**Lifetime** in physics is the

mean time of

*existence* of any member of an assembly of identical physical objects, each instance of which lives for a

duration that follows a given

distribution of which the

mean is the

*lifetime* (sometimes calls for this reason the

*mean lifetime*).

Typically one speaks of lifetime for unstable particles, for which the time elapsed before the disintegration follows the exponential distribution (resulting from the fact that a particle has no *memory* of its past in regard of its lifetime):

with the associated

**decay constant** (cf.

exponential decay). Stated another way,

*P(t)dt* is the infinitesimal probability that the particle disintegrates at time

*t*. Therefore, in an assembly of particles, is the rate of particles decaying at time

*t*. (This result is in decays per time unit.)

Computing the average links the lifetime to the decay constant constant :

The probability for one given atom to disintegrate in any infinitesimal interval of time is .

Another quantity of common use is the half life time , which is the time required for half of the population to disintegrate. It is linked to the lifetime through

## See also