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 (excl
usion 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.
At a seminal meeting of low-temperature researchers from many countries, held at the Royal Society in 1935, Fritz London summed up all the post-1911 theorizing by asking the scientists to take a dazzling imaginative leap—to stop thinking about superconductiv ity in terms of yesterday's classical physics and to instead consider superconductivity solely in terms of quantum physics and wave motions. Conceive of a superconductor, London pleaded, not as a collection of unrelated atoms but as one huge atom—and the problem will be more easily attacked. The big atom's interior order—the pattern synonymous with superconductivity—could be described, London said, by a single wave function. In other words, the superconducting state had to be produced by the electrons of this giant atom behaving coherently, or in unison.
Just when this notion was being put forth, a third ultra-low-temperature puzzle was discovered. This time it was made by a leading physicist who was unable to attend the meeting in 1935 at the Royal Society, though he desperately wanted to be there: Pyotr Kapitsa.
Previously, the Russian émigré had been the director of the Mond Laboratory in Cambridge, using equipment built expressly for him there at the suggestion of his teacher and mentor, Ernest Rutherford. The British had gone to this trouble because Kapitsa had demonstrated theoretical brilliance and practical ingenuity. For instance, he had invented a faster process for liquefying helium, and was working on a device that employed discharges of electricity to produce intense magnetic fields. By 1930 Kapitsa had accomplished so much that he had been elected a Fellow of the Royal Society, the first foreigner so honored in the previous two hundred years, and was doing important research on both magnetism and low temperatures. Each year he would take a trip back to Russia with his wife to visit their relatives, but when he did so in 1934, the couple were prevented from returning to Great Britain.
Two years of negotiating ensued until, in 1936, the Kapitsas, the Soviet government, and Rutherford completed a three-way deal. Kapitsa's wife agreed to go briefly to England to fetch the couple's two children and bring them back to the Soviet Union with her. In exchange for having his family reunited, Kapitsa accepted the direc torship of a new laboratory in the U.S.S.R., and Rutherford arranged for some of the Mond's equipment to be shipped to Moscow for use at the Institute for Physical Problems.
Once his machinery had arrived, in very short order Kapitsa succeeded in identifying and describing a third, new, and entirely unexpected aspect of matter in the region of ultracold. The discovery was galvanized into existence by a paper written by W. H. Keesom and his daughter Anna Petronella, which suggested that at the lambda point, the thermal conductivity of helium II increased over that of helium I by a factor of 3 million. This meant that helium II became a better conductor of heat than copper or silver, the best normal-temperature metallic conductors of heat.
Kapitsa, fascinated by helium II, used his imagination to make sense out of some odd things happening in research labs. Helium II had not been behaving like all other earthly liquids. It had escaped from containers dense and impermeable enough to prevent the leakage of any other fluid, even helium I. This ability of helium II had resulted in contamination of other fluids, making a shambles of experiments in several laboratories. Also, if a container of helium II was placed in a bath of helium II and filled to a level higher than the bath, the levels inside and outside the container would gradually equalize. Creeping up and over walls, defying friction and gravity, it seemed to refuse to adhere to normal physical rules of flow. "Helium," Kapitsa later wrote, "moves faster than a bullet."
Taking these Houdini-like effects as his starting point, Kapitsa tried to determine the parameters of helium II's escape artistry. Researchers at Cambridge and Leiden were also working on the problem, and the three groups kept in touch with one another by mail, telephone, and personal meetings, carrying on what Kurt Mendelssohn characterized as "ruthlessly searching discussions into the validity of each other's methods." In articles published beginning in 1938, Kapitsa seized the theoretical high ground and made order out of what had been perplexing chaos by describing what helium II was doing as exhibiting superfluidity,.* He attributed superfluidity to changes in the viscosity that were intimately related to what had initially prevented Onnes and Dana from publishing their data in 1922: the sharp rise in helium II's specific heat, later identified by Keesom and Petronella as being 3 million times greater than that of helium I. This fantastic ability of helium II to conduct heat and its ability to move about as though nothing could stand in its way, Kapitsa suggested, were aspects of the same phenomenon.
He devised an experiment that demonstrated the effects of these behaviors. Inside a large dewar of liquid helium II, he placed a smaller one also filled with liquid helium, and in that "bulblet" Kapitsa inserted a capillary tube with one end sealed inside and the other open to the helium vapor. The outer dewar was necessary to keep the inner one at the proper low temperature. Kapitsa set up a weathervane sort of instrument near the open end of the capillary and applied heat to the bottom of the bulblet. A submerged jet of invisible liquid helium issued from the top and turned the weathervane. The experiment went on for hours, with the vane spinning and the bulblet of liquid helium as full at the end as it had been before the start. Kapitsa figured out that the heat transformed some of the superfluid to normal fluid, which produced the submerged jet. He concluded that helium II had no entropy and a viscosity 10,000 times lower than that of liquid hydrogen, that is, an almost unmeasurably small viscosity, virtually none at all.
"At first sight," wrote Russian physicist Lev Landau of Kapitsa's weathervane experiment, liquid helium's properties "seem completely absurd, like the story of the giraffe which evoked the exclamation, 'There ain't no such animal!'"
No viscosity.
No inner magnetic field.
No electrical resistance.
The trio of unusual phenomena at the far edge of the ultracold was complete: superconductivity, superdiamagnetism, and superfluidity.
The discovery of this trio of phenomena meant the final eclipse of the old clockwork universe that obeyed Newtonian laws of motion. No adequate explanation of the new phenomena could be made by means of the old laws. Fortunately, though, this trio of puzzles surfaced at a time when quantum physics had matured enough to provide cogent attempts to explain superconductivity, superfluidity, and superdiamagnetism. Kapitsa was fond of saying that trying to detect the quantum nature of physical processes at room temperature was like trying to investigate the physical laws governing the collision of billiard balls on a table aboard a ship going through rough seas. And Landau would explain to his classes the advantages of lowering temperatures into the arena of the ultracold, where all sorts of processes slowed down and became more amenable to study. "As the temperature falls," Landau said, "the energy of the atomic particles decreases, the conditions in which classical mechanics are valid are eventually violated, and classical mechanics has to be replaced [as a tool for understanding] by quantum mechanics."
A protégé of Niels Bohr, and a man acknowledged as one of the great teachers of physics in the twentieth century, Landau was Kapitsa's closest colleague. An irascible perfectionist who liked to deny he had been a child prodigy even though he published a brilliant paper in quantum mechanics at the age of nineteen, Landau was arrested and imprisoned in the 1930s, charged with anti-Soviet activity and with being a Nazi spy, though he was Jewish. Kapitsa wrote directly to Stalin seeking Landau
's release. "I beg you to give orders that his case should be very carefully considered"; Kapitsa acknowledged that his colleague's character was "bad," that he was "not easy to get on with [and] enjoys looking for mistakes in others [which] has made him many enemies," but denied that Landau could ever do anything seriously dishonest. When this did not produce results, he wrote to the foreign minister and then to the head of the secret police, demanding the release of Landau and personally guaranteeing that his colleague would "not engage in any counterrevolutionary activities." He also threatened that if Landau was not freed, he, Kapitsa, would resign. Landau later wrote that Kapitsa's activism on his behalf required "superb courage, great humanity, and crystalline integrity." Released from prison, Landau returned to his laboratory at the Institute for Physical Problems, where he shortly began to examine helium II and its strange antics in a new way.
Landau suggested that helium II be considered one large molecule, akin to a crystal. At absolute zero, Landau believed, helium II was 100 percent superfluid. As the temperature rose, "elementary excitations"appeared on the superfluid, particles known as phonons (quantized sound waves) and rotons (which move in exotic ways, such as in the reverse direction of their momentum). These particles, Landau guessed, comprised the normal fluid. From these ideas, Landau was able to formulate equations for the motion of the two fluids, normal and superfluid. He was also able to define viscosity as "the ability of a liquid to oppose movement" and the effective absence of viscosity as the inability to oppose movement. He reasoned that if anything were able to retard the flow of helium II into the capillary from the larger dewar in Kapitsa's experiment, the kinetic energy of the liquid would be reduced, its temperature would rise, and it would behave like (or become) helium I. But since the capillary walls did not impede the movement or change the energy level of the phonons, the fluid remained in the form of helium II, and only after heating did it exit the top of the capillary and turn the weathervane. According to Landau, the velocity of helium II as it penetrated into the capillary was low enough to permit the unimpeded flow through certain walls, and against gravity. In other words, helium II was not faster than a bullet, as Kapitsa had contended; precisely the opposite was true. Helium II was the slowest yet the most persistently moving and unstoppable substance on Earth.
Absolute Zero and the Conquest of Cold Page 22