see the attachment referring to the pic 1 simultaneity was explained to me like this there is a moving box car. At point A there is an observer in the boxcar at rest wrt the box car his name is Dash. There is another observer not at rest wrt the box car at point C his name is Still Bill. Lightening strikes both ends of the boxcar at point X and Z at time To. Point C and A are equidistant from the ends of the boxcar say a distance L Time for photons to reach point C from point X and Z is L/c so Still Bill thinks lightening strikes are simultaneous As the boxcar is moving Dash, who was originally at point A, will some time later, say T1, be located at point B. It is assumed Dash is moving relative to the photons coming from the lightening strikes. Dash will see the photons from lightening strike Z before photons from lightening strike X so Dash will not think strikes are simultaneous. Simple! Can I emphasise that Dash is assumed to be moving relative to the photons from the lightening strikes. now look at picture 2 again moving boxcar person in the box car at rest wrt the box car standing at point A again his name is Dash lightening strikes at To at point A. ie the lightening strike is at the feet of Dash. Again at T1 Dash, originally at point A, will be located at point B. The photons from the lightening strike will move out in a perfect circle. The concentric circles show the photon front moving out from the lightening strike at various times. I have not included the observer not at rest wrt the boxcar, Still Bill, as I don’t care what he sees. Theory says Dash should see a sphere of photons moving away from him in all direction, clearly this is not the case. Dash was originally located at point A . At T1 he will be located at point B. If Dash at T1 when located at point B was to take a tape measure and measure the distance to the wave front of photons clearly he would get different measurements and therefore assume he is NOT in a sphere of photons? Still referring to picture 2 The distance Dash measures to the photon front using his tape measure will not be a function of the velocity of the boxcar it will be a function of his absolute velocity. So if you are in a moving frame of reference at rest wrt a point light source and you turn the light source on at say T0. If after a short delay, at say time T1, you measure the distance to the wave front from the point source of light. If the distance to the wave front is the same in all directions then you can conclude you are absolutely stationary. As you are absolutely stationary the time on your clock is absolute time and the distance you measure is absolute distance.