(Offtopic from another thread) So I have a super-duper laser which can emit a very short but extremely powerful pulse in some direction. This pulse is so powerful that the energy of light is say 1Kg * c**2, or the relativistic mass of light is 1Kg. What is a gravity from that light according to GR? I was thinking about it and it looks weird. So, this 'body' is moving at speed of light, so the gravity can not 'outrun' it, contrary to any other moving bodies. So if we have a test body close to the path of the beam. Until the beam arrives, the test body does not move, because gravity does not arrive yet. Gravity hits the test body at the same time the light passes by. After it is passed, test body is attracted into the direction where the beam is gone. So contrary to the flyby of any other body, this process is assymetric (!!!). The test body is pulled into the direction where the beam is gone, getting some of the momentum of the pulse. Hence, light in the beam loses energy and momentum and becomes redder. Finally, everything which is applicable to the powerful pulse is also applicable to the normal pulse or light from distant stars. They also can become redder. Do you see anything wrong so far?