How does the special theory of relativity relate to spacetime?
Special relativity begins with the idea that space and time are not fixed, but change depending on how fast and in what direction the observer is moving. In other words, they depend on an observer's reference frame. For example, imagine that you and a friend are on a train traveling at a constant 60 miles (97 kilometers) per hour and, to pass the time, you're playing catch. You are standing at the front end of the train and your friend is standing at the rear. When the ball is thrown, it appears to travel between the two of you at 30 miles (48 kilometers) per hour. Yet to an observer standing beside the railroad tracks watching the train go by, the ball appears to travel at 30 miles (48 kilometers) per hour (60 - 30) when you throw it and 90 miles per hour (60 +30) when your friend throws it. Thus, the ball appears to take different periods of time to travel between the two ball-players, depending on one's reference frame. The event cannot be described as "simultaneous" by the observer on the train and the one beside the tracks. It can only be adequately described by each observer by using the dimension of time as well as dimensions of space.
