text: what is relativity?   a train is moving from station a to station b; the distance between them is 10km.  when the distance is 2m from station a, a man at station a will say that the train is 2km from a; whereas the man at b will say that the train is at 8km.  therefore, relative to man at a, the distance is 2km and relative to the man at b the distance is 8km.  this is called relativity.  in physics we measure the quantities like mass, length and time.  these quantities can be measured in a laboratory which is at rest.  the same quantities can be measured in a laboratory which is set up in a fast moving train.  will the observations will be the same or different?  the theory of relativity deals about such things.