Some friends and I were out at a pub last night and this question came up. If you were in space and you had two balls. You threw one ball in one direction at half the speed of light. You through the other ball in exactly the opposite direction then relative to each other they would be travelling the speed of light. As the observer in the center I would be able to see light from both object but it would red shifted quite a bit. But what would an observer on each of the balls see of the opposite ball? Does light propagate through space at the same speed regardless of the speed of it source relative to another object?