If ever a competition were held for the most unrecognized genius of twentieth century astronomy, the winner surely would be Fritz Zwicky (1898–1974). A bold and visionary scientist, Zwicky was far ahead of his time in conceiving of supernovas, neutron stars, dark matter, and gravitational lenses. His innovative work in any one of these areas would have brought fame and honors to a scientist with a more conventional personality. But Zwicky was anything but conventional. In addition to his brilliant insights that turned out to be right, he also entertained notions that were merely eccentric. To his senior colleagues he could be arrogant and abrasive. He referred contemptuously to “the useless trash in the bulging astronomical journals.” He once said, “Astronomers are spherical bastards. No matter how you look at them they are just bastards.” His colleagues did not appreciate this aggressive attitude and, mainly for that reason, despite Zwicky’s major contributions to astronomy, he remains virtually unknown to the public.
Zwicky was born in Bulgaria of Swiss parents. He earned his Ph.D. in physics from the Federal Institute of Technology in Zurich, and then spent most of his life in the United States. He joined the California Institute of Technologyin 1925 to work on the physics of crystal structure. But he was soon caught up in the excitement of astronomical research at Caltech. He became fascinated with cosmic rays—the high energy subatomic particlesthat shoot through space at nearly the speed of light. No one could suggest a plausible candidate for the source of the mysterious particles. Then Zwicky made an astonishing conceptual leap. He decided that cosmic rays are produced in catastrophic explosions of massive stars. No one had previously imagined such a phenomenon. In a 1931lecture course at Caltech, Zwicky introduced the term “super-nova” to distinguish the explosion of an entire star from the more common and much less powerful nova, which involved violent and repeated outbursts on the surface of an unstable star.
Zwicky teamed up with the German-American astronomer Walter Baade to work on the supernova idea. Baade knew of several historical accounts of “new stars” that had appeared as bright naked eye objects for several months before fading from view. The Danish astronomer Tycho Brahe, for example, had made careful observations of one in 1572. Zwicky and Baade thought that such events must be supernova explosions in our own Galaxy. At a scientific conference in 1933, they advanced three bold new ideas: (1) massive stars end their lives in stupendous explosions which blow them apart, (2) such explosions produce cosmic rays, and (3) they leave behind a collapsed star made of densely-packed neutrons.
Zwicky reasoned that the violent collapse and explosion of a massive star would leave a dense ball of neutrons, formed by the crushing together of protons and electrons. Such an object, which he called a “neutron star,” would be only several kilometers across but as dense as an atomic nucleus. This bizarre idea was met with great skepticism. Neutrons had only been discovered the year before. The notion that an entire star could be made of such an exotic form of matter was startling, to say the least.
Zwicky made a persuasive case that supernovas actually occur and ought to be observable in other galaxies. Around 1935, he convinced George Ellery Hale, the Director of Mount Wilson Observatory, to build him an 18-inch Schmidt telescope, which had an unusually wide field of view, ideal for photographing many galaxies at once. In three years, Zwicky used it to discover twelve supernovas. He then persuaded Hale to build the 48-inch Schmidt telescope at Mt. Palomar. Its primary purpose was to photograph the entire northern sky, and the resulting Palomar Observatory Sky Survey became a major cornerstone of astronomy for the next fifty years. But Zwicky also used the 48-inch telescope for “supernova patrols.” He eventually discovered 122 supernovas, still a record for any one observer.
Astronomers readily accepted supernovas, but remained doubtful about neutron stars. Zwicky persisted and was ultimately vindicated. In the late 1930s, theoreticians showed that neutron stars were compatible with nuclear physics. Then, in 1967, radio astronomers discovered the first pulsars, and the next year Thomas Gold of Cornell University showed that such objects could only be rapidly spinning neutron stars produced in supernovas.
Zwicky’s independent mind also emboldened him to challenge the general assumption that the mass of the universe consists mostly of stars. In 1933, while investigating the great Coma cluster of galaxies, he stumbled upon a major discrepancy between theory and observation. The average speed of galaxies within a cluster depends on the total mass of the cluster, since each galaxy is attracted by the gravity of all the others. From the observed speeds of galaxies moving within the Coma cluster, Zwicky calculated its total mass. He then added up all the light from the galaxies in the cluster and used it to calculate the mass in the form of luminous stars. To his surprise, the mass of the cluster based on the speed of its galaxies was about ten times more than the mass of the cluster based on its total light output. He concluded that the Coma cluster must contain an enormous quantity of unseen matter, with enough gravity to keep the rapidly moving galaxies from flying apart. Zwicky in effect discovered that most of the mass in the universe is invisible. He called it “dark matter.” (See the profile on Vera Rubin)
In 1937 Zwicky thought of another way to investigate dark matter. If by chance a massive galaxy lies along our line of sight to a more distant galaxy, it could act as a “gravitational lens,” warping the surrounding space to magnify, distort, and even multiply the image of the background galaxy. This was a direct application of Einstein’s Theory of General Relativity. The bending of starlight by the gravity of the Sun had already been demonstrated in 1919. Zwicky predicted that massive galaxies would similarly distort the light rays from background objects and that the distortion could be used to “weigh” the lensing galaxies. Most astronomers did not take this idea seriously. But in 1979, five years after Zwicky died, the first of many gravitational lenses was discovered, and a cottage industry has since emerged to find and study them. The lensing effect is now used to measure the cosmological parameters of the universe, and to reveal distant objects otherwise too faint to see.
Zwicky also predicted the existence of low mass galaxies and then discovered the first such “dwarf” galaxies with the 100-inch telescope at Mt. Wilson. He anticipated the discovery of quasars by predicting that compact blue galaxies with high luminosity might be mistaken for nearby stars. He was one of the first astronomers to emphasize that the distribution of clusters in the universeis far from uniform on the largest scale.
A diligent worker, Zwicky published hundreds of papers in a wide range of topics. He produced a six-volume catalog of some 30,000 galaxies based on the Palomar Observatory Sky Survey, which remains a standard reference on galaxy clusters. He also published a catalog of bright compact galaxies, which proved invaluable in leading astronomers to find so-called active galaxies. In the book’s introduction, however, Zwicky included an intemperate rant describing other astronomers by name or allusion as “fawners” and “thieves” who stole his ideas and hid their own errors.
What is one to make of such an irascible character? Some of Zwicky’s contemporaries regarded him as an irritating buffoon. Others had a more balanced assessment. Jesse Greenstein, former chairman of the Caltech Astronomy Department, said “I disliked him as a human being. He was vain and very self-centered. Zwicky had an enormous facility to produce radical new ideas, some of which proved to be correct, but a lot of us wish he had not been so rough in the process.” Greenstein recalled the feud between Zwicky and Walter Baade that blew up during World War II. “Zwicky called Baade a Nazi, which he wasn’t, and Baade said he was afraid that Zwicky would kill him. They became a dangerous pair to put in the same room.” But Greenstein acknowledged that Zwicky “also had a humanitarian side. He stockpiled astronomy books in the basement of our department after the war. Without my knowledge… he bought these books using department funds and shipped them off to the war-torn astronomy libraries in Europe. This was quite gracious, but it cost us a lot of money, which we didn’t have at the time.”
Zwicky’s combativeness was apparently reserved for his peers. He was friendly toward students and administrative staff, whom he didn’t regard as competitors. Younger astronomers today tend to regard him as something of a hero, an eccentric genius who advanced astrophysics by proposing truly brilliant and outrageous new ideas without being worried in the least about the consequences. Such an ability is extremely rare, and we should not necessarily be surprised to find it coupled with so peculiar a character.