Fritz Zwicky was a Swiss astronomer who worked most of his life at Caltech in the US. He had a good reputation as an astronomical observer but his real passion was for astronomical theory based on applications of physics. He was in fact one of the first true astrophysicists from the 1920s onwards. But during most of his lifetime he was very underappreciated for his theories of cosmology and stellar physics. In fact that is really putting it mildly. Many of his colleagues were very hostile towards him and his theories. Of the scientists described in this series he is arguably the one who was most regarded as a “crackpot”. That is until many of his ideas were proved right many years later.
Zwicky had a remarkable ability to consider a problem from a fresh perspective and disregard any misguided preconceptions of the time. Because of this he was capable of coming up with what seemed like wild theories to others. With hindsight it seems like about half of these ideas turned out to be right while the others really were just too wild, or perhaps some of them are still ahead of their time.
In 1935 Zwicky published his theory in collaboration with Walter Baade that when supernovae explode they leave behind them a star with the density of nuclear matter made of neutrons. They predicted that these neutron stars were responsible for cosmic rays and proposed Supernovae as standard candles for measuring distances to other galaxies.
Today these ideas are so much a standard part of our astrophysics that it is hard to appreciate just how revolutionary they were at the time. Neutrons had just been discovered two years earlier while cosmic rays had only been observed since 1912. Even the term “supernova” had only been coined in 1926 by Zwicky himself. To other scientists of the time, putting these new ideas together in such a way must have seemed like just a historical trick that was too much to swallow.
In fact the theory was based on sound reasoning and built on the theory of white dwarfs as a Fermi gas which had developed over the previous decade. At the same time as Zwicky and Baade proposed their theory of supernovae another controversy was raging on the other side of the Atlantic between Chandrasekhar and Eddington. Chandrasekhar predicted that there was a limit to how heavy a white dwarf could be before it must collapse to form a black hole. Eddington could not accept that nature would include black holes and argued that relativity must be modified in such extreme circumstances to avoid the Chandrasekhar limit. Astronomers now believe that the neutron star is the densest stable state before this limit is reached.
The resistance towards these ideas persisted so long that when pulsars were observed 32 years later, few people were prepared for the discovery. The pulsing radio signals observed by Jocelyn Bell in Cambridge were at first thought to be interference and then alien signals. Bell’s supervisor Antony Hewish could not accept the observation at first because the strength, rapidity and regularity of the signal meant that it had to come from a small dense source. It was not until the following year that Thomas Gold and Franco Pacini proposed that pulsars were rotating neutron stars. When Stephen Hawking heard of the discovery that neutron stars exist he told Hewish that now they must accept that black holes too are out there in space waiting to be found.
There is a story that in the 1950s a woman member of the public viewed the Crab Nebula source at the University of Chicago’s telescope, and pointed out to the astronomer Elliot Moore that it appeared to be flashing. Moore, told her that it was just the star twinkling due to atmospheric waves. The woman protested that as a qualified pilot she understood scintillation and this was something else. We now know that it is a neutron star that flashes 30 times a second. At the time most astronomers could not have accepted such an explanation.
Neutron stars were not Zwicky’s only successful theory. He also believed that galaxies were held together in clusters with unseen dark matter accounting for the gravitational forces needed. he predicted on this basis that such clusters could act as gravitational lenses producing effects that would be observed. he was of course right on all counts but it is only in the last few decades that the theory of dark matter has finally become widely accepted over alternative explanations.
Not everything Zwicky thought of turned out to be right. His notable failures include his theory of tired light which he invented because he did not accept the expanding universe theory. Even though such ideas our now discounted, at the time they were not so unreasonable and such alternative theories are important in the development of cosmology and physics as counterfoils against which observations can be used.
Nevertheless, in his time almost all of Zwicky’s theories were rejected by his colleagues. He garnered respect only for his careful astronomical observations which included the discovery of over a hundred supernovae, more than any other individual has found. He lived just long enough to see neutron stars become excepted but it took longer for other astronomers to admit he had been right and credit him with the greatness he deserves. He received very few honours for his scientific work but was awarded a gold medal of the Royal Astronomical Society.