By the beginning of the twentieth century, however, the tide of opinion had turned. The primary objection was geometric. Most of the spiral nebulae appear in parts of the sky far away from the band of the Milky Way. If these were external galaxies, the reasoning went, they should appear scattered evenly across the sky. The fact that they appear to flee from the Milky Way suggests they are secondary objects ejected from our galaxy. The real answer, discovered much later, is that the Milky Way contains light-absorbing gas and dust that blots out the light from any object lying behind it. A few researchers suspected as much at the time. But in those early days of the new sci/religion, many scientists still carried a subtle reactionary agenda in their hearts. In 1905, Agnes Clerke, a highly influential astronomer and scientific historian, wrote, “No competent thinker, with the whole of the available evidence before him, can now, it is safe to say, maintain any single nebula to be a star system of co-ordinate rank with the Milky Way.” There is a whiff of the haughtiness of old religious cosmologies in these words: if our galaxy isn't the only one, at the very least it is by far the greatest. Sci/religion would soon establish the exact opposite notion as its approved doctrine. Until astronomers could determine the distance scale of the universe, they could make no connections between what they were seeing and the exciting but abstract cosmological ideas that Einstein had inspired.
In April 1920, the National Academy of Sciences in Washington, D.C., convened a debate (now recalled simply as the “Great Debate” because of its historical importance) to shed some light on the issue. Heber Curtis of the Lick Observatory in Santa Cruz, California, argued that the Milky Way was small in comparison to the vastness of space, and that the spiral nebulae were surely other galaxies lying at tremendous distances. Harlow Shapley from the Mount Wilson Observatory, just a couple of hundred miles south in Pasadena, took the opposing side. His view was that the Milky Way was about ten times as large as previously thought, so huge that it dominated the universe. Those vexing nebulae were just some gaseous junk spinning away from us, though Shapley understandably could offer only vague suggestions of how they got there or why they were in such a hurry to get away.
Curtis and Shapley argued their positions in styles as different as their viewpoints. Curtis was forty-eight, thirteen years older than Shapley, and with his round spectacles and dapper suit he looked rather patrician. He already had a well-established career at Lick, and he spoke with an orator's polish as he delved through a fairly technical presentation somewhat enlivened by a number of slides. Shapley, who resembled a rumpled Peter Lorre, took a more direct approach. As a former newspaperman, he knew better than to bludgeon his congregation with data. He delivered instead a directly spoken, broadly themed homily—although he, too, quickly introduced the many specific observational details he needed to press his case. Members of the National Academy of Sciences who wished to have a few drinks to ease them through the debate were out of luck. The Nineteenth Amendment had taken effect earlier that year, so the academy had to dispense with the wine it normally served before such lectures. Historians of science have come to view the Great Debate as a crucial prelude to the dramatic astronomical discoveries that occurred later in the decade. Once scientists understood the size and nature of the spiral nebulae, they quickly understood a great deal about the size and structure of the universe writ large. For Shapley, too, this was a pivotal event: he was bucking for the directorship of Harvard College Observatory, knowing that one of the key members of the observatory's visiting committee was attending the debate. The actual contretemps, however, depended on a series of intricate arguments that sailed serenely over the heads of most of the audience, few of whom were astronomers. It garnered little attention in either the popular press or the scientific literature of the time. Sometimes great moments in history make themselves known only in retrospect. “I had forgotten about the whole thing long ago,” Shapley wrote in his 1969 autobiography. “To have it come up suddenly as an issue, and as something historic, was a surprise.” Eventually the Great Debate became known as pivotal confrontation of ideas, the Diet of Worms to the cosmological revolution that lay just a few years ahead.
At first glance, Curtis would seem the more progressive thinker of the two men. He maintained that “the spirals are not intra-galactic objects but island universes, like our own galaxy, and that the spirals, as external galaxies, indicate to us a greater universe into which we may penetrate to distances of ten million to a hundred million light years.” He was right and even a bit conservative. The deepest images from the Hubble Space Telescope show objects roughly thirteen billion light-years distant. In a series of clever back-and-forth comparisons, Curtis showed that the properties of the spiral nebulae seemed strange if they were located in our galaxy but made perfect sense if they lay in far reaches of space. He reiterated that the spectra of these nebulae—that is, the patterns of their light when passed through a prism—strongly resembled those of stars, not clouds of gas. He correctly guessed that obscuring matter in our galaxy explains why no spirals appear along the band of the Milky Way. And he cited studies of the enormous speeds of the spiral nebulae. “Their space velocity is one hundred times that of the galactic diffuse nebulae. . . such high speeds seem possible for individual galaxies,” he said. In closing, he added a personal comment that shows how deeply mystical concerns had penetrated the world of observational astronomy: “There is a unity and an internal agreement in the features of the island universe theory which appeals very strongly to me.”
But the debate was not as black and white as it appeared on the surface. While Curtis was largely correct in his views about the reality of other galaxies, he was in many other ways a scientific conservative. A year after the eclipse expedition that seemed to prove general relativity, Curtis remained skeptical of Einstein's radical theory and its cosmological implications. In the Great Debate, he severely underestimated the size of the Milky Way because he still placed his trust in star-counting measurements, an old-fashioned reckoning technique dating back to William Herschel. Curtis distrusted Shapley's brand-new, and much more accurate, measurement techniques that relied on the study of certain types of variable stars.
Here we must have some sympathy for Curtis, however, because nature had played a nasty trick on him. In 1885 a bright star had flared up in the Andromeda, one of the brightest of the spiral nebulae. Assuming this was a nova, an erupting star similar to the ones seen in our galaxy, the Andromeda nebula couldn't be very far away. That in turn meant that it had to be fairly small. If nebulae were galaxies similar to our own, then the Milky Way had to be small as well. But Shapley's variable stars indicated the Milky Way is huge. Curtis was backed into a corner, so he had to argue against the way his opponent measured distances. What he didn't know—what nobody understood at that time—was that the 1885 star wasn't a nova. It was a supernova, a much brighter type of stellar explosion that is readily visible from much greater distances.
It is harder to excuse Curtis for his other bit of pigheadedness. Again following convention, he believed that the Earth lies almost at the exact center of the Milky Way. Such had been the party line ever since Herschel conducted a galactic census, tediously cataloging every star he could observe. In fact, we lie way out in the celestial suburbs, some two-thirds the way to the edge of the disk, but dusty gas clouds blot out much of the light from the dense concentration of stars at the center. The clouds create an illusion that the galaxy is equally bright in all directions, as if we were smack in the middle.
Shapley had found a way to part the interstellar mists and glimpse our true place in the Milky Way, using a technique that four years later definitely settled the entire dispute over the nature of the spiral nebulae. Here he drew on an amazing discovery by Henrietta Swan Leavitt, one of Harvard College Observatory's “computers”—the people who did the mundane but crucial work of surveying thousands upon thousands of stars and cataloging their properties. Starting around 1908, the meticulous Leavitt turned her attention to a u
nique class of stars called Cepheid variables, so called because the best-studied star of this type lies in the constellation Cepheus. These stars grow brighter and dimmer in a predictable, endlessly repeating manner. More extraordinary, she found that the period of variation correlates directly with the star's real, intrinsic luminosity: the longer the period, the more luminous the star. Ejnar Hertzsprung, a Danish astronomer who developed the still-standard scheme for classifying stars by color and luminosity, completed Leavitt's work by determining the approximate distances to a few nearby Cepheids, thereby anchoring the whole system of measurement. Now astronomers had reasonable hope that they could use the mathematical rigor of sci/religion to quantify the once-unfathomable depths of the cosmos.
Leavitt's intellectual process was roughly the inverse of Einstein's. He started with his visionary theory and worked his way down; she started with photographic plates from Harvard's twenty-four-inch refracting telescope and built up conclusions from what she saw. But humble sky analysts like Leavitt, a mostly female group laboring away like nuns in the cloister, were every bit as important as the high-profile cosmologists in extending the mystical power of modern science. Her research rested on the same kinds of optimistic assumptions as Einstein's 1917 cosmology. She believed in the constancy of physical law across time and space. She believed in the ultimate knowability of the universe. If Einstein was the prophet, Leavitt and her ilk were the quiet upholders of the faith who went out and witnessed science's miraculous power to bring the distant galaxies within reach. They spread the gospel by systematically attaching numbers to the different parts of the universe. Leavitt was a champion number cruncher.
Before she discovered the clocklike predictability of the Cepheids, astronomers were like mariners squinting at a light on the horizon. Not knowing its distance, they could not say for sure what they were looking at. It could be a mighty bonfire in a distant city, or it might be a puny lantern in the window of a seaside cottage right off the ship's bow. Leavitt found what scientists call a “standard candle”—a lighthouse beacon of known luminosity that allows an accurate plotting of the celestial seas and shoals. These beacons followed a remarkably regular pattern in which the period indicates the true luminosity. If all slowly flashing lighthouses had very powerful bulbs while the rapidly flashing ones had weak ones, you could estimate how far you are from shore just by timing the pulses of light. Similarly, a careful observer can determine a Cepheid's true luminosity and hence its distance just by plotting its changing brightness over several weeks. Cepheids are what makes the scale of the universe comprehensible. They were the instrument of Shapley's greatest triumph and later his greatest disgrace. Soon they unraveled Einstein's cosmology as well. Shapley used the new Cepheid technique, along with the mighty eye of the sixty-inch reflecting telescope at Mount Wilson, to map the size and shape of our galaxy. He got the size of the Milky Way wrong—he thought it was about three hundred thousand light-years across, three times larger than the currently accepted number, due to a misreading of his variable stars—but he got the layout dead on. We live not at the center of the Milky Way, as everyone from Herschel on had believed, but far off to one side. Shapley cast himself as a modern Copernicus, weaning mankind from one more delusion about its central importance. “The significance of man and the earth in the sidereal scheme has dwindled with advancing knowledge of the physical world. . . we have reached an epoch, I believe, when another advance is necessary,” he told his audience at the National Academy of Sciences. He implicitly accepted the modern view that our knowledge of the universe, not our place in the universe, is what makes us special. “Our conception of the galactic system must be enlarged to keep in proper relationship the objects our telescopes are finding; the solar system can no longer maintain a central location,” Shapley said.
Curtis could not bring himself to accept these findings. Partly he questioned Shapley's measurements because they made the Milky Way improbably enormous, “at least ten times greater than formerly accepted.” If our galaxy were that large, it would seemingly engulf the nearer spiral nebulae and hence cast doubt on the island universe interpretation that Curtis found so appealing. Such a drastic size revision demanded airtight proof, he insisted, and he distrusted the new and relatively unproven method of measuring distances with Cepheids. He questioned the basic assumption behind this technique, that the Cepheid variable stars behave the same way “anywhere in the universe.” Curtis was a cautious researcher, the type who typically ended statements with the mantra “More data are required.” But in his arguments there's also a hint of deeper skepticism regarding Shapley's optimistic belief in the uniformity of nature. Curtis just didn't have the faith.
In many ways it was Shapley, not Curtis, who was the more devout disciple of sci/religion in the Great Debate. Where Curtis kept calling for additional data, Shapley unflinchingly exploited up-to-the-minute techniques. Shapley's mistake was primarily one of recklessness. He put so much faith in his Cepheids and in the principle of uniformity that he didn't recognize the sources of error that caused him to overstate their distances. Based on those measurements, “the spiral nebulae can hardly be comparable galactic systems,” he baldly claimed. They could scarcely compare to his puffed-up picture of the Milky Way. Possibly they were minor stellar groupings basking in the grand glory of our galaxy or, more likely, they were nothing more than the gaseous whirlpools they appeared to be.
At the very end of his lecture, Shapley added an interesting caveat. “There maybe elsewhere in space stellar systems equal to or greater than ours—as yet unrecognized and possibly quite beyond the power of existing optical devices and present measuring scales,” he said. Many astronomers at the time similarly believed that part of the universe lay beyond human perception. Einstein's 1917 cosmology, which boldly asserted that the rules of general relativity hold everywhere through a finite reality, inaugurated a shift away from this line of thinking. Even then, Lambda represented a kind of hedge in Einstein's nascent cosmic religion. To embrace the cosmos—truly everything—he needed an extra factor that was justified neither by theory nor by observation. But just a few years later, theory and observation began to merge, Lambda fell by the wayside, and cosmologists rapidly expanded the empire of numbers throughout all of space and time.
The Great Debate called attention to some of the novel astronomical techniques and observations that would make this possible. Yet one of the most remarkable discoveries of the day occupied only a minor role in the event. As Curtis had briefly mentioned, some of the spiral nebulae appeared to be moving at very high speeds, more than seven hundred miles per second. He took these tremendous velocities as evidence that these objects could not reside within our galaxy, where far more sedate motions are the norm. But there was much, much more to these peculiar motions, as soon became clear. Measurements of the anomalous motions of the spiral nebulae provided a key piece of the evidence that discredited Lambda, blew apart the age-old notion of the static universe, and ushered in the first creation story rooted in science.
In 1920, however, tracking the flirtings of spiral nebulae seemed a tedious and not terribly exciting job. Basically the task fell to one man, a hardworking and little-known astronomer by the name of Vesto Melvin Slipher. He was one of those brilliant second-string players who lay the groundwork for somebody else to come along and grab all the glory—in this case, Edwin Hubble. There is no Slipher Space Telescope, there are no stirring biographies to inspire future generations of scientists. Yet Slipher's studies marked the essential first step toward the discovery that we live in an expanding universe. Had he worked with better instruments or possessed a more flamboyant personality, Slipher might be the one enshrouded in fame. Instead he is remembered, like Leavitt, as a diligent number cruncher and a historical footnote.
Slipher started down the road to near fame in 1901 when he left Indiana University to take on a “temporary” position at the notorious Lowell Observatory in Flagstaff, Arizona. He ended up staying for fifty-three year
s. Percival Lowell, a member of the aristocratic Lowell family of Boston, had built this lavish observatory as a shrine to his quixotic scientific fixations. Most famously, he believed that Mars was inhabited by an intelligent, dying race that had crisscrossed the planet with canals to distribute a dwindling supply of water. He drew wildly detailed maps of these canal systems, which he swore he could see through his telescope. The canals of Mars found their way into popular lore; Lowell's hyperbolic books became best-sellers. When NASA's modern spacecraft visited Mars, they spotted a giant canyon and various dark markings that Lowell might have seen and misinterpreted. The planet abounds with unique geological features, including giant volcanoes, ancient riverbeds, and windswept craters. There are, needless to say, no canals.
Despite the Mars fiasco, Lowell's cranky obsessions yielded some serious results regarding the pantheon of our solar system. His unbending belief in a planet beyond Neptune led to the serendipitous discovery of Pluto by Clyde Tombaugh, who was working under Slipher's direction. And Lowell's fascination with how planetary systems form inspired him to direct Slipher to study the spiral nebulae. Lowell was of the opinion that those nebulae were swirling clouds of gas caught in the act of condensing into stars and planets, a view also shared by a number of saner scientists of the day. He wanted to document this process and instructed Slipher to measure the presumed whirlpool-like motions of the nebulae. The meek Slipher and flamboyant Lowell made an odd pair, but they proved strangely suited to each other. Slipher's perpetually formal attire appealed to his boss's patrician tastes, and his measured approach to research counterbalanced Lowell's reckless, eccentric speculations. Lowell had such faith in his protege that he named Slipher his successor as head of the observatory. After Lowell's death in 1916, the observatory and all its resources were at Slipher's disposal.
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