We show that the time delay of a scattering process, defined as the difference of interacting and free sojourn times for increasing spatial regions, can only exist for sequences of dilated balls. The transformation properties of the Eisenbud-Wigner formula under translations are discussed.

Martin, Ph.A. & Sassoli de Bianchi, M. (1992). On the definition of time-delay in scattering theory. Helv. Phys. Acta 65, pp. 1119-1126 (1992). Doi: 10.5169/seals-116525.