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Physicists tune the laser for the ALPHA antihydrogen experiment, in the background.

Maximilien Brice/CERN

Atoms and antiatoms haven’t crashed Albert Einstein’s theory of relativity—yet

As any Star Trek fan knows, antimatter is supposed to be the exact opposite of matter—so that if the two touch they annihilate each other in a flash of pure energy. Now, after decades of trying, physicists have precisely compared atoms and antiatoms. The two appear to behave the same way to within a tiny uncertainty, and in a convoluted way the result supports the foundation of Albert Einstein's theory of special relativity. It also opens the way to more stringent comparisons of matter-antimatter—and the possibility that the two aren't exact opposites.

"We've been waiting for this for 30 years," says Thomas Udem, an experimental physicist at the Max Planck Institute for Quantum Optics in Garching, Germany, who works on precision measurements of hydrogen. "I consider it an incredible achievement." The measurement is "a piece of art," says Stefan Ulmer, an experimenter at Japan's RIKEN research institute in Wako who also was not involved in the new work.

An antihydrogen atom consists of an antielectron (or positron) bound to an antiproton. Since 2002, a few groups have been studying antihydrogen at the European particle physics laboratory CERN near Geneva, Switzerland, the world's only major source of antiprotons. Such work won't lead to anything like Star Trek’s warp drive, but it may enable physicists to determine whether hydrogen and antihydrogen atoms have exactly the same masses, spins, and other basic properties.

If not, the result would spoil physicists’ standard model of fundamental particles and forces. The theory possesses a kind of mathematical symmetry that requires particles and antiparticles to be mirror opposites. Ruin that symmetry and, working back through the theory, the implications would overturn the central premise of Einstein’s special theory—essentially the concept that there's no way to tell if you're stationary or moving relative to the universe. Thus, any difference between matter and antimatter would require a rethink of all modern physics.

One key test of matter-antimatter symmetry is to compare the frequencies of light absorbed by hydrogen and antihydrogen atoms. According to quantum mechanics, an atom can only absorb a photon of particular energies and colors as the electron within the atom hops from a lower energy state to a higher energy state. According to the standard model, hydrogen and antihydrogen ought to have the exact same states and absorb photons of the exact same energies.

Now, Jeffrey Hangst, an experimental physicist at Aarhus University in Denmark, and his 48 colleagues at the ALPHA collaboration at CERN have precisely measured the energy difference between antihydrogen's lowest energy state, called the 1S, and a higher energy state known as the 2S, by far the most precisely measured transition in ordinary hydrogen.

Were the experimenters working with ordinary hydrogen atoms, they could have used laser light to boost atoms to their 2S state and then tickle them with an electric field to make them fluoresce. Tuning the laser frequency to maximize the fluorescence would then home in on the exact transition energy. This is how the 1S-2S transition in hydrogen has been measured to four parts in a quintillion, about 1000 times more precise than the new antihydrogen result, Udem says.

However, that method won't work for antihydrogen because ALPHA researchers typically trap about 40 atoms per trial, too few to produce detectable fluorescence. So they rely on another scheme. Thanks to the quirks of quantum theory, to make the 1S-2S jump, antihydrogen (or hydrogen) has to absorb two photons with half the energy required for the 1S-2S transition. When excited, an atom can absorb a third photon, which strips away its positron entirely. The antiproton then floats out of the trap and into a surrounding array of particle detectors, in which it is annihilated and produces a subatomic blast. By counting the escaping antiprotons, researchers estimated how many atoms they were exciting.

Last year, the ALPHA researchers reported their first observation of the 1S-2S transition in antihydrogen. Now, they've shown that it matches hydrogen's to within two parts in a trillion. The precise shape of the absorption line also matches that seen in hydrogen, as the researchers report today in Nature. "We've actually done laser spectroscopy in antihydrogen," Hangst says. "This has been a life-long goal."

The experiment appears to tighten limits on possible violations of relativity by a factor of 10 to 100, says Alan Kostelecky, a theorist at Indiana University in Bloomington. "It's certainly a spectacular result," he says. However, within the standard model, a violation of relativity could manifest itself in many ways, and some are already more tightly limited by other types of experiments, Kostelecky says.

Hangst says the ALPHA team can go further and make its measurement of the 1S-2S transition as precise as the one currently achieved in hydrogen. "It won't be next year, but it won't be 10 years either," he says. As for whether antihydrogen will really be different than hydrogen, most physicists probably consider that a long shot. But it's still worth testing, Ulmer says: "The only way to learn is to look [for new physics] where we never looked before." In one of the simplest comparisons, Hangst and colleagues would like to see whether antihydrogen atoms "fall" upward under Earth's gravity—don't bet on it—a test that Hangst says they might be able to do this year.