Starting around 1910, Slipher set out on the delicate task of measuring the motions of the spiral nebulae. He did so by looking at slight changes in the frequency of light caused by motion. Light waves from an approaching object pile up on top of one another, causing the waves to compress to slightly shorter, bluer colors. Light waves from a receding object behave in an opposite way: they stretch out, moving to the longer, redder end of the spectrum. Sound waves do the same thing, which is why the pitch of an ambulance siren or a car horn suddenly changes when the vehicle whips by, from a slightly higher (shorter wavelength) tone to a deeper (longer wavelength) one. This effect of motion on wavelength is known as a Doppler shift, in honor of the Czech mathematician Christian Johann Doppler, who in 1842 described the effect for sound and correctly predicted it would affect light as well. You needn't wait for an ambulance to pass to experience the phenomenon. You can hear the Doppler shift in action by warbling your voice in the comfort of your own home if you sing a steady pitch into the spinning blades of a box fan.
By the time Slipher set to work, examining Doppler shifts had become a well-established astronomical technique. The difficulty lay in gathering enough light from the nebulae, whose brightness is spread out over a sizable patch of sky rather than concentrated in one spot like a star. First Slipher zeroed in on his target with the Lowell Observatory's twenty-four-inch refracting telescope. Then he passed the light through a prism that split the light into its separate colors and looked for characteristic markings within the spectrum: darker markings produced by, say, light absorbed by wisps of gas around stars or by the atmosphere around Venus. These markings identified the various elements in the objects, following the same technique that Kirchhoff and Bunsen had used a half century earlier to determine the composition of stars. Finally, Slipher compared the apparent color, or wavelength, of those markings with a reference spectrum of a stationary source in a laboratory. The Doppler shift, the amount of displacement in the spectrum, directly indicated how quickly the object was moving toward or away from the earth. Slipher spent much of his first decade at the Lowell Observatory picking apart the rays from Mars and other planets. He sniffed out the composition of their atmospheres and tried to measure the rotation of Venus and Uranus. Then he was ready to tackle the much fainter glow of the spiral nebulae.
Assuming the nebulae were spinning clouds of gas, Lowell expected Slipher would find one side of the spiral blueshifted, or approaching, and one side redshifted, or receding. Slipher discerned such turning motions all right—galaxies do, after all, rotate—but little else that dovetailed with Lowell's thinking. First of all, the spectra looked like those of stars, not of gas clouds. Second and more surprising, the nebulae as a whole were moving at breakneck speeds. In 1912, Slipher analyzed his first spectral measurement and discovered to his amazement that the Andromeda nebula is racing earthward at two hundred miles per second, much faster than any other object previously studied. Now Slipher started changing his focus, “investigating not only the spectra of the spirals but their velocities as well.” Andromeda turned out to be an anomaly, which leads to the third and most unexpected discovery. An overwhelming majority of the nebulae he observed had their spectra shifted to the red. They were retreating from us at speeds of up to seven hundred miles a second, or some forty times the speed at which the Earth circles the sun.
Slipher's colleagues were bowled over by these giddy nebulae. When he finished reading his report at the 1914 meeting of the American Astronomical Society in Chicago, the audience gave him a standing ovation. Even among professional scientists, who are more easily brought to tears by spectral data than most ordinary people, this was a rare show of enthusiasm. At first the astronomers thought Slipher's measurements might reveal the net motion of the Milky Way with respect to the nebulae. That hope dimmed as Slipher pressed on with his study. By 1917, he had examined twenty-five galaxies and found twenty-one of them were redshifted; if we were drifting past them, half should be shifted to the blue. From then on out, every new galaxy showed the same trend. When Slipher wrapped his project in 1926, having tapped out the Lowell Observatory's modest telescopic power, his tally read four nebulae approaching, forty-one receding.
These runaway nebulae were moving so swiftly that they would quickly escape from our galaxy if they had started out nearby. It seemed only logical, then, that they had never been a part of the Milky Way in the first place. Slipher's 1914 presentation and his follow-up reports therefore buoyed the spirits of those who had never lost faith in Herschel and his belief in a universe full of other island universes. Hertzsprung praised Slipher for his work and took it as proof that the spiral nebulae are separate systems, each comparable to our own. Hertzsprung's support carried particular authority, as he had just completed his demonstration on the use of Cepheid variable stars for determining the distances to cosmic objects. Using his new calibrations, he had deduced that the Magellanic Clouds, hazy patches of light in the southern sky, are independent, satellite galaxies of the Milky Way. Slipher's findings on the nebulae dovetailed perfectly with that view.
Slipher stopped short of declaring that he had solved the riddle of the spiral nebulae. In private, he had no doubt that these nebulae must be other galaxies, but he lacked the absolutely crucial information to prove this interpretation: He did not know how far away they were. Because he couldn't say where the nebulae were, he also couldn't say for sure what they were. He couldn't rule out the possibility that they were small, nearby objects shooting out from our galaxy at high speeds. Einstein could grab the entire universe with his equations but Slipher, saddled with a smallish telescope, could only grope at the periphery of the glory of sci/religion.
Similarly, Slipher couldn't determine the meaning of the enormous and ubiquitous motions of the spiral nebulae. Only a few of the brightest ones, most notably the prominent Andromeda nebula, appeared to be approaching. The fainter ones all showed the strong redshifts indicative of receding objects, and the faintest spirals were moving fastest of all. From this pattern, Slipher was on the verge of discovering that the speeds of the nebulae are proportional to their distance, the indelible signature of an expanding universe. But again, the lack of any way to measure the distances to the nebulae stopped him in his tracks. Slipher had neither the equipment nor the kind of training that could give him the final bit of information, that clinching evidence that would have inscribed him in the history books. So despite the hubbub surrounding Slipher's work, some of the sharpest astronomers of the day still sided with Shapley and refused to accept the existence of other galaxies.
Looking back, the gap between theory and observation is painfully obvious both in the Great Debate and in Slipher's studies. For the moment, the visual astronomers didn't have the conceptual tools they needed to extend the reach of their telescopes. Einstein, on the other hand, had all the reach he needed—he had effortlessly spread the power of general relativity across one hundred million light-years of unknown space—but lacked the numbers that would give his cosmological ideas full sci/religious authority. Had he seriously questioned his colleagues, he would have very likely learned of Slipher's observations—they had, after all, been the hit at one of the world's leading astronomy conferences. He might then have paused before insisting on his static universe. Clearly there was more out there than the slow-moving stars of the Milky Way. But he was seeking God in the equation and was content to let others work out the details of his divine vision. Amazingly, this revolution didn't take very long. Four years after the Great Debate, the question of “the scale of the universe” was decisively settled, Slipher was vindicated, and Shapley's arguments about the spiral nebulae were swept aside. Within a decade, Einstein's static universe was defeated once and for all, and science got into the creation business. All it took was the work of a latter-day Galileo, a scientific heavyweight named Edwin Hubble.
5. EINSTEIN'S PROPHECY FULFILLED
IF EINSTEIN WAS the Abraham of the new sci/religion, Edwin Powell Hubble was its M
oses. While Einstein meditated on his equations, Hubble demonstrated the miraculous nature of the universe as revealed on his photographic plates. While Einstein pondered “the Old One's secrets,” Hubble laid down the law: “Not until the empirical results are exhausted need we pass on to the dreamy realms of speculation.” In the end it was Hubble more than any other who realized Einstein's mystical goal of bringing all the stars and all of space into the domain of sci/religious understanding. Following his observational approach to cosmology, Hubble set out toward the boundaries of Einstein's finite universe. Then he went a step beyond, collecting the key evidence that the universe is also bounded in time—that astronomers might be able to reconstruct the origin of the world. Hubble was a scientific juggernaut who flattened all in his path. During the Great Debate, Harlow Shapley had been able to claim persuasively that the Milky Way is the only galaxy in the universe. Within four years Hubble obliterated this view and reframed our galaxy as one of thousands, perhaps millions. Vesto Slipher had slaved for more than a decade to understand the motions of the spiral nebulae. Three years after he abandoned his research, Hubble cracked the code of the nebulae and collected the decisive evidence that our universe is expanding. After this bombshell, Einstein retreated from his static cosmology and eventually denounced Lambda as his “biggest blunder.”
The man behind these heroic victories was a dashing enigma: a methodical observer, tenacious and frequently brilliant, who was also a pompous celebrity hound, as inclined to magnify his accomplishments as to magnify the stars. Hubble's intricate, extremely private personality has confounded the most dogged biographers. Historian Gale Christiansen, who has written an admirably thorough treatment of Hubble's life, lamented that “the astronomer revealed almost nothing of his inner universe.” Again in a pointed contrast to Einstein, Hubble kept his motivations opaque. His acute instincts guided him to the most fruitful problems in astronomy. He worked his way to Mount Wilson, then the greatest observatory in the world, where he rapidly emerged as the star player among a brilliant team. Yet all the fame and professional success were not enough. He worried that others would try to take some credit for the discoveries he claimed as his own. And he was forever plagued with insecurities that some of the fabricated details of his past would come to light and humiliate him. As a result, he never wrote a memoir, and his wife, Grace, spent years after his early death in 1953 editing his letters and her journals to make them accord with his official history.
Hubble claimed to have rejected a lucrative job as a lawyer; in actuality, he never passed the bar. He was a competent heavyweight boxer in college, but his much repeated tale that promoters in Chicago urged him to go professional was most likely a youthful fantasy. The objectivity of Grace's recollections can be judged from her description of the first time she saw her future husband: “The astronomer looked an Olympian, tall, strong and beautiful, with the shoulders of the Hermes of Praxiteles. . . there was a sense of power, channeled and directed in an adventure that had nothing to do with personal ambition and its anxieties and lack of peace.” Fortunately, she is not the only source of information about him. Edwin Hubble was born in 1889 in Marshfield, Missouri, the son of a lawyer. At the University of Illinois he studied astronomy under Forest Ray Moulton, a leading astronomer of the day, who was convinced the spiral nebulae were the birthplaces of stars. Hubble grew determined to solve the riddle of the nebulae but avoided mentioning this intention to his family, who counted on his finding a career in law. He continued on as a Rhodes scholar at Oxford University, where he majored in jurisprudence. Although his interest in the legal practice did not stick, the culture of Oxford did. The man from Missouri retained a penchant for British attire and British pronunciations—spitting out an occasional exclamation of “Bah Jove!”—for the rest of his life. Upon returning home, he taught Spanish and coached high school basketball in New Albany, Indiana, seemingly uninspired by the career choices that lay ahead. With Moulton's encouragement, Hubble returned to the University of Chicago to earn a Ph.D. in astronomy, but he got sidetracked again. Just as he was offered a position at Mount Wilson, the United States entered World War I and Hubble enlisted. He asked George Ellery Hale, the observatory's director, to hold the job for him. Hubble knew that his studies of the nebulae would depend on access to a powerful telescope, and there was none more powerful than the one-hundred-inch reflector about to begin service on Mount Wilson. Perched atop the chalky mountains near Pasadena, 5,700 feet above sea level, Mount Wilson boasted crystal-clear air and a relaxed commute to the observatory's nerve center at nearby Caltech. The one-hundred-inch Hooker telescope—funded by businessman John Hooker and industrialist turned philanthropist Andrew Carnegie—was the premier example of America's burgeoning wealth and scientific prestige. Even its primary mirror had a noble pedigree: it was cast from green wine-bottle glass in the same factory that had fabricated the mirrors at Versailles for King Louis XIV. Much to Hubble's relief, Hale agreed to keep the position open. Hubble then set off to battle, having hastily completed a dissertation on the classification of faint nebulae by form, brightness, and size. Meanwhile, Shapley remained behind at Mount Wilson, analyzing the languid pulsing of Cepheid variable stars in order to trace the outlines of the Milky Way.
In pointed contrast to Einstein, Friedmann, and Lemaitre, Hubble found the military life exhilarating. He was forever thrilled at being called “Major,” perhaps because he had been spared the worst horrors of war. Although he served in France, he never saw any serious combat action, much to his naive regret. “I barely got under fire and altogether I am disappointed in the matter of the war,” he wrote to Edwin Frost, his adviser at the University of Chicago's Yerkes Observatory. After the war he stayed on for a while in England, which afforded him the opportunity to sit in on lectures by Arthur Eddington, cosmology's great matchmaker, at Trinity College. Finally he could delay the inevitable no longer. In 1919 he returned to life in the United States and, more happily for him, to the job waiting for him at the new observatory. By Christmas he was puffing his pipe in the darkness, working the guide controls of the new Hooker telescope among the pine forests of Mount Wilson.
Hubble quickly established a reputation as punctual, patient, and vain—he still swaggered around in jodhpurs and military boots after starting work at the observatory. He clashed with Shapley, who, like Hubble, was born in Missouri (just seventy miles from Hubble's birthplace) but was worlds apart in his outlook. In the Great Debate, Shapley felt anxious at having to make a public presentation to an audience unfamiliar with astronomy. Here at Mount Wilson he was in his element. Combative, antiwar, and determinedly down-to-earth, Shapley was Hubble's bete noire. While Hubble served in Europe, Shapley had developed a reputation as one of Mount Wilson's top observers. His prestige, and his ego, presented natural obstacles to Hubble, though not for long. Immediately after the Debate, Shapley gambled and took the director's position at Harvard College Observatory. The job brought great prestige, but it meant losing access to the bucketfuls of starlight gathered up each night by Mount Wilson's one-hundred-inch mirror. If any telescope could expose the identity of the spiral nebulae and establish the true construction of the cosmos, surely this was it. According to one story, Shapley had seen photographs of the Andromeda nebula that revealed variable stars, proving the nebula was actually a distant galaxy, but he refused to believe the evidence before his eyes. Once he left there was no second chance. The job now fell to Hubble.
At Mount Wilson, Hubble returned to basics and began consolidating his earlier work on the classification of nebulae. Photographic surveys kept turning up more and more of these fuzzy blobs. By now the celestial catalog listings bulged with tens of thousands of nebula entries. There were “planetary” nebulae, named by William Herschel because their round shapes suggested ghostly echoes of the disks of the planets when viewed through the telescope. There were irregular smears of incandescence that intermingled with stars. There were dark patches that blotted out the light of the Milky Way. An
d then, of course, there were the infernal spiral nebulae that went their own way. To complicate things further, Slipher had found that some nebulae shine by reflected light, so their spectrum naturally looks much like that of a star. That discovery somewhat negated the commonsense argument that spiral nebulae had to be galaxies because their light appeared so much like that of a mass of stars. On the other hand, Slipher had found that the spirals appear to be receding at enormous velocities, which he considered near conclusive evidence that they are separate systems similar to the Milky Way. The Great Debate had produced no immediate movement on this issue. Eager but tentative, Hubble classified the spiral nebulae separately from the others without overtly endorsing the still-controversial “island universe” theory.
Part of Hubble's genius lay in tackling the right problem at the right time, and his instinct told him that following the spiral nebulae would lead to a scientific bonanza. But he was operating on more than pure intuition. In 1922—the same year Alexander Friedmann developed his first mathematical description of an expanding universe—the Swedish astronomer Knut Lundmark observed what he believed were individual stars in the arms of M33, a bright spiral nebula in a small, faint constellation with the dryly geometric name of Triangulum. Shortly thereafter, John Duncan at Mount Wilson spotted dots of light that grew fainter and brighter in the same nebula. To all appearances these were variable stars, similar to ones in the Milky Way but far dimmer owing to their great distance.
Sensing the answer was at hand, Hubble stepped up his efforts. He spent long nights on his bentwood chair, guiding the movements of the riveted-steel mount of the Hooker telescope to cancel out the Earth's rotation and stay true to the stars. The effort paid off with highly detailed, long-exposure images of the Andromeda nebula. Now the mottled light of Andromeda began to resolve itself into a multitude of luminous points, not a smear of gas but a vast hive of stars. Clinching proof came in October of 1923, when Hubble found the telltale flickering of a lone Cepheid variable among the grainy stellar multitude in one of Andromeda's arms. He watched the star's brightness peak, then dip, then rise again on a thirty-one-day cycle. Hubble then looked up the relationship between period and luminosity for Cepheids, which Shapley had refined while mapping the Milky Way, to derive the distance to Andromeda. His estimate was 930,000 light-years, less than half the current value, but a shockingly large number at the time. That distance placed Andromeda, one of the brightest and presumably the nearest of the nebulae, far outside Shapley's “big galaxy” model.
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