by Les Johnson
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 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.
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.
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 Rockets
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!
***
Further Reading
Early antimatter history has been discussed in many archival sources. One such is H. A. Boorse and L. Motz, ed., The World of the Atom, Basic Books, NY (1966).
The story of the antiproton is eloquently told by L. Yarris in “The Golden Anniversary of the Antiproton,” Science @ Berkeley Lab (Oct. 27, 2005), http://newscenter.lbl.gov/feature-stories/2005/10/27/
the-golden-anniversary-of-the-antiproton/
For further information regarding possible biomedical antiproton applications, check out L. Gray and T. E. Kalogeropoulos, “Possible Biomedical Applications of Antiproton Beams: Focused Radiation Transfer,” Radiation Research, 97, 246-252 (1984).
Many sources have speculated on possible military applications of antiprotons. Two web references on this topic, both by Andre Gsponer and John-Pierre Hurni, “Antimatter Underestimated,” arXiv:physics/0507139v1 [physics.soc-ph] 19 Jul 2005 and “Antimatter Weapons,” http://cul.unige.ch.isi/sscr/phys/antim-BPP.html
Many astronomy texts consider the early moments of the universe when matter (and antimatter) formed. One readable text, authored by Eric Chaisson and Steve McMillan, is Astronomy Today, 3rd ed., Prentice-Hall, Upper Saddle River, NJ (1999).
Sanger’s photon rocket is described by Eugene Mallove and Gregory Matloff in The Starflight Handbook, Wiley, NY (1989). This book also discusses the decay scheme for the proton-antiproton annihilation reaction.
Robert Forward’s work is reviewed in The Starflight Handbook and other interstellar monographs. His final report to the US Air Force Rocket Propulsion Laboratory is entitled AFRPL TR-83-067, “Alternate Propulsion Energy Sources.” Many of Bob Forward’s ideas regarding antimatter (and a host of other subjects) are also published in a more accessible form: R. Forward, Indistinguishable from Magic, Baen, Riverdale, NY (1995).
Antimatter production by black holes is described by C. Bambi, A. D. Dogov and A. A. Petrov in “Black Holes as Antimatter Factories,” which was published in Sept. 2009 in the Journal of Cosmology and Astroparticle Physics, which is an on-line journal. This paper is also available from a physics archive as arXiv.org/astro-ph>arXiv:086.3440v2.
A NASA web publication, titled “Antimatter Factory on Sun Yields Clues to Solar Explosions,” describes the discovery of gamma rays in solar flares. http://www.nasa.gov/vision/universe/solarsystem/rhessi
_antimatter.html.
To learn more about the surprising discovery of positrons associated with terrestrial lightning discharges, consult R. Cowen, “Signature of Antimatter Detected in Lightning,” www.wired.com/wiredscience/2009//11/antimatter-lightning/.
Information regarding the current capabilities of the Tevatron was obtained from Wikipedia and the Fermilab website. Operational details regarding the Large Hadron Collider are available on the CERN website.
Many books on SETI (the Search for Extraterrestrial Intelligence) deal with the Kardashev scheme for categorizing the capabilities of advanced technological civilizations. A very readable and authoritative one is W. Sullivan’s We Are Not Alone, revised edition, Dutton, NY (1993).
A number of researchers have considered the application of solar-sail technology to the construction of huge planetary sunshades or solar collectors. Analysis by Robert Kennedy, Ken Roy and David Fields is discussed and reviewed by L. Johnson, G. L. Matloff and C Bangs in Paradise Regained: The Regreening of Earth, Springer-Copernicus, NY (2009).
The cited antimatter-storage paper by S. D. Howe and G. A. Smith is entitled “Development of High-Capacity Antimatter Storage.” It was delivered at the Space-Technology and Applications International Forum-2000, University of New Mexico, Albuquerque, NM, July 30-February 3, 2000 and is available on line.
LUCY
Jack McDevitt
Jack McDevitt is a former English teacher (the first of three in this anthology), naval officer, Philadelphia taxi driver, customs officer and a motivational trainer. He is a Nebula Award-winning author and John W. Campbell Memorial Award winner. J
ack also served as one of the editors of this anthology.
In “Lucy,” Jack merges two favorite themes of futurists—artificial intelligence and deep space travel—into a story that actually makes you care deeply about the fate of a sentient computer.
***
“We’ve lost the Coraggio.” Calkin’s voice was frantic. “The damned thing’s gone, Morris.”
When the call came in, I’d been assisting at a simulated program for a lunar reclamation group, answering phones for eleven executives, preparing press releases on the Claymont and Demetrius projects, opening doors and turning on lights for a local high-school tour group, maintaining a cool air flow on what had turned into a surprisingly warm March afternoon, and playing chess with Herman Mills over in Archives. It had been, in other words, a routine day. Until the Director got on the line.
Denny Calkin is a small, narrow man, in every sense of the word. And he has a big voice. He was a political appointment at NASA, and consequently was in over his head. He thought well of himself, of course, and believed he had the answers to everything. On this occasion, though, he verged on hysteria. “Morris, did you hear what I said?” He didn’t wait for an answer. “We’ve lost the Coraggio.”
“How’s that again, Denny? What do you mean, lost the Coraggio?”
“What do you think I mean? Lucy isn’t talking to us anymore. We haven’t a clue where she is or what’s going on out there.”
Morris’s face went absolutely white. “That’s not possible. What are you telling me, Denny?”
“The Eagle Project just went over the cliff, damn it.”
“You have any idea what might be wrong?”
