Abstract
The theory of line width due to dissipative interactions, such as collisions, is examined for the case of weakly coupled systems (weak collisions). It is shown that the apparent paradox that the width of the energy of the total system is of the order of Avogadro's number is resolved by a substraction procedure. The width of a spectral line is the difference in widths of the two macroscopic states before and after the transition. This should be contrasted to the case of natural line width where the width of the line is the sum of the widths of the two states involved. The physical reason for the difference in sign is discussed.