antihydrogen and trapping antimatter
Ordinary hydrogen, the simplest atom, is one electron orbiting one proton. Antihydrogen is its complete antimatter version: a positron orbiting an antiproton. Building it is a remarkable feat, because antimatter annihilates the instant it touches ordinary matter — including the walls of any container. So physicists must make antihydrogen and then hold it in mid-vacuum, touching nothing, long enough to study it. Doing so lets them ask a deep question: does antimatter obey exactly the same physics as matter, or is there a hidden difference?
Making antihydrogen means producing the ingredients separately and coaxing them together gently. At CERN, antiprotons are created in collisions, slowed down dramatically in a decelerator, and cooled to very low energies; positrons are gathered from radioactive sources. The two are merged in a vacuum chamber so cold and empty that some positrons settle into orbit around antiprotons, forming neutral antihydrogen atoms. Because the atoms are electrically neutral, they cannot be held by electric fields; instead experiments use a magnetic trap that grips the atom's tiny magnetic moment, suspending the coldest atoms away from any surface for minutes at a time. Experiments such as ALPHA, ATRAP, and others have trapped antihydrogen and begun measuring its properties.
The payoff is precision tests of the symmetry between matter and antimatter. The CPT theorem of quantum field theory predicts that antihydrogen should have exactly the same spectrum of light-absorption frequencies as ordinary hydrogen; any tiny difference would be a revolutionary crack in our deepest theories. So far the spectral lines match to extraordinary precision, and in 2023 experiments confirmed that antihydrogen falls downward under gravity, just like matter. A common misunderstanding is that antimatter is hard to keep simply because it is rare; the deeper problem is that it self-destructs on contact with ordinary matter, which is why levitating it in a vacuum is the whole game.
CERN's ALPHA experiment traps antihydrogen atoms in a magnetic bottle and shines laser light on them; in 2018 it measured an antihydrogen spectral line and found it matched ordinary hydrogen's to about one part in a trillion.
Comparing antihydrogen with hydrogen tests the deepest symmetry in physics.
Trapped antihydrogen exists only as a handful of atoms at a time held for minutes, not as a usable substance; the idea of antimatter as a bulk fuel or weapon ignores that we can make only vanishingly tiny amounts at enormous energy cost.