Antihydrogen Antics
As StarTrek fans know well, the fuel for warp drive is antimatter. No science fiction stuff this—antimatter was predicted by Paul Dirac in 1928. At the time, the only known elementary particles were the positively charged proton and the negatively charged electron. Dirac combined the recently discovered quantum physics with special relativity and produced an equation that seemed to predict two kinds of particles: those with positive energies, as expected, and those with negative energies, which didn’t make any sense. After pondering this result for several years, Dirac realized that these negative energies actually correspond to “antiparticles” of positive energy with the same mass as protons and electrons but with opposite charge. (see The Buzz about Antimatter). More broadly, he theorized that for every particle there should exist a corresponding antiparticle.
Particle tracks in a cloud chamber
Positron-electron annihilation, before and after: Before, a positron and an electron approach each other. After, the two have annihilated into two gamma rays.
As Dirac was working out the significance of his equation, experimentalists were investigating particles of higher and higher energies. In those days there were no machines to accelerate particles, so investigators turned to Mother Nature. Raining down on Earth from all distant parts of the universe are particles called cosmic rays, which hit the atmosphere and set off a shower of secondary particles. A young physicist at Caltech, Carl Anderson, unaware of Dirac’s prediction, observed these particles by studying their tracks as they moved in a magnetic field. (See photo) One day Anderson noticed tracks that seemed to correspond to a positively charged electron. After doing more experiments to confirm this result, he reported in 1932 the existence of the positron, with the same mass as an electron but a positive charge. Only much later, in the 1950s, did physicists find the much more massive antiproton, because its discovery required a powerful particle accelerator.
When particle and antiparticle meet, they mutually annihilate. An electron and positron annihilate into two gamma ray photons (units of radiation), as shown in the diagram, and the gamma rays carry away the mass-energy of the particles according to E = mc2. For a proton and antiproton, annihilation produces four particles called pions. So the signature of antihydrogen annihilation is four pions and a pair of gamma rays, all coming from the same place, and with the right directions and energies.
