Originally Posted by RussT

Because here is how this "Really" goes!!!

The light from the Halfway light source hits both M31 and the Milky Way observers simultameously, setting each observers clock to 0, and right when the beam hits the M31 observer, the SN explodes, so our clock and his clock are BOTH 0 when the SN explodes, and Then it takes 2.5 million light years for the light from the SN to reach us. No fifth grader remark needed...