Physics of the impossible Michio Kaku quantum physics-Audio book PHASERS AND DEATH STARS
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machines are impossible. X-rays will prove to be a hoax. -PHYSICIS T LOR D KELVIN, 189 9 the (atomic) bomb will never go off. I speak as an expert in explosives. -ADMIRA L WILLIA M LEAH Y The Death Star is a colossal weapon, the size of an entire moon. Firing point-blank at the helpless planet Alderaan, home world of Princess Leia, the Death Star incinerates it, causing it to erupt in a titanic explosion, sending planetary debris hurtling throughout the solar system. A billion souls scream out in anguish,
creating a disturbance in the Force felt throughout the galaxy. But is the Death Star weapon of the Star Wars saga really possible? Could such a weapon channel a battery of laser cannons to vaporize an entire planet? What about the famous light sabers wielded by Luke Skywalker and Darth Vader that can slice through reinforced steel yet are made of beams of light? Are ray guns, like the passers in Star Trek, viable weapons for future generations of law enforcement officers and soldiers? In Star Wars millions of moviegoers were dazzled by these original, stunning special effects, but they fell flat for some critics, who panned them, stating that all this was in good fun, but it was patently impossible. Moon-sized, planet-busting ray guns are outlandish, and so are swords made of solidified light beams, even for a galaxy far, far away, they chanted. George Lucas, the master of special effects, must have gotten carried away this time. Although this may be difficult to believe, the fact is there is no physical limit to the amount of raw energy that can be crammed onto a light beam. There is no law of physics preventing the creation of a Death Star or light sabers. In fact, planet-busting beams of gamma radiation exist in nature. The titanic burst of radiation
from a distant gamma ray burster in deep space creates an explosion second only to the big bang itself. Any planet unfortunate enough to be within the crosshairs of a gamma ray buster will indeed be fried or blown to bits. BEA M WEAPON S THROUG H HISTORY The dream of harnessing beams of energy is actually not new but is rooted in ancient mythology and lore. The Greek god Zeus was famous for unleashing lightning bolts on mortals.
The Norse god Thor had a magic hammer, Mjolnir, which could fire bolts of lightning, while the Hindu god Indra was known for firing beams of energy from a magic spear. The concept of using rays as a practical weapon probably began with the work of the great Greek mathematician Archimedes, perhaps the greatest scientist in all of antiquity, who discovered a crude version of calculus two thousand years ago, before Newton and Leibniz. In one legendary battle against the forces of Roman general Marcellus during the Second Punic War in 214 BC, Archimedes helped to defend the kingdom of Syracuse and is believed to have created large batteries of solar reflectors that focused the sun's rays onto the sails of enemy ships, setting them ablaze. (There is still debate even today among scientists as to whether this was a practical, working beam weapon; various teams of scientists have tried to duplicate this feat with differing results.) Ray guns burst onto the science fiction scene
in 1889 with H. G. Wells's classic War of the Worlds, in which aliens from Mars devastate entire cities by shooting beams of heat energy from weapons mounted on their tripods. During World War II, the Nazis, always eager to exploit the latest advances in technology to conquer the world, experimented with various forms of ray guns, including a sonic device, based on parabolic mirrors that could focus intense beams of sound. Weapons created from focused light beams entered the public imagination with the James Bond movie Gold finger, the first Hollywood film to feature a laser. (The legendary British spy was strapped onto a
metal table as a powerful laser beam slowly advanced, gradually melting the table between his legs and threatening to slice him in half.) Physicists originally scoffed at the idea of the ray guns featured in Wels’s novel because they violated the laws of optics. According to Maxwell's equations, the light we see around us rapidly disperses and is incoherent (i.e., it is a jumble of waves of different frequencies and phases). It was
once thought that coherent, focused, uniform beams of light, as we find with laser beams, were impossible to create. THE QUANTUM REVOLUTION All this changed with the coming of the quantum theory. At the turn of the twentieth century it was clear that although Newton's laws and Maxwell's equations were spectacularly successful in explaining the motion of the planets and the behavior of light, they could not explain a whole class of phenomena. They failed miserably to explain why materials conduct electricity, why metals melt at certain temperatures, why gases emit light when heated, why certain substances become superconductors at low temperatures-all of which requires an understanding of the internal dynamics of atoms. The time was ripe for a revolution. Two hundred and fifty years
of Newtonian physics was about to be overthrown, heralding the birth pangs of a new physics. In 1900 Max Planck in Germany proposed that energy was not continuous, as Newton thought, but occurred in small, discrete packets, called "quanta." Then in 1905 Einstein postulated that light consisted of these tiny discrete packets (or quanta), later dubbed "photons." With this powerful but simple idea Einstein was able to explain the photoelectric effect, why electrons are emitted from metals when you shine a light on them. Today the photoelectric
effect and the photon form the basis of TV, lasers, solar cells, and much of modern electronics. (Einstein's theory of the photon was so revolutionary that even Max Planck, normally a great supporter of Einstein, could not at first believe it. Writing about Einstein, Planck said, "That he may sometimes have missed the target... as for example, in his hypothesis of light quanta, cannot really be held against him.") Then in 1913 the Danish physicist Niels Bohr
gave us an entirely new picture of the atom, one that resembled a miniature solar system. But unlike in a solar system in outer space, electrons can only move in discrete orbits or shells around the nucleus. When electrons "jumped" from one shell to a smaller shell with less energy, they emitted a photon of energy. When an electron absorbed a photon of a discrete energy, it "jumped" to a larger shell with more energy. A nearly complete theory of the atom emerged in 1925, with the coming of quantum mechanics and the revolutionary work of Erwin Schrödinger, Werner Heisenberg, and many others. According to the quantum
theory, the electron was a particle, but it had a wave associated with it, giving it both particle- and wavelike properties. The wave obeyed an equation, called the Schrödinger wave equation, which enabled one to calculate the properties of atoms, including all the "jumps" postulated by Bohr. Before 1925 atoms were still considered mysterious objects that many, like philosopher Ernst Mach, believed might not exist at all. After 1925 one could actually peer deep into the dynamics of the atom and actually predict its properties.
Astonishingly, this meant that if you had a big enough computer, you could derive the properties of the chemical elements from the laws of the quantum theory. In the same way that Newtonian physicists could compute the motions of all the celestial bodies in the universe if they had a big enough calculating machine, quantum physicists claimed that they could in principle compute all the properties of the chemical elements of the universe. If one had a big enough computer, one could also write the wave function of an entire human being MASER S AN D LASER S In 1953 Professor Charles Townes of the University of California at Berkeley and his colleagues produced the first coherent radiation in the form of microwaves. It was christened the "maser" (for microwave
amplification through stimulated emission of radiation). He and Russian physicists Nikolai Basov and Aleksandra Prokhorov would eventually win the Nobel Prize in 1964. Soon their results were extended to visible light, giving birth to the laser. (A phase, however, is a fictional device popularized in Star Trek.) In a laser you first begin with a special medium that will transmit the laser beam, such as a special gas, crystal, or diode. Then you pump energy into this medium from the outside, in the form of electricity, radio, light, or a chemical reaction. This sudden influx of energy pumps up the atoms of the medium, so the electrons
absorb the energy and then jump into the outer electron shells. In this excited, pumped-up state, the medium is unstable. If one then sends in a light beam through the medium, the photons will hit each atom, causing it to suddenly decay down to a lower level, releasing more photons in the process. This in turn triggers even more electrons to release photons, eventually creating a cascade of collapsing atoms, with trillions upon trillions of photons suddenly released into the beam. The key is that for certain substances, when this avalanche of photons is occurring all the photons are vibrating in unison, that is, they are coherent. (Picture a line of dominoes. Dominoes in their lowest energy state lie flat on a table. Dominoes
in a high-energy, pumped-up state stand up vertically, similar to the pumped-up atoms in the medium. If you push one domino, you can trigger a sudden collapse of all this energy at once, just as in a laser beam.) Only certain materials will "lase," that is, it is only in special materials that when a photon hits a pumped-up atom a photon will be emitted that is coherent with the original photon. As a result of this coherence, in
this flood of photons all the photons are vibrating in unison, creating a pencil-thin laser beam. (Contrary to myth, the laser beam does not stay pencil-thin forever. A laser beam fired onto the moon, for example, will gradually expand until it creates a spot a few miles across.) A simple gas laser consists of a tube of helium and neon gas. When electricity is sent through the tube the atoms are energized. Then, if the energy is suddenly released all
at once, a beam of coherent light is produced. The beam is amplified by using two mirrors, one placed at either end, so the beam bounces back and forth between them. One mirror is completely opaque, but the other allows a tiny amount of light to escape on each pass, producing a beam that shoots out one end. Today lasers are found almost everywhere, from grocery store checkout stands, to fiber-optic cables carrying the Internet, to laser printers and CD players, to modern computers. They are also used in eye surgery, to remove tattoos, and even in cosmetic salons. Over $5.4 billion worth of lasers were sold worldwide in 2004.
TYPES OF LASERS AND FUSION New lasers are being discovered almost every day as new materials are found that can lase, and as new ways are discovered for pumping energy into the medium. The question is, are any of these technologies suitable for building a ray gun or a light saber? Is it possible to build a laser powerful enough to energize a Death Star? Today a bewildering variety of laser sexist, depending on the material that lases and the energy that is injected into the material (e.g., electricity, intense beams of light, even chemical explosions). Among them are • Gas lasers. These lasers include helium-neon lasers, which are very common, creating a familiar red beam. They
are energized by radio waves or electricity. Helium-neon lasers are quite weak. But carbon dioxide gas lasers can be used for blasting, cutting, and welding in heavy industry and can create beams of enormous power that are totally invisible. • Chemical lasers. These powerful lasers are energized by a chemical reaction, such as a burning jet of ethylene and nitrogen difluoride, or NF3. Such lasers are powerful enough to be used in military applications. Chemical lasers are used in the U.S. military's airborne and ground lasers, which can produce millions of watts of power, and are designed to shoot down short-range missiles in midflight. • Excimer lasers. These lasers are also powered by chemical
reactions, often involving an inert gas (e.g., argon, krypton, or xenon) and fluorine or chlorine. They produce ultraviolet light and can be used to etch tiny transistors onto chips in the semiconductor industry, or for delicate Lasik eye surgery. • Solid-state lasers. The first working laser ever made consisted of a chromium-sapphire ruby crystal.
A large variety of crystals will support a laser beam, in conjunction with yttrium, holmium, thulium, and other chemicals. They can produce high-energy ultrashort pulses of laser light. • Semiconductor lasers. Diodes, which are commonly used in the semiconductor industry, can produce the intense beams used in industrial cutting and welding. They are also often found in checkout stands in grocery stores, reading the bar codes of your grocery items. • Dye
lasers. These lasers use organic dyes as their medium. They are exceptionally useful in terms of creating ultrashort pulses of light, often lasting only trillionths of a second. LASERS AND RAY GUNS? Given the enormous variety of commercial lasers and the power of military lasers, why don't we have ray guns available for use in combat and on the battlefield? Ray guns of one sort or another seem to be standard-issue weaponry in science fiction movies. Why aren't we working to create them? The simple answer is the lack of a portable power pack. One would need miniature power packs that contain the power of a huge electrical power station yet are small enough to fit on your palm. At present the only way to harness the power of a large commercial power station is to build one. At present
the smallest portable military device that can contain vast amounts of energy is a miniature hydrogen bomb, which might destroy you as well as the target. There is a second, ancillary problem as well-the stability of the lasing material. Theoretically, there is no limit to the energy one can concentrate on a laser. The problem is that the lasing material in a handheld ray gun would not be stable. Crystal lasers, for example, will overheat and crack
if too much energy is pumped into them. Hence to create an extremely powerful laser, the kind that might vaporize an object or neutralize a foe, one might need to use the power of an explosion. In that case, the stability of the lasing material is not such a limitation, since such a laser would be used only once. Because of the problems in creating a portable power pack and a stable lasing material, building a handheld ray gun is not possible with today's technology. Ray guns are possible, but only if they are connected by a cable to a power
supply. Or perhaps with nanotechnology we might be able to create miniature batteries that store or generate enough energy to create the intense bursts of energy required of a handheld device. At present, as we have seen, nanotechnology is quite primitive. At the atomic level, scientists have been able to create atomic devices that are quite ingenious, but impractical, such as an atomic abacus and an atomic guitar. But it is conceivable
that late in this century or the next, nanotechnology may be able to give us miniature batteries that can store such fabulous amounts of energy. Light sabers suffer from a similar problem. When the movie Star Wars first came out in the 1970s and light sabers became a best-selling toy among children, many critics pointed out that such a device could never be made. First, it is impossible to solidify light. Light always travels at the speed of light; it cannot be made solid. Second, light beams do not terminate in midair as do the light sabers
used in Star Wars. Light beams keep on going forever; a real light saber would stretch into the sky. Actually there is a way to construct a kind of light saber using plasmas, or superhot ionized gas. Plasmas can be made hot enough to glow in the dark and also slice through steel. A plasma light saber would consist of a thin, hollow rod that slides out of the
handle, like a telescope. Inside this tube hot plasmas would be released that would then escape through small holes placed regularly along the road. As the plasma flowed out of the handle, up the rod, and through the holes, it would create a long, glowing tube of superhot gas, sufficient to melt steel. This device is sometimes referred to as a plasma torch.
So it is possible to create a high-energy device that resembles a light saber. But as with ray guns, you would have to create a high energy portable power pack. Either you would need long cables connecting the light saber to a power supply, or you would have to create, via nanotechnology, a tiny power supply that could deliver huge amounts of power. So while ray guns and light sabers are possible to create in some form today,
the handheld weapons found in science fiction movies are beyond current technology. But late in this century or the next, with new advances in material science and also nanotechnology, a form of ray gun might be developed, making it a Class I impossibility. ENERGY FOR A DEATH STAR To create a Death Star laser cannon that can destroy an entire planet and terrorize a galaxy, such as that described in Star Wars, one would need to create the most powerful laser ever conceived. At present some of the most powerful lasers on Earth are being used to unleash temperatures found only in the center of stars.
In the form of fusion reactors, they might one day harness the power of the stars on Earth. Fusion machines try to mimic what happens in outer space when a star first forms. A star begins as a huge ball of formless hydrogen gas, until gravity compresses the gas and thereby heats it up; temperatures eventually reach astronomical levels. Deep inside a star's core, for example, temperatures can soar to between 50 million and 100 million degrees centigrade, hot enough to cause hydrogen nuclei to slam into each other, creating helium nuclei and a burst of energy. The fusion of hydrogen into helium, whereby a small amount of mass is converted into the explosive energy of a star via Einstein's famous equation E = mc 2, is the energy source of the stars. There are two ways in which scientists are currently attempting to harness fusion on the Earth. Both have proven to be much more difficult
to develop than expected. INERTIAL CONFINEMENT FOR FUSION The first method is called "inertial confinement." It uses the most powerful lasers on Earth to create a piece of the sun in the laboratory. A neodymium glass solid-state laser is ideally suited to duplicate the blistering temperatures found only in the core of a star. These laser
systems are the size of a large factory and contain a battery of lasers that shoot a series of parallel laser beams down a long tunnel. These high-power laser beams then strike a series of small mirrors arranged around a sphere; the mirrors carefully focus the laser beams uniformly onto a tiny, hydrogen-rich pellet (made of substances such as lithium deuteride, the active ingredient of a hydrogen bomb). The pellet is usually the size of a pinhead and weighs only 10 milligrams. The blast of laser light incinerates the surface of the pellet, causing the surface to vaporize and compress the pellet. As the pellet collapses, a shock wave is created that reaches the core of the pellet, sending temperatures soaring to millions of degrees, sufficient to fuse hydrogen nuclei into helium. The temperatures and pressures are so astronomical that "Lawson's criterion" is satisfied, the same criterion that is satisfied in hydrogen bombs and in the core of stars. (Lawson's criterion states
that a specific range of temperatures, density, and time of confinement must be attained in order to unleash the fusion process in a hydrogen bomb, in a star, or in a fusion machine.) In the inertial confinement process vast amounts of energy are released, including neutrons. (The lithium deuteride can hit temperatures of 100 million degrees centigrade and a density twenty times that of lead.) A burst of neutrons is then emitted from the pellet, and the neutrons strike a spherical blanket of material surrounding the chamber, and the blanket is heated up.
The heated blanket then boils water, and the steam can be used to power a turbine and produce electricity. The problem, however, lies in being able to focus such intense power evenly onto a tiny spherical pellet. The first serious attempt at creating laser fusion was the Shiva laser, a twenty-beam laser system built at the Lawrence Livermore National Laboratory (LLNL) in California that began operation in 1978. (Shiva is the Hindu goddess with
multiple arms, which the laser system design mimics.) The performance of the Shiva laser system was disappointing, but it was sufficient to prove that laser fusion can technically work. The Shiva laser system was later replaced by the Nova laser, with ten times the energy of Shiva. But the Nova laser also failed to achieve proper ignition of the pellets. Nonetheless, it paved the way for the current research in the National Ignition Facility (NIF), which began construction in 1997 at the LLNL. The NIF, which is supposed to be operational in 2009, is a monstrous machine, consisting of a battery of 192 laser beams, packing an enormous output of 700 trillion watts of power (the output of about700,000 large nuclear power plants concentrated in a single burst of energy). It is a state-of-the-art laser system designed to achieve full ignition of the hydrogen-rich pellets. (Critics have also pointed out its
obvious military use, since it can simulate the detonation of a hydrogen bomb and perhaps make possible the creation of a new nuclear weapon, the pure fusion bomb, which does not require a uranium or plutonium atomic bomb to kick-start the fusion process.) But even the NIF laser fusion machine, containing the most powerful lasers on Earth, cannot begin to approximate the devastating power of the Star Wars Death Star. To build such a device we must look to other sources of power. MAGNETIC CONFINEMENT FOR FUSION The second method scientists could potentially use to energize a Death Star is called "magnetic confinement," a process in which a hot plasma of hydrogen gas is contained within a magnetic field. In fact, this method could actually provide the prototype for the first commercial fusion reactors. Currently the most advanced fusion project of this type is the International Thermonuclear Experimental Reactor (ITER). In 2006 a coalition of nations (including
the European Union, the United States, China, Japan, Korea, Russia, and India) decided to build the ITER in Cucaracha, in southern France. It is designed to heat hydrogen gas to 100 million degrees centigrade. It could become the first fusion reactor in history to generate more energy than it consumes. It is designed to generate 500 megawatts of power for 500 seconds (the current record is 16 megawatts of power for 1 second). The ITER should generate its first plasma by 2016 and be fully operational in 2022. At a cost of $12 billion, it is the third most expensive scientific project in history (after the Manhattan Project and the International Space Station). The ITER looks like a large doughnut, with hydrogen gas circulating
inside and huge coils of wire winding around the surface. The coils are cooled down until they become superconducting, and then a huge amount of electrical energy is pumped into them, creating a magnetic field that confines the plasma inside the doughnut. When an electrical current is fed inside the doughnut, the gas is heated to stellar temperatures. The reason scientists are so excited by the ITER is the prospect of creating a cheap energy source.
The fuel supply for fusion reactors is ordinary seawater, which is rich in hydrogen. At least on paper, fusion may provide us with an inexhaustible, cheap supply of energy. So why don't we have fusion reactors now? Why has it taken so many decades to make progress after the fusion process was mapped out in the 1950s? The problem has been the fiendish difficulty of compressing the hydrogen fuel in a uniform manner. In stars, gravity compresses hydrogen gas into a perfect sphere, so that gas is heated evenly and cleanly. In NIF's laser fusion, the concentric
beams of laser light incinerating the surface of the pellet must be perfectly uniform, and it is exceedingly difficult to achieve this uniformity. In magnetic confinement machines, magnetic fields have both north poles and south poles; as a result, compressing gas evenly into a sphere is extremely difficult. The best we can do is to create a doughnut-shape magnetic field. But compressing the gas is like squeezing a balloon. Every time you squeeze
the balloon at one end, air bulges out somewhere else. Squeezing the balloon evenly in all directions simultaneously is a difficult challenge. Hot gas usually leaks out of the magnetic bottle, eventually touching the walls of the reactor and shutting down the fusion process.
That is why it has been so hard to squeeze the hydrogen gas for more than about one second. Unlike the current generation of fission nuclear power plants, a fusion reactor will not create large amounts of nuclear waste. (Each traditional fission plant produces 30 tons of extremely high-level nuclear waste per year. By contrast, the nuclear waste created by a fusion machine would be mainly the radioactive steel left over when the reactor is finally decommissioned.) Fusion will not completely solve the Earth's energy crisis anytime in the near future; Pierre-Gilles de Genes, French Nobel laureate in physics, has said, "We say that we will put the sun into a box. The idea is pretty. The problem is, we don't know how to make
the box." But if all goes well, researchers are hopeful that within forty years the ITER may pave the way for commercialization of fusion energy, energy that can provide electricity for our homes. One day, fusion reactors may alleviate our energy problem, safely releasing the power of the sun on the Earth. But even magnetic confinement fusion reactors would not provide enough energy to energize a Death Star weapon. For that we would need an entirely new design. NUCLEAR-FIRED X-RAY LASERS
There is one other possibility for simulating a Death Star laser cannon with today's known technology, and that is with a hydrogen bomb. A battery of X-ray lasers harnessing and focusing the power of nuclear weapons could in theory generate enough energy to operate a device that could incinerate an entire planet. The nuclear force, pound for pound, releases about 100 million times more energy than a chemical reaction. A piece of enriched uranium no bigger than a baseball is enough to incinerate an entire city in a fiery ball-even though only 1 percent of its mass has been converted to energy. As we discussed, there are a number
of ways of injecting energy into a laser beam. By far the most powerful of all is to use the force unleashed by a nuclear bomb. X-ray lasers have enormous scientific as well as military value. Because of their very short wavelength they can be used to probe atomic
distances and decipher the atomic structure of complicated molecules, a feat that is extraordinarily difficult using ordinary methods. A whole new window on chemical reactions opens up when you can "see" the atoms themselves in motion and in their proper arrangement inside a molecule. Because a hydrogen bomb emits a huge amount of energy in the X-ray range, X-ray lasers can also be energized by nuclear weapons. The person most closely associated
with the X-ray laser is the physicist Edward Teller, father of the hydrogen bomb. Teller, of course, was the physicist who testified before Congress in the 1950s that Robert Oppenheimer, who had headed the Manhattan Project, could not be trusted to continue work on the hydrogen bomb because of his politics. Teller's testimony led to Oppenheimer's being disgraced and having his security clearance revoked; many prominent physicists never forgave Teller for what he did. (My own contact with Teller dates from when I was in high school. I conducted a series
of experiments on the nature of antimatter and won the grand prize in the San Francisco science fair and a trip to the National Science Fair in Albuquerque, New Mexico. I appeared on local TV with Teller, who was interested in bright young physicists. Eventually I was awarded Teller's Hertz Engineering Scholarship, which paid for my college education at Harvard. I got to know his family fairly well through visits to the Teller household in Berkeley several times a year.) Basically, Teller's X-ray laser is a small nuclear bomb surrounded by copper rods. The detonation of the nuclear weapon releases a spherical shock wave of
intense X-rays. These energetic rays then pass through copper rods, which act as the lasing material, focusing the power of the X-rays into intense beams. These beams of X-rays could then be directed at enemy warheads. Of course, such a device could be used only once, since the nuclear detonation causes the X-ray laser to self-destruct. The initial test of the nuclear-powered X-ray laser was called the Cabra test, and it took place in 1983 in an underground shaft. A hydrogen bomb was detonated whose flood of incoherent X-rays was then focused into a coherent X-ray laser beam. Initially, the test was deemed a success,
and in fact in 1983 it helped to inspire President Ronald Reagan to announce, in a historic speech, his intent to build a "Star Wars" defensive shield. Thus was set in motion a multibillion dollar effort that continues even to this day to build an array of devices like the nuclear-powered X-ray laser to shoot down enemy ICBMs. (Later investigation showed that the detector used to perform the measurements during the Cabra test was destroyed; hence its readings could not be trusted.) Can such a controversial device in fact be used today to shoot down ICBM warheads? Perhaps. But an enemy could use a variety of simple, inexpensive
methods to nullify such weapons (for example, the enemy could release millions of cheap decoys to fool radar, or spin its warheads to disperse the X-rays, or emit a chemical coating to protect against the X-ray beam). Or an enemy might simply mass-produce warheads to penetrate a Star Wars defensive shield. So a nuclear-powered X-ray laser today is impractical as a missile defense system. But would it be possible to create a Death Star to use against an approaching asteroid, or to annihilate an entire planet? THE PHYSICS OF A DEATH STAR Can weapons be created that could destroy an entire planet, as in Star Wars? In theory, the answer is yes. There are several ways in which they might be created. First, there is no physical limit to the energy that can
be released by a hydrogen bomb. Here's how this works. (The precise outlines of the hydrogen bomb are top secret and classified even today by the U.S. government, but the broad outlines are well known.) A hydrogen bomb is actually built in many stages. By properly stacking these stages in sequence, one could produce a nuclear bomb of almost arbitrary magnitude. The first stage is a standard fission bomb, using the power of uranium-235 to release a burst of X-rays, as was done in the Hiroshima bomb. In the fraction of a second before the blast from the atomic bomb blows everything apart, the expanding sphere of X-rays outraces the blast (since it travels at the speed of light) and is then refocused onto a container of lithium deuteride, the active substance of a hydrogen bomb. (Precisely how this is
done is still classified.) The X-rays striking the lithium deuteride causes it to collapse and heat up to millions of degrees, causing a second explosion, much larger than thefirst. The burst of X-rays from this hydrogen bomb can then be refocused onto a second piece of lithium deuteride, creating a third explosion. In this way, one could stack lithium deuterated
side by side and create a hydrogen bomb of unimaginable magnitude. In fact, the largest hydrogen bomb ever built was a two-stage bomb detonated by the Soviet Union back in 1961, packing the energy of 50 million tons of TNT, although it was theoretically capable of a blast of over 100 million tons of TNT (or about five thousand times the power of the Hiroshima bomb). To incinerate an entire planet, however, is something of an entirely different magnitude? For this, the Death Star would have to launch thousands of such X-ray lasers into space, and they would then be required to fire all at once. (By comparison, remember that at the height of the cold war the United States and the Soviet Union each accumulated about thirty thousand nuclear bombs.) The collective energy from such an enormous number
of X-ray lasers would be enough to incinerate the surface of a planet. So it would certainly be possible for a Galactic Empire hundreds of thousands of years into the future to create such a weapon. For a very advanced civilization, there is a second option: to create a Death Star using the energy of a gamma ray buster. Such a Death Star would unleash a burst of radiation second only to the big bang. Gamma ray busters occur naturally in outer space, but it is conceivable that an advanced civilization could harness their vast power. By controlling
the spin of a star well before it undergoes a collapse and unleashes a hyper nova, one might be able to aim the gamma ray buster at any point in space. GAMMA RAY BURSTERS Gamma ray busters were actually first seen in the 1970s, when the U.S. military launched the Vela satellite to detect "nuke flashes" (evidence of an unauthorized detonation of a nuclear bomb). But instead of spotting nuke flashes, the Vela satellite detected huge bursts of radiation from space. Initially
this discovery set off a panic in the Pentagon: were the Soviets testing a new nuclear weapon in outer space? Later it was determined that these bursts of radiation were coming uniformly from all directions of the sky, meaning that they were actually coming from outside the Milky Way galaxy. But if they were extragalactic, they must be releasing truly astronomical amounts of power, enough to light up the entire visible universe. When the Soviet Union broke apart in 1990, a huge body of astronomical data was suddenly declassified by the Pentagon, overwhelming astronomers. Suddenly astronomers realized that a new, mysterious phenomenon was staring them in the face, one that would require rewriting the science textbooks. Since
gamma ray busters last from only a few seconds to a few minutes before they disappear, an elaborate system of sensors is required to spot and analyze them. First, satellites detect the initial burst of radiation and send the exact coordinates of the buster back to Earth. These coordinates are then relayed to optical or radio telescopes, which zero in on the exact location of the gamma ray buster. Although many details must still be clarified, one theory about the origins of gamma ray busters is that they are "hyper novae" of enormous strength, which leave massive black holes in their wake. It appears as if gamma ray busters are monster black holes in formation. But black holes emit two "jets" of radiation, one from the North Pole and one from the South Pole, like a spinning top. The radiation seen
from a distant gamma ray buster is apparently one of the jets that is aligned toward the Earth. If the jet of a gamma ray buster were aimed at the Earth, and the gamma ray buster were in our galactic neighborhood (a few hundred light-years from Earth), its power would be enough to destroy all life on our planet. Initially the gamma ray buster’s X-ray pulse would create an electromagnetic pulse that would wipe out all electronics equipment on the Earth. Its intense beam of X-rays and gamma rays would be enough to damage the atmosphere
of the Earth, destroying our protective ozone layer. The jet of the gamma ray buster would then heat up temperatures on the surface of the Earth, eventually setting off monster firestorms that would engulf the entire planet. The gamma ray buster might not actually explode the entire planet, as in the movie Star Wars, but it would certainly destroy all life, leaving a scorched, barren planet. Conceivably, a civilization hundreds of thousands to a million years more advanced than ours might be able to aim such a black hole in the direction of a target. This could be done by deflecting the path of planets and neutron stars into the dying star at a precise angle just before it collapses. This deflection would be enough to change the spin axis of the star so that it could be aimed in a certain direction.
A dying star would make the largest ray gun imaginable. In summary, the use of powerful lasers to create portable or handheld ray guns and light sabers can be classified as a Class I impossibility-something that is possible in the near future or perhaps within a century. But the extreme challenge of aiming a spinning star before it erupts into a black hole and transforming it into a Death Star would have to be considered a Class II impossibility-something that clearly does not violate the laws of physics (such gamma ray busters exist) but something that might be possible only thousands to millions of years in the future.