Absolute Zero and the Conquest of Cold (28 page)

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 remain 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.

Scientists were generating additional questions about the behavior of atoms in the vicinity of absolute zero, based on the possibilities raised by Walther Nernst's third law of thermodynamics. If Nernst was correct, as one approached ever closer to absolute zero, the atoms ought to increasingly align themselves in a formation approaching perfect order. In 1925 Einstein turned his thoughts once again to this area of inquiry, spurred by the work of Satyendra Nath Bose, an Indian physicist. As atoms slowed down and approached a virtual standstill, Einstein argued, they would be close enough together to cause their wave functions to overlap, merge, and cooperate, producing a state of matter unlike any already known. This hypothetical new phase or state of matter came to be labeled the Bose-Einstein condensate, and during the next seventy years, physicists would try unsuccessfully to create it to prove Einstein's contention.

Nernst's perfect-order idea, as refined by Planck and others to suggest that entropy measured the randomness of the microscopic state of a solid or liquid, also informed the separate inquiries of two other physicists, Dutch-born German theorist Pieter Debye and Canadian-born physical chemist William F. Giauque, newly ap
pointed to the University of California at Berkeley faculty. It was understood, thanks to the work of Pierre Curie, that the magnetic susceptibility of a substance is inversely proportional to the absolute temperature—that at low temperatures, materials are more readily magnetized. It was also understood that when a material was magnetized, the magnetizing field worked on those of its atoms known as magnetic ions, turning them to face all the same way. Making them do so produced heat energy.

Near the end of 1925, Debye asked rhetorically "whether an effort should be made to use such a process in approaching absolute zero" and concluded that someone ought to do experiments to prove or disprove the theory. Giauque proposed the same process at virtually the same moment in time, but he wasn't content to stop at theory; he tried to construct an apparatus to achieve the goal.

Giauque magnetized a weakly magnetic salt at liquid helium temperatures, then surrounded it with a vacuum and demagnetized it, which caused the electronic magnets of the ions in the lattice to become disordered. Doing that removed heat energy from the salt, which caused its temperature to fall. Giauque first published the theoretical basis for "adiabatic demagnetization" in the fall of 1924, and after solving technical problems such as construction of a thermometer that could register within a few thousandths of a degree of absolute zero, performed the first adiabatic demagnetization in 1933.

The new demagnetization process represented an entirely new way of effecting low temperatures—beyond liquefaction, beyond Joule-Thomson expansion, beyond pressure. With it, scientists were essentially manipulating subatomic structures to produce cold. Giauque would receive the Nobel Prize in 1949 for chemistry; "adiabatic demagnetization" was among his contributions cited. Many practical applications have resulted from the ability to produce very low temperatures.

Meanwhile, work continued on superconductivity. Was the phenomenon specific to a few metals or common to many? Onnes and his successors had demonstrated superconductivity only in relatively soft metals that had low melting points; in a Berlin laboratory, Walter Meissner determined that some metals among the harder group, such as the rare metals niobium and titanium, could be induced to become superconducting. Later on, using the new technique of magnetic cooling, Meissner was able to show that other metals—aluminum, cadmium, and zinc—became superconductors at temperatures below 1 K.

A geographic explorer coming across a form of life never seen before, one that could be either a plant or an animal, has to decide how to describe and investigate its properties. Treating it as a plant mandates one line of inquiry; as an animal, another. As more and more metals—but not all metals—were shown to be superconductors, an analogous basic question arose: Was the vanishing of electrical resistance due to the microscopic properties of the substance, a change to the electrons or to the nucleus, or was the onset of superconductivity a change of thermodynamic state similar to the change from a gas to a liquid? In the late 1920s, the betting favored a microscopic-properties change, partly because no one other than Einstein and Bose had been able to describe mathematically a state of matter beyond those of gas, liquid, and solid.

When Kelvin and Clausius had written of states of matter while constructing the laws of thermodynamics in the 1850s, they had done so in terms of pressure, volume, and temperature. Van der Waals in the 1870s had added molecular density as a descriptive. By the late 1920s, yet another factor was thought relevant: a substance's degree of magnetization. Taking into consideration the growing evidence that superconductivity and magnetism were related, Meissner and his graduate student Robert Ochsenfeld decided to investigate whether the change in a material as it became superconducting was accompanied by a change in its ability to become magnetized—or, as they put it in technical terms, by the degree to which magnetization permeated the substance.

Metals such as tin and lead could be readily magnetized at normal
temperatures. Would that ability change as the metal was cooled down to the temperature at which it became a superconductor? Meissner and Ochsenfeld cooled two adjacent long cylinders of single crystals of tin and at the same time introduced a magnetic field. Just at the moment that the solid tin became a superconductor, they removed the external magnetic field, then took readings of the cylinders' residual magnetism. They found none. The metal seem to have expelled all traces of a magnetic field from its interior.

Shades of Faraday! Ninety years earlier, Faraday had investigated the magnetic properties of all sorts of materials—metals, carrots, apples, meat—and found that all of them possessed, to a small degree, a property he labeled diamagnetism. Now Meissner and Ochsenfeld had shown that a solid crystal of tin could be perfectly diamagnetic, expel magnetism totally, just as Onnes had shown that supercooling mercury wires to 4.19 K totally eliminated their electrical resistance. It was now clear that the extreme changes in a substance's ability to conduct electricity, which occurred at very low temperatures, also had something to do with extreme changes in a substance's magnetic susceptibility.

This second instance of the vast transformative powers of the country of extreme cold, called
superdiamagnetism,
was also a significant puzzle whose solution would take many years. But, its identification in 1933 seemed to insist that the onset of superconductivity might best be considered akin to a thermodynamic change of state similar to what happened when a gas became a liquid or a liquid changed to a solid. How and why this change occurred, no one yet knew.

Theories to explain the how and why kept cropping up at a rate estimated by Kurt Mendelssohn of several each year; most were soon dismissed because they did not explain both superconductivity and superdiamagnetism. Between 1933 and 1935, however, several sets of scientists made good guesses about the nature of superconductivity that included possible explanations of superdiamagnetism.

In 1934 C. J. Gorter and H. B. G. Casimir, colleagues and successors of Keesom at the Leiden laboratory, suggested a model for superconductivity in which two fluids of electrons existed simultaneously. One was an ordered, condensed fluid of the sort Bose and Einstein had thought about, with zero entropy, which meant it could not transport heat (the product of resistance); this they called the "superfluid." The other was composed of electrons that behaved normally. When the temperature of helium was lowered beneath the transition point, more of the electrons entered the superfluid state, and that change, Gorter and Casimir postulated, was what caused superconductivity.

Picking up on the two-fluids idea, the brothers Fritz and Heinz London, at Oxford—where they had fled after escaping the Nazis in their native Germany—theorized how a superconductor might produce superdiamagnetism. In his earlier doctoral thesis, Heinz had figured the depth to which a current on the surface of a superconductor penetrates to the interior of the metal. All currents coursing on the surface of a metal produce a magnetic field. Fritz used this fact to explain the Meissner effect (exclusion of a magnetic field from the interior of a superconductor), by showing that when the current on the surface of the superconductor partially penetrated the surface to produce a magnetic field, that surface field canceled out the already existing field, so that the interior of the superconductor remained field-free, that is, it had perfect diamagnetism. Based on his previous research charting the depth of penetration, Heinz could predict the shape of the curve describing the falloff of the magnetic field within a superconductor, and he could express it in terms of the number and density of the superconducting electrons.