Going Interstellar (10 page)

Natural Antimatter Factories

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.

The Best Existing Human-Constructed Antimatter Factories

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!

Future Antimatter Factories in Sol Space

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 10
¹⁷
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 10
¹⁶
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 10
¹⁸
Joules of energy is converted into antimatter each year. Every year, this antimatter factory will convert about 4 X 10
¹⁹
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 = 2M
c
²
), 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.

How Do We Store Antimatter??—VERY, VERY CAREFULLY!!!

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.