  In the usual formulation of quantum theory, time is a global classical
evolution parameter, not a local quantum observable. On the other hand, both
canonical quantum gravity (which lacks fundamental time-evolution parameter)
and the principle of spacetime covariance (which insists that time should be
treated on an equal footing with space) suggest that quantum theory should be
slightly reformulated, in a manner that promotes time to a local observable.
Such a reformulated quantum theory is unitary in a more general sense than the
usual quantum theory. In particular, this promotes the non-unitary Hawking
radiation to a unitary phenomenon, which avoids the black-hole information
paradox.
