Physics of the Impossible.

Kaku, Michio.

PREFACE.

If at first an idea does not sound absurd, then there is no hope for it.

-ALBERT EINSTEIN.

One day, would it be possible to walk through walls? To build starships that can travel faster than the speed of light? To read other people"s minds? To become invisible? To move objects with the power of our minds? To transport our bodies instantly through outer s.p.a.ce?

Since I was a child, I"ve always been fascinated by these questions. Like many physicists, when I was growing up, I was mesmerized by the possibility of time travel, ray guns, force fields, parallel universes, and the like. Magic, fantasy, science fiction were all a gigantic playground for my imagination. They began my lifelong love affair with the impossible.

I remember watching the old Flash Gordon reruns on TV. Every Sat.u.r.day, I was glued to the TV set, marveling at the adventures of Flash, Dr. Zarkov, and Dale Arden and their dazzling array of futuristic technology: the rocket ships, invisibility shields, ray guns, and cities in the sky. I never missed a week. The program opened up an entirely new world for me. I was thrilled by the thought of one day rocketing to an alien planet and exploring its strange terrain. Being pulled into the orbit of these fantastic inventions I knew that my own destiny was somehow wrapped up with the marvels of the science that the show promised.

As it turns out, I was not alone. Many highly accomplished scientists originally became interested in science through exposure to science fiction. The great astronomer Edwin Hubble was fascinated by the works of Jules Verne. As a result of reading Verne"s work, Hubble abandoned a promising career in law, and, disobeying his father"s wishes, set off on a career in science. He eventually became the greatest astronomer of the twentieth century. Carl Sagan, noted astronomer and bestselling author, found his imagination set afire by reading Edgar Rice Burroughs"s John Carter of Mars novels. Like John Carter, he dreamed of one day exploring the sands of Mars.

I was just a child the day when Albert Einstein died, but I remember people talking about his life, and death, in hushed tones. The next day I saw in the newspapers a picture of his desk, with the unfinished ma.n.u.script of his greatest, unfinished work. I asked myself, What could be so important that the greatest scientist of our time could not finish it? The article claimed that Einstein had an impossible dream, a problem so difficult that it was not possible for a mortal to finish it. It took me years to find out what that ma.n.u.script was about: a grand, unifying "theory of everything." His dream-which consumed the last three decades of his life-helped me to focus my own imagination. I wanted, in some small way, to be part of the effort to complete Einstein"s work, to unify the laws of physics into a single theory.

As I grew older I began to realize that although Flash Gordon was the hero and always got the girl, it was the scientist who actually made the TV series work. Without Dr. Zarkov, there would be no rocket ship, no trips to Mongo, no saving Earth. Heroics aside, without science there is no science fiction.

I came to realize that these tales were simply impossible in terms of the science involved, just flights of the imagination. Growing up meant putting away such fantasy. In real life, I was told, one had to abandon the impossible and embrace the practical.

However, I concluded that if I was to continue my fascination with the impossible, the key was through the realm of physics. Without a solid background in advanced physics, I would be forever speculating about futuristic technologies without understanding whether or not they were possible. I realized I needed to immerse myself in advanced mathematics and learn theoretical physics. So that is what I did.

In high school for my science fair project I a.s.sembled an atom smasher in my mom"s garage. I went to the Westinghouse company and gathered 400 pounds of sc.r.a.p transformer steel. Over Christmas I wound 22 miles of copper wire on the high school football field. Eventually I built a 2.3-million-electron-volt betatron particle accelerator, which consumed 6 kilowatts of power (the entire output of my house) and generated a magnetic field of 20,000 times the Earth"s magnetic field. The goal was to generate a beam of gamma rays powerful enough to create antimatter.

My science fair project took me to the National Science Fair and eventually fulfilled my dream, winning a scholarship to Harvard, where I could finally pursue my goal of becoming a theoretical physicist and follow in the footsteps of my role model, Albert Einstein.

Today I receive e-mails from science fiction writers and screenwriters asking me to help them sharpen their own tales by exploring the limits of the laws of physics.

THE "IMPOSSIBLE" IS RELATIVE.

As a physicist, I have learned that the "impossible" is often a relative term. Growing up, I remember my teacher one day walking up to the map of the Earth on the wall and pointing out the coastlines of South America and Africa. Wasn"t it an odd coincidence, she said, that the two coastlines fit together, almost like a jigsaw puzzle? Some scientists, she said, speculated that perhaps they were once part of the same, vast continent. But that was silly. No force could possibly push two gigantic continents apart. Such thinking was impossible, she concluded.

Later that year we studied the dinosaurs. Wasn"t it strange, our teacher told us, that the dinosaurs dominated the Earth for millions of years, and then one day they all vanished? No one knew why they had all died off. Some paleontologists thought that maybe a meteor from s.p.a.ce had killed them, but that was impossible, more in the realm of science fiction.

Today we now know that through plate tectonics the continents do move, and that 65 million years ago a gigantic meteor measuring six miles across most likely did obliterate the dinosaurs and much of life on Earth. In my own short lifetime I have seen the seemingly impossible become established scientific fact over and over again. So is it impossible to think we might one day be able to teleport ourselves from one place to another, or build a s.p.a.ceship that will one day take us light-years away to the stars?

Normally such feats would be considered impossible by today"s physicists. Might they become possible within a few centuries? Or in ten thousand years, when our technology is more advanced? Or in a million years? To put it another way, if we were to somehow encounter a civilization a million years more advanced than ours, would their everyday technology appear to be "magic" to us? That, at its heart, is one of the central questions running through this book; just because something is "impossible" today, will it remain impossible centuries or millions of years into the future?

Given the remarkable advances in science in the past century, especially the creation of the quantum theory and general relativity, it is now possible to give rough estimates of when, if ever, some of these fantastic technologies may be realized. With the coming of even more advanced theories, such as string theory, even concepts bordering on science fiction, such as time travel and parallel universes, are now being re-evaluated by physicists. Think back 150 years to those technological advances that were declared "impossible" by scientists at the time and that have now become part of our everyday lives. Jules Verne wrote a novel in 1863, Paris in the Twentieth Century, which was locked away and forgotten for over a century until it was accidentally discovered by his great-grandson and published for the first time in 1994. In it Verne predicted what Paris might look like in the year 1960. His novel was filled with technology that was clearly considered impossible in the nineteenth century, including fax machines, a world-wide communications network, gla.s.s skysc.r.a.pers, gas-powered automobiles, and high-speed elevated trains.

Not surprisingly, Verne could make such stunningly accurate predictions because he was immersed in the world of science, picking the brains of scientists around him. A deep appreciation for the fundamentals of science allowed him to make such startling predictions.

Sadly, some of the greatest scientists of the nineteenth century took the opposite position and declared any number of technologies to be hopelessly impossible. Lord Kelvin, perhaps the most prominent physicist of the Victorian era (he is buried next to Isaac Newton in Westminster Abbey), declared that "heavier than air" devices such as the airplane were impossible. He thought X-rays were a hoax and that radio had no future. Lord Rutherford, who discovered the nucleus of the atom, dismissed the possibility of building an atomic bomb, comparing it to "moonshine." Chemists of the nineteenth century declared the search for the philosopher"s stone, a fabled substance that can turn lead into gold, a scientific dead end. Nineteenth-century chemistry was based on the fundamental immutability of the elements, like lead. Yet with today"s atom smashers, we can, in principle, turn lead atoms into gold. Think how fantastic today"s televisions, computers, and Internet would have seemed at the turn of the twentieth century.

More recently, black holes were once considered to be science fiction. Einstein himself wrote a paper in 1939 that "proved" that black holes could never form. Yet today the Hubble s.p.a.ce Telescope and the Chandra X-ray telescope have revealed thousands of black holes in s.p.a.ce.

The reason that these technologies were deemed "impossibilities" is that the basic laws of physics and science were not known in the nineteenth century and the early part of the twentieth. Given the huge gaps in the understanding of science at the time, especially at the atomic level, it"s no wonder such advances were considered impossible.

STUDYING THE IMPOSSIBLE.

Ironically, the serious study of the impossible has frequently opened up rich and entirely unexpected domains of science. For example, over the centuries the frustrating and futile search for a "perpetual motion machine" led physicists to conclude that such a machine was impossible, forcing them to postulate the conservation of energy and the three laws of thermodynamics. Thus the futile search to build perpetual motion machines helped to open up the entirely new field of thermodynamics, which in part laid the foundation of the steam engine, the machine age, and modern industrial society.

At the end of the nineteenth century, scientists decided that it was "impossible" for the Earth to be billions of years old. Lord Kelvin declared flatly that a molten Earth would cool down in 20 to 40 million years, contradicting the geologists and Darwinian biologists who claimed that the Earth might be billions of years old. The impossible was finally proven to be possible with the discovery of the nuclear force by Madame Curie and others, showing how the center of the Earth, heated by radioactive decay, could indeed be kept molten for billions of years.

We ignore the impossible at our peril. In the 1920s and 1930s Robert G.o.ddard, the founder of modern rocketry, was the subject of intense criticism by those who thought that rockets could never travel in outer s.p.a.ce. They sarcastically called his pursuit G.o.ddard"s Folly. In 1921 the editors of the New York Times railed against Dr. G.o.ddard"s work: "Professor G.o.ddard does not know the relation between action and reaction and the need to have something better than a vacuum against which to react. He seems to lack the basic knowledge ladled out daily in high schools." Rockets were impossible, the editors huffed, because there was no air to push against in outer s.p.a.ce. Sadly, one head of state did understand the implications of G.o.ddard"s "impossible" rockets-Adolf Hitler. During World War II, Germany"s barrage of impossibly advanced V-2 rockets rained death and destruction on London, almost bringing it to its knees.

Studying the impossible may have also changed the course of world history. In the 1930s it was widely believed, even by Einstein, that an atomic bomb was "impossible." Physicists knew that there was a tremendous amount of energy locked deep inside the atom"s nucleus, according to Einstein"s equation E = mc2, but the energy released by a single nucleus was too insignificant to consider. But atomic physicist Leo Szilard remembered reading the 1914 H. G. Wells novel, The World Set Free, in which Wells predicted the development of the atomic bomb. In the book he stated that the secret of the atomic bomb would be solved by a physicist in 1933. By chance Szilard stumbled upon this book in 1932. Spurred on by the novel, in 1933, precisely as predicted by Wells some two decades earlier, he hit upon the idea of magnifying the power of a single atom via a chain reaction, so that the energy of splitting a single uranium nucleus could be magnified by many trillions. Szilard then set into motion a series of key experiments and secret negotiations between Einstein and President Franklin Roosevelt that would lead to the Manhattan Project, which built the atomic bomb.

Time and again we see that the study of the impossible has opened up entirely new vistas, pushing the boundaries of physics and chemistry and forcing scientists to redefine what they mean by "impossible." As Sir William Osler once said, "The philosophies of one age have become the absurdities of the next, and the foolishness of yesterday has become the wisdom of tomorrow."

Many physicists subscribe to the famous dictum of T. H. White, who wrote in The Once and Future King, "Anything that is not forbidden, is mandatory!" In physics we find evidence of this all the time. Unless there is a law of physics explicitly preventing a new phenomenon, we eventually find that it exists. (This has happened several times in the search for new subatomic particles. By probing the limits of what is forbidden, physicists have often unexpectedly discovered new laws of physics.) A corollary to T. H. White"s statement might well be, "Anything that is not impossible, is mandatory!"

For example, cosmologist Stephen Hawking tried to prove that time travel was impossible by finding a new law of physics that would forbid it, which he called the "chronology protection conjecture." Unfortunately, after many years of hard work he was unable to prove this principle. In fact, to the contrary, physicists have now demonstrated that a law that prevents time travel is beyond our present-day mathematics. Today, because there is no law of physics preventing the existence of time machines, physicists have had to take their possibility very seriously.

The purpose of this book is to consider what technologies are considered "impossible" today that might well become commonplace decades to centuries down the road.

Already one "impossible" technology is now proving to be possible: the notion of teleportation (at least at the level of atoms). Even a few years ago physicists would have said that sending or beaming an object from one point to another violated the laws of quantum physics. The writers of the original Star Trek television series, in fact, were so stung by the criticism from physicists that they added "Heisenberg compensators" to explain their teleporters in order to address this flaw. Today, because of a recent breakthrough, physicists can teleport atoms across a room or photons under the Danube River.

PREDICTING THE FUTURE.

It is always a bit dangerous to make predictions, especially ones set centuries to thousands of years in the future. The physicist Niels Bohr was fond of saying, "Prediction is very hard to do. Especially about the future." But there is a fundamental difference between the time of Jules Verne and the present. Today the fundamental laws of physics are basically understood. Physicists today understand the basic laws extending over a staggering forty-three orders of magnitude, from the interior of the proton out to the expanding universe. As a result, physicists can state, with reasonable confidence, what the broad outlines of future technology might look like, and better differentiate between those technologies that are merely improbable and those that are truly impossible.

In this book, therefore, I divide the things that are "impossible" into three categories.

The first are what I call Cla.s.s I impossibilities. These are technologies that are impossible today but that do not violate the known laws of physics. So they might be possible in this century, or perhaps the next, in modified form. They include teleportation, antimatter engines, certain forms of telepathy, psychokinesis, and invisibility.

The second category is what I term Cla.s.s II impossibilities. These are technologies that sit at the very edge of our understanding of the physical world. If they are possible at all, they might be realized on a scale of millennia to millions of years in the future. They include time machines, the possibility of hypers.p.a.ce travel, and travel through wormholes.

The final category is what I call Cla.s.s III impossibilities. These are technologies that violate the known laws of physics. Surprisingly, there are very few such impossible technologies. If they do turn out to be possible, they would represent a fundamental shift in our understanding of physics.

This cla.s.sification is significant, I feel, because so many technologies in science fiction are dismissed by scientists as being totally impossible, when what they actually mean is that they are impossible for a primitive civilization like ours. Alien visitations, for example, are usually considered impossible because the distances between the stars are so vast. While interstellar travel for our civilization is clearly impossible, it may be possible for a civilization centuries to thousands or millions of years ahead of ours. So it is important to rank such "impossibilities." Technologies that are impossible for our current civilization are not necessarily impossible for other types of civilizations. Statements about what is possible and impossible have to take into account technologies that are millennia to millions of years ahead of ours.

Carl Sagan once wrote, "What does it mean for a civilization to be a million years old? We have had radio telescopes and s.p.a.ceships for a few decades; our technical civilization is a few hundred years old...an advanced civilization millions of years old is as much beyond us as we are beyond a bush baby or a macaque."

In my own research I focus professionally on trying to complete Einstein"s dream of a "theory of everything." Personally, I find it quite exhilarating to work on a "final theory" that may ultimately answer some of the most difficult "impossible" questions in science today, such as whether time travel is possible, what lies at the center of a black hole, or what happened before the big bang. I still daydream about my lifelong love affair with the impossible, and wonder when and if some of these impossibilities might enter the ranks of the everyday.

ACKNOWLEDGMENTS.

The material in this book ranges over many fields and disciplines, as well as the work of many outstanding scientists. I would like to thank the following individuals, who have graciously given their time for lengthy interviews, consultations, and interesting, stimulating conversations: Leon Lederman, n.o.bel laureate, Illinois Inst.i.tute of Technology Murray Gell-Mann, n.o.bel laureate, Santa Fe Inst.i.tute and Cal Tech The late Henry Kendall, n.o.bel laureate, MIT Steven Weinberg, n.o.bel laureate, University of Texas at Austin David Gross, n.o.bel laureate, Kavli Inst.i.tute for Theoretical Physics Frank Wilczek, n.o.bel laureate, MIT Joseph Rotblat, n.o.bel laureate, St. Bartholomew"s Hospital Walter Gilbert, n.o.bel laureate, Harvard University Gerald Edelman, n.o.bel laureate, Scripps Research Inst.i.tute Peter Doherty, n.o.bel laureate, St. Jude Children"s Research Hospital Jared Diamond, Pulitzer Prize winner, UCLA Stan Lee, creator of Marvel Comics and Spiderman Brian Greene, Columbia University, author of The Elegant Universe Lisa Randall, Harvard University, author of Warped Pa.s.sages Lawrence Krauss, Case Western University, author of The Physics of Star Trek J. Richard Gott III, Princeton University, author of Time Travel in Einstein"s Universe Alan Guth, physicist, MIT, author of The Inflationary Universe John Barrow, physicist, Cambridge University, author of Impossibility Paul Davies, physicist, author of Superforce Leonard Susskind, physicist, Stanford University Joseph Lykken, physicist, Fermi National Laboratory Marvin Minsky, MIT, author of The Society of Minds Ray Kurzweil, inventor, author of The Age of Spiritual Machines Rodney Brooks, director of MIT Artificial Intelligence Laboratory Hans Moravec, author of Robot Ken Croswell, astronomer, author of Magnificent Universe Don Goldsmith, astronomer, author of Runaway Universe Neil de Gra.s.se Tyson, director of Hayden Planetarium, New York City Robert Kirshner, astronomer, Harvard University Fulvia Melia, astronomer, University of Arizona Sir Martin Rees, Cambridge University, author of Before the Beginning Michael Brown, astronomer, Cal Tech Paul Gilster, author of Centauri Dreams Michael Lemonick, senior science editor of Time magazine Timothy Ferris, University of California, author of Coming of Age in the Milky Way The late Ted Taylor, designer of U.S. nuclear warheads Freeman Dyson, Inst.i.tute for Advanced Study, Princeton John Horgan, Stevens Inst.i.tute of Technology, author of The End of Science The late Carl Sagan, Cornell University, author of Cosmos Ann Druyan, widow of Carl Sagan, Cosmos Studios Peter Schwarz, futurist, founder of Global Business Network Alvin Toffler, futurist, author of The Third Wave David Goodstein, a.s.sistant provost of Cal Tech Seth Lloyd, MIT, author of Programming the Universe Fred Watson, astronomer, author of Star Gazer Simon Singh, author of The Big Bang Seth Shostak, SETI Inst.i.tute George Johnson, New York Times science journalist.

Jeffrey Hoffman, MIT, NASA astronaut.

Tom Jones, NASA astronaut.

Alan Lightman, MIT, author of Einstein"s Dreams.

Robert Zubrin, founder of Mars Society.

Donna Shirley, NASA Mars program.

John Pike, GlobalSecurity.org.

Paul Saffo, futurist, Inst.i.tute of the Future.

Louis Friedman, cofounder of the Planetary Society.

Daniel Werthheimer, [email protected], University of California at Berkeley.

Robert Zimmerman, author of Leaving Earth.

Marcia Bartusiak, author of Einstein"s Unfinished Symphony.

Michael H. Salamon, NASA"s Beyond Einstein program.

Geoff Andersen, U.S. Air Force Academy, author of The Telescope.

I would also like to thank my agent, Stuart Krichevsky, who has been at my side all these years, shepherding all my books, and also my editor, Roger Scholl, whose firm hand, sound judgment, and editorial experience have guided so many of my books. I would also like to thank my collegues at the City College of New York and the Graduate Center of the City University of New York, especially V. P. Nair and Dan Greenberger, who generously donated their time for discussions.

1: FORCE FIELDS.

I. When a distinguished but elderly scientist states that something is possible, he is almost certainly right. When he states that something is impossible, he is very probably wrong.

II. The only way of discovering the limits of the possible is to venture a little way past them into the impossible.

III. Any sufficiently advanced technology is indistinguishable from magic.

-ARTHUR C. CLARKE"S THREE LAWS.

"Shields up!"

In countless Star Trek episodes this is the first order that Captain Kirk barks out to the crew, raising the force fields to protect the starship Enterprise against enemy fire.

So vital are force fields in Star Trek that the tide of the battle can be measured by how the force field is holding up. Whenever power is drained from the force fields, the Enterprise suffers more and more damaging blows to its hull, until finally surrender is inevitable.

So what is a force field? In science fiction it"s deceptively simple: a thin, invisible yet impenetrable barrier able to deflect lasers and rockets alike. At first glance a force field looks so easy that its creation as a battlefield shield seems imminent. One expects that any day some enterprising inventor will announce the discovery of a defensive force field. But the truth is far more complicated.

In the same way that Edison"s lightbulb revolutionized modern civilization, a force field could profoundly affect every aspect of our lives. The military could use force fields to become invulnerable, creating an impenetrable shield against enemy missiles and bullets. Bridges, superhighways, and roads could in theory be built by simply pressing a b.u.t.ton. Entire cities could sprout instantly in the desert, with skysc.r.a.pers made entirely of force fields. Force fields erected over cities could enable their inhabitants to modify the effects of their weather-high winds, blizzards, tornados-at will. Cities could be built under the oceans within the safe canopy of a force field. Gla.s.s, steel, and mortar could be entirely replaced.

Yet oddly enough a force field is perhaps one of the most difficult devices to create in the laboratory. In fact, some physicists believe it might actually be impossible, without modifying its properties.

MICHAEL FARADAY.

The concept of force fields originates from the work of the great nineteenth-century British scientist Michael Faraday.

Faraday was born to working-cla.s.s parents (his father was a blacksmith) and eked out a meager existence as an apprentice bookbinder in the early 1800s. The young Faraday was fascinated by the enormous breakthroughs in uncovering the mysterious properties of two new forces: electricity and magnetism. Faraday devoured all he could concerning these topics and attended lectures by Professor Humphrey Davy of the Royal Inst.i.tution in London.

One day Professor Davy severely damaged his eyes in a chemical accident and hired Faraday to be his secretary. Faraday slowly began to win the confidence of the scientists at the Royal Inst.i.tution and was allowed to conduct important experiments of his own, although he was often slighted. Over the years Professor Davy grew increasingly jealous of the brilliance shown by his young a.s.sistant, who was a rising star in experimental circles, eventually eclipsing Davy"s own fame. After Davy died in 1829 Faraday was free to make a series of stunning breakthroughs that led to the creation of generators that would energize entire cities and change the course of world civilization.

The key to Faraday"s greatest discoveries was his "force fields." If one places iron filings over a magnet, one finds that the iron filings create a spiderweb-like pattern that fills up all of s.p.a.ce. These are Faraday"s lines of force, which graphically describe how the force fields of electricity and magnetism permeate s.p.a.ce. If one graphs the magnetic fields of the Earth, for example, one finds that the lines emanate from the north polar region and then fall back to the Earth in the south polar region. Similarly, if one were to graph the electric field lines of a lightning rod in a thunderstorm, one would find that the lines of force concentrate at the tip of the lightning rod. Empty s.p.a.ce, to Faraday, was not empty at all, but was filled with lines of force that could make distant objects move. (Because of Faraday"s poverty-stricken youth, he was illiterate in mathematics, and as a consequence his notebooks are full not of equations but of hand-drawn diagrams of these lines of force. Ironically, his lack of mathematical training led him to create the beautiful diagrams of lines of force that now can be found in any physics textbook. In science a physical picture is often more important than the mathematics used to describe it.) Historians have speculated on how Faraday was led to his discovery of force fields, one of the most important concepts in all of science. In fact, the sum total of all modern physics is written in the language of Faraday"s fields. In 1831, he made the key breakthrough regarding force fields that changed civilization forever. One day, he was moving a child"s magnet over a coil of wire and he noticed that he was able to generate an electric current in the wire, without ever touching it. This meant that a magnet"s invisible field could push electrons in a wire across empty s.p.a.ce, creating a current.

Faraday"s "force fields," which were previously thought to be useless, idle doodlings, were real, material forces that could move objects and generate power. Today the light that you are using to read this page is probably energized by Faraday"s discovery about electromagnetism. A spinning magnet creates a force field that pushes the electrons in a wire, causing them to move in an electrical current. This electricity in the wire can then be used to light up a lightbulb. This same principle is used to generate electricity to power the cities of the world. Water flowing across a dam, for example, causes a huge magnet in a turbine to spin, which then pushes the electrons in a wire, forming an electric current that is sent across high-voltage wires into our homes.

In other words, the force fields of Michael Faraday are the forces that drive modern civilization, from electric bulldozers to today"s computers, Internet, and iPods.

Faraday"s force fields have been an inspiration for physicists for a century and a half. Einstein was so inspired by them that he wrote his theory of gravity in terms of force fields. I, too, was inspired by Faraday"s work. Years ago I successfully wrote the theory of strings in terms of the force fields of Faraday, thereby founding string field theory. In physics when someone says, "He thinks like a line of force," it is meant as a great compliment.

THE FOUR FORCES.

Over the last two thousand years one of the crowning achievements of physics has been the isolation and identification of the four forces that rule the universe. All of them can be described in the language of fields introduced by Faraday. Unfortunately, however, none of them has quite the properties of the force fields described in most science fiction. These forces are 1. Gravity, the silent force that keeps our feet on the ground, prevents the Earth and the stars from disintegrating, and holds the solar system and galaxy together. Without gravity, we would be flung off the Earth into s.p.a.ce at the rate of 1,000 miles per hour by the spinning planet. The problem is that gravity has precisely the opposite properties of a force field found in science fiction. Gravity is attractive, not repulsive; is extremely weak, relatively speaking; and works over enormous, astronomical distances. In other words, it is almost the opposite of the flat, thin, impenetrable barrier that one reads about in science fiction or one sees in science fiction movies. For example, it takes the entire planet Earth to attract a feather to the floor, but we can counteract Earth"s gravity by lifting the feather with a finger. The action of our finger can counteract the gravity of an entire planet that weighs over six trillion trillion kilograms.

2. Electromagnetism (EM), the force that lights up our cities. Lasers, radio, TV, modern electronics, computers, the Internet, electricity, magnetism-all are consequences of the electromagnetic force. It is perhaps the most useful force ever harnessed by humans. Unlike gravity, it can be both attractive and repulsive. However, there are several reasons that it is unsuitable as a force field. First, it can be easily neutralized. Plastics and other insulators, for example, can easily penetrate a powerful electric or magnetic field. A piece of plastic thrown in a magnetic field would pa.s.s right through. Second, electromagnetism acts over large distances and cannot easily be focused onto a plane. The laws of the EM force are described by James Clerk Maxwell"s equations, and these equations do not seem to admit force fields as solutions.

3 & 4. The weak and strong nuclear forces. The weak force is the force of radioactive decay. It is the force that heats up the center of the Earth, which is radioactive. It is the force behind volcanoes, earthquakes, and continental drift. The strong force holds the nucleus of the atom together. The energy of the sun and the stars originates from the nuclear force, which is responsible for lighting up the universe. The problem is that the nuclear force is a short-range force, acting mainly over the distance of a nucleus. Because it is so bound to the properties of nuclei, it is extremely hard to manipulate. At present the only ways we have of manipulating this force are to blow subatomic particles apart in atom smashers or to detonate atomic bombs.

Although the force fields used in science fiction may not conform to the known laws of physics, there are still loopholes that might make the creation of such a force field possible. First, there may be a fifth force, still unseen in the laboratory. Such a force might, for example, work over a distance of only a few inches to feet, rather than over astronomical distances. (Initial attempts to measure the presence of such a fifth force, however, have yielded negative results.) Second, it may be possible to use a plasma to mimic some of the properties of a force field. A plasma is the "fourth state of matter." Solids, liquids, and gases make up the three familiar states of matter, but the most common form of matter in the universe is plasma, a gas of ionized atoms. Because the atoms of a plasma are ripped apart, with electrons torn off the atom, the atoms are electrically charged and can be easily manipulated by electric and magnetic fields.

Plasmas are the most plentiful form of visible matter in the universe, making up the sun, the stars, and interstellar gas. Plasmas are not familiar to us because they are only rarely found on the Earth, but we can see them in the form of lightning bolts, the sun, and the interior of your plasma TV.

PLASMA WINDOWS.

As noted above, if a gas is heated to a high enough temperature, thereby creating a plasma, it can be molded and shaped by magnetic and electrical fields. It can, for example, be shaped in the form of a sheet or window. Moreover, this "plasma window" can be used to separate a vacuum from ordinary air. In principle, one might be able to prevent the air within a s.p.a.ceship from leaking out into s.p.a.ce, thereby creating a convenient, transparent interface between outer s.p.a.ce and the s.p.a.ceship.

In the Star Trek TV series, such a force field is used to separate the shuttle bay, containing small shuttle craft, from the vacuum of outer s.p.a.ce. Not only is it a clever way to save money on props, but it is a device that is possible.

The plasma window was invented by physicist Ady Herschcovitch in 1995 at the Brookhaven National Laboratory in Long Island, New York. He developed it to solve the problem of how to weld metals using electron beams. A welder"s acetylene torch uses a blast of hot gas to melt and then weld metal pieces together. But a beam of electrons can weld metals faster, cleaner, and more cheaply than ordinary methods. The problem with electron beam welding, however, is that it needs to be done in a vacuum. This requirement is quite inconvenient, because it means creating a vacuum box that may be as big as an entire room.

Dr. Herschcovitch invented the plasma window to solve this problem. Only 3 feet high and less than 1 foot in diameter, the plasma window heats gas to 12,000F, creating a plasma that is trapped by electric and magnetic fields. These particles exert pressure, as in any gas, which prevents air from rushing into the vacuum chamber, thus separating air from the vacuum. (When one uses argon gas in the plasma window, it glows blue, like the force field in Star Trek.) The plasma window has wide applications for s.p.a.ce travel and industry. Many times, manufacturing processes need a vacuum to perform microfabrication and dry etching for industrial purposes, but working in a vacuum can be expensive. But with the plasma window one can cheaply contain a vacuum with the flick of a b.u.t.ton.