Spaceship Alpha is travelling at 0.9c. It shines a light beam. An observer that is stationary observes that the light beam is travelling at c, not 1.9c. Explain. How can I answer this question? I don't want to just say because Einstein said c is the same regardless of frame of reference. Imagine that the spaceship was travelling at c (which is impossible) and shines a light, does this mean the person will not see the light? What led Einstein initially to come up with the idea that light speed is constant?