Limits on Weak Equivalent Principle Violations using Fermi LAT Data

Gamma-ray bursts (GRBs) may be useful probes of the gravitational equivalence principle on cosmological scales. GRBs are unique probes of the distant universe in general because they vary so rapidly, typically sub-second, and because they are so far away, typically past redshift one. The gravitational equivalence principle states, most generally, that "everything falls the same". In the case of high energy photons in the Fermi range, the weak equivalence principle (WEP) asserts that gravitation should affect them all equally so that they should move along indistinguishable paths across the universe. Previous cosmological limits on the WEP have focused on the gravitational potential of our Milky Way galaxy, but it is shown here that WEP limits from GRB photons moving past distant clusters of galaxies would be substantially stronger. Toward this end, the relative timing of distant Fermi-LAT detected GRBs are used to place preliminary limits on WEP violations, potentially confirming that "everything falls the same" in a new regime -- the distant universe, with new objects -- super-GeV photons, and to a very high tolerance.