What does half-life mean?
Half-life is the time required, on average, for half the unstable nuclei in a large sample to decay.
Physical mechanism and core principles
The decay time of one nucleus is unpredictable, but large populations follow stable probabilities. Each half-life leaves one-half, one-quarter and then one-eighth in an exponential decline.
Evidence, applications and limits
Radiometric dating requires a suitable isotope and geological context. Risk depends not only on half-life but also radiation type, activity, amount and exposure route. Quantitative interpretation must state boundary conditions, units and ideal-model assumptions.
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