Fig. 1 - An imaginary "magic" mirror shows the differences
between normal matter and antimatter.
Antimatter is our first example of a new technology which, despite being almost indistinguishable from magic, is nevertheless real. Once found only on the Starship Enterprise, antimatter is now being made daily at laboratories around the world. Despite the proclivity of antimatter to mutually annihilate with anything it contacts, antimatter has been made, stopped, captured, and kept for months at a time in "bottles" made of electric and magnetic fields. Soon, bottles filled with antimatter will be transported around the world to be used by physicists for basic research into the fundamental mysteries of nature, by doctors for finding and treating cancer tumors, and by engineers for a multitude of practical applications, such as isotopic imaging, nuclear transmutation, and, one of these days, space propulsion.
What is this almost magical stuff called antimatter?
Our world is made of normal matter. The matter is in the form of atoms. The atoms have a heavy nucleus at the center made up of particles called protons and neutrons. Surrounding the nucleus of each atom is a cloud of electrons. Everything we normally experience is made up of these three stable particles; protons, neutrons, and electrons. Each of these three particles consists of a bundle of raw energy, wrapped up by nature into a compact, stable ball that we call matter. We don't really know why the proton and neutron weigh about the same, and yet are 1840 times heavier than the electron. We don't really know why the electrical charge on the electron is exactly equal and opposite in sign to the electrical charge on the proton. We don't really know why all the other characteristics of each particle are the way they are. That is still a mystery. It is as if each of the different particles had some special kind of quantum-mechanical "glue" to hold it together. The type of glue defines the amount of energy that can be bundled up into that particular kind of particle, and the various properties, such as mass, spin, and charge, that the resulting bundle should have.
For a long time after the electron and proton were discovered in the atom, scientists were puzzled at the asymmetry of nature. Why did the carrier of the positive electric charge, the proton, weigh so much more than the carrier of the negative electric charge, the electron? Then, in the mid-1900s, the scientists solved the puzzle by finding that each particle had a "mirror" twin. The mirror particle for the electron was a particle that had exactly the same mass as the electron, but the sign of its electric charge was reversed. Like the electron, the positron also acted as if it were spinning like a top, generating a detectable magnetic field. Since the positron had a positive charge while the electron had a negative charge, the magnetic fields were oppositely directed to the spin axis in the two particles. Thus the positron is the mirror image of the electron as seen in a "magic" mirror that reverses the charge on the mirror particle as well as reversing right and left handedness as does a normal mirror. [See Figure 1].
The mirror particle for the proton is called the antiproton. It has the same mass as the proton, but its charge and magnetic moment are reversed. There is also a mirror twin for the neutron called the antineutron. Since both the neutron and antineutron have no charge, it is hard to tell them apart. The neutron spins about its axis, however, and even though it is electrically neutral, it does have a magnetic field. The mirror neutron also has a magnetic field, but its spin is in the opposite direction to the magnetic field direction.
Since normal matter is made up of atoms built from electrons, protons, and neutrons, then it should be possible to make antimatter out of atoms built up from positrons, antiprotons, and antineutrons. Antimatter would be just like normal matter but with the charges reversed. For example, normal hydrogen is made of a single electron orbiting a proton, while antihydrogen would be made of a positron orbiting an antiproton.
There is an important difference between the two forms of matter. Whereas a particle made of normal matter is a bundle of energy held together with quantum-mechanical "glue", the mirror particle is a similar bundle of energy held together with "antiglue". Each "glue" turns out to be a solvent for the other! Thus, when an antiparticle meets a normal particle, the two glues dissolve each other, and the energy contained in the two particles is released in a micro-explosion. The mass of both particles is completely converted into energy. The amount of energy that is released is given by the famous Einstein equation E=mc2 (one of the few equations you will ever find in a newspaper). This complete conversion of mass to energy makes antimatter a highly efficient, compact, light-weight, almost magical source of energy. One milligram of antimatter combined with one milligram of normal matter produces the same energy as twenty tons of the most energetic chemical fuel in use today, liquid oxygen and liquid hydrogen.
According to the known laws of physics, particles and antiparticles should always be created in equal and opposite pairs. If matter and antimatter are always created in equal amounts, then half the universe should be made of antimatter. There should be antimatter galaxies containing antimatter stars illuminating antimatter planets populated with antimatter beings. If such antimatter galaxies existed, they would have large, tenuous clouds of antihydrogen gas surrounding them, just as our galaxy sits inside its own cloud of normal hydrogen gas. If there were regions where the normal matter galaxies and the antimatter galaxies lie near each other, their hydrogen clouds would overlap. In this overlapping region, the antihydrogen positrons would annihilate with the hydrogen electrons, releasing two gamma rays with exactly 511 million volts of energy. Astronomers using gamma-ray detectors flown on spacecraft such as the High Energy Astronomical Observatory have looked carefully for these characteristic gamma rays which indicate that matter is annihilating with antimatter. Once in a while, they detect some of these gamma rays, but every source found can be identified with a known neutron star or supernova, which occasionally produce small numbers of antielectrons. No source has been found which indicates that large amounts of antimatter exist anywhere in the universe. This mystery of the missing antimatter is one of the major unsolved problems of physics. Whatever the reason, the experimental fact is that only regular matter seems to occur in nature. If we want antimatter, we will have to make it.
Antimatter is now being made with the aid of huge "atom smashers". These machines use combinations of electric, magnetic, and radio fields to push against the electric charge on an electron or proton to accelerate them up to velocities close to that of light. The unit of energy that is used in particle accelerators is called the electron-volt or eV. If a metal plate has a positive voltage of one volt (a regular flashlight battery produces 1.5 volts), then an electron will be attracted to that plate. Just before the electron reaches the plate, it will have an energy of one electron-volt (1 eV).
Your television set produces about 20,000 volts inside (that is why there is a notice telling you not to open the back of the set). The electrons in the TV tube therefore reach 20 keV (20 kilo-electron-volts), and have enough energy when they strike the back of the screen to make the phosphor glow. A million volt machine can accelerate electrons (or protons) up to energies of 1 MeV (1 million electron-volts). At 1 MeV, an electron is moving at 94% the speed of light, while the heavier proton is only moving at 1/20th the speed of light. (The proton is 1840 times heavier than the electron so it doesn't have to move as fast to have the same energy as the electron.)
To get energies greater than a few million electron volts with just an electric field is difficult, because high voltages have a tendency to leak off into the air or emit corona discharge from sharp points even in a good vacuum. Once an electron or proton beam has been set moving using electric fields, however, it is possible to send radio waves traveling along in the same direction as the beam of particles. If the radio waves are properly tuned, the charged particles can gain energy from the moving radio waves just as a surfboard gains energy from a water wave. By this technique, energies of thousands of millions of volts, or giga-electron-volts (GeV) have been reached.
The basic method of making antiprotons is to start by accelerating normal matter protons to high energies in a proton accelerator. [See Figure 2.]
Scientists at the European antimatter factory at CERN in Switzerland accelerate their protons until their kinetic energy is 26 GeV. At the US antimatter factory at Fermilab in the USA, the proton accelerator is operated at 120 GeV. The amount of energy bound up into the mass of a proton is 0.938 GeV. Thus, any proton with a kinetic energy greater than 1 GeV has more energy in its motion than it has in its internal mass. A proton with a total energy of 120 GeV has within itself enough energy to make 120 protons, or 60 proton-antiproton pairs.
The high-energy "bullet" protons are then slammed into a target made of a thick tungsten or copper wire. In the wire, the bullet protons collide head-on into one of the metal nuclei. The kinetic energy of the bullet proton is released at a single point in space as a tiny fireball of energy. The released energy turns into a spray of gamma rays, and particle and antiparticle pairs of many different kinds. Unfortunately, only a small fraction are proton-antiproton pairs. At CERN, only four tenths of one percent of the bullet protons manage to produce a proton-antiproton pair. At Fermilab, the production efficiency is five percent, or five antiprotons produced for each one hundred bullet protons.
Not only antiprotons are produced by this process, but heavier antiparticles as well. Antineutrons are produced in almost the same numbers as antiprotons, while for each 10,000 antiprotons produced, about one antideuteron is generated. (An antideuteron contains in one nucleus an antiproton bound to an antineutron.) Even heavier antinuclei have been generated and detected. In one experiment carried out in 1978, physicists at CERN in Switzerland used their Super Proton Synchrotron to produce large numbers of antideuterons, 99 antitritium nuclei and 94 antihelium-3 nuclei. (Antitritium has one antiproton and two antineutrons, while antihelium-3 has two antiprotons and one antineutron.) These heavier antiparticles were sensed by sending them into a detector where they, unfortunately, were immediately annihilated. It would be another decade before methods were developed so that antimatter particles could be kept after they had been generated. So far, this has only been done with antiprotons.
It isn't easy to capture the antiprotons. They come out of the target in a broad spray at a wide range of angles and a wide spread in energy. But, with the aid of a lens made of magnetic fields and a magnetic particle selector, the negatively charged antiprotons can be separated from the remainder of the debris consisting of particles which have a different charge and mass than the antiproton. The antiprotons that are collected are then directed into a magnetic ring where they are accumulated. The best capture efficiency anyone has been able to accomplish to date is only one percent. Ninety-nine percent of the antiprotons that are generated are lost.
The antiproton collector ring also contains methods to "cool" the captured antiprotons. When the antiprotons are first made, they have a wide spread of energies and velocities. Some antiprotons are moving faster, and some slower, than the average velocity. Particle physicists then use various techniques to get all the antiprotons to travel at the same speed in the collector ring. The antiprotons still have lots of kinetic energy because of their high average speed, but their "temperature" is low because the variations in the energy are small. They are "cooler" than before.
Once the antiprotons have been captured and cooled in the collecting ring, most of the hard work has been done. The vacuum level in the collecting rings is so low that the antiprotons can be stored in the rings for days at a time. Scientists can then add more high-energy antiprotons at any time and increase the number being stored. A typical antiproton collector ring holds as many as a trillion antiprotons at any one time. This many antiprotons has a mass of about two trillionths of a gram. This may not sound like much antimatter, but if the two trillionths of a gram of antimatter were annihilated with an equal amount of matter, the total energy released would be about three hundred joules of energy, enough to light a 100-watt bulb for three seconds—enough energy to see.
Now that the antiprotons have been collected and cooled, they are sent to an antiproton decelerator, which is really nothing more than the proton accelerator run backwards. The slowed antiprotons are then captured and stored in a ring machine called a low energy antiproton accumulator. Once the engineers have all the machines tuned up correctly, the efficiency of cooling, transferring, decelerating, and storing the antiprotons at low energy is quite high—more than ninety percent. But considering the terrible losses during the initial processes of making and capturing antiprotons, the overall losses are quite high.
The collection efficiency at Fermilab is presently about 0.04% (only one antiproton is captured for every 2,500 antiprotons that are made). If we add in the five percent production efficiency of the target, that means that only one antiproton is captured for every 50,000 bullet protons sent into the target. In terms of energy, the efficiency is even worse. One antiproton, when annihilated with a proton, will produce about 2 GeV of energy. But, to get that one antiproton required 50,000 bullet protons, each with 120 GeV of energy—for a particle energy efficiency of one part in three million. Then, since the proton accelerator that makes the bullet protons is only five percent efficient at converting wall-plug energy into proton energy, the overall energy efficiency is only one part in sixty million. As a result, antimatter is presently a very expensive synthetic fuel—about ten trillion dollars per milligram—or two cents per million antiprotons. Fortunately, there are ways to improve this efficiency. Besides, there are some things, that only antiprotons can do, that make them worth even this exorbitant cost.
When first introduced to the concept of antimatter, one of the questions incredulous people have is: "How do you 'hold' onto this 'stuff' that disappears in a burst of energy the instant it touches normal matter?"
The scientists at CERN in Switzerland and Fermilab in the USA have demonstrated one solution to this problem. Their "bottle" is an evacuated tube bent into a ring. As the beam of antiprotons goes around inside the ring at high speed, it is directed and focused by magnetic fields to keep it from hitting the walls of the tube. Antiprotons have been kept for weeks in such a ring. The rings can also slow down the antiprotons so they can be put into a trap that is more portable than a ring.
The trap presently being used to hold antiprotons is the Penning trap. [See Figure 3.] It has side walls made of a carefully machined metal ring about five centimeters (two inches) in diameter. The inside surface has a hyperbolic shape. Above and below the hole in the ring are two domed metal end caps, also with hyperbolic shapes. This trap is placed in a vacuum chamber inside the bore of a superconducting magnet in a large thermos jug containing liquid helium. The magnetic field from the superconducting magnet runs along the axis of the trap from one end cap to the other. There is a small hole in one end cap to allow the antiprotons to enter the trapping region in the center.
To capture an antiproton once it has been inserted into the trap, the end caps are given a negative charge and the ring a positive charge. The negative charge on the caps repel the negatively charged antiproton, keeping it from going in the axial direction. The antiproton will attempt to move radially outward toward the positively charged ring electrode, but the magnetic field will keep it moving in a circle. If the magnetic field is strong enough, and the trap is cold enough, the antiproton will never get to the ring, and it will be permanently trapped in a tiny circular orbit.
Since 1986, the TRAP collaboration, led by Gerald Gabrielse of Harvard University, has used Penning traps at CERN to capture antiprotons. In order to improve the antiproton capture efficiency, they modified the design of the traps by using a multiplicity of elongated cylindrical electrodes and flat end windows instead of hyperbolically shaped electrodes. The multiple electrodes allow them to make a trap inside a trap. The inner trap is designed to hold a cloud of electrons, while the outer trap is designed to hold antiprotons, which have the same negative electric charge as the electrons, but are 1840 times more massive. CERN's low-energy antiproton accumulator ring provides the antiprotons, sending them in short intense bunches through an evacuated pipe to the trap, which is positioned at the end of the pipe. The antiproton bunches are only a quarter of a microsecond long and contain one hundred million antiprotons each.
When the bunched antiprotons first hit the aluminum entrance window of the trap, their speed is so high they pass right by the protons in the aluminum nuclei without having time to annihilate. But as they penetrate the entrance window and continue to encounter atoms, their negative charge repels the electrons belonging to those atoms and knocks some of the electrons out of their orbits. For each electron that an antiproton removes from an atom, the antiproton loses a little energy and slows down. The total speed lost by the antiproton in traversing the entrance window depends on the window's thickness, which is critical to the success of the trap. If the window is too thick, all of the antiprotons will be stopped in the window and annihilate. On the other hand, if the window is too thin, the antiprotons will enter the trap with too much energy and will escape out the other side. If the window's thickness is chosen correctly, about ten thousand of the one hundred million antiprotons that entered the window will exit the window with a velocity that is low enough that the antiprotons will be turned around by the three thousand volt negative potential of the exit window at the other end of the trap. The reflected antiprotons then head back down the twelve centimeter long trap toward the entrance window by which they entered the trap. The entrance window had a small positive voltage on it that allowed the antiprotons to enter the trap at low energy. Just before the quarter-microsecond pulse of antiprotons returns from the other end of the trap some third of a microsecond later, the trap is "slammed shut" by applying three thousand volts to the entrance window in less than a fiftieth of a microsecond. The antiprotons are now trapped.
The TRAP collaboration has used a Penning trap to hold about a hundred thousand antiprotons for two months without losing any. If a single one of the trapped antiprotons had collided with a left-over air molecule, or had touched the trap walls, it would have instantly annihilated and produced a large number of high energy particles and gamma rays, which would have been easily detected. One of these Penning traps could easily hold up to a trillion antiprotons. The traps are very small in size, and, when powered by nine-volt batteries, can be transported from site to site, or even across continents and oceans. (They do require large thermos-like containers full of liquid helium to keep them cold, so they are more pickup-truck portable than hand portable.)
Some of the most exciting and near-term uses for antimatter will be in medicine. The numbers of antiprotons that are being produced in one day's production at the present facilities are more than adequate not only for research, but for treatment and cure of hundreds of cancer patients. Ted Kalogeropoulos, a physicist at Syracuse University and his colleague Levi Gray, have found that a small number of antiprotons can be used to create density images of the interior of an object—including human bodies. They have used computers to conduct simulated experiments which have led them to believe that this new form of imaging will have several significant advantages over current methods of taking pictures of the inside of the human body, such as CAT scans, which use X-rays. Their antiproton imaging technique should give better pictures than CAT scans, with one hundred times less radiation dose to the patient. With antimatter, the same beam that images a tumor could also zap it. By increasing the number of antiprotons beamed at some particular point, doctors could deposit enough energy right into the tumor to destroy it.
The number of antiprotons needed is not large. A million antiprotons will produce a high quality picture of a single plane through the body, and a billion antiprotons will produce a high quality three-dimensional image of a large volume such as the head or chest area. More antiprotons would be needed to kill the tumors, depending upon the size. A trap containing one day's antiproton production from CERN or Fermilab would be sufficient to image a thousand patients, or treat many dozens of cancer tumors.
The next step in developing an antimatter technology is to make and store antimatter in the form of antihydrogen atoms and molecules. Conceptually, this is easy. You merely combine antiprotons and positrons to form antihydrogen. Being of opposite charge, they will automatically attract each other. Unfortunately, like a comet being attracted to the Sun, as the positron is attracted toward the antiproton, it gains so much speed during its infall that most of the time the positron just zooms around the antiproton and flies back out again, avoiding capture. What is needed is a third particle nearby to steal away a little of the kinetic energy of the incoming positron so it becomes captured in an orbit around the antiproton, creating an antihydrogen atom.
In July 1992, the Second Antihydrogen Workshop was held in Munich, Germany. It was attended by over one hundred scientists. In one of the papers presented at the meeting, one of the scientists estimated that about thirty antihydrogen atoms had already been produced as an accidental byproduct of an experiment at Fermilab, but had gone undetected at the time. It is interesting to note that there were more scientists at this meeting on antihydrogen that there had been antihydrogen atoms made to date!
Most of the papers discussed ongoing experiments to be the first to make antihydrogen at low energies where it could be trapped and studied. Most of the approaches involve first trapping a large number of positrons, which are relatively easy to make and trap, then putting an antiproton in the same trap. It is expected that the antiproton will capture one positron to form antihydrogen, while the other positrons will act as the "third body" to take away the excess energy. It is not easy, however, to design a trap that will hold heavy negatively charged antiprotons and light positively charged positrons at the same time.
Others at the conference described their techniques for holding onto the antihydrogen atoms once they are made. Since an antihydrogen atom has no net charge, it cannot be stored in the Penning traps used to hold positrons and antiprotons. One proposed technique uses a trap made of ultraviolet laser beams. The other trap techniques take advantage of the fact that an antihydrogen atom is slightly magnetic and can be captured in a trap made of opposing magnetic field coils with a magnetic field minimum at the center. This same type of trap can also hold antihydrogen molecules and antihydrogen ice. Scientists already have demonstrated magnetic field traps that can levitate and hold balls of liquid and frozen normal hydrogen, so the traps should work equally well with antihydrogen.
Electric fields can also be used to store and manipulate antimatter. If a ball of antihydrogen ice has a slight excess electric charge, electric fields can be used to move it around. A weak beam of ultraviolet light can be used to drive positrons off the antihydrogen ice ball to keep it charged. Experimenters at the Jet Propulsion Laboratory have been experimenting with such traps for use in zero gravity laboratories on Spacelab and the Space Station. They have already used such traps to stably levitate balls of water ice in one Earth gravity. Since antihydrogen ice has a much lower density than water ice, the same apparatus should be able to levitate a similar sized ball of antihydrogen against thirteen gravities.
Thus, experiments are underway that are expected to result in the production, cooling, and trapping of small amounts of antihydrogen. But once made, can it be kept? Will the residual air left in the trap annihilate with the antihydrogen ice ball, heating it up and causing a catastrophic chain reaction? Will antihydrogen atoms evaporate from the ice ball, travel to the walls of the trap and annihilate, starting a chain reaction?
The experiment involving the trapping of a hundred thousand antiprotons for two months with no annihilations occurring, showed that a trap kept at liquid helium temperatures has a vacuum good enough to store antimatter for months at a time without any loss. Experiments on the evaporation rate of normal hydrogen ice versus temperature, indicate that if the antihydrogen ice ball in the trap is kept below two degrees above absolute zero, only a few antihydrogen molecules a day would evaporate from the surface. Even if these escaped the trap fields and annihilated on the walls of the trap, calculations show that the energy released would not be enough to cause a heat chain reaction to start. Thus, once we have made and trapped antimatter, we can be confident that we can keep it for long periods of time until it is used. One exciting future use for antimatter is in space propulsion. Not for powering the space warp drives on the Starship Enterprise, but for powering more pragmatic engines called antimatter rockets.
To travel you must move. To move you must have energy. To get energy you must convert mass. Every time you burn a tankful of gasoline to take you and your automobile further down the highway, mass has disappeared. In the process of burning gasoline, some of the mass of the gasoline has been converted into energy. With chemical fuels, like gasoline and rocket fuels, the amount of mass that gets converted is parts in a thousand million. With fission energy, using uranium or plutonium as fuels, the amount of mass converted becomes parts in a thousand. With fusion fuels like hydrogen and deuterium, the mass conversion ratio reaches almost one percent. Yet all of these fuels are eclipsed by antimatter, for when antimatter meets normal matter all of the mass in both the antimatter and the normal matter gets converted into energy. Since normal matter is easy to come by, one could say that antimatter is a magical fuel that is two hundred percent efficient!
You may have heard that mixing antimatter with normal matter produces gamma rays. The first antimatter that was produced was the positron, the mirror particle to the electron. When positrons are mixed with electrons they do produce all of their energy as gamma rays, which are very difficult to cope with. When antiprotons are mixed with normal protons, however, the resulting energy does not come out as gamma rays. Instead, the proton and its mirror twin turn into a collection of particles called pions. On the average there will be three charged pions and two neutral pions. The neutral pions almost instantly produce gamma rays. These gamma rays can be stopped by a shield to produce heat to run the auxiliary systems.
The other sixty percent of the energy released, however, is in the form of highly energetic charged pions. The charged pions have a short life, but since they are traveling at 94% of the speed of light they cover a distance of twenty-one meters (sixty feet) during their lifetime, which is more than enough to extract the kinetic energy from them. [See Figure 4.] After they decay, they turn into other charged particles called muons. The muons will travel nearly two kilometers (over a mile) before they decay, turning into positrons and electrons. The charged particles can be used to heat normal matter, like water or hydrogen, to produce a hot plasma. The hot plasma can be ejected out a rocket nozzle to provide thrust or used to power electric generators.
An even simpler first-generation antimatter rocket engine consists of a cylinder of tungsten about thirty centimeters (one foot) in length and diameter, weighing about a third of a ton. Antiprotons are injected into a cavity in the center of the cylinder, where they annihilate with the hydrogen gas kept there. Most of the gamma rays and charged pions that emerge from the annihilation process are stopped in the tungsten block, depositing their energy in the tungsten and heating it up.
There are holes running through the cylinder, through which pass the ordinary hydrogen gas. The hydrogen enters the tungsten heat exchanger cold, and emerges at high temperatures, in the process keeping the tungsten from melting. This hot hydrogen gas is then expelled from a nozzle to create thrust. The speed of the emerging hydrogen is two or three times the speed obtainable from chemical rocket fuels. These antimatter rocket engines are limited in their performance by the melting point of tungsten. Reaction chambers made of magnetic fields are now being studied that would allow even higher exhaust velocities to be obtained for the propellant. [See Figure 4.]
Studies of the optimal design of antimatter rocket systems have come up with some unexpected results. First, for missions around the Earth and throughout the solar system, the analyses indicate you should not use equal parts of matter and antimatter. The best arrangement is to use few milligrams of antimatter to heat tons of propellant material—hydrogen, methane, or water—which becomes a hot gas for the rocket to spew out the exhaust nozzle. Second, only one size of antimatter rocket is needed, no matter how difficult the mission. With normal chemical rockets, the more difficult the mission, the more fuel the rocket must carry, and the larger the rocket becomes, especially the propellant tanks. This is not true for optimized antimatter rocket missions. Going up to low Earth orbit, orbiting the Moon, making a round trip journey to the surface of Mars, or traveling to the stars, can all be done with the same sized vehicle!
The optimum antimatter vehicle contains a fuel tank that holds an amount of propellant that has only four times the "dry" mass of the vehicle. (Most chemical rockets carry twenty to sixty times their dry mass in fuel.) If the mission is easy, the vehicle will carry only a few milligrams of antimatter and use it to heat the propellant to a hot gas, which is expelled out the nozzle to provide thrust to the vehicle. If the mission is difficult, the vehicle will take along more antimatter, but the same amount of propellant, and use the antimatter to heat the propellant until it is a blazing hot plasma that has a much higher exhaust velocity. Even for a mission to the stars, the amount of antimatter needed is measured in kilograms, not tons.
Still, since the present production rate is one trillionth of a gram a day, while space propulsion needs many milligrams per mission and grams per year, it will be decades before antimatter becomes a product that is bought and sold like gasoline or diesel fuel. There are many, many, many difficult problems left to solve in the production, capture, cooling, storage, transport, and use of this extremely potent, extremely expensive, nearly magic source of raw energy. But as one skeptic after another takes a look at the problems that have been overcome and the problems still left to be solved, it begins to look as if there are no "showstoppers". There is no physical reason why antimatter, in some form, cannot be made and stored in enough quantity to produce the kilowatts and megawatts of prime power and propulsion power needed for rapid space travel.
The production efficiency of the present machines is abysmally low. Fortunately, the low efficiency is not due to any fundamental limitation, and can be improved by orders of magnitude. The reason for the low efficiencies of the present machines is that all of the antimatter production facilities built to date have been built under limited budgets. Production efficiency has been sacrificed to such considerations as speed, cost, science requirements, and national pride.
In a study I carried out for the Air Force Rocket Propulsion Laboratory, I showed that if an antiproton factory were designed properly by engineers, instead of by scientists with limited budgets and in a hurry to win a Nobel prize, the present energy efficiency (electrical energy in compared to antimatter annihilation energy out) could be raised from a part sixty million to a part in ten thousand or 0.01%, while at the same time, the cost of building the factory could be substantially lowered compared to the cost of the high precision scientific machines. From these studies, I estimated the cost of the antimatter at ten million dollars per milligram. This may sound like a lot, but at ten million dollars per milligram, antimatter is already cost effective for space propulsion and power. At the present subsidized price of a Space Shuttle launch, it costs about five million dollars to put a ton of anything into low Earth orbit. Since a milligram of antimatter produces the same amount of energy as twenty tons of the most energetic chemical fuel available, then a milligram of antimatter costing ten million dollars would be a more cost effective fuel in space than twenty tons of chemical fuel costing ten times as much.
Antimatter is no longer science fiction. It is real, and is being made daily for scientific purposes. Within a few years, traps containing antiprotons will be transported around the world for research and medical purposes. Before the century is out, antihydrogen, and perhaps other antiatoms, will be made and kept in storage. As the 21st century progresses, new applications will be found for antimatter. Its price will come down as production starts.
Antimatter at the present cost of ten trillion dollars per milligram has already been proven cost effective for scientific experiments to win Nobel prizes. Antimatter at a hundred million dollars per milligram would definitely be cost effective for unmanned probe missions to the rings of Saturn and manned missions to Mars. When the cost of antimatter starts to drop below ten million dollars per milligram, then many new applications come to the fore, for now antimatter is cheaper in energy delivered to orbit than any chemical fuel, and possibly even cheaper than nuclear fuel.
Where will we get the energy to run these magic matter factories? Some of the prototype factories will be built on Earth, but for large scale production we certainly don't want to power these machines by burning fossil fuels on Earth. There is plenty of energy in space. At the distance of the Earth from the Sun, the Sun delivers over a kilowatt of energy for each square meter of collector, or a gigawatt (1,000,000,000 watts) per square kilometer. A collector array of one hundred kilometers on a side would provide a power input of ten terawatts (10,000,000,000,000 watts), enough to run a number of antimatter factories at full power, producing a gram of antimatter a day.
We know how to make antimatter. We know how to store antimatter. With a fully developed magic matter technology the solar system and the nearby stars can be ours. There is no question about the feasibility of the technology, it is only a matter of scaling and cost. The question is not: "Can we do it?" It is: "Do we want to do it?"
B.W. Augenstein, et al., editors, Antiproton Science and Technology (World Scientific, 1988).
David B. Cline, Low Energy Antimatter (World Scientific, 1986).
John Eades, editor, Proceedings of the Antihydrogen Workshop, Hyperfine Interactions, Vol. 76, pp. 1-402 (1993).
Robert L. Forward, "Antimatter Revealed," Omni, Vol. 2, No. 2, pp. 44-48 (1979).
Robert L. Forward, "Antiproton Annihilation Propulsion," Journal of Propulsion and Power, Vol. 1, pp. 370-374 (1985).
Robert L. Forward and Joel Davis, Mirror Matter: Pioneering Antimatter Physics (John Wiley & Sons, 1988).
Gerald Gabrielse, "Extremely Cold Antiprotons", Scientific American, pp. 74-85 (December 1992).