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Physicists have made the first precise measurements of the properties of antihydrogen – an atom consisting of an antiproton and positron. The observations match those of an ordinary hydrogen atom. Although this was what theorists had predicted, it leaves unexplained one of physics' greatest mysteries, which we'd have been on the way to solving if the results had been different.
In most ways, antimatter is like a mirror image of matter, with the same mass but opposite electric charges and some other fundamental aspects, a feature known as symmetry. We've studied antimatter produced in particle accelerators and by cosmic rays, but since an antimatter particle annihilates on encountering matter, they don't usually last very long, so measurements of how different antimatter particles interact with each other have been sparse.
Professor Jeffrey Hangst of Aarhus University and colleagues have started addressing this, observing the radiation released when a positron in antihydrogen changes energy levels. In Nature, they report the energy levels are within 2 percent of those in hydrogen if there is any difference at all. The measurements cover what is known as the Lamb shift, a difference in the energy levels of two excited electron states, which were once expected to be identical. The Lamb shift's discovery in 1947 inspired the development of quantum electrodynamics.
One of the hardest things to explain about the universe is why there is so much matter, and so little antimatter. Models of the Big Bang suggest equal amounts of each should have been created, leaving the obvious question of where all the antimatter went.
Physicists believe the near-absence of antimatter today indicates symmetry can't be perfect – in some way we don't yet understand why the properties of antimatter don't exactly reflect those of matter. It was hoped studying the transitions between energy levels for a proton might give a hint of these deviations.
A universe with equal amounts of matter and antimatter would not be a safe place to have a planet, if one could even form while constantly threatened by annihilation. Scientists, however, find “it's necessary for life” a very unsatisfying explanation for why the universe has the composition it does. So far, however, we haven't found anything better.
Hangst's methods demonstrate the difficulties of working with antimatter above the subatomic scale. To produce just 20 antihydrogens to study he had to trap 90,000 antiprotons at temperatures of less than 1 Kelvin and drift 3 million positrons past. The products were then zapped with 72,000 laser pulses at 12 different frequencies before being allowed to escape to annihilate themselves by running into surrounding matter. By counting the particles given off in the annihilation events for each different laser frequency Hangst inferred the size of the antimatter energy transitions.
Three years ago Hangst's team were the first to observe antihydrogen's light spectrum.
