Borderlands of Science

by Charles Sheffield

  This is an inspired piece of naming, comparable with John Archibald Wheeler's introduction of the term "black hole." Even people who have never heard of chaos theory pick up on it. It is also an appropriate name. The paths traced out in phase space in the region of a strange attractor are infinitely complex, bounded in extent, never repeating; chaotic, yet chaotic in some deeply controlled way. If there can be such a thing as controlled chaos, it is seen around strange attractors.

  We now address the basic question: Can strange attractors exist mathematically? The simple pendulum cannot possess a strange attractor; so far we have offered no proof that any system can exhibit one. However, it can be proved that strange attractors do exist in mathematically specified systems, although a certain minimal complexity is needed in order for a system to possess a strange attractor. We have this situation: Simple equations can exhibit complicated solutions, but for the particular type of complexity represented by the existence of strange attractors, the system of equations can't be too simple. To be specific, a system of three or more nonlinear differential equations can possess a strange attractor; less than three equations, or more than three linear equations, cannot. (The mathematical statement of this fact is simpler but far more abstruse: A system can exhibit a strange attractor if at least one Lyapunov exponent is positive.)

  If we invert the logic, it is tempting to make another statement: Any physical system that shows an ultra-sensitive dependence on initial conditions has a strange attractor buried somewhere in its structure.

  This is a plausible but not a proven result. I am tempted to call it the most important unsolved problem of chaos theory. If it turns out to be true, it will have a profound unifying influence on numerous branches of science. Systems whose controlling equations bear no resemblance to each other will share a structural resemblance, and there will be the possibility of developing universal techniques that apply to the solution of complicated problems in a host of different areas. One thing in common with every problem that we have been discussing is nonlinearity. Nonlinear systems are notoriously difficult to solve, and seem to defy intuition. Few general techniques exist today for tackling nonlinear problems, and some new insight is desperately needed.

  If chaos theory can provide that insight, it will have moved from being a baffling grab-bag of half results, interesting conjectures, and faintly seen relationships, to become a real "new science." We are not there yet. But if we can go that far, then our old common sense gut instinct, that told us simple equations must have simple solutions, will have proved no more reliable than our ancestors' common sense instinctive knowledge that told them the Earth was flat. And the long-term implications of that new thought pattern may be just as revolutionary to science.

  Today, we are in that ideal time for writers, where what can be speculated in chaos theory far exceeds what is known. I still consider the ultimate importance of chaos theory as not proven, but it has certainly caused a change of outlook. Today you hear weather forecasters referring to the "butterfly effect," in which a butterfly flapping its wings in the East Indies causes a hurricane in the Caribbean—a powerful illustration of sensitive dependence on initial conditions.

  Science fiction writers long ago explored the idea of the sensitive dependence on initial conditions in time travel stories. In "A Sound of Thunder" (Bradbury, 1952), a tiny change in the past produces a very different future.

  Is time really like that? Or would small changes made in the past tend to damp out over time, to produce the same present? Stating this another way, if we were to rerun the history of the planet, would the same life-forms emerge, or a totally different set? This is a hot topic at the moment in evolutionary biology.

  CHAPTER 12

  Future War

  Pessimists, gloomy about the future, point out that the only continuous progress in human history seems to be in methods of warfare. Optimists point out that humans are still here, most of us living far better than our ancestors ever dreamed. Pessimists reply, ah, but wait until the next war.

  Einstein, asked about the weapons of the Third World War, said that he did not know what they would be; but that the Fourth would be fought with sticks and stones.

  Einstein died in 1955. Were he alive today, I think he would be both gratified and horrified. Gratified, because the all-consuming fear of the 1950s was of large-scale nuclear war. Not only have we escaped that, but after the end of the Second World War we have avoided the use of nuclear weapons in combat. But Einstein would surely be dismayed at the continuing conflicts, all around the world, and the increased vulnerability of cities and civilians to terrorist acts.

  Dismayed, too, at the potential of today's science for the creation of new weapons. Weapons drive, and are driven by, technological advances. If the scale of war remains small, weapons are likely to become more tricky, more deadly—and more personal.

  Anyone who reads my stories may suspect that war and military affairs are not among my main interests. That is true. However, military science fiction is a big component of the field, and many people read little else. Regard this chapter, then, not as a compendium of military knowledge, or a source book on the writing of military science fiction. Think of it as a discussion of a few story ideas with military potential that seem to have been overlooked.

  12.1 The Invisible Man. The best weapon of all is one which the adversary never realizes has been used.

  Wouldn't it be wonderful if you could put on your tarnhelm and, like the old Norse heroes, become invisible to your enemies? You could sneak through lines of defense, sit in on private strategy meetings, steal battle plans, and even kill selected people. I have no doubt that our leaders in the Pentagon and CIA Headquarters, gnashing their teeth over their inability to get to Saddam Hussein, would have given a fair number of those teeth for a good cloak of invisibility.

  Can it be done?

  H.G. Wells took a shot at the problem in his novel The Invisible Man (Wells, 1897). His solution, a drug to make every part of the human body of the same refractive index as air, possesses a number of difficulties that I feel sure Wells knew about. Let us put aside the improbability of the drug itself, and examine other effects.

  First, if no part of your body absorbs light, that includes the eyes. Light will simply pass through them. But if your eyes do not absorb light, you will be blind. Seeing involves the absorption of photons by your retinas.

  Then there is the food that you eat. What happens to it while it is being digested? It would be visible in your alimentary canal, slowly fading away like the Cheshire Cat as it went from esophagus to stomach and to intestines.

  I think Wells could have done better with a little thought; and we can do a lot better, because we have technology unknown in his day.

  Consider how Nature tackles the problem of invisibility. The answer is not drugs, but deception. Animals do their best to be invisible to their prey or their predators. But they don't do it by fiddling around with their own internal optical properties. They are invisible if they look exactly like their background. The chameleon has the right idea, but it's hardware-limited. It can only make modest color and pattern adjustments. Humans disguise their presence with camouflage, but that, too, is a static and simple-minded solution.

  What we need is a whole-body suit. The rear part of the suit is covered with tiny imaging sensors, admitting light through apertures no bigger than pinholes. Their outputs feed charge-coupled devices, which pass an electronic version of the scene behind you, in full detail, to the suit's central processing unit (cpu). The front of the suit contains millions of tiny crystal displays. The cpu sends to each of these displays an appropriate signal, assigning a particular color and intensity. Seen from in front, the suit now mimics its own background (the scene behind it) perfectly.

  So far this is straightforward, comfortably within today's technology. The difficulty comes because the suit has to reproduce, when viewed from any angle, the background as seen from that angle. Someone behind you, for example,
has to see an exact match to the scene in front of you. To get the right effect from all angles, you have to use holographic methods and generate multi-angle reflectances. The computing power to do all this is considerable—far more than anything in today's personal computers. However, we have seen where that technology is going. Twenty or thirty more years, and the computing capacity we need will probably fit into your wristwatch.

  The problem of vision, never addressed by Wells, is also easy enough. The signal received from in front of the suit is pipelined to goggles contained within the suit's helmet. We would anticipate that a suit like this would work only with uniform, low-level illumination and relatively uniform backgrounds.

  Could we build one, today? I don't know, but if we could and we tried to sell it, I bet that its use would be banned in no time.

  12.2 Death rays. The death ray was introduced to science fiction in its early days. H.G. Wells was responsible for this one, too. The beam of intense light, flashing forth to set fire to everything in its path, is something we all remember from The War of the Worlds (Wells, 1898). Wells called it a heat ray, and he described it in the language of weapons: "this invisible, inevitable sword of heat."

  For the next half century, every respectable scientist knew that such a ray was impossible. Science fiction writers of the 1930s, however, continued to use it freely. And hindsight proves that they were right to so do. The three scientific papers that permit the death ray—we now call such a thing a laser—to exist had been published in 1916 and 1917, ironically at the height of the "war to end wars." The papers were by Einstein, and they established balances among the rates at which electrons orbiting an atom can move to higher or lower energy levels.

  This requires a little explanation. In Chapter 5, we noted that electrons around an atomic nucleus sit in "shells," and that an element's freedom to react chemically depends on whether it has a filled or a partially empty outer electron shell.

  In addition to the locations where electrons are normally situated, there are other possible sites where an electron can reside temporarily. An electron can be boosted to occupy such a site, provided that it is supplied with energy in the form of radiation. If an electron is in such a higher-energy position, it is said to be in an excited state. An electron with no extra energy is said to be in its ground state.

  Left to itself, an electron in an excited state will drop back to its ground state, emitting radiation as it does so. This is known as spontaneous emission. The return to the ground state normally happens quickly, but that is not always the case. Sometimes an electron can be at an excited energy level where other physical parameters, such as orbital angular momentum or spin, are incompatible with a straightforward return to the ground state. Such a transition is known as a forbidden transition, and the effect is to make the electron remain longer in the excited state.

  Even a forbidden transition normally takes place in a fraction of a second. The phenomenon that most of us have seen, called phosphorescence, in which a material continues to glow after it has been removed from sunlight, is a more complicated process. In phosphorescence, the electron usually becomes "trapped" in a dislocation in a crystal lattice structure. Only after it leaves that trap (which may take minutes or even hours) can it finally undergo spontaneous emission to the ground state.

  An electron can be induced to make a forbidden transition from a higher energy state to a ground state, by providing to it radiation of a suitable wavelength. This is known as induced or stimulated emission, because the electron as it drops back to the ground state gives out radiation of an energy appropriate to that state change.

  We now have all the ingredients for our basic death ray. We pump energy into a material, raising large numbers of electrons to excited states. They will fall back by spontaneous emission to a lower energy. However, if we have picked the right material many of them do not go at once to the ground state. Instead, they drop to another excited state from which the transition to the ground state is forbidden. There they stay, increasing in numbers, until at last we supply radiation of the right wavelength to induce a fall to the ground state. They do this in large numbers, producing a huge pulse of released energy in the form of light. The emitted light is monochromatic (of a single, precise wavelength) and coherent (all of the same phase).

  This is today's laser—light amplification by stimulated emission of radiation. The first one was built in 1960, and they are used now for everything from data transmission to eye surgery.

  This may seem somewhat disappointing for something billed as a "death ray." However, lasers are certain candidates for future wars. The first lasers were of low power, but that has changed. Great power (kilowatts and more) can be delivered into very small areas. A laser beam will burn almost instantly through any known material, and by 1968 it had already been used to initiate thermonuclear reactions. Perhaps even more relevant for the purposes of war, a laser beam can be made very narrow, with little spread over large distances. Since the power is delivered to the target at the speed of light, high-energy lasers are good in either offense or defense and have been proposed as the most effective form of protective shield against missile attack. They also, because they remain as a tight beam over great distances, have been suggested as the best way of launching spacecraft, or of sending power to them anywhere in the solar system.

  The first lasers employed electrons in the outer atomic shells of the atom, and the radiation they produced was normally in the visible or near-infrared wavelength regions of the spectrum. However, there is no reason in principle why an electron in the inner electron shells should not go through the same processes of energy absorption, spontaneous emission to a forbidden state, and final stimulated emission. Because the inner electron shells are more tightly bound, the energy released on the final return to ground state is higher, and the wavelength of the radiation produced will be shorter. The result is an X-ray laser: invisible in its output, and considerably more deadly.

  12.3 The ultimate personal weapon. War isn't what it was. In ancient times, one rational and economical way of deciding the outcome of a battle was through the use of champions.

  You select the best fighter in your army. I do the same in mine. We let those two fight it out, while the rest of us stand around, watch, and cheer on our guy. The individual serves as a surrogate for the whole army. If he wins, we all win; if he loses, we admit defeat.

  I don't think this was ever a common method—suppose I have ten times as many soldiers as you, but you have one huge chap twice the size of any of my people? Do you think I am going to risk losing the whole war with a one-on-one fight? Even if it worked against Goliath, I don't want anything to do with it.

  Individual combat, by chosen champions, will certainly not work today. For one thing, our weapons make personal strength in combat rather irrelevant. But the combat of champions makes a point that is as valid now as it ever was: in war, as in all other human activities, individuals make a difference. Some wars arise because of the ambitions of a single human.

  Such a person is usually well-protected, and capture is difficult. Killing is easier. A 20-megaton hydrogen bomb in downtown Baghdad would almost certainly have taken care of Saddam Hussein. But how many hundreds of thousands of innocents would have been killed along with him?

  The sledgehammer-on-the-ant solution is no solution at all. Too many would die. But suppose we could, neatly and cleanly, dispose of the major troublemaker.

  It was tried with Adolf Hitler, and failed. A bomb carried into a conference exploded as planned, but he was shielded from the blast by the leg of a table. It was tried by the CIA with Fidel Castro, and produced a variety of failures that read like a catalog of ineptitude. (Poisoned cigars, no less. Shades of Snow White.)

  But does it have to fail? Or can we suggest ways to guarantee the death of a single, chosen individual?

  Let us go back to basic biology. In what way is El Supremo, busy causing so much trouble, different from every other person on Earth?
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  Forget photographs, forget fingerprints and retinal patterns. More reliable than either, and increasingly recognized as such in court cases, is the uniqueness of an individual's DNA. Unless you happen to have an identical twin, your DNA is yours and yours alone.

  It should be a lot easier to obtain a sample of El Supremo's DNA than it would be getting close to the man himself. An old hat, a sock, or a dirty shirt will contain little flakes of skin, a razor may have a tiny drop of dried blood. Remember, a single cell should be enough. The entire genome is in every nucleus.

  Inside El Supremo's body, just as in your or my body, there are defense cells known as T cells. Their job is to mop up viruses that have invaded the body. This happens all the time, because viruses are small and light enough to be airborne. We take in viruses with every breath, and our T cells destroy them. If you have few T cells, your body will lose its resistance to outside infection, which is exactly what happens to people with AIDS.

  However, the T cells can't go around destroying everything in sight. They have to be able to distinguish foreign matter, which is not supposed to be there, from the cells of your own body. If something looks like you, in the right kind of way, your T cells will not touch it.

  What kind of way is that? Well, among other things our DNA contains a sequence that codes for the production of a molecule called a major histocompatibility complex (MHC). The MHC in your body, like your DNA, is unique and specific to you. The MHC, which is safe from T cell attack because it is recognized as part of you, can carry other things to a T cell. Those other things will then also be judged as part of you, and left in peace; or as alien to you, and destroyed.

  Now we are ready to go to work. Recall, from Chapter 6, that a virus is little more than a package of DNA wrapped in a coat of protein. We will take a virus that contains the DNA of a lethal disease. There are plenty of those. However, we will give to that virus a protein coat matching the MHC profile of a specific person—say, El Supremo.