by Isaac Asimov
Certain light metals, such as lithium, boron, magnesium, aluminum, and, particularly, beryllium, deliver more energy on combining with oxygen than even hydrogen does. Some of these are rare, however, and all involve technical difficulties in the burning—difficulties arising from smokiness, oxide deposits, and so on.
There are also solid fuels that serve as their own oxidizers (like gunpowder, which was the first rocket propellant, but much more efficient). Such fuels are called monopropellants. since they need no separate supply of oxidizer and make up the one propellant required. Fuels that also require oxidizers are bipropellants (two propellants). Monopropellants would be easy to store and handle and would burn in a rapid but controlled fashion. The principal difficulty is probably that of developing a monopropellant with a specific impulse approaching those of the bipropellants.
Another possibility is atomic hydrogen, which Langmuir put to use in his blowtorch. It had been calculated that a rocket engine operating on the recombination of hydrogen atoms into molecules could develop a specific impulse of more than 1,300. The main problem is how to store the atomic hydrogen. So far the best hope seems to be to cool the free atoms very quickly and very drastically immediately after they are formed. Researches at the National Bureau of Standards seem to show that free hydrogen atoms are best preserved if trapped in a solid material at extremely low temperatures—say, frozen oxygen or argon. If we could arrange to push a button, so to speak, to let the frozen gases start warming up and evaporating, the hydrogen atoms would be freed and allowed to recombine. If such a solid could hold even as much as 10 percent of its weight in free hydrogen atoms, the result would be a better fuel than any we now possess. But, of course, the temperature would have to be very low indeed—considerably below that of liquefied hydrogen. These solids would have to be kept at about −272° C, or just 1 degree above absolute zero.
In another direction altogether lies the possibility of driving ions backward (rather than the exhaust gases of burned fuel). The individual ions, of tiny mass, would produce tiny impulses, but could be continued over long periods. A ship placed in orbit by the high but short-lived force of chemical fuel could then, in the virtually frictionless medium of space, slowly accelerate under the long-lived lash of ions to nearly light’s velocity. The material best suited to such an ionic drive is cesium, the substance that can most easily be made to lose electrons and form cesium ion. An electric field can then be made to accelerate the cesium ion and shoot it out the rocket opening.
SUPERCONDUCTORS AND SUPERFLUIDS
But to return to the world of low temperature. Even the liquefaction and solidification of hydrogen did not represent the final victory. By the time hydrogen yielded, the inert gases had been discovered; of these the lightest, helium, remained a stubborn holdout against liquefaction at the lowest temperatures attainable. Then, in 1908, the Dutch physicist Heike Kammerlingh Ormes finally subdued helium. He carried the Dewar system one step further. Using liquid hydrogen, he cooled helium gas under pressure to about −255° C (18° K) and then let the gas expand to cool itself further. By this method he liquefied the gas. Thereafter, by letting the liquid helium evaporate, he got down to the temperature at which helium could be liquefied under normal atmospheric pressure (4.2° K), a temperature at which all other substances are solid, and even to temperatures as low as 0.7° K. For his low-temperature work, Ormes received the Nobel Prize in physics in 1913. (Nowadays the liquefaction of helium is a simpler matter. In 1947, the American chemist Samuel Cornette Collins invented the cryostat, which, by alternate compressions and expansions, can produce as much as 2 gallons of liquid helium an hour.)
Ormes, however, did more than reach new depths of temperature. He was the first to show that unique properties of matter existed at those depths. One of these properties is the strange phenomenon called superconductivity. In 1911, Onnes was testing the electrical resistance of mercury at low temperatures. It was expected that resistance to an electric current would steadily decrease as the removal of heat reduced the normal vibration of the atoms in the metal. But at 4.12° K the mercury’s electrical resistance suddenly disappeared altogether! An electric current coursed through it without any loss of strength. It was soon found that other metals also could be made superconductive. Lead, for instance, became superconductive at 7.22° K. An electric current of several hundred amperes set up in a lead ring, kept at that temperature by liquid helium, went on circling through the ring for two and a half years with absolutely no detectable decrease in quantity.
As temperatures were pushed lower and lower, more metals were added to the list of superconductive materials. Tin became superconductive at 3.73° K; aluminum, at 1.20° K; uranium, at 0.8° K; titanium, at 0.53° K; hafnium, at 0.35° K. (Some 1,400 different elements and alloys are now known to display superconductivity.) But iron, nickel, copper, gold, sodium, and potassium must have still lower transition points—if they can be made superconductive at all—because they have not been reduced to this state at the lowest temperatures reached. The highest transition point found for a metallic element is that of technetium, which becomes superconductive at temperatures under 11.2° K.
A low-boiling liquid can easily maintain substances immersed in it at the temperature of its boiling point. To attain lower temperatures, the aid of a still-lower-boiling liquid must be called upon. Liquid hydrogen boils at 20.4° K, and it would be most useful to find a superconducting substance with a transition temperature at least this high. Only then can superconductivity be studied in systems cooled by liquid hydrogen. Failing that, only the one lower-boiling liquid, liquid helium—much rarer, more expensive, and harder to handle—must be used. A few alloys, particularly those involving the metal niobium, have transition temperatures higher than those of any pure metal. Finally, in 1968, an alloy of niobium, aluminum, and germanium was found that remained superconductive at 21° K. Superconductivity at liquid-hydrogen temperatures became feasible—but just barely.
A useful application of superconductivity suggests itself at once in connection with magnetism. A current of electricity through a coil of wire around an iron core can produce a strong magnetic field: the greater the current, the stronger the field. Unfortunately, the greater the current, the greater the heat produced under ordinary circumstances; and thus, there is a limit to what can be done. In superconductive wires, however, electricity flows without producing heat; and, it would seem, more and more electric current could be squeezed into the wires to produce unprecedentedly strong electromagnets at only a fraction of the power that must be expended under ordinary conditions. There is, however, a catch.
Along with superconductivity goes another property involving magnetism. At the moment that a substance becomes superconductive, it also becomes perfectly diamagnetic: that is, it excludes the lines of force of a magnetic field. This phenomenon was discovered by the German physicist Walther Meissner in 1933 and is therefore called the Meissner effect. By making the magnetic field strong enough, however, one can destroy the substance’s superconductivity and the hope for supermagnetism, even at temperatures well below its transition point. It is as if, once enough lines of force have been concentrated in the surroundings, some at last manage to penetrate the substance; and then, gone is the superconductivity as well.
Attempts have been made to find superconductive substances that will tolerate high magnetic fields. There is, for instance, a tin-niobium alloy with the high transition temperature of 180 K. It can support a magnetic field of some 250,000 gauss, which is high indeed. This fact was discovered in 1954, but it was only in 1960 that techniques were developed for forming wires of this ordinarily brittle alloy. A compound of vanadium and gallium can do even better, and superconductive electromagnets reaching field intensities of 500,000 gauss have been constructed.
Another startling phenomenon at low temperatures was discovered in helium itself. It is called superfluidity.
Helium is the only known substance that cannot be frozen solid, even at absol
ute zero. There is a small irreducible energy content, even at absolute zero, which cannot possibly be removed (so that the energy content is “zero” in a practical sense) but is enough to keep the extremely “nonsticky” atoms of helium free of each other and, therefore, liquid. Actually, the German physicist Hermann Walther Nernst showed, in 1905, that it is not the energy of a substance that becomes zero at absolute zero, but a closely related property: entropy. For this work he received the 1920 Nobel Prize in chemistry. I do not mean, however, that solid helium does not exist under any conditions: in 1926, it was produced at temperatures below 1° K, by a pressure of about 25 atmospheres.
In 1935, Willem Hendrik Keesom, who had managed the solidification of helium, and his sister, A. P. Keesom, working at the Ormes laboratory in Leyden, found that liquid helium at a temperature below 2.2° K conducts heat almost perfectly, It conducts heat so quickly—at the speed of sound, in fact—that all parts of the helium are always at the same temperature. It will not boil—as any ordinary liquid will by reason of localized hot spots forming bubbles of vapor—because there are no localized hot spots in the liquid helium (if you can speak of hot spots in connection with a liquid below 2° K). When it evaporates, the top of the liquid simply slips off quietly—peeling off, so to speak, in sheets.
The Russian physicist Peter Leonidovich Kapitza went on to investigate this property and found that the reason helium conducts heat so well was that it flows with remarkable ease, carrying the heat from one part of itself to another almost instantaneously, at least 200 times as rapidly as copper, the next best heat conductor. It flows even more easily than a gas, having a viscosity only 1/1,000 that of gaseous hydrogen, and leaks through apertures so tiny that they stop a gas. Furthermore, the superfluid liquid forms a film on glass and flows along it as quickly as it pours through a hole. If an open container of the liquid is placed in a larger container filled to a lower level, the fluid will creep up the side of the glass and over the rim into the outer container, until the levels in both are equalized.
Helium is the only substance that exhibits this phenomenon of superfluidity. In fact, the superfluid behaves so differently from the way helium itself does above 2.2° K that it has been given a separate name, helium II, to distinguish it from liquid helium above that temperature, called helium I.
Since only helium permits investigation of temperatures close to absolute zero, it has become a very important element in both pure and applied science.
The atmospheric supply is negligible, and the most important sources are natural gas wells into which helium, formed from uranium and thorium breakdown in the earth’s crust, sometimes seeps. The gas produced by the richest known well (in New Mexico) is 7.5 percent helium.
CRYOGENICS
Spurred by the odd phenomena discovered in the neighborhood of absolute zero, physicists have naturally made every effort to get down as close to absolute zero as possible and expand their knowledge of what is now known as cryogenics. The evaporation of liquid helium can, under special conditions, produce temperatures as low as 0.5° K. (Temperatures at such a level, by the way, are measured by special methods involving electricity—for example, by the size of the current generated in a thermocouple, by the resistance of a wire made of some nonsuperconductive metal, by changes in magnetic properties, or even by the speed of sound in helium. The measurement of extremely low temperatures is scarcely easier than their attainment.) Temperatures substantially lower than 0.5° have been reached by a technique first suggested in 1925 by the Dutch physicist Peter Joseph Wilhelm Debye. A paramagnetic substance (that is, a substance that concentrates lines of magnetic force) is placed almost in contact with liquid helium, separated from it by helium gas, and the temperature of the whole system is reduced to about 1° K. The system is then placed within a magnetic field. The molecules of the paramagnetic substance line up parallel to the field’s lines of force and, in doing so, give off heat. This heat is removed by further slight evaporation of the surrounding helium. Now the magnetic field is removed. The paramagnetic molecules immediately fall into a random orientation. In going from an ordered to a random orientation, the molecules must absorb heat, the only source of which is the liquid helium. The temperature of the liquid helium therefore drops.
This process can be repeated over and over, each time lowering the temperature of the liquid helium—a technique perfected by the American chemist William Francis Giauque, who received the Nobel Prize for chemistry in 1949 in consequence. In this way, a temperature of 0.00002° K was reached in 1957.
In 1962, the German-British physicist Heinz London and his co-workers, suggested the possibility of using a new device to attain still lower temperatures. Helium occurs in two varieties, helium 4 and helium 3. Ordinarily they mix perfectly; but at temperatures below about 0.8° K, they separate, with helium 3 in a top layer. Some of the helium 3 is in the bottom layer with the helium 4, and it is possible to cause helium 3 to shift back and forth across the boundary, lowering the temperature each time in a fashion analogous to the shift between liquid and vapor in the case of an ordinary refrigerant such as Freon. Cooling devices making use of this principle were first constructed in the Soviet Union in 1965.
The Russian physicist Isaak Yakovievich Pomeranchuk suggested, in 1950, a method of deep cooling using other properties of helium 3; while as long ago as 1934, the Hungarian-British physicist Nicholas Kurti suggested the use of magnetic properties similar to those taken advantage of by Giauque, but involving the atomic nucleus—the innermost structure of the atom—rather than entire atoms and molecules.
As a result of the use of these new techniques, temperatures as low as 0.000001° K have been attained. And as long as physicists find themselves within a millionth of a degree of absolute zero, might they not just get rid of what little entropy is left and finally reach the mark itself?
No! Absolute zero is unattainable—as Nernst demonstrated in his Nobel-Prize-winning treatment of the subject (sometimes referred to as the third law of thermodynamics). In any lowering of temperature, only part of the entropy can be removed. In general, removing half of the entropy of a system is equally difficult regardless of what the total is. Thus it is just as hard to go from 300° K (about room temperature) to 150° K (colder than any temperature Antarctica attains) as to go from 20° K to 10° K. It is then just as hard to go from 10°K to 5° K and from 5°K to 2.5° K, and so on. Having attained a millionth of a degree above absolute zero, the task of going from that to half-a-millionth of a degree is as hard as going from 300° K to 150° K, and if that is attained, it is an equally difficult task to go from half-a-millionth to a quarter-of-a-millionth, and so on forever. Absolute zero lies at an infinite distance no matter how closely it seems to be approached.
The final stages of the quest for absolute zero has, by the way, resulted in the close study of helium 3, an extremely rare substance. Helium is itself not at all common on Earth; and when it is isolated, only 13 atoms out of every 10,000,000 are helium 3, the remainder being helium 4.
Helium 3 is a somewhat simpler atom than helium 4 and has only three-fourths of the mass of the more common variety. The liquefaction point of helium 3 is 3.2° K, a full degree below that of helium 4. What’s more, it was at first thought that whereas helium 4 becomes superfluid at temperatures below 2.2° K, helium 3 (a less symmetrical molecule, even though simpler) shows no sign of superfluidity at all. It was only necessary to keep trying. In 1972, it was discovered that helium 3 changes to a superfluid helium II liquid form at temperatures below 0.0025° K.
HIGH PRESSURES
One of the new scientific horizons opened up by the work on liquefaction of gases was the development of an interest in producing high pressures. It seemed that putting various kinds of matter (not only gases) under great pressure might bring out fundamental information about the nature of matter and also about the interior of the earth. At a depth of 7 miles, for instance, the pressure is 1,000 atmospheres; at 400 miles, 200,000 atmospheres; at 2,000 mile
s, 1,400,000 atmospheres; and at the center of the earth, 4,000 miles down, it reaches 3,500,000 atmospheres. (Of course, Earth is a rather small planet. The central pressures within Saturn are estimated to be over 50,000,000 atmospheres; within the even larger Jupiter, 100,000,000.)
The best that nineteenth-century laboratories could do was about 3,000 atmospheres, attained by Emile Hilaire Amagat in the 1880s. But, in 1905, the American physicist Percy Williams Bridgman began to devise new methods that soon reached pressures of 20,000 atmospheres and burst the tiny metal chambers he used for his experiments. He went to stronger materials and eventually succeeded in producing pressures of half a million atmospheres. For his work on high pressure he received the Nobel Prize in physics in 1946.
Under his ultrahigh pressures, Bridgman was able to force the atoms and molecules of a substance into more compact arrangements, which were sometimes retained after the pressure was released. For instance, he converted ordinary yellow phosphorus, a nonconductor of electricity, into a black, conducting form of phosphorus. He brought about startling changes even in water. Ordinary ice is less dense than liquid water. Using high pressure, Bridgman produced a series of ices (ice-II, ice-III, and so on) that not only were denser than the liquid but were ice at temperatures well above the normal freezing point of water. Ice-VII is a solid at temperatures higher than the boiling point of water.