Absolute Zero and the Conquest of Cold

by Tom Shachtman

  In a way, this was Onnes's acknowledgment of a most important point that could not have been understood even a few years earlier: that the liquefaction of helium and the discovery of superconductivity were the last triumphs of what would shortly be referred to as "classical physics," the physics of Newton and Boyle, Kelvin and Clausius, the physics of the past. Classical physics described objects and their motion, while quantum physics described matter only in terms of motion, wave motion. More so than many in his age group, Onnes understood and accepted that a new generation of scientists—almost a new breed—able to embrace supremely sophisticated, complex, counterintuitive ideas, were in the process of supplanting the generation that he and James Dewar so well exemplified. In other articles composed around this time, he reiterated his belief in the quantum theory of Planck, asserting that it might provide the "mechanism" responsible for the disappearance of electrical resistance in the several superconductors discovered by Onnes to exist at a few degrees above absolute zero.

  To understand why Onnes would predict the ability of quantum theory to explain superconductivity, we must backtrack to 1907, a year before he liquefied helium. In that year, an examination of the "specific heat" of copper at extremely cold temperatures led to the solution of one set of troubling anomalies previously highlighted by near-absolute-zero research, and in the process, produced an important verification of the quantum theory that Planck had articulated in 1900. The problem solver was Albert Einstein.

  Specific heat had fascinated physicists and chemists since 1819. That year, French chemists Pierre-Louis Dulong and Alexis-Thérèse Petit defined it as a measure of the heat required to raise the temperature of a small quantity of a substance by a fraction of 1 degree and determined the specific heat of all sorts of materials. They produced a law, an equation that accurately predicted the specific-heat capacity of common materials such as lead and copper. But by 1875—that is, before Cailletet and Pictet liquefied nitrogen and oxygen—it had already become obvious from research in the region just below o°C that the Dulong-Petit law did not hold for all temperatures. The situation was similar to what Andrews and van der Waals had encountered when dealing with Boyle's law: an equation that explained things quite well at.room temperature proved untenable when the temperature was dropped well below the freezing mark. When liquid hydrogen became available, researchers noted that at about 20 K, the specific-heat capacity of copper dropped to a mere 3 percent of what it was at room temperature. That disproved Dulong-Petit, but now the problem was to come up with an explanation, and an equation, incorporating that old law's description of how matter behaved at normal temperatures and also encompassing how matter behaved at the newly reached lower temperatures.

  This was just the sort of problem Einstein liked to tackle. Dulong and Petit had described the action of individual atoms in a way that was later defined as the "equipartition of energy." Einstein realized that he had to replace their description with one that took into account the "quantization" of the atoms' vibrations. In the picture of thermal motion that had evolved by 1907, an atom was considered to be an oscillator with six "degrees of freedom," each one containing some energy. By then, Einstein had decided that Max Planck's work on "quanta," the small parcels into which many forms of energy are subdivided—work that Einstein had originally thought was in conflict with his own—was really complementary to his own. So he extended Planck's quantum theory, arguing that atomic vibrations were quantized, meaning that the atoms did not vibrate freely but in small, measurable, incremental steps. That was the solution to the specific-heat puzzle. As the temperature of the copper fell, Einstein suggested, more and more of its atoms were constrained from vibrating, leading to an exponential drop in the metal's specific-heat capacity. He wrote an equation that matched reasonably well—not perfectly, but fairly closely—the observed data for the specific heat of copper, all the way from the Dulong-Petit area of 80°F, down through the liquid-oxygen and liquid-hydrogen temperatures, to around 10 K. Not only did his equation predict the changes in specific heat as temperature fell, but by showing that quantum theory could explain something that had previously been beyond understanding, Einstein's proof also upheld the insight of Nernst's third law of thermodynamics and provided an early verification of the truth and worth of Planck's quantum theory.

  Einstein's successful explanation of the drop in specific heats had excited Onnes well before his discovery of superconductivity. At the outset of his resistance experiments, Onnes had cobbled together a working hypothesis that was a hodgepodge of classical and quantum ideas, combining, in addition to the business about the impurities in the metal, the equations of state of van der Waals, married to Planck's notions of vibrating particles. Then Onnes's experiments with mercury revealed the sudden fall in resistance at 4.19 K—a result, he wrote with characteristic aplomb, "not foreseen by the vibrator theory of resistance that I had framed." So he had to abandon that hypothesis as he had abandoned others, but he maintained his belief that eventually quantum physics would provide the key to understanding superconductivity. And when the English physicist J. J. Thomson—later a Nobel Prize winner himself—postulated an explanation for superconductivity that did not include quantum theory, Onnes went out of his way to reject it publicly, on just that basis.

  While Onnes had been on the glide toward his Nobel, between 1908 and 1913, Dewar had not faded graciously away. In 1911 he commissioned the refitting of the amphitheater at the Royal Institution, at his own expense, in celebration of his having held the Fullerian Chair of Chemistry even longer than Faraday. In his research after 1908, Dewar made several important contributions, among them the invention of a charcoal-based calorimeter, which he used to measure the heat capacities of many elements and compounds in the liquid-hydrogen range and below. In 1913 he discovered that at 50 K, the heat capacities of the solid elements were related to their atomic weights by a logarithmic equation. He also returned to several other matters that had intrigued him in earlier years, among them soap bubbles and thin films, on which he now did some important research, and explosives, building on his pioneering work with charcoal. Back in 1889 Dewar and Sir Frederick Abel had invented cordite, a gelatinized mixture of nitrocellulose and nitroglycerin used as a smokeless explosive.

  Regarding explosives, Dewar came to believe that some of the innovations he had introduced had been purloined, without credit or payment, by Alfred Nobel and his heirs, and he brought suit against them. The suit was eventually dismissed as having no merit.

  Dewar never received a Nobel Prize for his research, although his liquefaction of hydrogen had been the key experiment in the descent toward absolute zero, and although his invention of the cryostat was essential in all experiments conducted at ultra-low temperatures. He had no pure discoveries to his name, and no theories, and Nobels usually went to discoverers and theorists. There may also have been resentment against Dewar among the heirs of Alfred Nobel responsible for the administration of the prizes, though the recipients were always chosen by a committee of experts in the field; as for that, Dewar's confrontational style with Rayleigh, Ramsay, and Travers had also earned him black marks among the better-respected English chemists and physicists of the day. In the elaborate procedure of nomination for the Nobel, their overt support would have been necessary to put him on the final ballot.

  In August 1914 the Great War began, pitting the forces of countries from the British Isles to the Balkans against one another. Among the early collateral-damage casualties was Olszewski. Austrian soldiers invaded the building in which his laboratory and quarters were located and turned it into their dormitory. Already frail, Olszewski took to his bed; ever the scientist, on the night that death neared, he noted down its approaching symptoms, sandwiching the observations between his requests for funeral arrangements.

  Another casualty of the war was low-temperature research. The still-small supply of helium was conscripted for the military in the combatant countries, which started to use helium gas for dirigibles
and lighter-than-air espionage and antiaircraft balloons.

  And so the exploration of the country of the cold came to a temporary halt at the discovery of superconductivity, that first indication of the profound transformations of matter that the ultracold environment could produce. This was not using cold to make eggs glow in the dark—it was far more basic and interesting. Everyone hoped ultracold research would resume after the war, because so many things were yet to be learned, first among them the explanation of why the superconducting state was brought into existence at low temperatures. Until that resumption, the discovery of superconductivity was a beacon lit at a very far outpost of Frigor, a fitting fulfillment of the scientific explorers' long quest into its frigid realm. In their race toward absolute zero, the generation of Onnes, Dewar, Olszewski, von Wroblewski, Cailletet, Pictet, Linde, and Hampson had successfully explored a difficult field, and they had bequeathed to the next generation exciting and formidable tasks reminiscent of those that had faced the theorists of Salomon's House in Bacon's fable: to distill the "knowledge of Causes, and the secret motion of things," and to use what they learned for "the enlarging of the bounds of Human Empire, to the effecting of all things possible."

  12. Three Puzzles and a Solution

  WHAT KAMERLINGH ONNES AND his fellow turn-of-the-century researchers did not immediately realize, in the period before the onset of the Great War, was that in their work on the ultracold they had unlocked a treasure chest of information about previously unknown aspects dealing with the operation of the normal world as well as with that strange one in the vicinity of absolute zero. The baubles in this trove would eventually provide avenues of understanding to the primal secrets of the universe.

  The single most significant roadblock to reaping those understandings was the enigma of superconductivity, whose solution would take another sixty years and require the efforts of battalions of good scientists. During the height of the attempts to figure it out, Felix Bloch coined what he called an axiom: "Every theory of superconductivity can be proved wrong." For many years, that was the only correct statement in the field.

  A more appropriate saw would have been that in science, each new discovery raises more questions than it answers. Chemist Leo Dana, fresh from receiving his doctorate at Harvard in 1922, ran right into one of those new questions when he arrived at Leiden to spend a postdoctoral year with Onnes.

  Unfortunately for Dana, the day of his arrival was the day after the death of Onnes's intended successor, J. P. Kuenen. The laboratory was in shock. Among other reasons for the dismay, Dana learned, a battle likely to further upset the laboratory would now take place between Protestants and non-Protestants for the position of heir apparent; for centuries, Leiden had been a Protestant university. A compromise shortly saddled Wilhelmus H. Keesom, a Catholic, with a Protestant codirector for a time.

  Dana busied himself with learning the ropes. His questions had to do with Onnes's postwar research into the unusual density of liquid helium. At the boiling point, 4.2 K, it rose dramatically from what it had been at warmer temperatures and passed through a maximum at 2.2 K, but it gradually declined thereafter, even when the temperature was dropped to less than 1 degree above absolute zero. What accounted for this peak and change in density? Could the density of helium be tied to the onset of superconductivity in metals alone or in conjunction with the effect magnetization had on superconductivity?

  Dana wanted to work with Onnes on these questions. The director was now an old and sick man. He seldom visited the laboratory, though he maintained a correspondence with nearly everyone of importance in physics, from Einstein to W. C. Rontgen to lesser-known researchers in the low-temperature laboratories in the Soviet Union, and he worked assiduously to help build up the commercial liquefaction and refrigeration industries of the world. Onnes communicated with colleagues principally by telephone; emphysema made it difficult for him to speak, but when he did, a French correspondent later recalled, his brief comments were always on target and often insightful.

  Dana was invited to Ter Wetering. Its appearance, he later recalled, made it "evident" that Onnes was now a wealthy man. "I was ushered into his study, furnished with antique furniture, oriental rugs and paintings; looking out the window, one saw the lovely scenery of the Dutch countryside. He was dressed in a fancy velvet gown—the typical man of means." Onnes advised the young Harvard-trained chemist, "If you ever see a ripe plum on a tree, reach up and grab it."

  In the laboratory, Onnes put Dana to work in trying to grab hold of something they both thought of as a ripe plum, the latent heat of liquid helium as it vaporized. What Onnes and Dana together found in their investigations of this phenomenon they labeled "remarkable." "Near the maximum density," they wrote in an article, "something happens to the helium, which within a small temperature range is even discontinuous."

  It was discontinuous in the same remarkable way as the "abrupt" drop in resistivity when mercury became superconducting. Next they found a similar change in the specific heat of liquid helium, at the same transition temperature they'd identified in the latent-heat experiment. When the temperature dropped below that line, the specific heat became much larger than expected, or than predicted by any theory, including Einstein's. Onnes did not want to report the figures, because they were too large and because they contradicted Einstein's work. The joint paper Dana yearned to have published had to wait for several years.

  Onnes and Dana could not figure out why the specific heat of the helium changed so markedly, but they determined the precise point at which the discontinuity started, 2.2 K, which a visitor to Leiden, Paul Ehrenfest, labeled the "lambda point," because the shape of the curve describing the specific heat resembled the form of the Greek letter lambda. Identifying the lambda point, however, did not mean anyone could yet understand the "something" happening to helium at 2.2 K.

  His year of postgraduate study almost up, Dana made ready to return to the United States and was invited to a farewell dinner at Ter Wetering. One moment stuck with him: at the dinner table, when Elisabeth Onnes wanted to have the next course served, rather than summon the waiters herself, she followed tradition and asked her husband to ring the bell; with great difficulty, Onnes got out of his chair and walked to a side panel to pull the bell to summon them.

  For Onnes, 1923 was a year of great losses—the deaths of van der Waals and Dewar. "One of the great figures of modern physics and physical chemistry," Onnes described his friend van der Waals, who died at age eighty-five, in an obituary in Nature, expressing admiration for his "severe culture of the ideal" and repeating Dewar's characterization of van der Waals as "the master of us all." The theorist had begun to deteriorate in 1913, Onnes wrote, and "[a]t last, only short visits allowed us to show to the venerated and beloved friend, whose heart we felt unchanged, what he had done for us."

  Nineteen days after van der Waals's death, Sir James Dewar died, on March 27,1923. A decade had elapsed since Dewar had first begun to reveal to Onnes in letters his bemused amazement that, though ill most of his life, he had actually reached the age of seventy. By 1923 Dewar was eighty-one, and he had continued to do important work on films and with a charcoal-gas thermoscope he constructed to measure infrared radiation. From the basement of the Royal Institution, where he had conducted his low-temperature experiments, Dewar had moved up to the attic, whence he measured the radiation from the sky. He took his last readings just a few nights before he was confined to his bed by his final illness.

  Onnes survived another three years, becoming less able to draw breath with each passing week. At his death in 1926, he was mourned throughout the world. The outpouring accompanying Onnes's passing was greater than for Dewar, a loner who left no school of successors, since the heirs of Onnes were everywhere in the laboratories of Europe and the British Isles.

  Just three weeks after Onnes died, his last collaborator, W. H. Keesom, completed what Onnes had worked toward for fifteen years, the solidification of helium. Because helium seemed to rema
in liquid as far down toward absolute zero as Keesom could reach while keeping the pressure moderate, he was able to make crystals form in the helium only by applying greater amounts of external pressure.

  The solidification of helium led to Keesom's refining of the Onnes-Dana data on specific heats. Keesom found that while liquid helium boiled at 4.2 K, when it descended to the lambda point of 2.2 K the boiling ceased, the bubbles stopped, and the liquid helium became completely still. These dramatic shifts at the lambda point suggested to Keesom that the liquid from 4.2 K down to 2.2 K must be treated as a distinct phase called helium "I," while the liquid below 2.2 K was very different and should be regarded as another separate phase called helium "II." Compared with helium I, helium II had a smaller density, a greater heat of vaporization, and a smaller surface tension.