Absolute Zero and the Conquest of Cold (27 page)

This was a good dream. But reality soon intruded. As Onnes continued to experiment in the ultracold environment, he tried to determine what effect magnetism would have on materials at very low temperatures, and found—to his dismay—that a magnetic field of a few hundred gauss, the strength exhibited by an ordinary household magnet, was enough to eliminate the superconductive state in materials such as mercury, tin, and lead. The moment a magnetic field was turned on in the vicinity of the material that had been rendered superconductive by liquid helium, the superconductive state appeared to vanish. This seemed to mean it would never be possible to have superconducting wires that would revolutionize the use of electrical power in the world.

This almost immediate dashing of his dream, and the inability of other contemporary scientists to realize the importance of superconductivity just then, may help to explain why, when the Nobel committee awarded Heike Kamerlingh Onnes the 1913 prize in physics, it cited the seventy-year-old scientist "for his investigations on the properties of substances at low temperatures, which investigations, among other things, have led to the liquefaction of helium," and did not specifically mention his discovery of superconductivity.

In his speech accepting the Nobel Prize in Stockholm, Onnes offered no philosophic ruminations on how the world had changed because of his discoveries. Rather, he treated the Nobel address as though it were a routine though nostalgic lecture to a scientific conference. He reported on the "Leiden und Freuden," the disappointments and joys, not only of his own research but also that of Dewar, Olszewski, von Wróblewski, Pictet, Cailletet, and Linde over the past thirty-five years. He recounted in detail the events of July 10,1908, when helium had first been liquefied, and his wish to have shown the liquid helium immediately to van der Waals. Onnes made certain to mention superconductivity, expressing again his wonder that "the disappearance [of electrical resistance] did not take place gradually but
abruptly,
" an occurrence that "brought quite a revelation." In a similar awestruck manner, Onnes detailed his findings about the extremely low density of helium. Explanations for these phenomena had still not been made, and in a fervent prediction, Onnes suggested to the Nobel audience that when explanations for these strange phenomena were made, they "could possibly be connected with the quantum theory."

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."

W
HAT 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?