Half-Life
The time it takes for half of a quantity of a radioactive isotope to decay — and, metaphorically, the time it takes for anything to lose half its potency.
Origin
The concept comes from Ernest Rutherford's early-20th-century work on radioactive decay. Each radioactive isotope has a characteristic half-life: carbon-14 is 5,730 years (the basis of radiocarbon dating); plutonium-239 is 24,100 years (a problem for nuclear waste); a free neutron is about 10 minutes. The same math applies to anything that decays exponentially: drug levels in the body, advertising effectiveness, the half-life of a meme.
Modern usage
'The half-life of [X]' is a standard way to talk about how fast information, attention, or relevance fades. 'A news cycle has a half-life of about 36 hours.' 'The half-life of facts' is the title of a 2012 Samuel Arbesman book. Also the title of one of the most influential video games of the 1990s.
In the wild
Outrage tweets have a half-life of about three hours.— common usage
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