In a normal hydrogen atom, an electron orbits a nucleus composed of a single proton. The electron has a negative electrical charge, the proton is positive. An antihydrogen atom is the opposite: a positively charged positron orbits a negative antiproton.

According to the Standard Model, the two types of atom are equivalent, like mirror images. They are also incompatible: when matter meets antimatter, they annihilate each other in a burst of energy.

The equivalence of hydrogen and antihydrogen rests on an assumption called CPT invariance. This states that if one were to take any piece of matter and simultaneously reverse all the charges of its elementary particles, the direction of time's flow, and another property of particles called parity, the substance would obey exactly the same laws of physics.

Just as, if one reversed the threads of all the nuts and bolts in a steel bridge, the bridge would hold together just as well, but if one switched only some of the threads, the parts would no longer fit together.

If CPT invariance holds, antihydrogen will behave just like hydrogen. In particular, it will absorb and emit light at identical frequencies. If this spectrum is different, the theory of CPT invariance will crumble.

If antihydrogen does have quirks, it will be immensely exciting. The differences might help to explain why there is more matter than antimatter in the Universe, even though the Big Bang should, in theory, have produced equal amounts of both.

And if antihydrogen responds differently to gravity, it will raise questions about the theory of relativity. It might even point to the long-sought unification of relativity and quantum theory. All of which makes antihydrogen worth looking for.

Elusive stuff

CERN's ATHENA scientists bombard atoms with protons from a particle accelerator to make antiprotons which they catch in a storage ring called the Antiproton Decelerator. This feeds slow-moving antiprotons into a magnetic trap, immobilizing them. Another trap accumulates positrons, which some radioactive materials emit.

The researchers combine these ingredients in a magnetic mixing trap. When some pair up to form antihydrogen, the electrically neutral anti-atoms drift out of the trap, hit the walls, annihilate and produce particles called pions. Pions signal the death of an anti-atom.

Judging from the number of pions, the ATHENA team calculates that their trap holds at least 50,000 anti-atoms, all cooled to within 15 degrees of absolute zero.