Special Relativity

Special relativity unified our concepts of space and time into the unified concept of space-time. In essence time is a fourth dimension and must be included with the three space dimensions when we talk about the location of an object or event. As a consequence of this unification of space and time the concept of simultaneous events has no absolute meaning. Whether or not two events occur simultaneously and the order in which different events occurs depends on the reference frame of the observer.

If, for example, you want to meet a friend for lunch, you have to decide both which restaurant to eat at and when to eat lunch. If you get either the time or the restaurant wrong you are not able to have lunch with your friend. You are in essence specifying the space-time coordinates of an event, a shared lunch. Note that both the space and time coordinates are needed, so space and time are unified into the single concept of space-time.