Let's say 1 Kilogram of matter is converted into light, and all this light is reflected into one direction. What is the momentum of the light? Mass of 1 kg times velocity c ? Let's say the same light enters a piece of glass where the speed of light is 1/2 c. What is the momentum of the light now? 1 kg times 1/2 c ? And did the piece of glass absorb half of the momentum? Let's say the light enters a gravity field where the gravitational time dilation factor is 1/2. Does the light give half of its momentum to the gravitating thing, similarly as light perhaps gave half of its momentum to the piece of glass?