Ok, let's imagine an observer on the ground, he witnesses a large spacecraft flying pass him relatively at 3 x 10^5m/s. Now let's imagine he saw through a large window, an alien shining a torch across the window in the spacecraft. By Einstein's second postulate on special relativity, we expect that the observer will see the light moving across the spacecraft at c. By considering the lorentz's factor, the spacecraft now appears to be travelling at 299999.7m/s to my observer. With respect to the observer, the alien still sees his light beam travelling at c. But to the observer, although the spacecraft appears to be travelling at a slower velocity than its actual velocity, it still doesn't account for the discrepancy between the 2 time frames! Someone please help me on understanding this concept!