by Les Johnson
“He’s stolen your job,” said Gregorian. “He’s made himself navigator.”
Nikki turned toward him.
Waving his free hand as nonchalantly as he could, Ignatiev said, “We’re maneuvering through the hydrogen clouds, avoiding the areas of low density.”
“He’s using the pulsars for navigation fixes,” Gregorian explained. He actually seemed to be impressed.
“Of course!” Nikki exclaimed. “How clever of you, Alex.”
Ignatiev felt his face redden.
The rest of the crew rose to their feet as they neared the table.
“Dr. Ignatiev,” said the redheaded engineer, in a tone of respect, admiration.
Nikki beamed at Ignatiev. He made himself smile back at her. So she’s in love with Gregorian, he thought. There’s nothing to be done about that.
The display screen above the table where the crew had gathered showed the optical telescope’s view of the star field outside. Ignatiev thought it might be his imagination, but the ruddy dot of Gliese 581 seemed a little larger to him.
We’re on our way to you, he said silently to the star. We’ll get there in good time. Then he thought of the consternation that would strike the mission controllers in about six years, when they found out that the ship had changed course.
Consternation? he thought. They’ll panic! I’ll have to send them a full report before they start having strokes.
He chuckled at the thought.
“What’s funny?” Nikki asked.
Ignatiev shook his head. “I’m just happy that we all made it through and we’re on our way to our destination.”
“Thanks to you,” she said.
Before he could think of a reply, Gregorian raised his glass of amber liquor over his head and bellowed, “To Dr. Alexander Alexandrovich Ignatiev. The man who saved our lives.”
“The man who steers across the stars,” added one of the biologists.
They all cheered.
Ignatiev basked in the glow.
They’re children, he said to himself. Only children.
But they’re my children. Each and every one of them. The idea startled him. And he felt strangely pleased.
He looked past their admiring gazes to the display screen and the pinpoints of stars staring steadily back at him. An emission nebula gleamed off in one corner of the view. He felt a thrill that he hadn’t experienced in many, many years.
It’s beautiful, Ignatiev thought. The universe is so unbelievably, so heart-brimmingly beautiful: mysterious, challenging, endlessly full of wonders.
There’s so much to learn, he thought. So much to explore. He smiled at the youngsters crowding around him. I have some good years left. I’ll spend them well.
ANTIMATTER
STARSHIPS
Dr. Gregory Matloff
Dr. Greg Matloff is a leading expert in possibilities for interstellar propulsion. He recently retired from his position as a tenured astronomy professor with the physics department of New York City College of Technology, CUNY. He has served as a consultant for NASA, a Hayden Associate of the American Museum of Natural History, a Fellow of the British Interplanetary Society, and a Corresponding Member of the International Academy of Astronautics.
Greg coauthored with Les Johnson and C Bangs Living Off the Land in Space, the monograph Deep-Space Probes, and he wrote The Starflight Handbook in collaboration with Eugene Mallove (1989). His papers on interstellar travel and methods of protecting Earth from asteroid impacts were published in The Journal of the British Interplanetary Society, Acta Astronautica, Spaceflight, Space Technology, The Journal of Astronautical Sciences, and Mercury. In 1998, he won a $5,000 prize in the international essay contest on Extraterrestrial Intelligence sponsored by the National Institute for Discovery Science.
In this, the first of his two essays for this anthology, Greg explains the fundamental physics of antimatter propulsion. Yes, antimatter is real but its use will be challenging indeed. . . .
***
Most readers of this book have heard of antimatter. Because of that fictional engineer Scotty on the Starship Enterprise in the original Star Trek, most readers know that it is both exceptionally energetic and very difficult to store. Scotty, in fact, spends a great deal of time trying to maintain the stability of the ship’s antimatter “core” and making sure that the stuff does not come in contact with the walls of the core’s containment vessel, which is composed of ordinary matter. If he were to fail in this endeavor, the ship would immediately explode and be visible across the galaxy as a miniature, short-lived supernova.
If you’ve read Dan Brown’s thriller Angels and Demons or seen the Hollywood movie version, you know that this material can be produced in nuclear accelerators such as the Large Hadron Collider in the CERN, located on the French-Swiss border. And you know that in the wrong hands, even a tiny quantity of antimatter could be used to commit terrorist acts such as blowing up the Vatican.
But what is this stuff? How do we know about it? Does it exist in nature? How can we produce and store it? And, how effective might it be in propelling an interstellar spacecraft?
Early Antimatter History
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 Se
gre, 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.
Antimatter in the Early Cosmos
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.
The Antimatter-Matter Interaction
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.
Antimatter Factories
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.
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 cl
oser 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!
