Quantum foam

It is possible that instead, in a quantum theory of gravity, spacetime would consist of many small, ever-changing regions in which space and time are not definite, but fluctuate in a foam-like manner.

[6] Subsequent experiments were, however, unable to confirm the supposed variation on the speed of light due to graininess of space.

[7][8] Other experiments involving the polarization of light from distant gamma ray bursts have also produced contradictory results.

But observations of radiation from nearby quasars by Floyd Stecker of NASA's Goddard Space Flight Center failed to find evidence of violation of Lorentz invariance.

X-ray and gamma-ray observations of quasars using NASA's Chandra X-ray Observatory, the Fermi Gamma-ray Space Telescope and ground-based gamma-ray observations from the Very Energetic Radiation Imaging Telescope Array (VERITAS) showed no detectable degradation at the farthest observed distances, implying that spacetime is smooth at least down to distances 1000 times smaller than the nucleus of a hydrogen atom,[14][15][16][17][18] setting a bound on the size of quantum fluctuations of spacetime.