In Newtonian physics, light is the fastest speed in the universe, attainable only by light. Speed in this case relates to motion through space only. Time is irrelevant, except in its ability to be used to measure motion through space.sarephina said:What is the difference between Newtonian and Einsteinian speeds of light?
In Einsteinian physics, light is the only speed in the universe. Motion through space and time is symmetrical, such that an increase in motion through one results in a symmetrical decrease in motion through the other.
To give the brief answer, you are correct that each went at the speed of light with respect to the other. Howver, the one in the space ship underwent accelearation with respect to the other, and this is the critical factor.sarephina said:why isn't the Earth twin younger with respect to the spaceship one? Since the spaceship twin was unmoving wrt himself, everyone else should have been travelling near the speed of light, so he should age and they should not.