I apologize for the the probably gross error on my behalf, but, not as a scientist in the GR /SR field, I am a bit confused. My question is why is c constant? I am not arguing why is c constant in a specific frame, I do not understand why you couldn't, for a lack of a better term, frame stack. So for example, if one throws a rock at 20mph in a platform going 10 mph, than the speed of the rock from a person stationary on the ground sees the rock going 30 mph. absurdly simple. Yet for a ship traveling at or near c, and the ship had a head light pointing in the same direction of the velocity of the ship. The out side observer will see the ship, and the head light of the ship traveling at the same speed, yet the Pilot of the ship sees the light, traveling away from him at c. What I do not understand is this. Why would the out side observer see the head light at all. Why would that light even "exist" to the out side observer. Wouldn't it make more sense, for the head light to only exist to the pilot. Since theoretically, nothing can travel faster than c, if and only if they are in the same frame, citing Einstein himself: Tests of Einstein's two Postulates 1. The laws by which the states of physical systems undergo change are not affected, whether these changes of state be referred to the one or the other of two systems of coordinates in uniform translatory motion. 2. Any ray of light moves in the “stationary” system of coordinates with determined velocity c, whether the ray be emitted by a stationary or by a moving body. But, since the spaceship is already nearing the speed of light, does that not define the spaceship as a new frame, which light is free to be a constant in, but breaks down when thought about from the arbitrary observer. If this were true, it would make lightspeed stacking possible.