“The Shapley–Curtis debate makes interesting reading, even today. It is important not only as a historical document but also as a glimpse into the reasoning processes of eminent scientists engaged in a great controversy for which the evidence on both sides is fragmentary and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. This debate illustrates forcefully how tricky it is to pick one’s way through the treacherous ground that characterizes research at the frontiers of science.” – Frank Shu

In 1919 George Hale, head of Mount Wilson Observatory, called for the US National Academy of Sciences to host a debate about either Einstein’s theory of relativity or “island universes” – galaxies outside of our own. The home secretary of the academy, C G Abbot, was sanguine that either would be worth pursuing on a public stage. He wrote to Hale saying: “You mentioned the possibility of a sort of debate. From the way the English are rushing relativity in Nature and elsewhere, it looks as if the subject would be done to death long before the meeting of the academy, and perhaps your first proposal to discuss the island universe would be more interesting. I have a sort of fear, however, that people care so little about island universes that unless the speakers took pains to make the subject very engaging, the thing would fall flat.”

And so it followed that at a meeting of the academy on 26 April 1920, scientists Harlow Shapley and Heber Curtis presented contrasting arguments that collectively came to be known as “the Great Debate”, though at the time it was officially titled “The Scale of the Universe”.

Shapley argued that there was nothing more to the universe than our Milky Way galaxy, and there were no other “island universes”. He could not swallow the notion that there was more to reality than our own galaxy, as that would imply that Andromeda (pictured above) was about 10⁸ light-years away from us. Furthermore, he claimed these observed “spiral nebulae” were merely nearby gas clouds within the Milky Way, rather than distinct galaxies in their own right. He also argued that our Sun was far from the centre of the Milky Way, yet another point of disagreement between Curtis and him.

The Great Debate was not only about the nature of the universe. The young, hungry Shapley had an agenda – he had hoped that by defeating the older Curtis, he would earn the directorship of Harvard College Observatory. It would be a tall order – Curtis had been known to be a skilled and precise orator. And his confidence was on full display even before the debate. In a letter to Shapley he wrote that “A good friendly scrap is an excellent thing, once in a while…sort of clears up the atmosphere.”

A good friendly scrap is an excellent thing, once in a while…sort of clears up the atmosphere

It is likely that Shapley knew he would be swimming upstream. He opened his argument by saying that “To Ptolemy and his school, the universe was geocentric; but since the time of Copernicus the Sun, as the dominating body of the solar system, has been considered to be at or near the centre of the stellar realm. With the origin of each of these successive conceptions, the system of stars has ever appeared larger than was thought before. Thus the significance of man and Earth in the sidereal scheme has dwindled with advancing knowledge of the physical world, and our conception of the dimensions of the discernible stellar universe has progressively changed. Is not further evolution of our ideas probable?”

Contrary to Shapley, Curtis argued that Andromeda and other spiral nebulae could, in fact, be other galaxies. In support of his hypothesis he appealed to the fact that Andromeda seemed to possess more novae than the Milky Way. Why would this be if Andromeda was merely a part of the Milky Way? A better explanation was that Andromeda was a distinct galaxy that simply possessed a different rate of nova occurrences than the Milky Way.

Shapley and Curtis were both given 40 minutes to make their case to an audience of academics, possibly including Albert Einstein. Shapley presented ideas he had written in one of his papers, emphasizing the scale of the Milky Way. Curtis, meanwhile, offered a slideshow to express his explanation that spiral nebulae were actually “island universes” – better known to us today as galaxies. The speakers were not really addressing each other’s core arguments, with Shapley focusing on the Milky Way’s size and Curtis on the possibility of island universes. Curtis’s eloquence and stage presence dwarfed Shapley’s, which may ultimately have contributed to Curtis’s victory in the eyes of the audience.

Fortunately for Shapley, he still earned the directorship of the Harvard College Observatory, while Curtis went on to run the Allegheny Observatory. In 1923 astronomer Edwin Hubble measured the changing brightness of what are called Cepheid variable stars. He demonstrated that they were so distant from us as to be outside of the Milky Way. With that, the Great Debate was settled, and Curtis’s apparent victory upgraded to a definitive one.

Shapley’s position that the Milky Way is the entirety of our universe might seem laughable to our contemporary minds. No modern scientist would admit to ever rejecting a good explanation of observations merely because it violates our intuitions about what reality should be like. But we should hesitate before judging Shapley. Today, there are debates about the existence of the multiverse, and future scientists may one day laugh at our unwillingness to accept that, just as we are tempted to judge Shapley’s resistance to accept the true size of the universe.

• This article was published in Lateral Thoughts, Physics World’s regular column of humorous and offbeat essays, puzzles, crosswords, quizzes and comics, which appears on the back page of the print edition. You can submit your own Lateral Thoughts. Articles should be 900–950 words, and can be e-mailed to pwld@ioppublishing.org