Gamma-ray burst provides test of foamy universe quantum gravity theory

Ars Technica : In 1955 John Wheeler proposed that at the minuscule Planck scale , the universe was “foamy"—that is, quantum fluctuations would cause tiny, imperceptible variations in spacetime. One result of spacetime foam is that two photons emitted at the same time from the same source would be affected by different variations and would not arrive at the same place simultaneously. Tsvi Piran of the Hebrew University of Jerusalem and his colleagues recently used the Fermi Gamma-Ray Telescope to examine a gamma-ray burst for evidence of any such alterations in photon travel time. Because the spread of arrival times is predicted to be stronger at higher energies, the researchers were looking for changes in the brightness over time across the burst’s spectrum. Their observations revealed that there were not enough high-energy photons to suggest that any dispersion was taking place above the Planck scale. That puts even further constraints on possible quantum gravity theories that predict effects on scales larger than the Planck length.