page_title: Positronium
section_title: History
hierarchical_section_title: Positronium / History
caption_attribution_description: English: This photo shows the Positronium Beam at University College London
context_page_description: Positronium is a system consisting of an electron and its anti-particle, a positron, bound together into an exotic atom, specifically an onium. The system is unstable: the two particles annihilate each other to predominantly produce two or three gamma-rays, depending on the relative spin states. The energy levels of the two particles are similar to that of the hydrogen atom. However, because of the reduced mass, the frequencies of the spectral lines are less than half of those for the corresponding hydrogen lines.
context_section_description: Stjepan Mohorovičić predicted the existence of positronium in a 1934 article published in Astronomische Nachrichten, in which he called it the "electrum". Other sources credit Carl Anderson as having predicted its existence in 1932 while at Caltech. It was experimentally discovered by Martin Deutsch at MIT in 1951 and became known as positronium.  Many subsequent experiments have precisely measured its properties and verified predictions of quantum electrodynamics.  There was a discrepancy known as the ortho-positronium lifetime puzzle that persisted for some time, but was eventually resolved with further calculations and measurements. Measurements were in error because of the lifetime measurement of unthermalised positronium, which was only produced at a small rate. This had yielded lifetimes that were too long. Also calculations using relativistic quantum electrodynamics are difficult to perform, so they had been done to only the first order.  Corrections that involved higher orders were then calculated in a non-relativistic quantum electrodynamics.
caption: The Positronium Beam at University College London, a lab used to study the properties of positronium
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