The Physics of War

by Barry Parker

  Stimulated emission.

  It was an interesting phenomenon, but for several years no one took any interest in it. During World War II, however, radar was developed and used extensively, and there was considerable interest in further developing it after the war. One of the areas of interest was the possibility of a microwave amplifier; in other words, a device that would increase, or amplify, microwaves. Joseph Weber of the University of Maryland became particularly interested in the device. After studying the problem in detail he came to the conclusion that it might be possible to build an amplifier using stimulated emission. He pointed out, however, that what is called a “population inversion” would be needed. Such an inversion occurs when high-energy levels of an atom contain more electrons than lower levels. This is not the normal situation; the electrons in an atom are usually distributed so that most are in lower energy levels, with fewer in upper energy levels.

  A typical energy diagram showing the number of electrons in each level.

  A population inversion.

  But how could a population inversion be created? Obviously an energy source would be required to force the electrons to higher energy levels, and appropriate energy sources were soon found. We now refer to them as pumps.

  Weber designed a microwave amplifier that he thought might work, but he didn't build it. This was left to Charles Townes of Columbia University. Townes was also studying microwaves and looking into the possibility of an amplifier. He decided to set up a population inversion with what he called a resonant cavity, which is a box with reflecting walls. He devised a method for pumping the electrons within this resonant cavity up to excited states, and by doing so he succeeded in creating a population inversion. In addition, he devised a method for allowing the electrons to suddenly fall to the ground state. The radiation they gave off when this occurred was “coherent” microwave radiation; in other words, the wavelengths were all lined up and had the same phase and frequency (see diagram). In the process he produced the first of what is now called a maser (whereas a maser uses microwave radiation, a laser uses visual light).

  Charles Townes.

  Soon after he created the maser, Townes began to look at the possibility of a similar device that used optical waves or visible light. This did not prove to be easy. Optical photons are quite different from microwave photons, and Townes worked on the device for several years before he managed to build one. The new device, called the laser (short for light amplification by stimulated emission of radiation), has now overshadowed the maser because it has many more uses in modern society.

  The basic principle of the laser is similar to that of the maser. A laser creates a beam of light in which all the photons are coherent. In an ordinary light beam the photons are of many different wavelengths (white light is composed of all colors, each of which has a different wavelength), and the waves are not lined up; as a result, they easily knock one another out of the beam so that the beam cannot be sharply focused. In a laser beam the photons (or waves) are coherent and of the same frequency so that they can be sharply focused.

  Top beam: coherent light. Bottom beam: incoherent light.

  As in the case of microwaves, a resonant cavity is also needed here; in the laser, though, it is usually called an optical cavity. The medium within the optical cavity is called the gain medium; it is a material that has the properties needed for amplification of the light by stimulated emission. For this amplification a pump is needed; it is usually an electrical circuit or a flash lamp. Mirrors are placed at either end of the optical cavity, one of which is partially transparent so that some light can pass through it. Light within the cavity reflects back and forth through the gain medium and is amplified each time it makes a pass. This medium is, in essence, a population of atoms that has been excited by an external source. The medium itself can be liquid, gas, solid, or plasma.

  The gain medium is “pumped” to an excited state; in other words, the atoms within it are in the excited state after the pumping occurs. Eventually a population inversion is achieved in which higher energy states are more densely populated than lower energy states. The reflected beam grows in intensity until finally it is powerful enough to break through the partially reflecting mirror. What emerges is a coherent laser beam.

  Townes, along with his student Arthur Schawlow, was the first to design a workable laser, but they did not build it. They did, however, publish a paper on it, and they filed a patent for the idea in July 1958. A research worker at TRG Incorporated by the name of Gordon Gould was also working on a similar device. Gould tried to patent his device in April 1959, but his application was turned down, even though Gould had described the construction of his laser in a notebook prior to Townes and Schawlow filing for their patent. Several court cases followed, and it took years to settle them.8 The two groups are now credited with having invented the laser independently.

  The first person to actually build a working laser, however, was Theodore Maiman of Hughes Research Laboratories in California. His device was quite different than that of Townes, Schawlow, and Gould; they had designed a device using gas as the gain medium. Maiman used a ruby rod with a helical flash lamp wound around it that acted as a pump.

  The next step was, of course, to use lasers as weapons of war. Laser-like devices such as ray guns had been used in science fiction for years. It turned out, however, to be much more difficult to make laser weapons than expected, and there's little chance that a laser-like weapon will replace small arms in the near future. The main problem is that lasers require a huge power source, and because of this, there are serious engineering problems. Larger weapons, however, are possible, and the navy has recently built one that could disable an enemy ship and knock down enemy drones. The biggest advantage of a laser such as this is that it doesn't require expensive ammunition. However, the laser itself would be relatively expensive.

  One form of laser that appears to have considerable potential is the x-ray laser. It produces a coherent beam of x-rays rather than an optical beam; as a result, it has much more energy. It was considered part of the Strategic Defense Initiative that was proposed in 1983 (sometimes referred to as “Star Wars”). Such lasers were to be powered by nuclear explosions. Tests eventually showed, however, that they were not feasible.

  TRANSISTORS, MICROCHIPS, AND COMPUTERS

  Many scientific breakthroughs have led to important developments in weaponry, but nothing approaches the invention of the transistor. All electronic devices now use transistors in one form or another, and hardly a form of weaponry exists that doesn't use electronics in some way. The electronic age came about early in the twentieth century with the invention of the triode, or vacuum tube. It gave us radar and many other electronic devices. But it was fragile in many ways, and relatively large. When John Bardeen, Walter Brattain, and William Shockley developed the transistor at Bell Labs in late 1947, however, the world of electronics underwent a revolution. Tiny radios, calculators of all types, and powerful computers soon followed. Today most transistors are actually found in integrated circuits, or microchips, as they are frequently called; nevertheless, it was the invention of the transistor that started the revolution.9

  A transistor is a device that can amplify, or switch, incoming electronic signals. It was developed by physicists working in solid-state physics. As the name implies, solid-state physics deals with solids. And, as you no doubt know, solids come in many varieties. Some are good conductors of electricity; others are insulators (nonconductors), and there is a group in between called semiconductors. Semiconductors have proven particularly important because solids of this type have made transistors possible.

  To understand things a little better, let's look at the atomic structure of metals and semiconductors. We'll begin with a gas. The atoms of a gas have a nucleus with a number of electrons whirling around it in various energy levels. Assume that we apply pressure to the gas or lower its temperature. What happens to it? The atoms begin to move closer together and eventually the gas
turns into a liquid as the atoms get closer and closer. At this point the energy levels of the various atoms are still completely separated, but as you continue applying pressure (or lowering the temperature), the liquid becomes solid, and the energy levels of the individual atoms begin to overlap. They will create what are called energy bands, which are continuous regions of energy.

  The exact way these bands form depends on the particular material being compressed or cooled. If you could look at these energy levels closely you would see that some of them contain electrons, and some are empty. There are also gaps between the bands. In most metals and semiconductors there are, in fact, two major bands with a gap between them. They are referred to as the valence band and the conduction band. The size of the gap between the two bands determines whether they are metals, semiconductors, or insulators.

  A current, as we saw earlier, is a group of electrons moving through the lattice created by the atoms of a metal or a semiconductor. In effect, the electrons jump from atom to atom. To move through the lattice, however, they have to have enough energy to overcome the “gap energy.” In other words, they somehow have to acquire enough energy to jump from the valence band up to the conduction band. Semiconductors have relatively small gaps, so it doesn't take a lot of energy for electrons in the valence band to jump up to the conduction band. Conductors such as copper, on the other hand, have little or no gap, and electrons flow very easily when a small voltage is applied.

  Conduction and valence bands. Note the gap between them. EF is called the Fermi levels.

  Two of the most important semiconductors, as far as electronic systems are concerned, are germanium and silicon. What makes these semiconductors particularly valuable is that they can be “doped” with impurity atoms such as boron and phosphorus. Impurity atoms have either a deficit or excess of valence electrons (valence electrons are responsible for the electrical conductivity of different elements). Doping is the process of inserting these impurities, which make new energy levels available within the gap, either just below the conduction band or just above the valence band. The levels just below the conduction band are created by donor impurity atoms; the levels just above the valence band are created by acceptor impurity atoms. Semiconductors doped with donor impurities are called n-type. Those doped with acceptor impurities are called p-type. When an electron jumps to an acceptor level it leaves a “hole” in the valence band, and this hole acts like a positive electron.10

  Energy-level diagram of a semiconductor with electrons in the acceptor levels and holes in the valence band.

  Armed with this information, Bardeen and Brattain began looking into how semiconductors could be used in electronics. One of the simplest electronic devices at the time was the rectifier—a device that would allow current to flow only in one direction. Having decided to look into the possibility of creating a rectifier using semiconductors, they found something that was of even more interest: a simple form of amplification. Amplification is an increase in the signal; it can be an increase in current, voltage, or power. In their experiments Bardeen and Brattain achieved current and power amplification but not voltage amplification. Their first device used point contacts on the surface of the semiconductor.

  Bardeen and Brattain continued to improve their device, but there were several problems with the contact probes. One of the major ones was that there was a surface layer on the semiconductor that appeared to be causing problems. William Shockley, who was the leader of the group, now became more involved. He suggested that a three-layered semiconductor structure would work just as well and would be simpler. This would, in essence, be two p-n junctions placed back to back to form either a p-n-p or an n-p-n device, which we now call a transistor.

  Several connections can be made to a transistor; usually an input signal through two connections is amplified and the resultant, or output signal, is obtained through two other connections. Over the years the size of transistors has decreased significantly; as a result they are now incorporated into very small circuits of various types. They soon became the central device for computers, and with increasing technology they became even smaller and smaller. As a result, computers also became very small.

  Eventually most transistors were integrated into tiny circuits called microchips. Tiny wafers began to hold hundreds, then thousands and even hundreds of thousands of tiny transistors and other electronic components. And surprisingly, as microchips became smaller they also became more reliable. Today literally billions of transistors can be placed on a tiny microchip. As a result, computers of all types now surround us in an incredible variety of devices, and they have revolutionized the weapons of war. They are found in tanks, airplanes, guided missiles, rockets, many types of guns, and almost all types of bombs.

  SATELLITES AND DRONES

  We don't normally think of satellites as weapons of war, and so far they haven't been involved in direct fighting, although in theory they could be equipped with many different types of weapons, including lasers of various types, particle-beam weapons, and even missiles. As we saw earlier, Sputnik was launched by the Soviets in 1957. Explorer 1, the first American satellite, was launched the following year, but for several years the United States trailed the Soviet Union in space technology. Although satellites are used for many commercial purposes, including transcontinental TV broadcasting, long-distance telephone transmission, weather prediction, and GPS navigation, one of their major uses is for spying, and we will direct our attention primarily to spy satellites.

  Within a few years after Sputnik, both the United States and the Soviet Union were launching satellites for spying. Early spy satellites recorded data then ejected it in canisters that had to be retrieved. It wasn't long, however, before radio came into use as a means of retrieving the information. The first series of spy satellites launched by United States in 1959 was called Corona. Since then a large number of spy missions have been initiated, as spying techniques have become more and more sophisticated. Many other nations, including Israel, the United Kingdom, France, Germany, and India, are now launching their own spy satellites.

  This sky is now full of spy satellites, most orbiting overhead at altitudes of one hundred to two hundred miles. They travel at approximately 17,500 miles per hour, taking snapshots of millions of different items of interest to the military and the Central Intelligence Agency. They are, in effect, giant digital cameras pointed at the earth. Everyone has heard of the amazing discoveries made by the Hubble Space Telescope, with its giant mirror. As it turns out, the United States has telescopes in satellites that are now just as large and powerful as Hubble, but they're pointed toward the earth. They are referred to as Keyhole-class (KH) spy satellites, and they provide very high-resolution images; they can, in fact, resolve objects down to five or six inches.11

  But high resolution isn't their only feature. The newer satellites can now take pictures in stereo (side-by-side images at a slightly different angle) that, with the help of computers, can give three-dimensional images. In addition to this, radio images and infrared images can be obtained. Infrared imagery is particularly helpful because it allows the cameras to see through clouds and also at night. Furthermore, intelligence collecting has now become so refined and fast that a single radio or cell phone can be located and pinpointed almost immediately, and orders for targeting its owner can be issued within minutes.

  Most satellites, in fact, now contain extensive, fast computers that can crunch huge amounts of data within a fraction of a second. This data can be quickly transmitted down to an operation center on earth.

  In addition to satellites, unmanned aircraft are now also being used extensively. They are usually referred to as drones. Drones have been used extensively in the wars in Iraq and Afghanistan. The two major types now being used by United States are the MQ-1 Predator and the MQ-9 Reaper (but others are also in use). They are referred to as UAVs (unmanned aerial vehicles) or RPVs (remotely piloted vehicles). And there's no doubt that they are changing the nature of m
odern aerial combat and combat in general. Their main advantage, of course, is that a pilot is no longer in danger. Nevertheless, the craft can still inflict considerable damage to an enemy. Another important advantage is that they are much cheaper to build than conventional fighter planes. The predator is only about twenty-seven feet long.12

  The “pilot” of these drones is usually thousands of miles away. For the ones being used in Iraq, Afghanistan, and Pakistan, the pilot is usually located at a military installation in the United States, where he or she is positioned in front of a screen that shows what a pilot in a plane would normally see, and manipulates the drone as if sitting in the drone's “cockpit.” Furthermore, the pilot is able to communicate with troops on the ground below the drone. In particular, he or she can give them information about the position and capability of the enemy.

  A predator drone.

  Most drones are considerably smaller than fighter planes, and they are not as well equipped. Predator drones usually have no armaments, since they are used mainly for spying; Reapers, however, are equipped with missiles. The British are designing a model they call Taranis, however, which will be about the size of a fighter plane. It will be equipped with weapons of several types, and it will be capable of defending itself from attacks by other aircraft. The Israeli air force also has drones called Hermes 450s, which are equipped with missiles. Many of the countries with drones use the Sperwer, which is produced in France. It is capable of twelve hours of sustained flight, and it is equipped with various electrical-optical devices including infrared and radar sensors; it also carries missiles and antitank weapons.