  Positronium is the simplest bound state, built of an electron and a positron.
Studies of positronium in vacuum and its decays in medium tell us about Quantum
Electrodynamics, QED, and about the structure of matter and biological
processes of living organisms at the nanoscale, respectively. Spectroscopic
measurements constrain our understanding of QED bound state theory. Searches
for rare decays and measurements of the effect of gravitation on positronium
are used to look for new physics phenomena. In biological materials positronium
decays are sensitive to the inter- and intra-molecular structure and to the
metabolism of living organisms ranging from single cells to human beings. This
leads to new ideas of positronium imaging in medicine using the fact that
during positron emission tomography (PET) as much as 40% of positron
annihilation occurs through the production of positronium atoms inside the
patient's body. A new generation of the high sensitivity and multi-photon
total-body PET systems opens perspectives for clinical applications of
positronium as a biomarker of tissue pathology and the degree of tissue
oxidation.
