The half-life of a substance is the time it takes for it to lose one half of its initial value. In nuclear physics, it’s the time it takes for unstable atoms to lose energy by emitting radiation. In biology, it usually refers to the time it takes to eliminate half of a substance (water, alcohol, pharmaceuticals) from the body. In chemistry, it is the time required to convert one half of a reactant (hydrogen or oxygen, for example) to product (water). In love, it’s the amount of time it takes for lovers to feel half of what they once did…Like hydrogen-7 or lithium-5 or boron-7, love has an infinitesimally small half-life that decays to nothing. And when it’s gone, it’s like it was never there at all.
— Nicola Yoon, The Sun is Also a Star