by Michio Kaku
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.
MASERS AND LASERS
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 Aleksandr Prokhorov would eventually win the Nobel Prize in 1964. Soon their results were extended to visible light, giving birth to the laser. (A phaser, 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 lasers exist, 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 trifluoride, 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 rod. 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=mc2, 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 about 700,000 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 Cadarache, 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 i
nto 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.