The time-energy uncertainty relation

A discussion is presented of the formulations of the time and energy uncertainty principle given by Mandelstam, Tamm, Wigner, and the one proposed recently by the present authors, based on the concept of the equivalent width. In the case of the free particle, all three formulations give essentially the same result, while in the problem of the lifetime-width relation for a decaying state, only the one based on the equivalent width concept is applicable. This is also the case in the energy-measurement process, when it is formulated as a transition produced by an external interaction of finite duration.