The Day We Found the Universe - Marcia Bartusiak (2009)
Chapter 14. Using the 100-Inch Telescope the Way It Should Be Used
While it appeared that Hubble had clinched astronomy's brass ring, solving the mystery of the spiral nebulae once and for all, a nagging problem remained: how to explain the galaxies' astounding velocities, first spotted by Vesto Slipher in the 1910s. Why were the spiraling disks speeding away from us? They “shun us like a plague,” exclaimed Eddington. It was a puzzle whose solution would prove to be even more momentous than Hubble's settling the island-universe controversy.
Hubble began to focus his full attention on the cosmic exodus in 1928. That summer the International Astronomical Union was holding its triennial general assembly in the picturesque city of Leiden, set along the Old Rhine in southern Holland. With fine weather to entice them, more than three hundred delegates attended the gathering, where they were entertained with boat excursions down the city's noted canals, gliding past scenery painted by Rembrandt three centuries earlier. It was the height of the Roaring Twenties, and Europe was overflowing with tourists. “Most of the Americans appear to be over here this summer, always on the run scooping up culture with both hands, buying walking sticks and spats, post cards,” noted Lowell astronomer Carl Lampland, who attended the meeting.
Hubble had been appointed acting chairman of the IAU Nebulae Commission, and in and around its July session, he took the opportunity to sit down with Willem de Sitter to discuss relativity and its application to cosmology. Hubble was undoubtedly familiar with (though hardly an expert on) the Einstein and de Sitter solutions to the universe's structure. At the very end of his magisterial 1926 paper “Extra-Galactic Nebulae,” Hubble had included a brief section titled “The Finite Universe of General Relativity,” in which he mentions both of them. Moreover, the following year at Hubble's direction, Milton Humason had remeasured the redshifts of two nearby galaxies. In his brief report, likely ghostwritten by Hubble, Humason specially noted that the galaxy speeds computed from those redshifts were unusually low, “consistent with the marked tendency already observed” for the closest galaxies to have the smaller pace.
Hubble was thus certainly aware of the general trend of galaxy velocities outward, but at Leiden he seemed to have finally grasped the tremendous hubbub the galaxy redshifts were generating among cosmologists and received some further lessons from one of the world's few experts on general relativity. Eager to have his model of the universe put to the test, de Sitter encouraged Hubble at this time to extend the red-shift measurements of the spiral nebulae begun by Slipher at the Lowell Observatory. With only a puny 24-inch refractor with which to work, Slipher had essentially come to the end of his search. He had been able to acquire the redshifts of the brightest spiral nebulae, over forty of them, but trying to obtain a reading from ever fainter and smaller galaxies was impossible. Slipher had exhausted the power of his telescope and could simply not gather enough photons. “The Flagstaff assault on these objects stopped just short of some great excitement,” Shapley later pointed out. Most figured that to reliably establish whether a galaxy's redshift was related to its distance in a predictable way would require a far bigger telescope, like the 100-inch reflector available to Hubble at Mount Wilson. It was the perfect match of problem to instrument. De Sitter knew this, and Hubble was obviously convinced as well.
Upon returning to California, Hubble immediately made this pursuit his top observational priority. Having conquered the mystery of the spiral nebulae, he was now commencing his next great challenge—to see if there truly was a definitive trend to the redshifts of the galaxies as they rushed headlong into distant space. It was at this time that Hubble forged his industrious partnership with Humason, each taking on a specific task to get the overall job done. While Hubble searched for Cepheid variables to determine the distances to a sample of galaxies, his colleague focused on getting the redshift data to figure out the galaxies' velocities (if that indeed was how the redshifts were to be interpreted). Hubble's plan was to put these two pieces of information together and determine if there was a law—a specific formula—that linked a galaxy's distance to its measured redshift.
Humason was not too happy at first upon hearing of his new assignment. Hubble had come home from Leiden quite excited and quickly suggested to Humason that he try to obtain a galaxy redshift that was not yet known. But the prism on the 100-inch spectrograph had started to yellow, and the photographic plates then on hand were considerably slow-acting for such work. Humason knew it would take several nights to get a decent spectrum. “I didn't feel much enthusiasm about these long exposures,” he later recalled. “But [Hubble] kept at me and encouraged me.” Hubble was after fainter and fainter objects, the ones too distant for Slipher to have studied with his smaller telescope, and some were low on the southern sky. “To get these,” said Humason, “you had to climb onto the 100 inch and sit on the iron frame during the long winter nights, which was extremely cold and uncomfortable.” For hours on end, through the freezing night, he would have to keep his guide star on the center of the cross wires, to make sure his image remained sure and steady. “The eye-strain, the monotony, the constant awareness—it was a test of endurance,” he said. But Humason's unusual entrée into astronomy offered him superb preparation for this arduous undertaking. He was accustomed to hard work.
Born in Minnesota in 1891, Humason as a boy moved to the West Coast with his family and one summer as a teenager enjoyed a camping holiday on Mount Wilson. He fell in love with the mountain and soon dropped out of grammar school to work as a bellboy and handyman at the newly opened Mount Wilson hotel, a popular resort spot for local residents. He washed dishes, corralled the horses, and shingled the cottages. Once the 60-inch telescope was under construction, he drove the mule trains that took the equipment, piece by piece, up the rugged path to the top of the peak. When a mountain lion was found feasting on a prized goat in the area, Humason tracked the animal down and shot him between the eyes with a .22-caliber rifle. Several years after Humason married the daughter of the observatory's chief engineer, his father-in-law arranged for him to work as the observatory's janitor. Gradually he was allowed to help the astronomers as a night assistant and over time won their respect and trust in making observations on his own, despite his eighth-grade education and lack of formal training in astronomy. Seth Nicholson took the young man under his wing and taught him some mathematics; Shapley mentored him as well. With his round face and round eyeglasses, the quiet and self-effacing Humason came to look like an academic. In 1920 he was promoted to a staff position in the photography department and two years later moved up to assistant astronomer. Known for his patience and conscientious attention to detail, he became especially skilled at taking long photographic or spectroscopic exposures of the most faint celestial objects. A likable fellow and an inveterate gambler, he relieved the pent-up tension from this grinding work by playing poker with the other night assistants and shop workers. If his schedule allowed, he'd catch the late-afternoon horse races at the nearby Santa Anita racetrack, taking any astronomer who wanted to go with him.
Over time, Humason was even put in charge of arranging telescope time. Sharing Hubble's strong loyalty for the Republican Party, Humason tried to get as many Democrat observers as possible on the mountain, away from the polls, on election days. Solar astronomer Nicholson, a staunch Democrat, evened the score by making sure only Republicans were scheduled on the solar telescopes at the same time. Hubble, whose status could never be threatened by Humason's humbler origins, got along fine with his devoted junior partner.
Milton Humason at Mount Wilson
(Courtesy of AIP Emilio Segrè Visual Archives)
• • •
By 1929 Hubble had determined the distances to twenty-four galaxies (including the Small and Large Magellanic Clouds), the most remote then judged to reside some 6 million light-years away. He accomplished this feat by establishing a ladder of measurements, one rung leading to the next. First he used Cepheid variables, his most reliable yardstick, to directly obtain the distances to six relatively nearby galaxies; then he judged the magnitude of the brightest stars in those galaxies. Figuring such stars were similarly bright in other, more distant galaxies, he proceeded to use them as standard candles. He sought out these radiant stars in more far-off galaxies—fourteen in all—and estimated each galaxy's distance based on the stars' apparent luminosities. Then, taking all twenty of these galaxies into account (the first six and the subsequent fourteen), he estimated the brightness for an average galaxy and used that value for judging the distance to four more remote galaxies. Hubble's moving outward like this, rung by rung, was similar to Shapley's strategy for his globular cluster distance measurements, but here Hubble was making an even braver leap into distant space.
Hubble then paired each galaxy's distance with its measured velocity to see if there was a connection, some sort of organized flow in which the galaxies flew outward into the depths of space. Humason by then had redone a number of the redshifts, but when Hubble prepared his first paper on the findings (“A Relation Between Distance and Radial Velocity Among Extra-Galactic Nebulae”), he primarily used Slipher's original measurements.
Hubble was more vigilant than usual in preparing this landmark 1929 publication, chiefly because of the checkered history of the subject. An earlier and rather clumsy attempt by the Polish-American mathematical physicist Ludwik Silberstein to find a relationship between a galaxy's distance and its redshift had been met with derision, especially by noted astronomers Knut Lundmark and Gustaf Ström-berg, who was Hubble's colleague at Mount Wilson. Silberstein had lumped globular clusters in with spiral nebulae, which led to a meaningless result. He was ridiculed for both his inept analysis and leaving out data that went against his prediction, which tainted everyone's outlook on the problem. To make sure this didn't happen to him, Hubble shrewdly sought out the advice of Silberstein's two harshest critics and specifically highlighted their contributions in his paper. “Mr. Strömberg has very kindly checked the general order of these values… Solutions of this sort have been published by Lundmark,” he wrote fawningly. Hubble knew he was dealing with a controversial finding, so he was taking every precaution. He was wooing potential enemies to his side. He didn't even like Lundmark, having earlier accused the man of plagiarizing his system for classifying the galaxies, and the point he had Strömberg verify was so simple it scarcely needed checking. As it was, Hubble held up publication of his data to make sure he had nailed down every argument, as well as gathered data on even fainter galaxies so he and Humason could quickly publish a follow-up and prevent others from jumping into what Hubble considered his field. He was being both careful and cunning; he was not just introducing an idea but selling it hard. Hubble knew he had to make an air tight case in order to convince his more skeptical colleagues. “There is more to the advance of science than new observations and new theories,” historian Norriss Hetherington has noted. “Ultimately, people must be persuaded.”
According to Hetherington, Hubble presented his first data on the problem as if he were standing before a judge and jury—again, not surprising given his legal training. Hubble even had witnesses. With Hubble citing their assistance, Strömberg and Lundmark were brought forward to serve as objective bystanders to verify his competence. What Hubble saw was a definite pattern to the galaxies' retreat, a rule that was simple and yet so elegant. The velocity of the galaxies was found to steadily increase—rise in a linear fashion, as scientists say—as astronomers peered ever deeper into space. At double the distance, a galaxy's speed doubles as well. A galaxy 10 million light-years away travels twice as fast as a galaxy 5 million light-years distant. Hubble also calculated the rate of that increase. This number has since been amended (as better and better measurements were made over the years), but at first Hubble found that for every million parsecs outward (around 3 million light-years), the velocity of a galaxy increased by 500 kilometers per second. He referred to this factor as K, the same term introduced by others in earlier analyses. By the late 1930s, though, astronomers were regularly referring to it as H, “Hubble's constant,” later shortened to the Hubble constant.
Hubble did not really “discover” this relation but rather demonstrated an effect already suspected, with data that at last convinced his fellow astronomers. In previous attempts the plotted measurements looked like scattershot across the page. But on Hubble's graph, even though there was still some scatter, the galaxies lined up far more tightly. The sure straight line he was able to draw through his points, a diagram that is now an honored icon in cosmology textbooks, gave everyone confidence in the results.
Hubble actually carried out two separate computations. In one, he calculated his rate of recession using all twenty-four of his galaxies. In the second approach, he figured out a rate of recession when he combined the galaxies into nine groups, according to their distance and direction on the sky. Both methods led to similar outcomes. “For such scanty material, so poorly distributed, the results are fairly definite,” concluded Hubble, almost with surprise.
In a clever move, Hubble didn't include on his historic graph the strongest bit of evidence then on hand. He left that for Humason to convey in a separate paper opportunely placed right before his in the Proceedings of the National Academy of Sciences. Humason's result was the attention grabber, setting up Hubble's paper, published under his name alone, for the fait accompli. After cutting his teeth on a few of Slipher's galaxies, Humason had gone after a fainter, previously unmeasured target, as Hubble directed. It was the galaxy NGC 7619 in the Pegasus constellation. “I agreed to try one exposure,” recalled Humason, who wanted to see whether it was even possible to venture farther out than Slipher. That exposure extended over a few nights, the sparse photons from the dim galaxy hitting the plate for a total of thirty-three hours. It was a lonely enterprise. Humason usually worked within the 100-inch dome with only the light of a tiny red bulb as company. For hours he would keep two crossed hairs—the guiding wires—smack-dab on his galaxy, a smudge of light barely visible through the barrel of the telescope. All this despite the fact, as one observer put it, “that the mountain itself is rolling eastward with the earth at ten times an express train's speed.” For assurance, Humason went back and did the measurement again, this time for forty-five hours.
Edwin Hubble's famous 1929 velocity-distance graph for a sample
of galaxies out to 2 million parsecs (∼6 million light-years),
evidence for what later came to be understood as an expanding
universe (From Proceedings of the National Academy of Sciences 15
: 172, Figure 1)
But making these observations was only the beginning. Back in the office, the spectrum on each photographic plate had to be put under a microscope and the shift of the spectral lines carefully measured. Humason often used the lines generated by calcium atoms glowing within the galaxy, which stood out sharply. Given Humason's lack of training in mathematics, though, he'd then take his measurements to an observatory “computer” (a person with a slide rule or adding machine), who used a formula to convert the physical measurement into a galaxy velocity. In the case of NGC 7619, Elizabeth Mac-Cormack calculated a final velocity of 3,779 kilometers per second, more than twice Slipher's highest velocity. The success spurred Mount Wilson officials to get Humason a better and faster spectrograph, appreciably reducing his bone-wearying exposure times, and the Kodak company was inspired by his problem to invent faster photographic plates, which was fortunate. Exhausted by his initial spectral observations, Humason was ready to quit Hubble's project if such improvements were not in the works.
With NGC 7619's speed record in hand, Humason was able to put a further gold star in Hubble's rising constellation by noting that his measurement meshed exactly with the relation that Hubble had just found between a galaxy's distance and its velocity. “The high velocity for N. G. C. 7619 derived from these plates,” Humason was able to report, “falls on the extrapolated line.” With that triumphant pronouncement, Humason was extending Hubble's findings even farther into space, out to 20 million light-years.
It seemed inevitable that the only astronomer to voice immediate doubts about Hubble's new law was his long-standing adversary, Harlow Shapley, who was concerned that distances could only be certain for the nearest galaxies. He was perhaps envious, “in part regretting a lost opportunity to pursue such a relation himself,” suggests historian Robert Smith. Shapley wrote a quick and pointed response to Hubble's paper, where he rightly argued that at a great distance a cluster of stars would be mistaken for a single star, making it a bad “standard candle.” But on the opening page of this article, he didn't miss the opportunity to steal away a bit of Hubble's thunder and proclaim that ten years earlier he had published a notice that “the speed of spiral nebulae is dependent to some extent upon apparent brightness, indicating the relation of speed to distance.” Shapley failed to disclose that in 1919 he still believed that the spiral nebulae were minor members of the Milky Way. So his claim, in the end, was meaningless.
Within two years, Hubble and Humason examined forty more galaxies beyond the several dozen that Slipher had earlier measured. They proceeded outward from what was then measured as 6 million light-years to as far out as 100 million light-years, a tremendous vault into the cosmos over such a short time. “Humason's adventures were spectacular,” recalled Hubble many years later. “When he was sure of his techniques, and confident of his results, he set forth. From cluster to cluster he marched with great strides right out to the limit of the 100-inch.” At one point Humason spent an entire week, night after night, gathering the light from just one faint galaxy in the Leo cluster to determine its redshift. Nicholas Mayall, a graduate student from Berkeley who was then assisting Hubble on a galaxy-counting project, was there as Humason developed the photograph at the end of the run. Holding the small plate up in front of the light box, Humason declared, “My God, Nick, this is a big shift!” The spectral lines, recalled Mayall, “were shifted way over to hell-and-gone from where they should have been. This proved to be a red shift of 20,000 kilometers per second, and it was probably more than twice the biggest one he had ever obtained before. He was simply jubilant.” This galaxy was racing outward at more than a twentieth the speed of light. Caught up in the moment, Humason announced it was time to celebrate and promptly went down to his room, swung open his closet door, and took out a bottle of his mysterious “panther juice,” an illicit alcoholic brew. After their toast at dawn and a brief nap, the two colleagues went for breakfast at the Monastery and called up Hubble on the phone with the news of the record-breaking redshift. “Milt,” replied Hubble, “you are now using the 100-inch telescope the way it should be used.” The link between a galaxy's velocity and its distance had been made even stronger. “You can't imagine how electric the atmosphere was,” said Mayall. “So many things were happening in astronomy and physics—they all came to focus at that time and place.”
Humason's velocities were so astounding that some astronomers were finding it difficult to believe that he could measure them at all. But Humason had the benefit of experience. By first pegging redshifts at relatively low velocities, he became as familiar with the spectral lines as with old friends, and found it easy to spot them as they shifted redward and got fainter once he pursued galaxies farther and farther into space.
A selection from Milton Humason's measurements showing how the
spectral lines for calcium (marked as KH) move farther to the right (red
end of the spectrum) as both the distance for the galaxy and its velocity
increase (1 parsec = 3.26 light-years) (From Astrophysical Journal 83
: 10-22, Plate III, courtesy of the American Astronomical Society)
So important did the Mount Wilson administration consider this endeavor that Hubble and Humason got almost all the “dark time” on the 100-inch throughout the 1930s, to the dismay of other galaxy researchers. Only during those precious few nights each month when the Moon, with its disruptive light, remained hidden below the horizon could the two investigators carry out the measurements on their extremely faint targets. “The intense publicity that swirled around Mount Wilson's nebular department, with Hubble the bright star at its core,” noted Allan Sandage, “was anathema to the spectroscopists [at Mount Wilson].” Most astronomers at this time, in the United States and elsewhere, were focused on determining the life histories of stars. “Here they were,” continued Sandage, “toiling away on stellar astrophysics—the most exciting and exotic facet of contemporary astronomy, to their eyes—yet the public seemed to find them, well, boring.” Even though less than 5 percent of Mount Wilson's major publications in this era involved cosmology, the topic dominated the news stories coming out of the observatory. “Some spectroscopists began to feel resentful,” said Sandage. Even to this day, the legend persists that the Mount Wilson Observatory's sole focus at that time was galaxies, so great was the attention focused on Hubble and his accomplishments.
But what did it all mean? What was causing the galaxies to flee from the Milky Way in such a methodical way? Were these swift velocities even genuine? It was easy to equate the redshifts with velocity, as that was the simplest interpretation and the most straightforward way to talk about the phenomenon in scientific papers. Everyone used the terms interchangeably. But perhaps some new law of physics was at work and the galaxies weren't truly racing away after all. Maybe the retreat was entirely a chimera.
Hubble, the consummate observer, did not consider that question his main concern. He was reluctant to speculate. He wanted only to weigh the data that the universe provided him. Given that leaning, Hubble devoted most of his 1929 paper to establishing the link between a galaxy's distance and its redshift, its six pages filled with tables of numbers, a few equations, and a single graph. Only in the very last paragraph did he bring up a potential explanation. “The outstanding feature,” he wrote, “is the possibility that the velocity-distance relation may represent the de Sitter effect,” the most active model then in play. Maybe the light waves were lengthening as they traveled, setting up the illusion of movement; or maybe matter was truly scattering outward due to the weird nature of de Sitter space. More significant to Hubble was that bona fide data could now be offered in discussions of cosmological models. For centuries cosmology was a realm of speculation and imagination alone. Anybody's vision of the cosmos could be entertained—its origin, its behavior, its structure—simply because there was no way to refute it. But now actual cosmic measurements could be brought to the debate. Theory now had to meet the test of observation. It was one of the triumphs of twentieth-century astronomy, and Hubble initiated the endeavor. The galaxies became his “great beacons scattered through space,” luminous markers for mapping the topography of the universe.
For the most part, Hubble remained focused solely on his observations, leaving theory to others. “The interpretation,” he told de Sitter at one point, “should be left to you and the very few others who are competent to discuss the matter with authority.” Hubble obviously had some serious doubts about what it all meant. “It is difficult to believe that the velocities are real—that all matter is actually scattering away from our region of space,” he told a Los Angeles Times reporter in 1929. In the very first paragraph of his discovery paper on this subject he referred to the velocities of the galaxies as “apparent.” He maintained this conceit for the rest of his career. As long as various theoretical explanations were under scrutiny, he didn't want to be caught on the wrong side. He was never comfortable in the robe of a theorist and so published his data in such a way that the measurements could remain unsullied, no matter what the interpretation. “Not until the empirical sources are exhausted, need we pass on to the dreamy realms of speculation,” he mused. This attitude rubbed off on his loyal helpmate as well. “I have always been rather happy that… my part in the work was, you might say, fundamental; it can never be changed—no matter what the decision is as to what it means,” said Humason.
Hubble was quite possessive of their legacy and kept close watch on it. When de Sitter in a 1930 review article casually referred to the link between velocity and distance (“It has been remarked by several astronomers that there appears to be a linear correlation”), Hubble immediately picked up his pen and reminded de Sitter who should be getting the lion's share of the credit. “The possibility of a velocity-distance relation among nebulae has been in the air for years—you, I believe, were the first to mention it,” he wrote. “But… I consider the velocity-distance relation, its formulation, testing and confirmation, as a Mount Wilson contribution and I am deeply concerned in its recognition as such.”
Hubble conveniently forgot to tell de Sitter that most of the galaxy velocities he first drew upon in his 1929 paper were actually Slipher's data, which Hubble used without direct citation or acknowledgment, a serious breach of scientific protocol. Hubble partially made up for this nefarious deed by briefly referring in his next big paper on the redshift law, published in 1931, to the “great pioneer work of V. M. Slipher at the Lowell Observatory.” More gracious amends were made in 1953. That year, as Hubble was preparing a talk on the “Law of Red-Shifts” to be given in England (the prestigious George Darwin Lecture of the Royal Astronomical Society), he wrote Slipher asking for some slides of his first 1912 spectrum of the radial velocity of the Andromeda nebula and in this letter at last gave the Lowell Observatory astronomer due credit for his initial breakthrough (albeit more than two decades late). “I regard such first steps as by far the most important of all,” wrote Hubble. “Once the field is opened, others can follow.” In the lecture itself, Hubble professed that his discovery “emerged from a combination of radial velocities measured by Slipher at Flagstaff with distances derived at Mount Wilson… Slipher worked almost alone, and ten years later…had contributed 42 out of the 46 nebular velocities then available.”
Privately, Slipher was bitter that he didn't get more immediate public credit but was too humble and reserved to demand his share of the glory in 1929. He was at least honored by his peers for his contributions. The Royal Astronomical Society presented its highest award, the Gold Medal, to him in 1933, with its president, Frederick Stratton, amusingly announcing that “if cosmogonists to-day have to deal with a Universe that is expanding in fact as well as in fancy, at a rate which offers them special difficulties, a great part of the initial blame must be borne by our medallist.” In many ways, Slipher's accomplishment resembled that of Arno Penzias and Robert Wilson several decades later. In 1964 the two Bell Laboratory researchers were calibrating a massive horn-shaped antenna in New Jersey in preparation for some radio astronomy observations and registered an unexpected cosmic radio noise wherever they looked on the sky, spending months trying to discover its source. Just as Slipher revealed a remarkable phenomenon that took others time to fully interpret, so too did Penzias and Wilson need fellow astronomers to tell them what they had found, that they had been listening to the faint reverberation of the Big Bang all along. But there any resemblance between the two cases ends. While Penzias and Wilson received the Nobel Prize for their serendipitous discovery, firming their rank among the scientific elite, Slipher as the years passed was reduced in the public's eye to a secondary role in the momentous saga of the fleeing galaxies.