Planck-Scale Gravity Test at PETRA

Quantum or torsion gravity models predict unusual properties of space-time at very short distances. In particular, near the Planck length, around 10-35 m, empty space may behave as a crystal, singly or doubly refractive. This hypothesis, however, remains uncheckable for any direct measurement since the smallest distance accessible in experiment is about 10-19 m at the LHC. Here we propose a laboratory test to measure space birefringence or refractivity induced by gravity. A sensitivity 10-31 m for doubly and 10-28 m for singly refractive vacuum could be reached with PETRA 6 GeV beam exploring UV laser Compton scattering.