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Ann Arbor, Ml 48106-1346 USA 800-521-0600 UMI LUNATIC ON A MOUNTAIN: FRITZ ZWICKY AND THE EARLY HISTORY OF DARK MATTER by Tricia Close Submitted in partial fulfilment of the requirements for the degree of Master of Science Saint Mary’s University Halifax, Nova Scotia June 2001. © Copyright by Tricia Close, 2001 National Library Bibliothèque nationale 1^1 of Canada du Canada Acquisitions and Acquisitions et Bibliographic Services sen/ices bibliographiques 395 WeWngton StrMt 395, me Wellington Ottawa ON K1A0N4 Ottawa ON K1A0N4 Canada Canada KourAto Votn réiêtwnc» OurtiÊ N o n référtne» The author has granted a non­ L’auteur a accordé une ücence non exclusive licence allowing the exclusive permettant à la National Library of Canada to Bibhothèque nationale du Canada de reproduce, loan, distribute or sell reproduire, prêter, distribuer ou copies of this thesis in microform, vendre des copies de cette thèse sous paper or electronic formats. la forme de microfrche/frhn, de reproduction sur papier ou sur format électronique. The author retains ownership of the L’auteur conserve la propriété du copyright in this thesis. NeiÂer the droit d’auteur qui protège cette thèse. thesis nor substantial extracts from it Ni la thèse ni des extraits substantiels may be printed or otherwise de celle-ci ne doivent être imprimés reproduced without the author’s ou autrement reproduits sans son permission. autorisation. 0-612-65728-0 CanadS Certificate of Examination Department of Astronomy and Physics Saint Mary’s University Dr. Gary Welch (co-supervisor) Associate Professor of Astronomy and Physics Saint Mary’s University Dr. Eric Mills (co-supervisor) Professor of History of Science Department of Oceanography Dalhousie University fk G~ ■ [ [AM Dr. David Turner Pro lessor of Astronomy and Physics Saint Mary’s University Dr. David Guenther Associate Professor of Astronomy and Physics Saint Mary’s University Lunatic on a Mountain: Fritz Zwicky and the Early History of Dark Matter Tricia Close Abstract. Fritz Zwicky is identified commonly as the discoverer of dark matter, the unobservable mass that is believed to occupy about 90 percent of the universe. In 1933 Zwicky found a mass discrepancy in the Coma cluster of galaxies, and in 1936 a similar discrepancy was found in the Virgo cluster. The startling news of a mass discrepancy in clusters did not raise many eyebrows. In fact, the astronomical community did not address the problem until the occasion of two conferences held in 1961. and it only gained astronomical prestige at the end of the 1970s, forty years after the initial postulation. Zwicky’s role in the history of this most important subject in astronomy is significant in tracing the progression of the understanding of extragalactic dynamics and observations. It also shows that it is not only scientific evidence that defines theories, but that social interactions are also of vital importance. Ill "What are we to make of such men as Democritus, of the sixth century B.C.. who imagined the universe to be populated with an infinity o f worlds like our own ? Shall we call him a dreamer or a brilliant philosopher? iVhatever we claim for Deniocriiiis and others like him. we must admit that he was out o f touch with his lime. If he deserved to he heard because he was interesting, he also deserved to be called irrational. What is the role of such a man in history? These men may have sounded like lunatics on a mountain, but they spoke from the deepest boundaries of their rational selves. They pushed the boundaries o f sanity slightly wider and made room for creativity. " Charles Whitney, 1971. 6-7. IV Acknowledgements I would like to thank all of the faculty of the Saint Mary’s Astronomy and Physics department for allowing me to pursue a thesis on the history of modem astronomy. I thoroughly enjoyed the research project and am thankful for the opportunity and the experience. I would like to thank Gary Welch and especially Eric Mills, who volunteered his time, for being my supervisors. Their patience is appreciated and I hope that they feel rewarded by the outcome of the project. Table of Contents Page Certificate of Examination Abstract 111 Epigraph IV Acknowledgements Table of Contents VI Photo of Fritz Zwickv IX Chapter 1 Introduction Section A. Introduction Section B. Dark Matter and Newton’s Law of Gravitation Section C. Astronomy at the Beginning of the Twentieth Century 9 L Big Science and Developments in Instrumentation 9 a. Extragalactic Astronomy 14 Hi. Galaxy Masses 15 Section D. Prospectus 16 Chapter 2 Fritz Zwicky: Conqueror of High Peaks Section A. Who was Fritz Zwicky? A Short Biography 18 Section B. Zwicky’s Switch from Physics to Astronomy 21 Section C. Zwicky’s Contribution to Two Great Dilemmas 23 /. Redshift a. Cosmic Rays 25 Section D. Zwicky’s Reputation 27 i. Supernovae: The Melding ofTheory and Observation 28 ii. Rocketry and Extraterrestrial Travel 32 VI ni. The Morphological Outlook and Method 33 Section E. Summary 35 Chapter 3 Fritz Zwicky’s Discovery of Dark Matter Section A. Something Amiss in the Coma Cluster 38 i. The Discovery 38 ii. The Solutions 39 Section B. The Virgo Cluster Examined by Sinclair Smith 42 Section C. Zwicky Reconsiders the Masses of Galaxies in Clusters 43 Section D. Zwicky Observes Many Clusters of Galaxies 45 Section E. Intergalactic Matter: A Possible Source of the Unobserved Mass 49 Section F. Faint Galaxies: Another Possible Source of Unobserved Mass 51 /. The Luminosity Function 51 ii. Zwicky's Observations of Faint GaUuxies 53 Section G. The Outcome of the First Unveiling of "Dark Matter” 54 Chapter 4 The Skeleton in the Cupboard Section A. The Years Following Zwicky's Discovery 57 i. The Mass Discrepancy in Individual Galaxies 57 ii. Clustering Tendency 60 Hi. Galaxy Counts in the Coma Cluster 62 Section B. The 1961 Santa Barbara Conferences 64 /. The Evidence: Rotation Curx’es and Mass-Luminosity Ratios 66 ii. Dynamical Solutions: Systems of Positive Total Energy. Subsystems and Supersystems 68 Hi. Obsen>ational Solutions: Faint Galaxies and Intergalactic Matter 69 /V. Concensus of the Conferences ” 1 Section C. The Popularization of the Dark Matter Problem Section D. Summary 73 Vll Chapter 5 Conclusions Section A. Zwicky’s Role in the Discovery of Dark Matter 75 Section B. Reactions to the Discovery 76 Section C. Lunatic on a Mountain 81 Appendix A The Virial Theorem 83 Bibliography 88 vm Photo by James McClanahaa'counesy Caltech Archives Fritz Zwicky ( 1898-1974) IX Chapter 1 Introduction "Astronomy is a science of matter — for it is matter that heats up. spins madly and darts swiftly throughout the universe, releasing the light waves that enable us to perceive the inner workings o f the cosmos. Marcia Bartusiak A. Introduction Objects in space are observable because they emit radiation. The glowing celestial bodies of the night sky have always been gazed upon and wondered at. providing a rich backdrop for mythical and religious dogma. The rise of science and the invention of the telescope led astronomy into the realm of fact rather than myth, and especially in the past century the increase in telescope power has introduced many new objects to astronomers. Yet despite the ability to observe deep into space in the visual. X-ray, radio, and infrared wavelengths there is much more than is seen. In fact, using the Newtonian law of gravitation has led astronomers to conclude that 90 to 99 percent of the universe is not detectable by present means. That conclusion originates from an inconsistency between luminous and dynamical mass measurements of galaxies. The luminous mass is estimated from the observed flux of radiation emitted." The dynamical mass is calculated from the law of gravitation and the observed motions of the stars in the ‘Bartusiak, 1993, p. .xiii. ^The luminosity L is determined using the inverse square law F = L /4jr, where F is the flu.x and r is the distance to the galaxy. The luminosity is converted into a mass estimate by multiplying by an assumed mass-to-luminosity ratio. The luminous mass of a galaxy cluster is the sum of the luminous masses of its members. galaxy. In addition, the dynamical mass of a cluster of galaxies is found using the motions of individual galaxies within it. The two mass measurements are not often equal for galaxies or for clusters o f galaxies; the dynamical mass exceeds the luminous mass. That is, much of the matter of the universe must be non-luminous; it does not emit or absorb electromagnetic radiation.