Interfering with time The interference pattern arising from light or particles passing through a double slit is a simple experiment that belies the subtleties of interpretation when attempting to describe and understand the effect. For example, determining “which path” the light or particles travel can result in the interference pattern disappearing. Margalit et al. present a new take on interferometry using time (see the Perspective by Arndt and Brand). A clock—i.e., the internal state of a cold atom condensate—was coherently split and brought back together to interfere. Making one-half of the clock tick at a different rate resulted in a change in the interference pattern, possibly as a consequence of the time being a “which path” witness. Science , this issue p. 1205 ; see also p. 1168