Going Interstellar (collection)

ANTIMATTER STARSHIPS Dr. Gregory Matloff

Antimatter belongs to a mirror world. The anti-electron or positron, for example, has the same mass as the electron but an opposite (positive) electrical charge. Because their electric charges are opposite and opposite charges attract, electrons and positrons attract each other. When they touch, they mutually annihilate one another and their energy appears in the form of a gamma-ray photon.

It was the British physicist Paul A. M. Dirac who predicted in the 1930s that such a mirror world would exist. In his development of a relativistic theory of the electron, Dirac may have been the first to realize that the vacuum is far from empty.

The concept of a dynamic vacuum is hard to swallow by most people schooled in classical physics. After all, we are all taught in secondary school that a perfect vacuum is totally empty—devoid of all matter. And everyone who has followed extra-vehicular activity in space or seen the science fiction movie 2001: A Space Odyssey, knows how quickly a human astronaut would die if exposed without a spacesuit to the hard vacuum of interplanetary space.

But Dirac chose to view the universal vacuum on the tiny scales of quantum mechanics. In very small portions of space and on infinitesimal time intervals, a better model for the vacuum is the dynamic sea. Think of an ocean wave—the peak of the wave corresponding to a positive vacuum energy state and the trough analogous to a negative vacuum energy state. In Dirac’s theory, every sub-atomic particle in the “positive-energy” universe that we inhabit has a “negative-energy” analog. The negative-energy analog of the electron (also considered a “hole” in Dirac’s “sea”) is the positron. When the two meet, the result is a neutral state corresponding to calm water in the ocean.

Science, unlike deductive philosophy, requires experimental or observational confirmation of brilliant theoretical ideas. It was the American physicist Carl Anderson, working at Caltech, who discovered the track of a positively charged electron in cloud chamber photographs of cosmic ray tracks in 1932. For this discovery, which was confirmed by others, Anderson shared the 1936 Nobel Prize in Physics.

Positrons actually can be found in other places. For example, they are produced when carbon-11 naturally decays into boron-11. But there are no known radioactive decay schemes that release the positron’s big brother, the antiproton.

Because protons and antiprotons are almost two thousand times as massive as electrons and positrons, a more energetic strategy was required to search for the antiprotons. The instrument used to do the trick was the 6.5 billion electron volt proton accelerator called the Bevatron at the Lawrence Radiation Lab, which was at University of California at Berkeley.

Antiprotons were initially produced by bombarding a stationary target with a high-energy proton beam accelerated by the Bevatron. The discovery was announced in the November 1, 1955 issue of Physical Review Letters by Owen Chamberlain, Emilio Segre, Clyde Wiegand and Thomas Ypsilantis. Chamberlain and Segre shared the 1959 Nobel Prize for this discovery.

It is known today that most or all particles have corresponding antiparticles. This is even true for electrically neutral particles such as the neutron. The antineutron is also electrically neutral, but it has other properties opposite that of the neutron.

Because of the inefficiencies involved in antimatter production, matter-antimatter reactors will almost certainly never be a solution to the energy requirements of our global civilization.

Since antimatter is essentially non-existent on Earth, one might hope that we will someday locate a cosmic repository for it. Unfortunately, since cosmic-ray studies put an upper limit on the universal antimatter/matter ratio under 0.0001, the odds do not look very good for locating such a source.

But this presents us with a cosmological mystery. According to the Big Bang Theory, which is well supported by observational evidence, all of the matter, energy and space/time in our universe originated from a fluctuation in the universal vacuum that somehow became stabilized approximately 13.7 billion years ago.

In this early universe, things were very compact and very hot. Three of the four universal forces—electromagnetic, nuclear strong and nuclear weak—were united in one “super force.” Instead of nucleons, atoms, stars and planets, the early universe’s matter was a soup of tremendously energetic subatomic particles called quarks and gluons.

As things cooled and inflated, the universe went through a number of phase changes. At some point, nucleons such as protons, deuterons and alpha particles were created.

Here is the rub. As these primeval nucleons were created out of the energetic stew of pre-nuclear matter, standard, well-established nuclear physics predicts an exactly equal number of nucleons and anti-nucleons. Many or all of these particles and anti-particles should have been converted into gamma rays as they annihilated each other. In fact, the universe should be absolutely empty as a result of these matter/antimatter annihilation events!

Clearly, this is not the case. Matter exists, but what became of the antimatter? Did the early universe divide during its inflationary phase into a matter-half and an antimatter-half? If so, then why don’t we detect annihilation gamma rays from regions where these two sub-universes come into contact?

The giant black holes that became luminous, quasi-stellar objects and now reside quietly at the centers of spiral galaxies (such as our Milky Way) also evolved in the early universe. Some suggest that in some unknown fashion, a bit more of the universe’s early antimatter fell into these cosmic maws than did normal matter. But no one can suggest a mechanism. If this hypothesis turns out to be correct, though, there are some interesting science-fiction concepts. How might we travel to the huge black holes? And how might we get the antimatter out of them?

Another possibility is that there is a slight asymmetry in the production scheme for matter and antimatter. This scheme might slightly favor the production of normal matter. Experimental evidence for such an asymmetry is sparse. One reason for the development and construction of the Large Hadron Collider at the CERN is to search for such asymmetries. But even this enormous and energetic proton accelerator may not have sufficient energy to duplicate conditions in the very early universe.

It was originally believed that the interaction of a particle and its antiparticle twin would instantaneously result in gamma ray photons. This would not be great for space travel since gamma rays are not easy to deflect. But nature is actually a bit kinder to us in this respect. Yes, gamma rays are the end product. But along the way, many of the intermediate, short-lived particles are electrically charged.

Early antimatter rocket pioneers had no idea regarding the charged-particle decay scheme for matter-antimatter annihilation products. In the early 1950s, the German rocket scientist Eugen Sanger proposed that a spacecraft propelled by the matter-antimatter reaction would be a photon rocket emitting gamma rays. But focusing these gamma rays so that they emerged as an exhaust seemed to be a nearly insurmountable problem. Sanger’s thought experiments centered upon an electron gas that might reflect the gamma rays. But he was never able to solve the problem.

It was a flamboyant and dynamic American physicist and science fiction author, Robert Forward, who brought the charged-particle decay scheme of the proton-antiproton annihilation reaction to the attention of the space propulsion community. An imposing figure, Forward was famous for his colorful vests. Legend has it that he never wore any of his vests more than once!

In 1983, Forward conducted a research effort on alternative propulsion techniques. This was published in a December 1983 report for the United States Air Force Rocket Propulsion Laboratory. According to this report, the immediate products of proton-antiproton annihilation are between three and seven electrically neutral and charged pions. (A pion is one of the many subatomic particles found to comprise the matter around us.)

A magnetic nozzle can be used to focus these electrically charged particles and expel them out the rear of a matter/antimatter rocket as exhaust. A large fraction of the energy produced in the proton/antiproton annihilation is transferred to the kinetic energy of this charged particle exhaust. Although an operational matter/antimatter annihilation rocket will not have the one hundred percent efficiency of Sanger’s photon rocket (probably thirty to fifty percent according to Forward), it will be much more effective than a fission or fusion rocket. And charged particles, even short-lived charged particles, are much easier to handle than gamma rays.

To date, no repositories of antiprotons or anti-hydrogen have been found. But antimatter is routinely produced in nature and also by humans. In this section, we deal with various types of antimatter factories.

First, let’s consider nature’s factories. Then, we will look at antimatter production in our largest existing nuclear accelerators. Finally, we treat antimatter production facilities that might be constructed by a future solar-system wide civilization.

It has been suggested that one source of antiparticles in nature is black holes. The process would work as follows. Protons have a higher mobility than electrons. In the case of a black hole immersed in a tenuous neutral plasma composed of electrons and protons, more protons than electrons might tend to disappear into the event horizon of a cosmic black hole. This would produce a positive charge on the black hole and a large electric field. If the field becomes enormous, a vacuum instability could be produced. This vacuum instability might result in the production of matter/antimatter pairs. It is conceivable that in the early universe, the preferential gathering of protons into black holes and the resulting positive charge on these singularities might have resulted in more negatively-charged antiprotons being absorbed by them than positively-charged protons (since opposite charges attract). But what then happened to the surplus positrons?

Another way that matter/antimatter pairs can theoretically be produced by black holes is Hawking Radiation, named after the world-famous British theoretical physicist. Black holes of all sizes may have been created in an early stage of the universe. As black holes age, they ultimately evaporate with the less massive ones suffering this fate sooner that their more massive compatriots. Primordial black holes of asteroid-planet mass are theoretically evaporating during the current universal epoch. As a black hole evaporates, much of its contained energy is radiated away. Some of this radiation should be converted to matter/antimatter pairs.

Closer to home, it has been noted that even stable, main-sequence stars like our Sun may be antimatter factories. In 2002, satellite observations of solar flares indicated that a large flare may release as much as half a kilogram of antimatter. Apparently, solar flares in some unknown manner sort particles by mass so that many of the antiparticles unexpectedly survive their passage through dense solar layers.

Even closer to home and more surprising are satellite observations of terrestrial lightning discharges. In 2009, it was reported that during its first fourteen months of operation, the NASA Fermi Gamma Ray Space Telescope had detected gamma ray bursts associated with seventeen lightning discharges. The positrons were detected in two of these.

Our most energetic particle accelerators can accelerate sub-atomic electrically charged particles to nearly the speed of light. When these energetic particle beams impact a target, some of the beam energy is converted to particle/antiparticle pairs.

When Robert Forward wrote his US Air Force report on advanced propulsion in 1983, there were three antimatter factories in the world. All were proton accelerators. One was in Russia, another was CERN, and the third was the Tevatron at the Fermi National Accelerator Laboratory near Chicago. None of these machines can be considered “small” by any standard. The Tevatron, for example, has a four mile circumference and is equipped with more than one thousand superconducting magnets operating at temperatures close to absolute zero.

Accelerated protons in the Tevatron circle the ring almost fifty thousand times per second at a peak velocity of 99.99999954 percent the speed of light in vacuum. To protect the surrounding environment from stray radiation, the Tevatron tunnel is 25 feet below ground.

Operating continuously, the Tevatron could produce and temporarily store, at enormous expense, about 1 nanogram per year of antiprotons. If all three of these devices were to be devoted to antimatter production and operated continuously, we might have a gram of the stuff after one hundred million years. We need to do a bit better for star flight!

Huge and imposing as it is, the Tevatron must be considered obsolete when it is compared to its cousin the Large Hadron Collider (LHC) at CERN. The LHC has a radius of over two and half miles and is equipped with 9,300 magnets for beam bending and focusing.

Within the fully operational LHC, particle beams will circulate 11,245 times each second. There will be up to six hundred million particle collisions per second and the best vacuum in the solar system will be maintained within this device.

One of the primary goals of the LHC is to produce, accumulate and store antiprotons. An AOL news item on November 18, 2010 reported that 38 anti-hydrogen atoms have been produced at the CERN by combining decelerated LHC-produced antiprotons with positrons produced by radioactive decay. (An article describing this experiment, by G. B. Andresen et al., is entitled “Trapped Antihydrogen” and was published November 17, 2010 in Nature online). These anti-atoms were stored for a record 0.2 seconds. Thirty-eight anti-atoms is a long way from what we will need to fuel a starship. And 0.2 seconds is a tiny duration compared with the months or years we will require the fuel to be stored. But it’s a good start!

It is very unlikely that a future terrestrial civilization will pepper the Earth’s surface with LHC-sized accelerators. Almost certainly, antimatter factories will be created in interplanetary space rather than on the Earth.

Although humanity has some significant space accomplishments—lunar landings, Mars rovers, a semi-permanent international space station, extra-solar probes—we are a very long way from having an in-space technological infrastructure capable of tapping cosmic energy sources and converting the energy obtained to quantities of antimatter sufficient for interstellar flight.

The possible development of such an off-planet industrial base might follow the model of the Russian astrophysicist Nikolai Kardashev. Kardashev was interested in the aspects of an extraterrestrial civilization that we might detect over interstellar distances. He hypothesized that ET’s cosmic signature would likely depend on his energy level.

Humanity is now probably about 0.7 on the Kardashev scale. When and if our civilization can utilize all the solar energy striking our planet, then we will have advanced to the point where we will be a Kardashev Type I civilization.

If our economies continue to develop at the current pace, in a few thousand years we might evolve into a Kardashev Type II civilization. At that point, we will control the resources of the solar system and be able to tap the Sun’s entire radiant output.

A Type II civilization would have sufficient energy at its disposal to launch starships on a regular basis to a wide variety of galactic destinations. Over a time scale of millions of years, it could entirely occupy its galaxy and be able to tap the energy output of all stars in its home galaxy. Then it will be a Kardashev Type III civilization.

With such enormous energy reserves, intergalactic travel would ultimately develop. If this civilization continues and expands long enough, it could become the ultimate Type IV civilization that occupies the entire universe and can tap all of its energy.

Clearly, a Kardashev Type IV civilization does not (yet) exist in our universe. If it did, we would be, by definition, part of it. If a Kardashev Type III civilization existed in the Milky Way, we would be part of it as well (unless ET was constrained by some moral code such as Star Trek’s Prime Directive from influencing the development of primitive humanity). So the most energetic extraterrestrial civilizations we can hope to detect are expanding Type IIs.

If humanity evolves into a solar-system wide civilization, it could approach the capabilities of a Kardashev Type II civilization. We might be able to accomplish planetary engineering feats throughout the solar system, such as the terraforming of Mars.

But Mars is not the best location for a huge antimatter factory because it is farther from the Sun than the Earth is and receives about half the solar power. A much better location for a planet-wide antimatter factory is Mercury, the innermost world of our solar system.

Mercury is in a rather elliptical solar orbit with an average distance of 0.39 Astronomical Units (forty percent of Earth’s solar distance) from the Sun. This parched and airless world has a radius thirty-eight percent that of the Earth or about two thousand four hundred forty kilometers. Let us assume that the entire surface of Mercury is covered with solar photovoltaic cells. These supply energy to a gigantic version of the LHC with the single task of creating, decelerating and storing antimatter.

At the Earth’s location in the solar system (1 Astronomical Unit or one hundred fifty million kilometers from the Sun), the amount of solar power striking a surface facing the Sun (called the Solar Constant) is about fourteen hundred watts per square meter. Because solar light intensity varies as the inverse square of solar distance, the Solar Constant at Mercury’s average distance from the Sun is about nine thousand watts per square meter.

The solar power striking Mercury is therefore about 1.7 X 10¹⁷ watts, or approximately ten thousand times the total electrical power produced by our global civilization from all sources.

We next assume a twenty percent energy conversion efficiency for the solar cells coating Mercury’s surface. The electrical energy input into the hypothetical antimatter factory constructed on this hot, small planet, is therefore about 3 X 10¹⁶ watts.

If our Mercury antimatter factory works continuously and 4 X10^-5 of the electrical energy input is converted into matter/antimatter pairs (as in the Tevatron), about 5 X 10¹⁸ Joules of energy is converted into antimatter each year. Every year, this antimatter factory will convert about 4 X 10¹⁹ Joules of energy into antiprotons.

Optimistically, we assume that all of these can be collected, decelerated, perhaps neutralized with positrons and safely stored until ready for use in the engines of a starship. The total antiproton annual production mass from this hypothetical antimatter factory can be calculated from a variation of Einstein’s famous equation (E = 2Mc²), where the factor 2 accounts for the fact that half the energy (E, in Joules) is converted into protons, M is the antimatter mass in kilograms and c is the speed of light in vacuum (three hundred million meters per second).

Even then, our hypothetical Mercury-based antimatter factory can produce only about five hundred kilograms of anti-hydrogen atoms. If the factory works continuously for a century, about fifty thousand kilograms of antimatter will be produced. This may be hardly enough for Eugen Sanger’s photon rocket, which requires equal amounts of matter and antimatter. But, as we shall see in the section on antimatter rockets below, an operational spacecraft propelled by antimatter/matter-annihilation may function quite well if antimatter is a very small fraction of the total fuel mass.

It should also be mentioned that it is not necessary that our antimatter factory or factories be located on a planet’s surface. Another location would be free space. Here, a huge parabolic, micron-thin reflector might be used to concentrate and focus solar energy on a bank of efficient, hyper-thin and low mass solar photovoltaic cells. Robert Kennedy, Ken Roy and David Fields have suggested that humans may ultimately construct approximately one thousand-kilometer solar-sail sunshades in space to slightly reduce the amount of sunlight striking the Earth and thereby alleviate global warming. Such in-space devices could also be used to concentrate solar energy on Mars. There is no inherent reason why these sunshades or solar concentrators could not serve a dual function and direct sunlight towards in-space antimatter factories.

Also, as Forward speculates, the antiproton conversion efficiency he quotes for the Tevatron may not be the ultimate. There is plenty of room for improvement if some of humanity’s brightest minds turn their attention to the problems of antimatter production and storage.

No matter where the antimatter is produced, the next challenge is the safe storage of the stuff until we are ready to use it in a starship engine. This is especially difficult since antimatter is the most volatile material in the universe and will disappear in a puff of radiation if brought into contact with normal matter.

As it turns out, there are a number of options. But none of these is especially easy. This section describes some candidate antimatter storage systems.

One possibility is magnetic storage rings. Using combinations of electric and magnetic fields, antiprotons would be spun continuously around one ring at constant velocity, positrons (if necessary) around another. When reaction with normal matter in the starship’s combustion chamber is required, an appropriate mass of antiparticles could be magnetically diverted towards the target without touching chamber walls. Antiparticles have been stored in such a manner after deceleration in existing antimatter factories. But we wonder what the limits are on antiparticle density in the ring. And is it possible to reliably alter field strength in parts of the storage ring as the ship changes its acceleration rate?

Many of the potential solutions to antimatter storage have been reviewed in a paper by the American physicists Steven Howe and Gerald Smith. They describe a version of the Penning trap they constructed at Pennsylvania State University. This device might be able to store one hundred billion antiprotons per cubic centimeter. That sounds like a lot of antiprotons, but a Penning trap at least a kilometer across would be required to store a kilogram of antiprotons!

Forward, in his Air Force report, expresses the opinion that antimatter engineers will store frozen anti-hydrogen rather than antiprotons or an antiproton-positron plasma. A ball of anti-hydrogen with an electric charge could be levitated using electric fields. Care must be taken, though, to adjust the field to compensate for the starship’s acceleration. And some mechanism must be developed to cleanly remove anti-hydrogen atoms from the ice ball and transfer them to the reaction chamber without prematurely and disastrously annihilating them.

The levitated ice ball concept might be workable in the frigid wastes of interstellar space. But frozen anti-hydrogen might be very hard to store in the much hotter environment of a near-Sun antimatter factory.

We are a long way away from being able to produce and store the amounts of antimatter needed for an interstellar voyage.

Antimatter technology is in its infancy. But as it matures, its application to space flight is a natural outcome. Figure 1 presents major features of an antimatter rocket. The payload rides ahead of the fuel tanks. The fuel consists of normal matter (probably hydrogen) and antimatter. Antimatter is fed into an “annihilation chamber” where it reacts with normal matter. An electromagnetic nozzle is used to expel the charged particles as exhaust.

Figure 1. Artist concept of an antimatter rocket. (Image courtesy of NASA.)

Let’s say we desire an interstellar cruise velocity of 0.09c after all the fuel is expelled, which allows a ship to reach Alpha Centauri in about fifty years (not counting the time required for acceleration and deceleration).

If our starship has a mass of about one million kilograms, then it would require twelve thousand eight hundred kilograms of antimatter. The hypothetical Mercury-based antimatter factory discussed in a previous section could produce this mass of antiprotons in about twenty-five years.

Instead of a crewed starship, let’s say we wish to launch a robotic probe with an unfueled mass of one thousand kilograms. In this case, only 12.8 kilograms of antimatter will be required! And if further miniaturization is possible, the antimatter mass required for an interstellar probe can be reduced still further.

We next consider the acceleration process. If the ship requires about 10 years to accelerate an average of about 10⁷ kilograms of matter will be converted into energy each second. The probe generates matter/antimatter annihilation energy at an approximate average rate of 10¹⁰ watts, roughly equivalent to that of a large city. The ship’s generated power level will be about one thousand times greater, approximating that of our entire global civilization! Antimatter propulsion is clearly not for the faint hearted!