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In quantum physics, every subatomic particle has a counterpart called its anti-particle. The anti-particle of an electron is the positron. When a negatively-charged electron and a positively-charged positron come together, there is a chance that they will spin around each other in the same direction – if they do, then this is termed 'ortho-positronium'. If they spin in opposite directions, it is called 'para-positronium'. The electron and positron do this simply because of their mutual electric force, with no intervention from gravity1.
Positronium (ortho- or para-) is very simple in that the structure is similar to that of a hydrogen atom, except that instead of a proton there is a positron. Both forms of positronium can be explained by the theory of quantum electrodynamics (QED)2 with a substantial degree of accuracy.
Positronium can be created artificially in the laboratory. To do this, a low-energy beam of positrons are fired into matter; some of the positrons move slowly enough to ionise the matter, thereby obtaining an electron that begins to orbit around the positron.
QED predicts that ortho-positronium should decay in 1.42*10-10 of a second, releasing three photons of gamma radiation. In 1990 at the University of Michigan at Ann Arbor, scientists tested the lifetime and the experiment was repeated in March 2000 by Gregory Adkins (Franklin and Marshall College in Pennsylvania), Richard Fell (Brandeis University in Massachusetts) and Jonathan Sapirstein (Notre Dame University in Indiana). In both experiments, there was a discrepancy in the lifetime – it was 0.1% too fast. The reason for this discrepancy has not yet been verified, but it may have something to do with mirror matter...