Putting a new spin on galaxies: Horace W. Babcock, the Andromeda Nebula, and the dark matter revolution

1. IntroductionWhen a scientist is the first to perform a difficult type of observation and correctly interprets the result as a significant challenge to then-widely accepted core theories, and the result is later recognized as seminal work in a field of major importance, it is a surprise to find that that work was essentially ignored by the scientific community for thirty years. Such was the fate of the doctoral research on the rotations of the Andromeda Nebula (M31) conducted by Horace Welcome Babcock (1912-2003), who went on to become a very prominent astronomer - in an entirely different sub-field, never working on the subject of his dissertation again. This paper seeks to explain the 'non-reception' of Babcock's work on galactic dynamics and the reasons he did no further work in that sub-field. In particular this paper shows that, contrary to the claims of some commentators, the non-reception of Babcock's work should not be understood as an example of the unjust treatment of a young scientist by the conservative establishment.In 1939, Horace Babcock published the first detailed spectrographic study of M31.1 Babcock's spectra of M31, originally obtained for his doctoral dissertation at the Lick Observatory in California,2 showed a quite unexpected pattem. This unexpected pattem turned out to have massive implications for our understanding of the formation and composition of galaxies. Today Babcock's result is interpreted as an instance of what may be called the 'dynamical discrepancy in astrophysics', popularly known as 'the dark matter problem'. It indicates the need for a thorough overhaul of some of the most fundamental theories and assumptions in contemporary physics and astronomy. In his own time, however, Babcock's result caused barely a ripple within the astronomical community. In fact, despite the consilience provided by several related results obtained by others in the 1930s, it was not until the 1970s that the astronomical community gave the dynamical discrepancy the attention that it deserved, as we realise with the benefit of hindsight. This delay is something of a mystery, not least because the evidence itself did not change significantly in type or quality in the intervening period. As a corollary to explaining the non-reception of Babcock's work on M31, this paper also gives an analysis of some of the historical, sociological and institutional factors responsible for the slow response of the astronomical community to the early evidence for the existence and importance of the dynamical discrepancy.2. The Conceptual Background to the Dark Matter ProblemThe dynamical discrepancy is a significant and surprising disagreement between two different but seemingly independenuy rehable ways of estimating the masses of largescale astrophysical systems such as galaxies and clusters of galaxies. The internal motions within such systems are inconsistent with the known laws of physics given the distribution of direcdy detectable matter in those systems - that is, given just the matter directly observable in these systems and the known laws of physics, the rotation rates observed in them cannot be explained. The dynamical discrepancy was first discovered in the 1930s; despite concerted efforts (especially since the 1980s) it remains unresolved today. According to our current view, there are two classes of possible solutions: either (1) there is a great deal more mass in large-scale astrophysical systems than can be detected by any direct means; or (2) our best theory of gravity, Einstein's General Theory of Relativity, does not apply to those systems and therefore needs to be replaced. On the first option, the excess mass is 'dark' in the sense that it neither emits nor absorbs electromagnetic radiation at any wavelength. This is one of the reasons for thinking that the excess mass is likely not any form of matter with which we are already familiar (protons, neutrons, etc.). On the second option, some new theory will be needed to replace General Relativity as the theory describing gravitational interactions taking place over distances equal to or greater than the radius of a galaxy. …