Chapter 7 - Debatable

The Great Debate 1920

Chapter 7 - Debatable

First, get the facts; then you can distort them at your leisure:                                Mark Twain 1835-1910

                     

 

The Great Debate, aka the Shapley-Curtis Debate, took place on the 26^th of April, 1920.  It is described by some as ‘much more than a debate �" rather the story of humanity’s discovery of the immensity of the universe, whose resolution amazed the world.’ Hosted by the National Academy of Sciences - Washington; the principal arguments centred on the nature of spiral nebulae and the size of the universe.  The rudimentary questions being debated were: are distant nebulae comparatively small, and do they lay within our galaxy - or are they huge, autonomous galaxies in their own right? 

 

Whilst the matters under debate took place in 1920, scientific opinions relating to the topics had been around for many years and would continue to cause dissention for many years hence.

 

Historically, the nature and background to the debate serves to demonstrate how far our knowledge in this field has progressed in a period of less than 100 years.  Likewise, retrospectively we begin to appreciate the incredible advances in technology that have occurred. The protagonists, now both long dead, were well-respected astronomers in their own rights, exemplifying the standard of those involved in the debate.  Typically, the disputants represented not only the views of their respective scientific factions, but also the divisions between established views and those who disputed the accepted entrenched interpretations.

 

As a young man, Harlow Shapley had ambitions to be a journalist, but the journalism course was cancelled.  A disappointed Shapley went back to the course list; at the top of the list was: a-r-c-h-a-e-o-l-o-g-y, which Harlow claimed he could not pronounce.  Over the page was: a-s-t-r-o-n-o-m-y, which he could pronounce, and so began a distinguished career.  At the time of the debate, Shapley was about 34 years old; and certainly the less confident of the two men, holding Curtis in some awe. 

At about age 48, Heber Doust Curtis was the older of the two and a long-established, well-respected astronomer.  He was described as having an aura of superiority and was well-known for speaking with authority and confidence in matters of astronomy.  It is recorded that both Shapley and Curtis actually shared a carriage on the 4000 km train journey from California to the debate venue in Washington.  The atmosphere between the two men remained extremely strained throughout their enforced time together.  By no stretch of the imagination could they have been considered ‘friends’.

 

On the night of the debate, each man was given 40 minutes to outline his case; a very short time for such a complex subject.  Shapley was the first to speak.  He kept his presentation broad, hoping to appeal to a scientific audience which lacked representation from astronomical fields.  It is worth noting here, at that time, Vesto Slipher (1875-1969), using spectroscopy, had already established the concept of red shift; and that the galaxies were moving rapidly away from each other.  Despite this, Shapley insisted the Milky Way was the whole universe.  Shapley estimated of the size of our galaxy to be ten-times larger than a previous estimate; made at the turn of the century by Jacobus Kapteyn (1851-1922).  Shapley predicted the diameter of the Milky Way to be some 300,000 light years, or 100 kilo-parsecs.  He also positioned the centre of our Galaxy approximately 20 kilo-parsecs from the Sun.  All of these estimates were reportedly made with the aid of Cepheid variables.  Having presented his *metagalaxy concept, Shapley suggested his model obviously precluded spiral galaxies being bodies in their own right.  A second element to Shapley’s argument concerned the 1885 nova in the Andromeda Nebula; however, this proved to be a major fault in his argument.  What Shapley failed to understand was the result of interstellar absorption on the apparent brightness of distant stars. (Terrestrial example: street lights on a foggy night).

 

Curtis elected to ignore the audience mix, and delivered an astronomically technical presentation.  As indicated, he spoke with great authority and self-confidence.  Heber Curtis firmly believed the Milky Way to be only a small part of a much larger universe.  Unlike Shapley, Curtis estimated the diameter of our galaxy to be 30,000 light years or 10 kilo-parsecs.  He also estimated the Sun to be very near the centre of the Milky Way.  In describing our galaxy, he suggested it might be shaped like a flat lens �" like two dinner plates laid face to face. 

In regard to spiral nebulae, he claimed them to be ‘island universes’ positioned outside our galaxy.  This idea had also been postulated by Immanuel Kant in the mid-18^th century.  The key reason Curtis gave for disputing Shapley’s metagalaxy model was that Cepheid variables were not good distance indicators. 

 

Anyone with a basic knowledge of astronomy today would quickly realise the debate was not settled decisively by either man.  If anything, the ‘debate’ merely emphasized the lack of substantial evidence for either man’s argument.  Whilst the dispute focussed attention on a question that, at that time, was far from resolved, it also accentuated the dangers of professional arrogance.  Both men had publically presented rival theories concerning matters at the limits of science, each prepared only with the weakest of evidence.

 

Oddly, each man was accurate on at least one major point, and both were incorrect on a key point.  Shapley was correct, when he stated the Sun was well away from the centre of our Galaxy.  Shapley had also been correct in suggesting the Galaxy was much larger than previous estimates.  As we are now aware, Harlow Shapley was also right about the usefulness of Cepheid variables as distance indicators.

 

Heber Curtis was of course right about spiral nebulae being outer galaxies, he had also been correct when discussing the inaccuracies of van Maanen’s measurements of these galaxies.  However �" both men were wrong about some things they actually agreed upon �" the most obvious being the importance of interstellar absorption of light.  Both men had stated this phenomenon was not important in determining the size of the universe.  Today, we understand this to be an extremely important issue.

 

Attending another Astronomy Forum, I recall the subject: "Slipher, and the discovery of galaxies and the expanding universe”.  Prof. John Peacock (Edinburgh Uni), explained in great detail how it is difficult, with the naked eye, to see anything that is not in our Milky Way.  Proverbially, one cannot see the wood for trees �" or in this instance, the stars for the cosmic dust etc.  This element was one aspect that caused confusion in the Shapley-Curtis Debate; and to some extent was still not fully resolved or understood in 1920.

 

Discussing the resolution of the Great Debate, Robert Nemiroff et al, specified how in 1924, Edwin Hubble found Cepheid variables in the nearest major spiral nebula, M31.  This had followed a period where stars in the nebula were being determined.   When challenged with this evidence Shapley immediately acknowledged he was wrong on the spiral nebulae question.  Furthermore, during the 1920s and 1930s, evidence grew that interstellar (light) absorption was a significant issue.  During that period the importance of Cepheid variables for distance authentication was also verified. 

 

Whilst the Great Debate was neither ‘Great’ nor really ‘a debate’ �" it was, nevertheless, an important intellectual event in the development of human understanding of the universe.  No individual person has all the answers.  Disagreement is as important as consensus, and mistakes are a vital part of our learning processes.  Both Shapley and Curtis lived to serve mankind as leaders in the fields of science, astronomy, physics and cosmology. 

 

Harlow Shapley’s ‘other love’ was Myrmecology �" the study of ants �" which was his lifetime hobby. He passed away two weeks before his 87^th birthday, in 1972. 

 

After a long, illustrious career, Heber Doust Curtis, astronomer extraordinaire, died in 1942 aged 69.  Their story is yet another chapter in humanity’s on-going unravelling of the wonders of the universe.

                      

 

 

Addendum:

Ø  In 1920 America’s national debt was $24.3 million �" (now $14.5 trillion), and the worlds’ population was two billion.

Ø  *Metagalaxy: the total system of galaxies and intergalactic space making up the universe

Ø  *Nebulae - German �" nebelflecken

Ø  'Some piously record "In the beginning God", but I say "In the beginning hydrogen". �" Harlow Shapley

Refs:

Gibbs, K. (2010). The Curtis-Shapley Debate - two different views of our universe. Retrieved June 25, 2011, from schoolphysics: http://www.schoolphysics.co.uk/age14-16/Astronomy/text/Curtis_shapley/index.html

Robert J Nemiroff et al. (n.d.). The Shapley - Curtis Debate in 1920 - The Scale of the Universe. Retrieved June 25, 2011, from http://apod.nasa.gov/diamond_jubilee/debate20.html

Singh, S. (2005). Big Bang. London: Harper Perennial.

Wikipedia. (2011, April 28). The Great Debate. Retrieved June 24, 2011, from http://en.wikipedia.org/wiki/The_Great_Debate