The ‘Great Debate’ of 1920 – Shapley vs Curtis
I was so busy at the weekend that although I had the date in my diary I forgot to write a post on 26th April, which was the centenary of the Great Debate that took place on 26th April 1920 at the Smithsonian Museum of Natural History.
The principal protagonists on the US debate were astronomers Harlow Shapley and Heber Curtis. It concerned the nature of so-called spiral nebulae (such as the Andromeda Nebula M31 shown above) and the size of the Universe.
Shapley argued the case that distant nebulae were relatively small and lay within the outskirts of Earth’s home galaxy, while Curtis held that they were in fact independent galaxies, implying that they were exceedingly large and distant.
The two scientists first presented independent technical papers about “The Scale of the Universe” during the day and then took part in a joint discussion that evening. Two papers outlining their opposing positions were subsequently published by Shapley and by Curtis in the May 1921 issue of the Bulletin of the National Research Council. The published papers each included counter arguments to the position advocated by the other scientist at the 1920 meeting.
Many at the time felt that Shapley had won the debate, interpretating the Milky Way as the entire Universe rather than just one of many galaxies. The spiral nebulae were relatively nearby, possibly solar systems in the process of formation.
A key piece of evidence in favour of the Shapley argument was provided by Adriaan van Maanen, who claimed to have measured the rotation a spiral nebula which implied the object had to be nearby. Van Maanen’s measurements were later shown to be incorrect. Moreover, within a decade, Edwin Hubble and others had established that the spiral nebulae are in fact large and enormously distant; they galaxies like our own Milky Way.
Two things struck me about this story. One is that it illustrates that its not unusual for a majority scientists to be wrong about something. Debates like this are really not very good for settling scientific arguments. In the end it the data count far more than opinions.
The second is that it is remarkable to think that just a century ago we knew so little about the Universe. Our modern view of the Universe may well turn out to be wrong in some important respects but I still think we can say we know more now than we did then!
I’m reminded of this quote:
Follow @telescoperWe have not succeeded in answering all our problems. The answers we have found only serve to raise a whole set of new questions. In some ways we feel we are as confused as ever, but we believe we are confused on a higher level and about more important things.
In the Dark 
April 29, 2020 at 11:18 am
An interesting addendum to this is that Hubble sent a letter to Shapley saying “You will be interested to hear that I have found a Cepheid variable in the Andromeda Nebula”. Shapley is then reported to have commented “Here is the letter that destroyed my universe”. Rather than being at all bitter, Shapley encouraged Hubble to publish.
And yes, that one observation of a star enlarged our understanding of the scale of the universe by… billions. And Hubble is not even remembered mainly for this…
April 29, 2020 at 12:05 pm
I often wonder whether Curtis referred to Slipher during the debate. After all, Slipher had specifically pointed out that, if the redshifts were velocities, the nebulae could not be bound by the Milky Way.
April 29, 2020 at 12:09 pm
Re renaming of Hubble’s law, some of us disagree strongly with this, mainly because it constitutes a revisionist interpretation of the original law, i.e., an empirical relation between redshift and distance for the nebulae. I have a paper on this on the ArXiv, have just revised it for a journal.
April 30, 2020 at 11:58 am
Yes, I have cited Harrison in my paper. It seems to be to be quite simple; there is a big difference between an empirical relation observed between two variables – which is not obeyed by the closest nebulae, as you know – and an exact law of cosmic expansion derived from relativity. One is a manifestation of the other.
April 29, 2020 at 9:31 pm
This is Robert W. Smith, historian of science, who was at Cambridge as a PhD student at about the same time as me. I used to play evening 20 over cricket with him for the Cavendish Lab team. He then went to the USA to write a history of the Hubble Space Telescope, and I lost track of him. According to Wikipedia he has for some years been Professor of History and Classics at the University of Alberta. He may even read this blog.
April 30, 2020 at 11:58 am
Robert W. Smith is a very well-known historian of astronomy