In the Great Debate of 1920, two astronomers argued about the size of the universe. The question was whether the universe is made up of a single galaxy, or whether there are many more. Eventually, modern science explored the possibility of a multiverse, suggesting that there may be more worlds beyond our own.
On April 26, 1920, about 300 scientists were invited to a National Academy of Sciences debate at the Smithsonian Natural History Museum’s Baird Auditorium. Here, two astronomers engaged in a debate that later went down in history as the Great Debate. The topic of the day-long debate was the question, “How big is the universe?” One astronomer believed the universe was a single galaxy. One astronomer argued that the universe consisted of only one galaxy, while the other argued that there were many more galaxies outside of our own. The two astronomers, Harlow Shapley and Heber Doust Curtis, were world-renowned as the directors of Harvard Observatory and Lick Observatory in California, respectively.
The controversy began in 1716, when British scientist Edmund Halley observed a giant haze. Later named Andromeda, this huge body of light attracted the attention of many scientists. “Harlow Shapley and Heber Doust Curtis had different opinions about Andromeda. “Harlow Shapley believed that Andromeda was just a nebula and belonged to our Milky Way galaxy, while Heber Doust Curtis believed that Andromeda was a galaxy and existed outside of our galaxy. The debate over the size and structure of the universe culminated in the Great Debate in April 1920.
The Great Debate ended in 1923 with Edwin Powell Hubble’s discovery of the Cepheid variable star. This discovery and the results of a study published in 1924 proved the existence of Andromeda beyond the size of the Milky Way as claimed by Harlow Shapley. Thus, Harlow Shapley’s claim that Andromeda was within our galaxy was completely destroyed.
As a result of the Great Debate, we realized that the universe is not simply limited to our galaxy, but that there are many galaxies beyond it. This discovery is attributed to Heber Doust Curtis, who challenged Harlow Shapley’s claim based on the rotation rate of the Pinwheel Nebula observed by Van Maanen at the time. Today, we take it for granted that there are many other galaxies outside of our own Milky Way. Heber Doust Curtis’s work deserves praise. He made arguments similar to those of modern cosmology with observational techniques and data that were not yet developed at the time.
Modern astronomy is now looking beyond our universe. Some scientists argue that there are other universes besides ours, while others argue that the multiverse does not exist due to unobservability. Just as we’ve discovered worlds beyond our own galaxy, it’s possible that there are other worlds outside our own universe. So, let’s think about the observability of the multiverse through the lens of the Great Debate of the past.
There are two ways to measure the mass of objects in the universe. There are two ways to measure the mass of objects in the universe: using optical observations (electromagnetic waves) and using observations of gravitational effects (gravitational waves). Scientists have found that the masses from optical observations are smaller than the masses from gravitational observations, which has led them to postulate the existence of matter that does not interact with electromagnetic waves but still has mass, i.e. dark matter. However, what particles dark matter is made of and where it comes from is still a mystery.
This situation is similar to the way we thought about Andromeda in the 1920s. At that time, people thought that Andromeda was a nebula within our galaxy, but several properties were discovered that contradicted this. For example, Heber Doust Curtis observed that the number of supernovae observed within Andromeda is higher than the number of supernovae in the Milky Way as a whole, leading to the idea that Andromeda is another galaxy outside of the Milky Way.
I think the same is true for dark matter. If we consider the universe to be all there is, it’s hard to explain the gravitational pull of dark matter. But if there is a new universe outside of ours, and dark matter is caused by it, that’s a different story. In a multiverse, there are an infinite number of universes, and gravity can act between them. Therefore, we can think of dark matter as a force exerted on our universe by other universes.
Of course, the multiverse has not yet been observed and may never be observed. Some people criticize the multiverse theory, arguing that it’s futile to believe in a theory that hasn’t been observed and has no solid evidence. However, if the Great Debate, which took place less than 100 years ago, is any indication, the possibility of multiple universes cannot be dismissed. “Heber Doust Curtis also argued for an island universe at the time, but modern science has proven him right.
However, some people argue that the multiverse is unobservable. They believe that the case of Heber Doust Curtis and multiverse theories are inherently different. “While the Heber Doust Curtis and Harlow Shapley debates were problems that could be resolved as observational technology improved, it is argued that the multiverse is not only difficult to observe, but may be impossible. This is based on the argument that it is only possible by traveling to another dimension beyond our own, which is impossible for humans who exist in three dimensions. The argument is that if the multiverses are independent of each other and unobservable, then it is not correct to say that they exist.
Nevertheless, I think it is highly likely that multiverses exist. Current cosmology is still in a quagmire. Even the Big Bang theory, which is accepted by many as orthodoxy, is still a completely unproven hypothesis, but I support multiverse theory because it can explain many different theories. Consider superstring theory, which has been proposed since the late 1960s to explain the universe. It proposes that all matter in the world is made of very small strings. There is a contradiction in organizing everything with strings in three dimensions plus time, but if we assume that strings are matter that exists in 10 dimensions, there could be an infinite number of universes.
As such, various theories proposed by modern science explain the possibility of multiple universes. Although multiverses have not been observed, and most likely will never be observed, this does not mean that they do not exist. More than 100 years ago, a scientist succeeded in predicting a new world beyond the one we know, with no observations and no solid evidence. I believe that the multiverse will allow us to go beyond the universe as we know it. Perhaps in the distant future, the multiverse will be accepted as orthodoxy, and we will be on our way to another world beyond the multiverse.
