Going Interstellar (9 page)

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 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.

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.