Observer (special relativity)

In special relativity, an observer is a frame of reference from which a set of objects or events are being measured. Usually this is an inertial reference frame or "inertial observer". Less often an observer may be an arbitrary non-inertial reference frame such as a Rindler frame which may be called an "accelerating observer". The special relativity usage differs significantly from the ordinary English meaning of "observer". Reference frames are inherently nonlocal constructs, covering all of space and time or a nontrivial part of it; thus it does not make sense to speak of an observer (in the special relativistic sense) having a location. Also, an inertial observer cannot accelerate at a later time, nor can an accelerating observer stop accelerating. Physicists use the term "observer" as shorthand for a specific reference frame from which a set of objects or events is being measured. Speaking of an observer in special relativity is not specifically hypothesizing an individual person who is experiencing events, but rather it is a particular mathematical context which objects and events are to be evaluated from. The effects of special relativity occur whether or not there is a sentient being within the inertial reference frame to witness them. Einstein made frequent use of the word "observer" (Beobachter) in his original 1905 paper on special relativity and in his early popular exposition of the subject. However he used the term in its vernacular sense, referring for example to "the man at the railway-carriage window" or "observers who take the railway train as their reference-body" or "an observer inside who is equipped with apparatus". Here the reference body or coordinate system—a physical arrangement of metersticks and clocks which covers the region of spacetime where the events take place—is distinguished from the observer—an experimenter who assigns spacetime coordinates to events far from himself by observing (literally seeing) coincidences between those events and local features of the reference body.