Antimatter, the fuel of the intergalactic starships of science fiction became a tangible if fleeting reality this month with the announcement that the first atoms of the element anti-hydrogen had been produced last autumn at the European Particle Physics Laboratory (CERN) near Geneva. Sub-atomic particles of antimatter have been around for years, but this is the first time that they have been put together to assemble complete atoms.
Only nine atoms of antimatter were produced, so the results are unlikely to herald an immediate boom in interstellar tourism. However Michael Charlton of University College London says that the results are exciting because antimatter atoms can be used to test the standard model of particle physics- which provides the most complete explanation for the origin of the universe- in a new way, and with the very precise methods of atomic physics.
The standard model predicts that every type of elementary particle has a corresponding antiparticle that is its mirror image and has opposite charge. Each atom of antihydrogen is made from two such particles, the negatively charged antiproton, and the positively charged anti-electron or positron. If it obeys the predictions of the standard model the single positron in an anti-hydrogen atom should be capable of occupying exactly the same energy levels as does the electron in a hydrogen atom.
This can be checked by testing whether anti-hydrogen absorbs and emits light in the same way as does ordinary hydrogen. When the electrons in an atom jump to lower or higher energy levels the atom emits or absorbs energy in the form of light. The wavelength of the light is exactly proportional to the energy difference between the two levels, so the spectrum of light emitted by antihydrogen should be identical to the hydrogen spectrum.
Testing whether the standard model predicts accurately how atoms of antimatter behave is extremely important. A scientific theory can never be proved to be correct- it simply gains acceptance if it predicts correctly how the world behaves. So a theory that makes no testable predictions is worse than one whose predictions are incorrect. Consequently the opportunity to test the standard model in new ways is welcome.
Fortunately only a few atoms of antimatter are needed for the tests. Antiprotons and positrons are relatively easy to produce from beams of high energy particles, radiation or, in the case of positrons, radioactive decay. But attaching a positron to an antiproton to make an atom is more difficult. In the experiments at CERN, a team of physicists from Germany Italy and Switzerland, fired a jet of xenon atoms into a beam of antiprotons moving at close to the speed of light.
Some of the collisions between protons and Xenon atoms produced positrons and electrons. Just occasionally a positron would be produced that was moving at the same speed as one of the protons, and the two would come together. 15 hours of experiments created 9 atoms of antihydrogen, each of which existed for about 30 billionths of a second before being annihilated.
High speed antihydrogen atoms that only last for 30 billionths of a second are not much use for testing the standard model. “The really exciting thing” says Charlton “would be if we could hold antihydrogen and store it.” He expects to do this within a few years by slowing down the antiprotons and positrons before bringing them together. The resulting antihydrogen “should be perfectly stable”- provided it doesn’t come into contact with any matter.
One of the puzzles about antimatter, according to Graham Thompson of Queen Mary & Westfield College in London is that there is so little of it about. The creation of matter and antimatter from energy, and their annihilation to produce energy are symmetrical processes. Matter and antimatter are produced in equal quantities, but according to current theories antimatter decays slightly faster.
The difference is minute- about 1 part in a billion- says Thompson. But the result is a universe that contains almost no antimatter. We can be quite sure of this he says, because even the radiation that arrives from outside our galaxy contains no antiparticles, and no radiation produced by their annihilation. According to Thompson the most exciting work waiting to be done on antimatter is the experiments to confirm the tiny assymmetry that led to its virtual elimination from the universe. “We have a lot of circumstantial evidence” he says “But it is like the difference between knowing that it could happen this way, and tying it into the standard model so that we know that it had to happen.”
However according to Charlton there is one thing that nobody knows about antimatter, and that is how much it weighs. “We know the mass of antiparticles to very high precision” he says, but “gravity isn’t tied into the standard model and we don’t really know how it acts on antimatter.” If his experiments to produce stationary atoms of antihydrogen are successful we could soon find out.