by Thomas Hager
In Tolman's classes Pauling heard some of the criticisms of the Bohr-Sommerfeld model, others from listening to talks given by visiting European physicists, especially a course on quantum physics he took from Paul Ehrenfest. As a graduate student, however, Pauling wasn't ready either to pass judgment on Bohr's model or create a new one. There was too much data on every side, too many changes happening, too many new ideas to digest. For the most part, he simply accepted what he was taught, including the general correctness of the Bohr-Sommerfeld atom. In the Caltech Seminar in Physical Chemistry, which Pauling considered the most important course Tolman taught, teacher and students worked their way chapter by chapter through the new fourth edition of Sommerfeld's influential text Atombau und Spektrallinien ("Atomic Structure and Spectral Lines"), in German, in which the German physicist detailed his ideas on the structure of the atom. Sommerfeld himself presented his model of the atom on a tour of the United States in 1922-23. Pauling heard him lecture at Caltech and became a believer: He buttonholed Sommerfeld as the professor left class one day and told him about his own ideas of atomic structure as they walked down a Caltech arcade; he even managed to show Sommerfeld some wire-and-wood models he had made demonstrating (incorrectly, as it turned out) how Bohr-Sommerfeld orbitals could explain the tetrahedral binding of carbon. At this point, Pauling was unable to differentiate between the many deficiencies in quantum theory and shortcomings in his own knowledge.
Those shortcomings were sometimes painfully evident. In one seminar Tolman asked Pauling why it was that most substances, when placed in a magnetic field, briefly develop a magnetism that is opposed to the field—a phenomenon called diamagnetism. The correct answer is that the magnetic field alters the orbital motion of the electrons in the substance. Pauling, however, unaware of the latest findings, answered that diamagnetism was simply "a general property of matter." That amused Tolman, and the teacher made a point of targeting Pauling for more questions. Once, when Tolman asked him another question he couldn't answer, Pauling gave a less amusing response. "I don't know," he said. "I haven't taken a course in that subject." After class an older, wiser postdoctoral fellow took Pauling aside and gave him some friendly advice. "Linus," he said, "you shouldn't have answered Professor Tolman the way you did. You are a graduate student now, and you're supposed to know everything."
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The kinds of things Pauling wanted to know were increasingly theoretical.
Roughly speaking, scientists can be divided into two broad groups: theorists and experimentalists. Experimentalists work in the laboratory, teasing empirically provable facts from nature one small bit at a time, eventually building large collections of data showing exactly how substances behave. Theorists work in their heads, trying to make larger sense of the facts gathered by experimentalists, searching for the underlying natural laws that guide individual events. Experimentalists determine what happens; theorists explain why. Under Tolman's influence, Pauling was becoming further seduced by the excitement of theoretical thinking. It suited his temperament and talents. His interests were wide-ranging, and theoretical scientists were rewarded for thinking in broad terms. His memory was phenomenal, allowing him to draw upon a huge store of facts in a number of fields. He enjoyed solving puzzles, and making theoretical sense of seemingly unrelated bits of experimental data was the greatest puzzle in the world. And he was ambitious. Good theorists were the stars of science, the Einsteins and Lewises and Bohrs. It was the route to the top.
This outlook, combined with his growing sense that the structure of molecules was critical to understanding their chemical behavior, led Pauling toward a goal: He wanted to discover the laws that guided the bonding of atoms in molecules. Atoms connected with specific numbers of other atoms at specific distances to form molecules with specific shapes. Why these numbers, distances, and shapes and not others? The chemical bond, whatever it was, was the key. And it was logical that quantum physics, with its growing string of successes in laying bare the inner workings of the atom, would also eventually explain the chemical bond. Whoever linked quantum theory to the chemical bond would also reconcile the physicists' dynamic atom and the chemists' static model. And whoever stole for the benefit of chemistry (still primarily a descriptive science) the fire of the new methods of mathematical physics would have the chance to reshape discipline, forging a truly "physical chemistry" in which chemical phenomena could be predicted quantitatively, directly from the laws of physics. It was an enormous and important prize. Pauling decided early in his graduate career to pursue it.
To succeed, he needed to know everything about the new physics. Pauling started reading other books and papers on quantum theory and never missed one of the lively research conferences held twice each week by the physics department or jointly by physics and astronomy. Some of the talks were given by graduate students, who were asked to read about and report on their latest findings. Others were given by faculty members on their own research or by visiting scholars. There was always discussion and debate. Pauling would remember the "feeling of excitement" at these conferences. It was here that he learned of de Broglie's idea that electrons could behave like waves as well as particles. On another occasion, a graduate student, Charlie Richter (who later developed the Richter scale) rushed in to announce that two young Dutchmen, Goudsmit and Uhlenbeck, had discovered that electrons had spin. "He was more excited than I've ever see him—not even an earthquake could make him that excited," remembered Pauling. "And everybody else was excited. Things were happening in the fields of physics and chemistry."
Thanks to Robert Millikan, the Caltech physics division was keeping up with—and increasingly contributing to—those changes. Part of the package promised to get Millikan to Caltech was money to import to Pasadena one of the young European physicists taking part in the quantum revolution. Millikan filled the position in 1921 with one of Sommerfeld's former assistants, the mathematical physicist Paul Sophus Epstein, who brought with him the outlook, the prestige, and some of the excitement of the great European scientific centers. Epstein knew practically everybody who was anybody in European physics. At the same time, Millikan's own reputation was approaching its peak, reached in 1923 when he learned (during a class Pauling was taking from him) that he had won the Nobel Prize in physics—only the second American to be so honored.
By the early 1920s, Caltech already had an international reputation that made it a regular stop for foreign physicists touring America. Pauling heard a veritable who's who during his student years. Besides Sommerfeld and Ehrenfest there was the grand old man of physics, Hendrick Lorentz, who had investigated the effects of magnetism on light; the brilliant German mathematical physicist Max Born, who was making his institute at Goettingen one of the world centers for quantum physics; Born's friend, the experimentalist James Franck, whose work bombarding gases with electrons would help confirm the quantum nature of the atom; C. G. Darwin (grandson of Charles Darwin and an accomplished mathematical physicist) from Great Britain; and C. V. Raman from India. Lorentz had already won a Nobel Prize in physics; Born, Franck, and Raman would eventually win theirs. During the time Pauling was a graduate student, Caltech was the best place in America to learn about the new physics.
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To understand quantum theory, Pauling also had to understand the advanced mathematics upon which it was based. He dived into every math course and seminar he could find at Caltech: advanced calculus, vector analysis, integral equations, complex numbers, and potential theory. Mathematics was fine as a tool, "But I never could get very interested in it," Pauling remembered. "Mathematicians try to develop completely logical arguments, formulating a few postulates and then deriving the whole of mathematics from these postulates. Mathematicians try to prove something rigorously. And I never have been very interested in rigor."
His interest was in using mathematics as a weapon to attack what he saw as more interesting problems: Pauling used his extraordinary memory to build three huge mental libraries: one of tr
aditional chemistry, mostly gathered during his time at OAC; another of atomic sizes, bond distances, and crystal structures from his x-ray work; and a third of the mathematical equations and themes of quantum physics. As he neared the end of his graduate years, under Tolman's influence all three areas of interest began to meld into new ideas: Pauling's first theories.
It started on several fronts simultaneously. By late 1924, Noyes was letting Pauling review prepublication drafts of some papers he was writing, an unusual honor to bestow on a graduate student, especially one ostensibly working under another professor. Noyes was interested in the behavior of electrically charged atoms, or ions, in solution, and was responding to a new theory of dilute ionic solutions put forward by the team of Peter Debye and Ernst Huckel. Pauling did not let his respect for King Arthur inhibit his editorial comments. "I found a lot of things to criticize [in Noyes's drafts], and I did point out some statements Noyes had made in them that I thought did not apply really to the theory, and he changed that," remembered Pauling. Then he used Noyes's work as the basis for his own theorizing. "I thought why not do a better job, make the theory applicable to concentrated solutions?"
Pauling worked on his theory of concentrated ionic solutions for several months before showing it to Noyes. Noyes in turn arranged for Pauling to present his ideas in person to Debye, whom he had invited to visit Pasadena in the spring of 1925. Debye was then at the apex of an astonishing career in science. Born in Holland, he had given up his first love, electrical engineering, for physics, earned a Ph.D. at Munich under Sommerfeld, then succeeded Einstein as professor of theoretical physics at Zurich. But Debye's research interests were chemical. He made his reputation by discovering a way to measure the polarity of molecules (molecules which carry a positive charge on one end and a negative charge on another are polar, or said to have a dipole moment; for decades the unit used in the measurement of dipole moments was called the debye). He also recognized quickly after its discovery the immense potential of x-ray crystallography and was the first to use it on powdered solids as well as whole crystals. Even before his theory of dilute ionic solutions was published in 1923, Millikan had been impressed enough to offer Debye a Caltech professorship. Debye had turned it down. Now, visiting Pasadena four years later, he was firmly established as one of the world's greatest physical chemists.
Nervous, Pauling stood in front of a small group, including Noyes and Tolman as well as Debye, and took two hours to carefully describe his new theory. When he finished, the room was uncomfortably quiet. Tolman broke the silence to note a few reservations that he had, and the group broke up. Debye never said a word to Pauling about the presentation. Pauling eventually gathered that the problem was with his mathematics. He had developed a strong physical sense of the answer to the problem, but the mathematics he had devised to support his points had been unsatisfactory; he had made too many unsupported assumptions. He continued working on his theory for two more years before finally giving it up in the face of continuing questions about his numbers from Tolman and mathematics professor Harry Bateman. He wrote Noyes, "My treatment is rather physical and intuitive than mathematical and rigorous; ... I have come to believe that even though I believe it to be correct, it should not be published so long as I am unable to defend it against significant adverse criticism." The experience left a bitter taste. Pauling knew he was on the right track; he could feel it.
Debye saw enough promise in Pauling to ask his help with other problems during his Pasadena visit. Pauling worked very hard for several weeks on a problem Debye had suggested concerning the behavior of a drop of liquid within another drop; it went nowhere. Then they worked together on the influence of an electric field on dilute ionic solutions; this resulted in a paper published in August 1925.
Cowriting a paper with one of the leading scientists in his field was a coup for Pauling, but it wasn't his only activity. His mind was ranging everywhere. At one point he even jotted down some thoughts about a possible connection between the Debye-Huckel theory and the structure of dwarf stars. He also began working with Tolman on another, quite different theoretical question, this time concerning an original idea of Pauling's on the residual entropy of supercooled liquids at absolute zero. The initial idea was Pauling's; he presented it "not... in a very sophisticated form" to Tolman, who then directed him to papers to read for more background. After Pauling wrote the first draft, Tolman asked him what he thought the order of names should be on their joint paper. The young student apparently still had something to learn about the fine points of deferring to faculty. He said that since the original idea had been his, well, his name should be first. The point was well taken—priority should be given to the person with the original insight—and Tolman, perhaps amused again by his impetuous student, agreed. Remembering the incident many years later, Pauling said that "at that time I was not very accustomed to thinking about other people, but more to thinking about myself."
Tolman would see more of that tendency in Pauling later, and his attitude would eventually turn from amusement to coolness. They would never again collaborate on a paper. Even when he became Tolman's director at Caltech, Pauling never called him "Richard"; it was always "Professor Tolman." Still, Tolman was a major figure in Pauling's development. While Pauling would later credit Dickinson with teaching him the deliberate methods of experimental work, Tolman played a more important role, guiding Pauling to the wide scientific landscape being opened for chemists by the latest findings of the quantum physicists. "It may be that Tolman was the one of my various teachers, many of whom were really excellent people, who influenced me more than anyone in my career," Pauling said.
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Noyes was a different case. Pauling never considered that Noyes, the man who made possible his graduate career, had as much to do with his scientific development as had Tolman and Dickinson. His attitude was rooted in part in differences in their scientific interests. Pauling referred disparagingly to Noyes's specialty of thermodynamics as a "black box" into which chemists fed data and received answers without knowing why things happened, and Noyes never did become proficient in the new approaches to chemistry based on quantum theory that so interested Pauling. Pauling also thought Noyes was not tough enough in negotiating for the chemistry division. Moreover, Pauling liked "men's men," and Noyes was in some ways too poetic, too careful, too effete for his tastes. (Pauling was not alone in that assessment; he would later recall Tolman calling Noyes "that old maid.") The result was a feeling, at least on Pauling's part, of distance and reserve.
But Noyes continued to grow more enthusiastic about Pauling. He had seen this young wunderkind quickly master the complexities of x-ray crystallography; he had seen him run the lab in Dickinson's absence. He had watched Pauling aggressively question Debye's theories, then switch gears to collaborate successfully with him. He had seen Pauling triumph in both experimental and theoretical work—an unusual accomplishment for a graduate student. In 1925, Noyes gave Pauling a final test, asking him to direct a dozen chemistry undergraduates in some original research—another part of Noyes's plan to expose budding chemists early to the realities of the laboratory. And Pauling succeeded here, too. The undergraduates did well, and one of them, a freshman named Edwin McMillan, got a publication out of his work, cowritten with Pauling. (McMillan would later win the Nobel Prize for chemistry—before Pauling won his.)
By the time he was in his third year of graduate school, Pauling's original thinking and abilities in the classroom and the laboratory had impressed not only Noyes but everyone at Caltech. He was good enough to be able to stitch together a doctoral dissertation, "The Determination with X-rays of the Structure of Crystals," from five previously published papers, and in June 1925 received his doctorate summa cum laude in chemistry, with minors in physics and mathematics.
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Pauling's brilliance was rooted in hard work. He remembered his schedule during the first year at Caltech: "I had, I think, forty-five hours (which at some universiti
es would be called fifteen hours) of classwork, the advanced courses I was taking, mostly in physics and mathematics. Later on, the department made a rule that a teaching fellow could only sign up for thirty hours. In addition, I spent a lot of time on research. After dinner I would go back to the lab and work until perhaps 11 o'clock at night. On Saturday and Sunday I'd just work all day."
His roommate that first year, Paul Emmett, remembered Pauling working even more. They shared one bed, using it sequentially. Pauling, according to Emmett, routinely returned from the lab around three in the morning, when Emmett would get up to start studying.
And every evening in the laboratory Pauling would write a letter to Ava Helen. Their steady correspondence brought them closer to each other, and he found himself missing her terribly. Within months they had agreed to cut short their engagement and, despite their parents' opposition, to marry at the end of his first year of graduate school.
In the spring of 1923, Pauling bought a seven-year-old Model T Ford from Roscoe Dickinson for fifty dollars and learned to drive by taking it around the block a few times. In June he headed north to Oregon to get married. "I was planning to stop when it got dark," he remembered, "but I was eager to get to Oregon, so I thought, ‘Why don't I just keep on driving?’" Rattling over a gravel road in the Siskiyou Mountains late at night, trying to keep his speed up so the headlights would work, he drove the car off the road into a pit. It came to rest upside down, a splintered wooden roof support jammed into Pauling’s leg. He pulled himself out, bound the gash, and waited all night for help. When it finally arrived the next morning, he got the car repaired well enough to drive and made it in time for the wedding.
