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Interstellar travel is just an antimatter of time
Energy from particle annihilation could cut voyages by light years Keay Davidson, Chronicle Science Writer
"Antimatter men" haunted comic books and TV science-fiction shows such as "Lost in Space" in the 1960s. On the TV show, the fictional Professor Robinson encountered little more than a nonsensical "antimatter" version of himself. Now, what used to be pure fantasy has become serious science. Real-life professors and scientists are grappling with real antimatter -- the particle physicists' "mirror image" of ordinary matter -- in today's laboratories. Antimatter might have practical uses, too, visionaries claim. Possibilities include antimatter-powered robotic aircraft that could remain aloft for months to provide military and weather surveillance. Antimatter beams could blast cancer tumors. Antimatter emitters could detect chemical weapons. Even more far out, antimatter-powered space cruisers could zip from Earth to Mars far quicker than conventional spacecraft -- perhaps even getting to the nearest star system in a few decades. If you saw a chunk of antimatter, it would look and weigh like ordinary matter -- just like the matter in your body, car, cat and toaster. It would take a close look at the fundamental particles to tell the key difference: In ordinary matter, the negative electric charge is carried by particles called electrons, while the positive core of the atom is the proton. So just as Tweedledee's counterpart is Tweedledum, electrons and protons have their antimatter counterparts, known as antielectrons, also called positrons, and antiprotons. One fundamental difference between matter and antimatter is that their subatomic building blocks carry opposite electric charges. For example, while an ordinary electron is negatively charged, an antielectron is positively charged (hence positrons, for "positive electrons"). And while an ordinary proton is positively charged, an antiproton is negative. Before antimatter was discovered, its existence was predicted by the physicist Paul Dirac in the late 1920s. A few years later, Cal Tech physicist Carl Anderson vindicated Dirac's forecast by spotting the first known antimatter, namely positrons (antielectrons) in the form of cosmic rays from outer space that shot through a lab detector. Antiprotons were discovered by Berkeley physicists in the 1950s, while they were firing protons through the Bevatron accelerator. If matter and antimatter collide, they "annihilate" each other, exploding into pure energy -- far more energy than would be extracted by any other known process, even nuclear fission or fusion. If matter and antimatter could be exposed to each other slowly, the reaction could theoretically be contained and used to power vehicles. Possible examples include an aircraft able to circle the globe many times on a microscopic "tank" of antimatter, or a starship able to fly to Alpha Centauri, a three-star system more than 20 trillion miles away, within a few decades. Meanwhile, two startup companies are investigating the possibility of transforming antimatter research from a "pure science" into one that pays off in terms of dollars, national security and medical care. While acknowledging a certain "giggle factor," Gerald A. Smith, head of Positronics Research LLC in Santa Fe, N.M., said it might be possible to develop practical uses for antimatter. Smith's firm is concentrating on nonspace applications, such as antimatter engines for military robotic aircraft and remote sensors for detecting terrorist weapons. The former chair of physics at Penn State University, Smith started the company in 2001; it has five employees and six consultants, including Ph.D.s and technicians. The other firm is Hbar Technologies LLC of West Chicago, Ill., a two- physicist company run by Smith's former colleague, the physicist Steve Howe of Los Alamos National Laboratory. Largely funded by NASA, Hbar is devoting special attention to possible spaceflight uses of antimatter. ("Hbar" alludes to the physics symbol for an "antihydrogen" atom: the letter H with a bar over it.) Among Howe's dreams is the development of ways to blast cancer tumors with antiprotons. Because antiprotons explode upon hitting ordinary atoms, an antiproton beam could explode tumor cells more effectively than existing medical therapies, he speculates. As for the sensing devices, Smith envisions a soldier carrying a portable device that emits positrons, which bathe a suspect chemical agent -- say, a cloud of nerve gas. In response, the gas molecules would emit a gamma-ray "signature" that is characteristic of the nerve gas molecule, thus allowing its dangerous nature to be identified. In the meantime, antimatter is the star enigma in one of the longest- running puzzles of modern astrophysics. According to physical theory, the Big Bang, some 13 billion to 14 billion years ago, should have generated equal amounts of matter and antimatter. Yet according to astrophysicists, the cosmos consists almost entirely of matter. Where's all the antimatter? In hopes of finding the answer, an international consortium of about 600 scientists, including researchers at the Stanford Linear Accelerator Center in Menlo Park, has been running for several years something known as the "BaBar" experiment. BaBar is laboratory shorthand for "B and B-bar," technical terms for the matter and antimatter versions of particles called "B-mesons." Naturally, the project's pet symbol is Babar, the cartoon elephant. For almost a half-century, scientists have theorized that nature contains built-in tendencies to generate less antimatter than matter. To test this hypothesis, experimenters have collided particles inside a SLAC accelerator. The collisions generate matter and antimatter particles called, respectively, B-mesons and anti-B-mesons. These quickly decay into lighter particles, such as kaons and pions. Last week, the physicists announced finding a "large difference" between the decay processes in B-mesons and anti-B-mesons: 910 instances of the B- meson decaying to a kaon and a pion, compared with only 696 instances of anti- B-meson decay. If verified, this finding reinforces the suspicion that when it comes to particle creation, nature plays with loaded dice: There is more matter. Antimatter poses an obvious challenge: How do you store it? Since matter and antimatter annihilate each other on contact, how can antimatter be stored in an ordinary container such as a fuel tank for an ultralight aircraft or starship? It can't, of course. Both firms are developing antimatter "traps" made not of matter but, rather, of electromagnetic fields inside extremely low- pressure vacuums. The invisible electromagnetic fields are nonmaterial; hence, they can "hang on" to antimatter particles without destroying them. The trick is to build an electromagnetic field that is strong enough, reliable enough and long-lasting enough (for months or years) to hang onto zillions of antimatter particles without them escaping and annihilating nearby matter. Smith's firm is trying to develop such a trap for positrons, and Howe's for antiprotons, but it's hard: Particles with like electric charges repel each other "like angry bees," and quickly squirt out of the container, Smith notes. Smith is studying the possibility of positron-powered ultralight robotic aircraft under contract to the Air Force. So far, he says, he has received a total of $3.7 million from the Air Force Research Laboratory Munitions Directorate, based at Eglin Air Force Base near Valparaiso, Fla. In theory, these aircraft could be used for remote TV surveillance of enemy forces or other military purposes. A millionth of a gram of antimatter is so small "you couldn't even see it on the head of the pin," Smith says. Yet it contains enough energy to propel an ultralight robotic aircraft "around the world three times without refueling. " Because of the safety issue, antimatter engines on such aircraft should be shielded with a thin layer of lead, he adds. Meanwhile, the stars await. "Matter/antimatter annihilation represents the 'ultimate' source of stored energy for space propulsion," says a November 2002 report from NASA's Marshall Space Flight Center in Alabama. Today's rockets use chemical fuel, which is too weak and heavy to support an interstellar mission. The nearest stars are more than 20 trillion miles away; a trip by chemical rocket would take thousands of years. By contrast, antimatter engines would accelerate so fast that the mission would be much shorter. Howe and his colleagues have calculated that with 17 grams of antimatter -- barely enough to hold in your hand -- a robotic space probe could get to Alpha Centauri in 40 years. To get there in a decade, the rocket would need at least four times as much antimatter. "Interstellar flight requires quantities of antiprotons that we can't even imagine producing at this point," acknowledges Howe, whose firm is largely funded by the NASA Institute of Advanced Concepts. But time might change everything; he notes that in the early 1940s, there were only "micrograms of enriched uranium (for nuclear bombs) available to the world. "At that time, if you said you'd need a ton of it, it would have seemed impossible. But nowadays, we have so many tons of it, we've quit making it." 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