" The lack of an absolute standard of rest meant that one could not determine whether two events that took place at different times occurred in the same position in space. For example, suppose our Ping-Pong ball on the train bounces straight up and down, hitting the table twice on the same spot one second apart. To someone on the track, the two bounces would seem to take place about thirteen meters apart, because the tram would have traveled that far down the track between the bounces. "

― Stephen Hawking , A Brief History of Time