  In this review we present the problem of time in quantum physics, including a
short history of the problem and the known objections about considering time a
quantum observable. The need to deal with time as an observable is elaborated
through some unresolved problems. The lack of a consistent theory of time is
currently hindering the formulation of a full-fledged theory of quantum
gravity. It is argued that the proposal set forth by several authors of
considering an intrinsic measurement of quantum time, besides having the
conventional external time, is compelling. Recently several suggestions have
been put forward to revive the proposal of Page and Wootters (1983),
elaborating and resolving some of the main ambiguities of the original proposal
and opening new scope for understanding its content. The approach followed in
these new contributions exposes the need to go beyond the limitations enforced
by the conventional approach of quantum physics. The attitude of covariant loop
quantum gravity, in which it is called to completely ignore time, is also
discussed. This review could be a step forward in an endeavour to reform our
outlook of the unification of the theory of relativity and quantum physics by
furnishing the conceptual ground needed for this goal. Intentionally, some
technical details are avoided since we aim to present the approaches to resolve
the problem in a simple way with the clearest possible outlook. These can be
looked up in the original references provided.
