This Month in Physics History

April 26, 1920: The Shapley-Curtis Debate

Harlow Shapley (left); Heber D. Curtis (right)

In April 1920, the Natural History Museum in Washington, DC, played host to an historic interchange on competing theories about the scale of the universe. At the center of the controversy were Harlow Shapley, a young ambitious rising star in astronomy who specialized in the properties of stars in binary systems of globular clusters, and Heber D. Curtis, a well-respected established authority on the properties of spiral nebulae known for his conservative approach and frequent skepticism of new theories. Their confrontation at the 1920 meeting of the National Academy of Science in Washington is widely held to be at the crux of a major shift of humanity's view of its place in the universe.

Ironically, given its historical significance to astronomers, the actual debate was neither well publicized nor well-attended at the time it occurred during the 1920 meeting of the National Academy of Sciences - nor was it a truly formal "debate" in the modern sense of the word. Shapley and Curtis each gave a 40-minute presentation and were allowed one opportunity to rebut the other's remarks, followed by commentary from the floor. Curtis argued that the universe is composed of many galaxies similar to our own, identified by astronomers of that period as "spiral nebulae." In contrast, Shapley believed that the spiral nebulae were merely nearby gas clouds, and that the universe was composed of a single large galaxy. In Shapley's model, the sun is far from the center of the galaxy, while Curtis located the sun near its center.

Astronomers still disagree about which man technically "won" the debate, but history ultimately has the final say, and it appears to be draw. A partial resolution to the standoff occurred in the mid-1920s, when Edwin Hubble identified Cepheid variable stars in the Andromeda Galaxy, which enabled him to demonstrate that the distance to Andromeda was even greater than Shapley proposed. In the 1930s, the further discovery of interstellar absorption, combined with an increased understanding of the distances and distribution of globular clusters, led to general acceptance that the Milky Way was much larger than previously estimated, and that the Sun was not near its center.

Thus, Shapley proved to be correct about the size of our galaxy and the sun's location in it, while Curtis correctly predicted that the universe is composed of many galaxies, among them the spiral nebulae which are very similar to our own galaxy - a point Shapley readily conceded when the new evidence came to light. Shapley was also correct about the usefulness of Cepheid variables as distance indicators, which continue to be cornerstones of our knowledge of distance to further objects today. Ironically, both men were mistaken regarding a point on which they were in agreement: the interstellar absorption of starlight, which they agreed was not important in determining the size of the galaxy.

According to the astronomer F. Shu, the Shapley-Curtis debate stands to this day as a fascinating "glimpse into the reasoning processes of eminent scientists engaged in a great controversy for which the evidence on both sides is fragmentary and partly faulty," and as an historical illustration of the difficulty of navigating through "the treacherous ground that characterizes research at the frontiers of science."

For more information on the Shapley-Curtis debate, see http://antwrp.gsfc.nasa.gov/diamond_jubilee/debate_1920.cfm.

Birthdays for April:

April 12: Igor Tamm (1895)
April 22: J. Robert Oppenheimer (1904)
April 23: Max Planck (1858)
April 29: Henri Poincar (1854)
April 30: Karl Friedrich Gauss (1777)