  Quantum or torsion gravity models predict unusual properties of space-time at
very short distances. In particular, near the Planck length, around 1E-35 m,
empty space may behave as a crystal, singly or doubly refractive. However, this
hypothesis remains uncheckable for any direct measurement since the smallest
distance accessible in experiment is about 1E-19 m at the LHC. Here I propose a
laboratory test to measure the space refractivity and birefringence induced by
gravity. A sensitivity from 1E-31m down to the Planck length could be reached
at existent GeV and future TeV energy lepton accelerators using laser Compton
scattering. There are already experimental hints for gravity signature at
distances approaching the Planck length by 5-7 orders of magnitude, derived
from SLC and HERA data.
