Countinguniverses ought to be easy. By definition, you can stop at 1.
Troubleis, definitions change. A century ago, the universe was defined as the MilkyWay galaxy. Heretics who disagreed had long been ridiculed until sciencestaged what became known as the Great Debate, on April 26, 100 years ago. Onthat date, American astronomers HarlowShapley and HeberCurtis articulated opposing views on the scope of the cosmos.
Todayastronomers know that the Milky Way, huge as it is, is a mere drop in thecosmic bucket. Just as the sun is only one of 100 billion or so stars swirlingwithin the Milky Ways pinwheel disk, the Milky Way is only one of hundreds ofbillions of such galaxies inhabiting a vast, expanding bubble of space.
Butin 1920, conventional wisdom dictated that the Milky Way was alone. Mostexperts insisted that the fuzzy patches of light known as nebulae residedwithin the Milky Way. Nebulae with a spiral structure might be solar systems inthe making, some astronomers suggested.
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Othersinsisted that the nebulae were far, far away, well beyond the Milky Waysborders. In fact, the heretics argued, the nebulae (at least some) containedstars in quantities comparable to our galaxy, and deserved recognition asisland universes.
Actually,the island universe idea had been a popular explanation for the nebulae in themid-19th century. (American astronomer OrmsbyMacKnight Mitchel coined the island universe label inthe 1840s, a translation from a German article referring to the nebulae as Weltinseln.)But by centurys end, the astronomical consensus had affirmed the Milky Way asthe sole and rightful universe. Irish astronomer and author Agnes Clerkedeclared in 1890 that no competent thinker believed the nebulae to be galaxiescomparable to the Milky Way. She later wrote that the island universe theoryhad passed into the realm of discarded and half-forgotten speculations.
Butduring the first two decades of the 20th century, new astronomical observationsraised doubts. Curtis, for one, maintained that the evidence favored islanduniverses. But Shapley insisted that the nebulae could not be far enough awayto be outside the Milky Way. He cited measurements (by Adriaan van Maanen) ofmotion of the spiral arms within some nebulae; such motion would beundetectable if the nebulae were actually distant galaxies.
In1919, leaders of the National Academy of Sciences decided it would be fun to holda debate on the dispute at the academys meeting the following April.
Technically,the topic of the debate was to be on the distance scale of the universe. Onthat issue, Curtis was the conservative and Shapley was the heretic. Curtismaintained the more traditional view that the visible Milky Way stretched onlyabout 30,000 light-years across at most, and was possibly much smaller. Shapleythought that the Milky Way had a diameter of 300,000 light-years (much bigger eventhan todays estimate of roughly 100,000 light-years or so).
AlthoughShapleys view of the Milky Ways size was radical, it did support theconsensus view opposing island universes.
If, as Shapley maintained,the Galaxy was much larger than had previously been thought, it would be moredifficult for Curtis to sustain the claim that the spiral nebulae wereindependent island universes, historian Michael Hoskin observed in a 1976 paper analyzing the debate.
Asit turned out, the debate was nothing that CNN would had televised. Eachastronomer just presented a 40-minute talk. Shapley, who went first, read froma typewritten script. Curtis, the better speaker, showed slides, a more powerfulway to make his point.
Shapleyrecounted a potpourri of recent astronomical observations, barely mentioningthe island universe theory. He insisted that Curtis interpretation of the observationsrequired abandoning the very foundations of modern astrophysics.But he acknowledged that if the Milky Way was really small, the island universeidea just maybe could be right.
Ifthe galactic system is as large as I maintain, the spiral nebulae can hardly becomparable galactic systems, Shapley declared. If it is but one-tenth aslarge, theremightbe a good opportunity for the hypothesisthat our galactic system is a spiral nebula, comparable in size with the otherspiral nebulae, all of which would then be island universes of stars.
Curtispresented data supporting his view of a smaller Milky Way, citing variousestimates of its diameter ranging from 10,000 light-years to 30,000light-years. He argued that the analysis of light from spiral nebulae indicatedthat they were clusters of stars (with similar features to the spectrum oflight from the Milky Way itself). The spectrum of the spiral nebulae offers nodifficulties in the island universe theory of the spirals, Curtis stated.Subsequent slides further built the case for the spirals as island universes.
Moredetailed arguments (deviating considerably from the original talks) appearedthe next year inpapers by Shapley and Curtis published jointly under the title TheScale of the Universe in the Bulletin of the National Research Council.Resolution of the debate came two years later: Astronomer Edwin Hubbledemonstrated that the Andromedanebula was truly an island universe full of stars at a distance farexceeding even Shapleys generous estimate of the Milky Ways girth.
Facedwith new findings, Shapley had to concede. When a letter arrived from Hubblereporting the Andromeda results, Shapley remarked: Here is the letter thatdestroyed my universe.
Shapleyhad been misled by van Maanens measurements they simply turned out to bewrong. Shapley said later that van Maanen was his friend, so of course he believed him,astronomer Virginia Trimble commented in a 1995 discussion of the debate.
ButShapley had not been entirely defeated. For on another important point, he wasright, and Curtis was wrong. In his smaller Milky Way, Curtis placed the sunvery near the center, as astronomical consensus dictated. Around the turn ofthe century, astronomer Simon Newcomb had wondered about that consensus, though,pointing out that ancient astronomers believed with equal confidence that theEarth sat at the center of the universe. Shapley declared that Newcomb wasright to be skeptical.
We havebeen victimized by the chance position of the sun near the center of asubordinate system, and misled by the consequent phenomena, to think that weare Gods own appointed, right in the thick of things, Shapley said at the1920 debate in much the same way ancient man was misled, by the rotation ofthe earth to believe that even his little planet was the center of theuniverse.
Today astronomers all know that Shapley was right about the sun; it is substantially displaced from the galactic center. And everybody knows that Curtis was also right: The Milky Way home to sun, Earth and humankind is not a single universe unto itself, but one of a myriad upon myriad of other galaxies no longer known as island universes, as the definition of universe had to be changed.
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