  We present a discussion of the fundamental loss of unitarity that appears in
quantum mechanics due to the use of a physical apparatus to measure time. This
induces a decoherence effect that is independent of any interaction with the
environment and appears in addition to any usual environmental decoherence. The
discussion is framed self consistently and aimed to general physicists. We
derive the modified Schroedinger equation that arises in quantum mechanics with
real clocks and discuss the theoretical and potential experimental implications
of this process of decoherence.
