I'm revising for an exam on particle physics and understand the lot, however I'd like clarification on the calculation of lifetimes of particles. I understand that particles decaying by the strong interaction last roughly 10^-23s because that is the minimum time that information can cross a nucleus of order a fermi (ie at light speed). Also I know that EM decays take about 10^-16 seconds (like in the pi-zero decay to 2 photons), but I'd like to know why? Also why does the weak decay take about 10^-10s? (give or take a few orders of magnitude!) Thanks.