But not all of them have been, so the study goes on.
Scientists came up with a notion to look at photons that were about 7 billion years old, having left a galaxy that many years ago at about the same time.
They wanted to see if they travelled at the same speed:
This strategy led Guiriec and his collaborators to examine photons generated by a gamma-ray burst that erupted in a galaxy 7.3 billion light-years from Earth. Recorded by the Fermi Gamma-ray Space Telescope on May 10, 2009, the burst produced an assortment of gamma-ray photons, including one with an energy of 31 billion electron-volts — about 13 billion times the energy of visible light — and another photon about one-tenth as energetic.(Science News). We had posted an article discussing one of the quantum gravity hypotheses which talked about the Lorentz invariance which allows that gravity can exceed the speed of light:
Although the two photons had significantly different energies and journeyed for more than 7 billion light-years, they arrived at the Fermi telescope less than nine-tenths of a second apart. That tiny time difference means that these photons traveled at almost exactly the same speed, just one part in 100 million billion apart, notes study coauthor Peter Michelson of Stanford University. That difference is small enough to suggest that the speed of light is constant regardless of the energy of the photon. The finding therefore rules out any theory of quantum gravity that predicts a large energy-dependent change in velocity, he adds.
Although faster-than-light force propagation speeds do violate Einstein special relativity (SR), they are in accord with Lorentzian relativity, which has never been experimentally distinguished from SR — at least, not in favor of SR. Indeed, far from upsetting much of current physics, the main changes induced by this new perspective are beneficial to areas where physics has been struggling, such as explaining experimental evidence for non-locality in quantum physics, the dark matter issue in cosmology, and the possible unification of forces.(Gravitate Towards Light Speed). While the gamma ray photon experiment mentioned in the Science News article, linked to above, does clearly support the notion of photons travelling at the uniform speed of light, it still does not answer the question of whether or not the speed of gravity can exceed the speed of light.