rested on a single cluster with a poorly known distance scale, and his neutron-star proposal came only two years after the neutron's discovery by James Chadwick (see James Chadwick). Confirmation arrived decades later through independent lines of evidence (rotation curves, X-ray gas, lensing). Today the Zwicky Transient Facility at Palomar Observatory carries his name. Historians note that the “dark matter” of 1933 and the particle hypotheses of today are linked by the observation but not by a known identity: what dark matter consists of remains unknown.