Suppose that an object explodes in outer space a distance d = 1 × 10^6 km from theEarth. Gamma ray photons with energies between 1 eV and 1 MeV leave the explosion and arrive at a detector on earth some time later. All of the rays arrive at the detector at the same time,within the detector's time resolution of ?t = 10^-9sec. Use this information to derive an upper limit on the photon mass.