Fritz Zwicky: An Extraordinary Astrophysicist

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Fritz Zwicky: An Extraordinary Astrophysicist
Norbert Straumann, Uni Z¨
urich
November 24, 2012
Introduction
As a boy and also later, I often listened to lengthy interviews with Fritz Zwicky,
which he regularly gave at the Swiss radio when he returned home for short vis-
its. I found them always interesting, although I was too young to understand the
importance of what he told the public about supernovae and other discoveries in
astronomy. Later, my scientific activities had for about two decades almost noth-
ing to do with astronomy. For this reason, I never met Fritz Zwicky persionally,
and saw him only once in action when he gave a general, but quite unusual physics
colloquium at ETH. Therefore, I can only comment on Zwicky’s outstanding con-
tributions in astrophysics. However, the reader is strongly invited to learn a lot
about Fritz Zwicky in all his colorful facets and activities from the biography:
Fritz Zwicky: Genie mit Ecken und Kanten [1], that has recently been translated
into English [2].
Gustav Andreas Tammann and I have written more extensively about Zwicky’s
most significant contributions to astrophysics and observational astronomy in a
special contribution to the cited books [1], [2]. Below, I shall give an abbreviated
version of our article.1
Zwicky as the father of dark matter
Fritz Zwicky was the first to recognize that in rich clusters of galaxies, a large
portion of the matter is not visible. In his pioneering work, which he published as
early as 1933 in Helvetica Physica Acta, he estimated the total mass of the COMA
cluster of galaxies from the motions of the galaxies within that cluster. Using the
virial theorem he came to the conclusion that the galaxies were on average moving
1Unfortunately, we can not recommend the English translation of our article. We were not
asked to correct its distortions and errors. For example, our German expression for “synchrotron
radiation of electrons” was translated as “synchrotron radiation of neutrons”.
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too fast for the COMA cluster to be held together only by the mass of the visible
matter. In Zwicky’s words:
“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.”
Figure 1: Fritz Zwicky
Zwicky’sanalysis at the time was of course quite rough. It was deduced from
very limited statistics, an uncertain radius of the cluster and on a distance scale,
based on a value of the Hubble constant that was for a long time about seven
times too large. Surprisingly, his results have stood the test of time as reasonable
estimates.
In recent times lots of investigations at all astrophysical length scales have
confirmed that the vast majority of material in the Universe is of totally unknown
nature. The ratio of known to unknown is about 5:1. (We do not discuss here at-
tempts to replace dark matter by modifications of general relativity.) One favored
hypothesis is that dark matter might consist of heavy elementary particles which
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interact, like neutrinos, only weakly with matter known to us. So far, direct, indi-
rect and collider searches have only led to upper limits. The goal of dark matter
identification has eluded us for long, but prospects for discovery in the coming
years remain intact.
Zwicky as the father of neutron stars
In 1931, Zwicky started working with the outstanding astronomer Walter Baade,
who had just come to the Mount Wilson Observatory, after working in Hamburg
and G¨ottingen. Soon they were closely collaborating. Their main topic were the
rare “novae ” in distant galaxies. Because of their great distances, compared to the
novae in the Milky Way, they must have enormous luminosities. For this reason
they coined the term “supernovae” for these bright events.
Over the past two millennia, eight long-duration “new stars” have been seen
with the naked eye and on the basis of the recorded observations they must cer-
tainly have been supernovae. The brightest was the supernova of 1006, which was
also registered in St. Gallen. The Chronicles of the Monastery record that the
very bright star appeared just over the rocky alpine horizon (Alpstein). Thanks to
this description of the position, the Einstein R¨ontgen Observatory was able to find
the remnants of this historical supernova. Its appearance is similar to that of the
famous Crab Nebula, shown in Fig. 2.
From existing data, Baade and Zwicky deduced that supernovae had an enor-
mous luminosities of about 108times that of the Sun. Their estimate is about 100
times too small because in the 1930s extragalactic distances were underestimated
by a factor of approximately 7. On the other hand, Baade and Zwicky did es-
timate the total energy release more or less correctly. Admittedly, the way that
they arrived at it was incorrect in that they relied on exaggerated expectations of
the proportion of ultraviolet light and X-rays which at the time had not yet been
observed. (Nowadays we know that 99% percent of the total energy released is in
the form of neutrinos. Incidentally, the gigantic neutrino pulse was first observed
in the supernova event of 1987 in the Large Magellanic Cloud by underground
detectors on Earth. This may be regarded as the birth of “neutrino astronomy”.)
What was the primary source of the enormous energy release in supernova
events? Zwicky’smost outstanding contribution to astrophysics may have been his
bold hypothesis, that when a supernova occurred, a neutron star was formed in the
centre. Recall that the neutron was discovered by James Chadwick in February
1932. Zwicky immediately developed the idea that when the central region of
a high-mass star collapses, a very dense core mainly consisting of neutrons is
formed. He estimated the released binding energy as the mass equivalent of about
10% of the solar mass. This was sufficient to cover the energy requirements of a
supernova. In fact only about 1% is released as explosion energy and the rest is
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Figure 2: Crab Nebula in the constellation of Taurus. This is the relict of a super-
nova from the year 1054, which was described in detail in the Chinese year books
of the Sung dynasty. The bluish glow from the central region of the nebula is due
to synchrotron radiation by high energy electrons moving in weak magnetic fields.
In the red filaments recombination radiation of electrons with protons in the hot
gas dominates. Near the centre of the nebula is a rapidly rotating pulsar.
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lost within seconds in a gigantic neutrino pulse. Zwickly correctly guessed that the
outer layers would be heated to high temperatures by the energy of the explosion
and would lead to an enormous eruption into the interstellar space. He quickly
came to the conviction that cosmic rays were also generated in supernova events.
Although there are other sources of cosmic rays, such as in the central regions of
active galaxies, supernovae presumably generate a significant proportion.
In autumn of 1933, Baade and Zwicky presented their considerations at a
meeting of the American Physical Society at Stanford University. An edited ver-
sion was published on 15 January 1934 in Physical Review. In the summary, the
two authors wrote:
“With all reserve we suggest the view that supernovae represent the
transition from ordinary stars to neutron stars, which in their final
stages consist of extremely closely packet neutrons.”
For decades, these far-sighted predictions were more or less ignored. Articles
relating to neutron stars could, for many years, be counted on the fingers of one
hand. This may in part be because to many questions Zwicky did not know the
answer. He did not know what the cause for the collapse was, nor did he have
detailed suggestions for the behavior of the collapsing core and its transformation
into a neutron star. To some extent this was simply because the necessary founda-
tions of physics were lacking. Nuclear physics was in its very early stage and even
today we still do not fully understand how the primary energy triggers a supernova
explosion. Current research centres on increasingly realistic simulations. Zwicky
missed the opportunity of underpinning some of his claims with detailed investi-
gations. For example, in 1934 it would have been possible to perform calculations
on the structure of neutron stars, which in fact is what Oppenheimer, Volkov and
Tolman did four years later, and thereby recognized that neutron stars can only
exist with masses less than a few solar masses. (The largest neutron star mass dis-
covered so far is close to two solar masses, a fact that has important implications
on the equation of state above nuclear densities.) It appears that Zwicky was not
interested in “technical details” of this type. For him it was sufficient to develop
a qualitative understanding of the phenomena, and he certainly had a good nose
for that. Another critical factor was his unusual ability to grasp the relationship
between different phenomena intuitively. So in addition to the conjecture that su-
pernovae obtain their energy from the collapse of a massive star whose central
region becomes a neutron star, he recognized at the same time that the energy re-
leased in supernovae events could account for high energy cosmic rays. Following
that, he worked for decades on a comprehensive search for supernovae. Zwicky
was strengthened by his exceptional self confidence and he never experienced the
fear of being seen to fail.
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It must, however, be admitted that some of Zwicky’s work shows obvious
weaknesses. For example, his article On the Theory and Observation of Highly
Collapsed Stars, written in 1939, is for the most part rather disappointing and
some of it is simply wrong. His use of simple things in general relativity shows
that he had too little understanding of it. It is also irritating that he does not cite
the brilliant work of Oppenheimer and Volkov.
Zwicky’s scenario has survived until the present day. It took decades until
neutron stars were discovered as radio pulsars. The Milky Way may well contain
about a billion of them.
Zwicky and gravitational lenses
Zwicky was also the first to recognize the astronomical potential of the gravita-
tional lens effect, on which he published in 1937 two remarkable short papers in
Physical Review.
It is not generally known that as early as 1912, Einstein realized that gravita-
tional fields can have a similar effect on light propagation as optical lenses. Beside
the lens effect, he also studied the induced changes of brightness, but did not pub-
lish his results since he saw no way to observe the predicted phenomena. It was
only in 1936 that he was persuaded by the Czech engineer R.W. Mandl to write
a short note on the subject. But even then, Einstein did not believe that a cosmic
Fata Morgana of this sort would ever be observed, if only because the telescopes
of the time were not capable of measuring the minute differences in angle between
possible double images.
Zwicky was familiar with Einstein’s two-page publication in Science and one
year later added that whole galaxies could be seen to act as lenses. In his first
short article on the topic, he put it in these terms:
“Last summer Dr. V.K. Zworykin (to whom the same idea had been
suggested by Mr. Mandl) mentioned to me the possibility of an image
formation through the action of gravitational fields. As a consequence
I made some calculations which show that extragalactic nebulae offer
a much better chance than stars for the observation of gravitational
lens effects.”
Crucial for this prognosis was the fact that Zwicky had estimated the masses
of galaxies as about 400 times larger than the estimates of his colleagues in as-
tronomy. Without a major amount of dark matter, the lens effect would have been
too small to form arcs and multiple images.
In his first article, Zwicky lists several reasons why the discovery of lens ef-
fects would be of considerable interest. Firstly, it would open the possibility of
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a new test of general relativity.2Secondly, Zwicky says, galaxies could be dis-
covered at far greater distances as a result of brightness amplifications, a fact that
indeed has become important. And finally, Zwicky’s hope that the total masses
of galaxies (and clusters of galaxies) could be determined using the gravitational
lens effect has been realized far more effectively than he could ever have dreamt
of. In a second equally short paper, Zwicky arrives on the basis of simple esti-
mations at the following almost visionary prediction, which was to prove correct
almost 40 years later:
“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.”
Fig. 3 shows an impressive example for this. Certain galaxies behind the
galaxy cluster Abell 2218 look like arcs because of the distortion which is centred
about the mid-point of the cluster. No doubt, Zwicky would have viewed images
like this enthusiastically and derived great satisfaction from them. In recent times,
gravitational lensing has become an important field with considerable potential
for studying the distribution of matter on all scales.
Fritz Zwicky as an observing astronomer
As an observing astronomer, Zwicky was both innovative and controversial. He
proposed a whole range of new methods, such as synthetic color photography of
galaxies, a process whereby he added or subtracted various color filters to pho-
tographs already taken, and in this way separated out young blue stars from old
red ones. Zwicky achieved two outstanding successes, namely the systematic
search for supernovae and his great Catalogue of Galaxies in the northern sky – a
milestone in extragalactic research. Space does not allow us to go into further de-
tails. More can be found in the article with Gustav Tammann in our contribution
to [1] and [2]. There, we also wrote about Zwicky’s relationships with colleagues
that often included virulent attacks, mostly connected to priority issues. Among
the many anecdotes he is “credited” with coining the term a spherical bastard, i.e.
“He’s a bastard no matter which way you look at him!”
There can be no doubt that Fritz Zwicky is one of the most original astro-
physicists of the twentieth century and today nobody would question his intuitive
genius in the field and his passion for science.
2So far this has not been possible, because astrophysical lenses are too complex.
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Figure 3: This color photograph of the rich galaxy cluster Abell 2218, taken by
the Hubble space telescope, shows a number of arcs around the centre of the
cluster, which is located near the high luminosity galaxy just below left of the
centre. The observed distortions have been used to determine the total mass in the
central region of the cluster. (Source:NASA/Andrew Fruchter and the ERO Team
[STScl]).
References
[1] A. St¨ockly and R. M¨uller, Fritz Zwicky: Genie mit Ecken und Kanten. Verlag
Neue Z¨urcher Zeitung (NZZ Libro) (2008).
[2] A. St¨ockly and R. M¨uller, Fritz Zwicky: An Extraordinary Astrophysicist.
Cambridge Scientific Publishers, Cambridge (2011).
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