 In most situations, only macroscopic objects are relevant to our experience. However, today’s technology has reached a point in which interference experiments are performed with single particles. In such situations a description of a world with states of only macroscopic objects, such as sources and detectors, is possible but cumbersome. Hence it is fruitful to add a description of some microscopic objects. Vaidman 2010 argues that the proper way to describe the relevant microscopic particles is by the two-state vector which consists of the usual, forward evolving state specified by the measurement in the past and a backward evolving state specified by the measurement in the future. Such a description provides a simple explanation of the weak trace the particles leave, Vaidman 2013.