Physics of the Impossible

by Michio Kaku

  The nanobot I’ve just described is sometimes called a von Neumann probe, named after the famed mathematician John von Neumann, who worked out the mathematics of self-replicating Turing machines. In principle, such self-replicating nanobot spaceships might be able to explore the entire galaxy, not just the nearby stars. Eventually there could be a sphere of trillions of these robots, multiplying exponentially as it grows in size, expanding at nearly the speed of light. The nanobots inside this expanding sphere could colonize the entire galaxy within a few hundred thousand years.

  One electrical engineer who takes the idea of nanoships very seriously is Brian Gilchrist of the University of Michigan. He recently received a $500,000 grant from NASA’s Institute for Advanced Concepts to explore the idea of building nanoships with engines no bigger than a bacterium. He envisions using the same etching technology used in the semiconductor industry to create a fleet of several million nanoships that will propel themselves by ejecting tiny nanoparticles that are only tens of nanometers across. These nanoparticles would be energized by passing through an electric field, just as in an ion engine. Since each nanoparticle weighs thousands of times more than an ion, the engines would pack much more thrust than a typical ion engine. Thus the nanoship engines would have the same advantages as an ion engine, except they would have much more thrust. Gilchrist has already begun etching some of the parts for these nanoships. So far he can pack 10,000 individual thrusters on a single silicon chip that measures 1 centimeter across. Initially he envisions sending his fleet of nanoships throughout the solar system to test their efficiency. But eventually these nanoships might be part of the first fleet to reach the stars.

  Gilchrist’s proposal is one of several futuristic proposals being considered by NASA. After several decades of inactivity, NASA has recently given some serious thought to various proposals for interstellar travel—proposals that range from the credible to the fantastic. Since the early 1990s NASA has hosted the annual Advanced Space Propulsion Research Workshop, during which these technologies have been picked apart by teams of serious engineers and physicists. Even more ambitious is the Breakthrough Propulsion Physics program, which has explored the mysterious world of quantum physics in relation to interstellar travel. Although there is no consensus, much of their activity has focused on the front-runners: the laser sail and various versions of fusion rockets.

  Given the slow but steady advances in spaceship design, it is reasonable to assume that the first unmanned probe of some sort might be sent to the nearby stars perhaps later in this century or early in the next century, making it a Class I impossibility.

  But perhaps the most powerful design for a starship involves the use of antimatter. Although it sounds like science fiction, antimatter has already been created on the Earth, and may one day provide the most promising design yet for a workable manned starship.

  10: ANTIMATTER AND ANTI-UNIVERSES

  The most exciting phrase to hear in science, the one that heralds new discoveries, is not “Eureka” (I found it!) but “That’s funny…”

  —ISAAC ASIMOV

  If the man doesn’t believe as we do, we say he is a crank, and that settles it. I mean, it does nowadays, because now we can’t burn him.

  —MARK TWAIN

  You can recognize a pioneer by the arrows in his back.

  —BEVERLY RUBIK

  In Dan Brown’s book Angels and Demons, the bestselling predecessor to The Da Vinci Code, a small band of extremists, the Illuminati, have hatched a plot to blow up the Vatican using an antimatter bomb, stolen from CERN, the nuclear laboratory outside Geneva. The conspirators know that when matter and antimatter touch each other the result is a monumental explosion, many times more powerful than a hydrogen bomb. Although an antimatter bomb is pure fiction, antimatter is very real.

  An atomic bomb, for all its awesome power, is only about 1 percent efficient. Only a tiny fraction of the uranium is turned into energy. But if an antimatter bomb could be constructed, it would convert 100 percent of its mass into energy, making it far more efficient than a nuclear bomb. (More precisely, about 50 percent of the matter in an antimatter bomb would be turned into usable explosive energy; the rest would be carried away in the form of undetectable particles called neutrinos.)

  Antimatter has long been the focus of intense speculation. Although an antimatter bomb does not exist, physicists have been able to use their powerful atom smashers to create minute quantities of antimatter for study.

  PRODUCING ANTI-ATOMS AND ANTI-CHEMISTRY

  At the beginning of the twentieth century, physicists realized that the atom consisted of charged subatomic particles with electrons (with a negative charge) circulating around a tiny nucleus (with a positive charge). The nucleus, in turn, consisted of protons (which carried the positive charge) and neutrons (which were electrically neutral).

  So it came as quite a shock in the 1930s when physicists realized that for every particle there is a twin, an antiparticle, but with an opposite charge. The first antiparticle to be discovered was the antielectron (called the positron), which has a positive charge. The positron is identical to the electron in every way, except that it carries the opposite charge. It was first discovered in photographs of cosmic rays taken in a cloud chamber. (Positron tracks are quite easy to see in a cloud chamber. When placed in a powerful magnetic field, they bend in the opposite direction from ordinary electrons. In fact, I photographed such antimatter tracks while I was in high school.)

  In 1955 the particle accelerator at the University of California at Berkeley, the Bevatron, produced the first antiproton. As expected, it is identical to the proton except that it has a negative charge. This means that, in principle, one can create anti-atoms (with positrons circulating around antiprotons). In fact, anti-elements, anti-chemistry, anti-people, anti-Earths, and even anti-universes are theoretically possible.

  At present the giant particle accelerators at CERN and the Fermilab outside Chicago have been able to create minute quantities of antihydrogen. (This is done by blasting a beam of high-energy protons into a target using particle accelerators, thereby creating a shower of subatomic debris. Powerful magnets separate out the antiprotons, which are slowed down to very low velocities and then are exposed to the antielectrons that are naturally emitted from sodium-22. When the antielectrons orbit around the antiprotons, they create antihydrogen, since hydrogen is made up of one proton and one electron.) In a pure vacuum, these anti-atoms might live forever. But because of impurities and collisions with the wall, these anti-atoms eventually strike ordinary atoms and they are annihilated, releasing energy.

  In 1995 CERN made history when it announced that it had created nine antihydrogen atoms. Fermilab soon followed suit by producing one hundred atoms of antihydrogen. In principle, there is nothing to prevent us from creating higher anti-elements as well, except for the staggering cost. Producing even a few ounces of anti-atoms would bankrupt any nation. The current rate of production of antimatter is between one-billionth to ten-billionths of a gram per year. The yield might increase by a factor of three by the year 2020. The economics of antimatter are very poor. In 2004 it cost CERN $20 million to produce several trillionths of a gram of antimatter. At that rate, producing a single gram of antimatter would cost $100 quadrillion and the antimatter factory would need to run continuously for 100 billion years! This makes antimatter the most precious substance in the world.

  “If we could assemble all the anti-matter we’ve ever made at CERN and annihilate it with matter,” reads a statement from CERN, “we would have enough energy to light a single electric light bulb for a few minutes.”

  Handling antimatter poses extraordinary problems, since any contact between matter and antimatter is explosive. Putting antimatter in an ordinary container would be suicide. When the antimatter touched the walls, it would explode. So how does one handle antimatter if it is so volatile? One way would be first to ionize the antimatter into a gas of ions, and then to safely confine it in a “magnetic b
ottle.” The magnetic field would prevent the antimatter from touching the walls of the chamber.

  To build an antimatter engine, a steady stream of antimatter would need to be fed into a reaction chamber, where it would be carefully combined with ordinary matter, creating a controlled explosion, similar to the explosion created by chemical rockets. The ions created by this explosion would then be shot out one end of the antimatter rocket, creating propulsion. Because of the antimatter engine’s efficiency in converting matter into energy, in theory it is one of the most appealing engine designs for future starships. In the Star Trek series, antimatter is the source of the Enterprise’s energy; its engines are energized by the controlled collision of matter and antimatter.

  AN ANTIMATTER ROCKET

  One of the main proponents of the antimatter rocket is physicist Gerald Smith of Pennsylvania State University. He believes that in the short term as little as 4 milligrams of positrons would be sufficient to take an antimatter rocket to Mars in just several weeks. He notes that the energy packed into antimatter is about a billion times greater than the energy packed into ordinary rocket fuel.

  The first step in creating this fuel would be to create beams of antiprotons, via a particle accelerator, and then store them in a “Penning trap,” which Smith is constructing. When built, the Penning trap would weigh 220 pounds (much of it being liquid nitrogen and liquid helium) and would store about a trillion antiprotons in a magnetic field. (At very low temperatures, the wavelength of the antiprotons is several times longer than the wavelength of the atoms in the container walls, so the antiprotons would mainly reflect off the walls without annihilating themselves.) He states that this Penning trap should be able to store the antiprotons for about five days (until they finally are annihilated when mixed with ordinary atoms). His Penning trap should be able to store about a billionth of a gram of antiprotons. His goal is to create a Penning trap that can store up to a microgram of antiprotons.

  Although antimatter is the most precious substance on Earth, its cost keeps dropping dramatically every year (a gram would cost about $62.5 trillion at today’s prices). A new particle injector being built at Fermilab outside Chicago should be able to increase the production of antimatter by a factor of ten, from 1.5 to 15 nanograms per year, which should drive down prices. However, Harold Gerrish of NASA believes that with further improvements the cost could realistically go down to $5,000 per microgram. Dr. Steven Howe, of Synergistics Technologies in Los Alamos, New Mexico, states, “Our goal is to remove antimatter from the far-out realm of science fiction into the commercially exploitable realm for transportation and medical applications.”

  So far, particle accelerators that can produce antiprotons are not specifically designed to do so, so they are quite inefficient. Such particle accelerators are designed primarily to be research tools, not factories for antimatter. That is why Smith envisions building a new particle accelerator that will be specifically designed to produce copious quantities of antiprotons to drive down the cost.

  If prices for antimatter can be lowered even further by technical improvements and mass production, Smith envisions a time when the antimatter rocket could become a workhorse for interplanetary and possibly interstellar travel. Until then, however, antimatter rockets will remain on the drawing boards.

  NATURALLY OCCURRING ANTIMATTER

  If antimatter is so difficult to create on Earth, might one find antimatter more easily in outer space? Unfortunately, searches for antimatter in the universe have turned up very little, which is rather surprising to physicists. The fact that our universe is made up mainly of matter, rather than antimatter, is difficult to explain. One might naïvely have assumed that at the beginning of the universe, there were equal, symmetrical quantities of matter and antimatter. So the lack of antimatter is puzzling.

  The most likely solution was first proposed by Andrei Sakharov, the man who designed the hydrogen bomb for the Soviet Union in the 1950s. Sakharov theorized that at the beginning of the universe there was a slight asymmetry in the amount of matter and antimatter in the big bang. This tiny symmetry breaking is called “CP violation.” This phenomenon is currently the center of much vigorous research. In effect, Sakharov theorized that all the atoms in the universe today are left over from a near perfect cancellation between matter and antimatter; the big bang caused a cosmic cancellation between the two. The tiny leftover matter created a residue that forms the visible universe of today. All the atoms in our bodies are leftovers from this titanic collision of matter and antimatter.

  This theory leaves open the possibility that small amounts of antimatter may occur naturally. If so, discovering that source would drastically reduce the cost of producing antimatter for use in antimatter engines. In principle, deposits of naturally occurring antimatter should be easy to detect. When an electron and an antielectron meet, they annihilate into gamma rays at an energy of 1.02 million electron volts or more. Thus by scanning the universe for gamma rays at this energy, one could find the “fingerprint” for naturally occurring antimatter.

  In fact, “fountains” of antimatter have been found in the Milky Way galaxy, not far from the galactic center, by Dr. William Purcell of Northwestern University. Apparently a stream of antimatter exists that creates this characteristic gamma radiation at 1.02 million electron volts as it collides with ordinary hydrogen gas. If this plume of antimatter exists naturally, then it might be possible that other pockets of antimatter exist in the universe that were not destroyed in the big bang.

  To look for naturally occurring antimatter more systematically, the PAMELA (Payload for Antimatter-Matter Exploration and Light-Nuclei Astrophysics) satellite was launched into orbit in 2006. It is a collaborative effort between Russia, Italy, Germany, and Sweden, designed to search for pockets of antimatter. Previous missions searching for antimatter were carried out using high-altitude balloons and the Space Shuttle, so the data was collected for no more than a week or so. PAMELA, by contrast, will stay in orbit for at least three years. “It is the best detector ever constructed and we will use it for a long period,” declares team member Piergiorgio Picozza of the University of Rome.

  PAMELA is designed to detect cosmic rays from ordinary sources, such as supernovae, but also from unusual ones, such as stars made entirely of antimatter. Specifically, PAMELA will look for the signature of anti-helium, which might be produced in the interiors of anti-stars. Although most physicists today believe that the big bang resulted in a near perfect cancellation between matter and antimatter, as Sakharov believed, PAMELA is based on a different assumption—that whole regions of antimatter universe did not undergo that cancellation and hence exist today in the form of anti-stars.

  If antimatter exists in minute quantities in deep space, then it might be possible to “harvest” some of that antimatter to use to propel a starship. NASA’s Institute for Advanced Concepts takes the idea of harvesting antimatter in space seriously enough that it recently funded a pilot program to study this concept. “Basically, what you want to do is generate a net, just like you’re fishing,” says Gerald Jackson of Hbar Technologies, one of the organizations spearheading the project.

  The antimatter harvester is based on three concentric spheres, each made out of a lattice wire network. The outermost sphere would be 16 kilometers across and would be positively charged, so that it would repel any protons, which are positively charged, but attract antiprotons, which are negatively charged. The antiprotons would be collected by the outer sphere, then slow down as they passed through the second sphere and would finally stop when they reached the innermost sphere, which would be 100 meters across. The antiprotons would then be captured in a magnetic bottle and combined with antielectrons to make antihydrogen.

  Jackson estimates that controlled matter-antimatter reactions inside a spacecraft could fuel a solar sail to Pluto using just 30 milligrams of antimatter. Seventeen grams of antimatter, says Jackson, would be enough to fuel a starship to Alpha Centauri. Jackson claims that there
might be 80 grams of antimatter between the orbits of Venus and Mars that might be harvested by the space probe. Given the complexities and cost of launching this huge antimatter collector, however, it probably won’t be realized until the end of this century, or beyond.

  Some scientists have dreamed about harvesting antimatter from a meteor floating in outer space. (The Flash Gordon comic strip once featured a rogue antimatter meteor drifting in space, which could create a terrifying explosion if it came in contact with any planet.)

  If naturally occurring antimatter is not found in space, we will have to wait decades or even centuries before we can produce significantly large quantities of antimatter on the Earth. But assuming that the technical problems of producing antimatter can be solved, this leaves open the possibility that one day antimatter rockets may take us to the stars.

  Given what we know of antimatter today, and the foreseeable evolution of this technology, I would classify an antimatter rocket ship as a Class I impossibility.

  FOUNDER OF ANTIMATTER

  What is antimatter? It seems strange that nature would double the number of subatomic particles in the universe for no good reason. Nature is usually quite sparing, but now that we know about antimatter, nature seems to be supremely redundant and wasteful. And if antimatter exists, can anti-universes also exist?