When we talk about a particle's lifetime we always mean its average lifetime. A particle that is not absolutely stable has, at every moment of its life, the same chance of decaying. Some particles live longer than others, but the average lifetime is a characteristic of any particle species.

One can also use the concept of ‘half-life’. If we have a large number of identical particles, the half-life is the time it takes for one-half of all the particles to decay. The half-life is 0.693 times the average lifetime.

One glance at Table 1 shows that some particles have a much longer average lifetime than others. The lifetimes differ enormously. A neutron, for example, lives 1013 times longer than a sigma-plus, and the sigma-plus has more than 109 times as long a lifetime as the sigma-zero. But if you observe that the ‘natural’ time scale for an elementary particle (which is the time it takes for their quantum mechanical state, or wave function, to evolve or oscillate) is somewhere around 10−24 seconds, you can safely state that all these particles are pretty stable. In our professional jargon, they are all called ‘stable particles’.

How is a particle's lifetime determined? Particles with long lifetimes, such as the neutron and the muon, have to be captured, preferably in great numbers, after which one registers the decays electronically. Particles with lifetimes around 10−10 to 10−8 seconds used to be registered in a bubble chamber; nowadays this happens more often in a spark chamber.