Variance of the Quantum Coordinates of an Event
  We study the variances of the coordinates of an event considered as quantum observables in a Poincare' covariant theory. The starting point is their description in terms of a covariant positive-operator-valued measure on the   Minkowski space-time. Besides the usual uncertainty relations, we find stronger inequalities involving the mass and the centre-of-mass angular momentum of the object that defines the event. We suggest that these inequalities may help to clarify some of the arguments which have been given in favour of a gravitational quantum limit to the accuracy of time and space measurements. 
M. Toller (Department of physics of the University, I. N. F. N.   Trento, Italy.)