GrailQuest and HERMES: hunting for gravitational wave electromagnetic counterparts and probing space-time quantum foam

GrailQuest(Gamma-ray Astronomy International Laboratory for Quantum Exploration of Space-Time) is an ambitiousastrophysical mission concept that uses a fleet of small satellites whose main objective is to search for a dispersion lawfor light propagationin vacuo. Within Quantum Gravity theories, different models for space-time quantization predictrelative discrepancies of the speed of photons w.r.t. the speed of light that depend on the ratio of the photon energy tothe Planck energy. This ratio is as small as 10−23for photons in theγ−ray band (100 keV). Therefore, to detect thiseffect, light must propagate over enormous distances and the experiment must have extraordinary sensitivity. Gamma-RayBursts, occurring at cosmological distances, could be used to detect this tiny signature of space-time granularity. This canbe obtained by coherently combine a huge number of small instruments distributed in space to act as a single detector ofunprecedented effective area. This is the first example of high-energy distributed astronomy: a new concept of modularobservatory of huge overall collecting area consisting in a fleet of small satellites in low orbits, with sub-microsecond timeresolution and wide energy band (keV-MeV). The enormous number of collected photons will allow to effectively searchthese energy dependent delays. Moreover,GrailQuestwill allow to perform temporal triangulation of impulsive events witharc-second positional accuracies: an extraordinary sensitive X-ray/Gamma all-sky monitor crucial for hunting the elusiveelectromagnetic counterparts of Gravitational Waves, that will play a paramount role in the future ofMulti-messengerAstronomy. A pathfinder ofGrailQuestis already under development through theHERMES(High Energy Rapid ModularEnsemble of Satellites) project: a fleet of six 3U cube-sats to be launched by the end of 2022.