The Speed of Light I know that the speed of light is constant for some reason. 


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*But why is it that when shining light from a spacecraft that is moving at (almost) the speed of this light, the speed of this light wouldn't seem to travel at (almost) twice the speed of light to an outside observer, or does it?

*I also would like to know why the speed of light is constant? 

*Finally, I also would like to know why the speed of light considered the fastest speed possible? I read that there is no explanation, it is just a physical law of our universe but is there a possible explanation?
 A: The idea that speed of light is measured to be the same constant, $c$, in all reference frames, is a postulate that Einstein put forth in his seminal paper, On the Electrodynamics of Moving Bodies. So according to the theory of special relativity, it doesn't really have a "reason" for being true; it's a first principle. It's true to the best of our knowledge because all the experiments we've done to measure the speed of light in various reference frames have returned the answer $c$.
A spacecraft couldn't travel at the speed of light in the first place, because as it goes faster and faster (in the observer's frame), its mass effectively gets bigger and bigger, meaning it requires more energy just to increase its speed a tiny bit. It effectively would take infinite energy for a spaceship to reach the speed of light.
If, however, the spaceship were going half the speed of light, and someone in the spaceship turned on a flashlight, the person on earth and the person in the spaceship would both measure the same speed of the light from that flashlight: $c$.
