According to Einstein, all observers experience and measure the same light speed c, whether they're in stationary position or moving in motion.
Okay, this is one of the fundamental parts of relativity, and I'm new at it, so it's still having some difficulty in fully digesting the idea.
Let's start at the basic "real world" level first. If I'm driving on the road and going 50mph, and a car from behind me is catching up at 100mph, that car's speed, as it passes me, will not seem as fast relative to my perspective since I'm already in the same forward motion at half the speed. In other words, when it passes me, it won't be traveling 100mph faster than me since I'm not stationary.
But going to the area of light speed and advanced theories, all this idea goes out the window?! Let's say I'm in a spaceship, and I'm traveling impossibly fast at 0.5c. If light is coming from behind me and zips right by, I will feel see light traveling at 1 C faster than me. Whether i'm not moving at all, or walking, or driving in a normal car, or riding in a spaceship, I will observe light to come and go at the same constant c...
But my question is.... going back to the spaceship -- if my ship was already moving at 0.5 C, and this light from behind zooms by at that same rate of c, then wouldn't that light speed actually be 1.5c???
Or am I looking at this totally the wrong way because light is not an object? And that is the whole purpose of relativity?
Okay, going to something even more theoretically bizarre and/or unrealistic, what if I was traveling in a spaceship that was going at c? Since I'd be traveling at light speed myself, would light from behind even catch up to me? or would light still zip by my point of refence at c when I see it??