= Half-life
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The <half-life> of <radioactive decay>, which as discovered a few years before <quantum mechanics> was discovered and matured, was a major mystery. Why do some nuclei fission in apparently random fashion, while others don't? How is the state of different nuclei different from one another? This is mentioned in <Inward Bound by Abraham Pais (1988)> Chapter 6.e Why a half-life?
The term also sees use in other areas, notably <biology>, where e.g. <RNAs> spontaneously decay as part of the <cell>'s <control system>, see e.g. mentions in <E. Coli Whole Cell Model by Covert Lab>.