Was Zeno Right? Quantum Clocks and the Discreteness in Quantum Gravity: Theoretical and Experimental Implications

  • Date June 28, 2017
  • Hour 3 pm
  • Room GSSI Main Lecture Hall
  • Speaker Luciano Burderi (University of Cagliari)


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 Δt that this clock can measure scales as the inverse of its size Δr. This implies an uncertainty relation between space and time: ΔrΔt ≥ Għc4 where G, ħ, and c are the gravitational constant, the reduced Planck constant, and the speed of light, respectively. We outline and discuss the implications of this uncertainty principle for: a) energy dependent speed of light; b) non-existence of singularities inside the Horizons of Black Holes; c) entropy of Black Holes. Finally, I discuss how several of the proposed models for space-time quantization predict an energy dependent speed for photons. Although the predicted discrepancies with the general speed of light are minuscule, I show how it is possible to detect this intriguing signature of space-time granularity with a new concept of modular observatory for photons in the energy band 10keV – 30 MeV, the Hermes Project (High Energy Rapid Modular Ensemble of Satellites). The observatory consists of a swarm of micro/nano-satellites on low orbits. Sub-microsecond time resolution and wide energy band allow to probe tiny energy dependent delays, expected to be the signature of the granular structure of space-time. Moreover this experiment allows to perform temporal triangulation of high signal to noise impulsive events with positional accuracies of few arcseconds, making this observatory a promising hunter for the elusive electromagnetic counterparts of Gravitational Waves.