by Isaac Asimov
Masers and Lasers
MASERS
Another recent advance of astonishing magnitude begins with investigations involving the ammonia molecule (NH3). The three hydrogen atoms of the ammonia molecule can be viewed as occupying the three apexes of an equilateral triangle, whereas the single nitrogen atom is some distance above the center of the triangle.
It is possible for the ammonia molecule to vibrate: that is, the nitrogen atom can move through the plane of the triangle to an equivalent position on the other side, then back to the first side, and so on, over and over. The ammonia molecule can, in fact, be made to vibrate back and forth with a natural frequency of 24 billion times a second.
This vibration period is extremely constant, much more so than the period of any artificial vibrating device-much more constant, even, than the movement of astronomical bodies. Such vibrating molecules can be made to control electric currents, which will in turn control time-measuring devices with unprecedented precision-as was first demonstrated in 1949 by the American physicist Harold Lyons. By the mid-1950s such atomic clocks were surpassing all ordinary chronometers. Accuracies in time measurement of I second in 1,700,000 years have been reached by making use of hydrogen atoms.
The ammonia molecule, in the course of these vibrations, liberates a beam of electromagnetic radiation with a frequency of 24 billion cycles per second. This radiation has a wavelength of 1.25 centimeters and is in the microwave region. Another way of looking at this fact is to imagine the ammonia molecule to be capable of occupying one of two energy levels, with the energy difference equal to that of a photon representing a 1.25-centimeter radiation. If the ammonia molecule drops from the higher energy level to the lower, it emits a photon of this size. If a molecule in the lower energy level absorbs a photon of this size, it rises to the higher energy level.
But what if an ammonia molecule is already in the higher energy level and is exposed to such photons? As early as 1917, Einstein had pointed out that, if a photon of just the right size struck such an upper-level molecule, the molecule would be nudged back down to the lower level and would emit a photon of exactly the size and moving in exactly the direction of the entering photon. There would be two identical photons where only one had existed before. This theory was confirmed experimentally in 1924.
Ammonia exposed to microwave radiation could, therefore, undergo two possible changes: molecules could be pumped up from lower level to higher or be nudged down from higher level to lower. Under ordinary conditions, the former process would predominate, for only a very small percentage of the ammonia molecules would, at anyone instant, be at the higher energy level.
Suppose, though, that some method were found to place all or almost all the molecules in the upper energy level. Then the movement from higher level to lower would predominate. Indeed, something quite interesting would happen. The incoming beam of microwave radiation would supply a photon that would nudge one molecule downward. A second photon would be released, and the two would speed on, striking two molecules, so that two more were released. All four would bring about the release of four more, and so on. The initial photon would let loose a whole avalanche of photons, all of exactly the same size and moving in exactly the same direction.
In 1953, the American physicist Charles Hard Townes devised a method for isolating ammonia molecules in the high-energy level and subjected them to stimulation by microwave photons of the correct size. A few photons entered, and a flood of such photons left. The incoming radiation was thus greatly amplified.
The process was described as “microwave amplification by stimulated emission of radiation”; and from the initials of this phrase, the instrument came to be called a maser.
Solid masers were soon developed—solids in which electrons could be made to take up one of two energy levels. The first masers, both gaseous and solid, were intermittent: that is, they had to be pumped up to the higher energy level first; then stimulated. After a quick burst of radiation, nothing more could be obtained until the pumping process had been repeated.
To circumvent this drawback, it occurred to the Dutch-American physicist Nicolaas Bloembergen to make use of a three-level system. If the material chosen for the core of the maser can have electrons in any of three energy levels-a lower, a middle, and an upper—then pumping and emission can go on simultaneously. Electrons are pumped up from the lowest energy level to the highest. Once at the highest, proper stimulation will cause them to drop down—first to the middle level, then to the lower. Photons of different size are required for pumping and for stimulated emission, and the two processes will not interfere with each other. Thus, we end with a continuous maser.
As microwave amplifiers, masers can be used as very sensitive detectors in radio astronomy, where exceedingly feeble microwave beams received from outer space will be greatly intensified with great fidelity to the original radiation characteristics. (Reproduction without loss of original characteristics is to reproduce with little “noise.” Masers are extraordinarily “noiseless” in this sense of the word.) They have carried their usefulness into outer space, too. A maser was carried on board the Soviet satellite Cosmos 97, launched 30 November 1965, and did its work well.
For his work, Townes received the 1964 Nobel Prize for physics, sharing it with two Soviet physicists, Nicolai Cennediyevich Basov and Aleksandr Mikhailovich Prochorov, who had worked independently on maser theory.
LASERS
In principle, the maser technique could be applied to electromagnetic waves of any wavelength, notably to those of visible light. Townes pointed out the possible route of such applications to light wavelengths in 1958. Such a light-producing maser might be called an optical maser. Or, this particular process might be called “light amplification by stimulated emission of radiation,” with the resultant popular term, laser (figure 9.15).
Figure 9.15. Continuous-wave laser with concave mirrors and Brewster angle windows on discharge tube. The tube is filled with a gas whose atoms are raised to high-energy states by electromagnetic excitation. These atoms are then stimulated to emit energy of a certain wavelength by the introduction of a light beam. Acting like a pipe organ, the resonant cavity builds up a train of coherent waves between the end mirrors. The thin beam that escapes is the laser ray. After a drawing in Science, 9 October 1964.
The first successful laser was constructed in 1960 by the American physicist Theodore Harold Maiman. He used, for the purpose, a bar of synthetic ruby, this being essentially aluminum oxide with a bit of chromium oxide added. If the ruby bar is exposed to light, the electrons of the chromium atoms are pumped to higher levels and, after a short while, begin to fall back. The first few photons of light emitted (with a wavelength of 694.3 millimicrons) stimulate the production of other such photons, and the bar suddenly emits a beam of deep red light four times as intense as light at the sun’s surface. Before 1960 was over, continuous lasers were prepared by an Iranian physicist, Ali Javan, working at Bell Laboratories. He used a gas mixture (neon and helium) as the light source.
The laser made possible light in a completely new form. The light was the most intense that had ever been produced, and the most narrowly monochromatic (single wavelength), but it was even more.
Ordinary light, produced in any other fashion—from a wood fire to the sun or to a firefly—consists of relatively short wave packets. They can be pictured as short bits of waves pointing in various directions. Ordinary light is made up of countless numbers of these.
The light produced by a stimulated laser, however, consists of photons of the same size and moving in the same direction. Hence, the wave packets are all of the same frequency; and since they are lined up precisely end to end, so to speak, they melt together. The light appears to be made up of long stretches of waves of even amplitude (height) and frequency (width). This is coherent light; because the wave packets seem to stick together. Physicists had learned to prepare coherent radiation for long wavelengths. It had never been done for light, though,
until 1960.
The laser was so designed, moreover, that the natural tendency of the photons to move in the same direction was accentuated. The two ends of the ruby tube were accurately machined and silvered so as to serve as plane mirrors. The emitted photons flashed back and forth along the rod, knocking out more photons at each pass, until they had built up sufficient intensity to burst through the end that was more lightly silvered. Those that did come through were precisely those that had been emitted in a direction exactly parallel to the long axis of the rod, for those would move back and forth, striking the mirrored ends over and over. If any photon of proper energy happened to enter the rod in a different direction (even a very slightly different direction) and started a train of stimulated photons in that different direction, these would quickly pass out the sides of the rod after only a few reflections at most.
A beam of laser light is made up of coherent waves so parallel that it can travel through long distances without diverging to uselessness. It could be focused finely enough to heat a pot of coffee a thousand miles away. Laser beams even reached to the moon, in 1962, spreading out to a diameter of only two miles after having crossed nearly a quarter of a million miles of space!
Once the laser was devised, interest in its further development wasnothing short of explosive. Within a few years, individual lasers capable of producing coherent light in hundreds of different wavelengths, from the near ultraviolet to the far infrared, were developed. Laser action was obtained from a wide variety of solids, from metallic oxides, fluorides, tungstates, from semiconductors, from liquids, from columns of gas. Each variety had its advantages and disadvantages.
In 1964, the first chemical laser was developed by the American physicist Jerome v. V. Kasper. In such a laser, the source of energy is a chemical reaction (in the case of the first, the dissociation of CF3I by a pulse of light). The advantage of the chemical laser over the ordinary variety is that the energy-yielding chemical reaction can be incorporated with the laser itself, and no outside energy source is needed. This is analogous to a battery-powered device as compared with one that must be plugged into a wall socket. There is an obvious gain in portability to say nothing of the fact that chemical lasers seem to be considerably more efficient than the ordinary variety (12 percent or more, as compared with 2 percent or less).
Organic lasers—those in which a complex organic dye is used as the source of coherent light—were first developed in 1966 by John R. Lankard and Peter Sorokin. The complexity of the molecule makes it possible to produce light by a variety of electronic reactions and therefore in a variety of wavelengths. A single organic laser can be tuned to deliver any wavelength within a range, rather than find itself confined to a single wavelength, as is true of the others.
The narrowness of the beam of laser light means that a great deal of energy can be focused into an exceedingly small area; in that area, the temperature reaches extreme levels. The laser can vaporize metal for quick spectral investigation and analysis and can weld, cut, or punch holes of any desired shape through high-melting substances. By shining laser beams into the eye, surgeons have succeeded in welding loosened retinas so rapidly that surrounding tissues have no time to be affected by heat. In similar fashion, lasers have been used to destroy tumors.
To show the vast range of laser applications, Arthur L. Shawlow developed the trivial (but impressive) laser-eraser, which in an intensely brief flash evaporates the typewriter ink of the formed letters without so much as scorching the paper beneath; at the other extreme, laser interferometers can make unprecedentedly refined measurements. When earth strains intensify, they can be detected by separated lasers, where shifts in the interference fringes of their light will detect tiny earth movements with a delicacy of one part in a trillion. Then, too, the first men on the moon left a reflector system designed to bounce back laser beams to earth. By such a method, the distance to the moon may be determined with greater accuracy than the distance, in general, from point to point on Earth’s surface.
One possible application that created excitement from the beginning has been the use of laser beams as carrier beams in communications. The high frequency of coherent light, as compared with that of the coherent radio waves used in radio and television today, holds forth the promise of being able to crowd many thousands of channels into the space that now holds one channel. The prospect arises that every human being on Earth may have his or her own personal wavelength. Naturally, the laser light must be modulated. Varying electric currents produced by sound must be translated into varying laser light (either through changes in its amplitude on its frequency, or perhaps just by turning it on and off), which can in turn be used to produce varying electric current elsewhere. Such systems are being developed.
It may be that since light is much more subject than radio waves to interference by clouds, mist, fog, and dust, it will be necessary to conduct laser light through pipes containing lenses (to reconcentrate the beam at intervals) and mirrors (to reflect it around corners). However, a carbon-dioxide laser has been developed that produces continuous laser beams of unprecedented power that are far enough in the infrared to be little affected by the atmosphere. Atmospheric communication may also be possible then.
Much more immediately practical is the possibility of using modulated laser beams in optical fibers, supertransparent glass tubes finer than a human hair, to replace insulated copper wires in telephone communications. Glass is tremendously cheaper and more common than copper and can carry far more information by way of laser light. Already, the bulky copper-wired cables in many places are giving way to the far less bulky optical fiber bundles.
A still more fascinating application of laser beams that is very here-and-now involves a new kind of photography. In ordinary photography, a beam of ordinary light reflected from an object falls on a photographic film. What is recorded is the cross-section of the light, which is by no means all the information it can potentially contain.
Suppose instead that a beam of light is split in two. One part strikes an object and is reflected with all the irregularities that this object would impose on it. The second part is reflected from a mirror with no irregularities. The two parts meet at the photographic film, and the interference of the various wavelengths is recorded. In theory, the recording of this interference would include all the data concerning each light beam. The photograph that records this interference pattern seems to be blank when developed; but if light is shone upon the film and passes through and takes on the interference characteristics, it produces an image containing the complete information. The image is as three-dimensional as was the surface from which light was reflected, and an ordinary photograph can be taken of the image from various angles that show the change in perspective.
This notion was first worked out by the Hungarian-British physicist Dennis Gabor in 1947, when he was trying to work out methods for the sharpening of images produced by electron microscopes. He called it holography, from a Latin word meaning “the whole writing.”
While Gabor’s idea was theoretically sound, it could not be implemented, because ordinary light would not do. With wavelengths of all sizes moving in all directions, the interference fringes produced by the two beams of light would be so chaotic as to yield no information at all. It would be like producing a million dim images all superimposed in slightly different positions.
The introduction of laser light changed everything. In 1965, Emmet N. Leith and Juris Upatnieks, at the University of Michigan, were able to produce the first holograms. Since then, the technique has been sharpened to the point where holography in color has become possible, and where the photographed interference fringes can successfully be viewed with ordinary light. Microholography promises to add a new dimension (literally) to biological investigations; and where it will end, none can predict.
Chapter 10
* * *
The Reactor
Energy
The rapid advances in technology in the twentieth century
have been bought at the expense of a stupendous increase in our consumption of the earth’s energy resources. As the underdeveloped nations, with their billions of people, join the already industrialized countries in high living, the rate of consumption of fuel will jump even more spectacularly. Where will we find the energy supplies needed to support our civilization?
We have already seen a large part of the earth’s timber disappear. Wood was our first fuel. By the beginning of the Christian era, much of Greece, northern Africa, and the Near East had been ruthlessly deforested, partly for fuel, partly to clear the land for animal herding and agriculture. The uncontrolled felling of the forests was a double-barreled disaster. Not only did it destroy the wood supply, but the drastic uncovering of the land meant a more or less permanent destruction of fertility. Most of these ancient regions, which once supported advanced cultures, are sterile and unproductive now, populated by a ground-down and impoverished people.
The Middle Ages saw the gradual deforestation of western Europe, and modern times have seen the much more rapid deforestation of the North American continent. Almost no great stands of virgin timber remain in the world’s temperate zones except in Canada and Siberia.
COAL AND OIL: FOSSIL FUELS
Coal and oil have taken wood’s place as fuel. Coal was mentioned by the Greek botanist Theophrastus as long ago as 200 B.C., but the first records of actual coal mining in Europe do not date back before the twelfth century. By the seventeenth century, England, deforested and desperately short of wood for its navy, began to shift to the large-scale use of coal for fuel, inspired perhaps by the fact that the Netherlanders had already begun to dig for coal. (They were not the first. Marco Polo, in his famous book about his travels in China in the late 1200s, had described coal burning in that land, which was then the most technologically advanced in the world.)