Antimatter: the stuff of science fiction

There isn’t much antimatter in the universe, but a team of Canadians is trapping atoms of it
The blue track at the centre represents a trapped antihydrogen. Its annihilation on the trap wall generates pions (golden trio of exiting tracks), which deposit energy on the silicon detectors. This signal is used to retrace the tracks to their point of origin, giving the time and position of the annihilation.
The stuff of science fiction
Chukman So/Alpha; Alpha/CERN

Particle physicist Makoto Fujiwara has been studying antimatter professionally for 12 years. But his interest in the exotic mirror-image of ordinary matter dates back to a Japanese boyhood full of science fiction and pop-sci expository literature. “Experimenting with antimatter means peeking into a missing other side of our universe,” he says. “Nobody, for instance, has ever been able to measure what happens to antimatter in a gravity field. When you drop an apple, it falls down. What would happen to an anti-apple?” Physicists, he says, would expect it to behave exactly like its twin; but until they watch it happen, they can never be certain.

Fujiwara (who is affiliated with the University of Calgary and the national TRIUMF particle-physics consortium) and other Canadians are moving ever closer to planting one of those anti-apple trees. There isn’t much antimatter around in our universe. Tiny amounts are being created all the time by cosmic-ray and radioactive-decay processes, but when antimatter comes into contact with ordinary “positive” matter, both are annihilated instantly. Beams of antimatter particles can be created in a vacuum, however, and the ALPHA Collaboration, an international team that includes the Canadians, has been working on the next step: combining those particles into atoms and molecules of actual antistuff, and magnetically trapping that stuff long enough to study it.

The natural starting point is with the simplest of the elements: hydrogen. A hydrogen molecule has just one electron and one proton, and so an antihydrogen atom can be made from one anti-electron (a “positron”) and one antiproton. These particles are exotic little beasts, but if you have ever had a PET scan you have already benefited from the routine use of antimatter. Positrons are what the “P” stands for.

Atoms are less easy to handle. Last year, ALPHA researchers at CERN in Geneva created the first significant amounts of antihydrogen ever observed. But combining antiparticles generated by radioactivity produces such hot, energetic antihydrogen atoms that it is hard to keep them for very long from colliding with the walls of their magnetic “bottle” and annihilating. Last year’s experiments created antihydrogen that remained in existence for only one-sixth of a second. Now, after developing superior cooling techniques, Fujiwara and his colleagues have been able to keep some antihydrogen around for 1,000 seconds.

The notion of a “negative” or “mirror” type of matter originated in the late 1920s, when developments in quantum physics led P.A.M. Dirac and others to postulate the existence of opposite-charged partners of the fundamental particles then known. The positron was observed by experimenters almost immediately; it was just two years between Dirac’s inspired guess and his physics Nobel. The discovery raised the question why, if there are two symmetrical kinds of matter, our cosmic neighbourhood is overwhelmingly made up of just one. Where did all the antimatter go?

Little progress has been made toward an answer. But now, thanks to ALPHA’s atom-trapping, experiments on antimatter atoms and molecules will be possible for the first time. Physicists will be able to test the assumption that antimatter is like matter in every way except for the reversed electrical charges.

Topping the agenda will be a comparison of antihydrogen’s absorption spectrum with regular hydrogen’s. Every element absorbs certain characteristic wavelengths of light, which is what enables astronomers to determine the chemical composition of the stars. One team will be using lasers to measure antihydrogen’s absorption of visible-light wavelengths, and another, led by pioneering spectroscopist Walter Hardy of the University of British Columbia, will be bouncing microwaves off of it. “We’re doing spectroscopy first because it is the thing we can do most accurately,” says Hardy. Even an infinitesimal difference between matter and antimatter would have major consequences for physics and cosmology.

Hardy praises the “patience and skilled detective work” of Fujiwara’s team, whose tuning of the finicky antimatter trap improved its performance by almost four orders of magnitude within a year. For Fujiwara, fundamental physics comes first. But there’s a reason antimatter is a popular choice for weapons and propulsion systems in science fiction; matter-antimatter collisions transform mass into energy even more efficiently than nuclear reactions. “The ultimate technological applications are far away,” says Fujiwara, “but that doesn’t stop me from thinking about them occasionally.”