I've been told contradicting ideas about this. I've been told that light doesn't travel at a constant speed everywhere (i.e. light slowing down in speed after entering a more dense medium). However, I've also read that light speed is constant everywhere (i.e. if you could travel close to the speed of light then you would experience warped space-time so light would still travel at light speed relative to you). So which is it or are both these ideas not the whole story?