The quantum clock: a critical discussion on space-time
  We critically discuss the measure of very short time intervals. By means of a "gedankenexperiment", we describe an ideal clock based on the occurrence of completely random events. We show that the minimum time interval Delta t that this clock can measure scales as the inverse of its size Delta r. This implies an uncertainty relation between space and time: Delta r Delta t > G \hbar / c^4, where G, \hbar, and c are the Gravitational constant, the reduced Planck constant, and the speed of light, respectively. We outline and briefly discuss the implications of this uncertainty principle. 
Luciano Burderi, Tiziana Di Salvo
gr-qc hep-th