Observer (special relativity)

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The term observer in special relativity refers most commonly to an inertial reference frame. Less often it may refer to an arbitrary non-inertial coordinate frame; in particular, a Rindler frame is sometimes called an "accelerating observer". In such cases an inertial reference frame may be called an "inertial observer" to avoid ambiguity. Note that these uses differ 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; rather, "inertial observer" and "accelerating observer" describe distinct coordinate systems which assign coordinates to the same events.

[edit] History

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^[1]. However he used the term in essentially its vernacular sense, referring for example to "an observer who takes the train as his reference body" or "an observer located at the origin of the coordinate system". 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. The key feature of Einstein's construction is that the observer's measurements are independent of his location or his state of motion. The observer who takes the train as his reference body need not be on the train, or moving at the same speed as the train, or even moving inertially, since he observes only coincidences of events and such observations are independent of the effects of his motion (Doppler shift and aberration).

This distinction was dropped by most later writers, leading to the modern identification of observer and coordinate system. Where Einstein referred to "an observer who takes the train as his reference body" or "an observer located at the origin of the coordinate system", later writers generally refer to "an observer on the train" or "an observer moving with velocity v", where v is the velocity of Einstein's coordinate system. In modern textbooks the use of observer in this sense is firmly established, with the verb "observe" normally used to refer to the assignment of coordinates and "see" to visual perception of light (or other local measurements).

[edit] Usage in other scientific disciplines

In general relativity the term "observer" refers more commonly to a person (or a machine) making passive local measurements, a usage much closer to the ordinary English meaning of the word. In quantum mechanics, "observation" is synonymous with quantum measurement and "observer" with a measurement apparatus. This conflict of usages within physics is sometimes a source of confusion, as when a person walking along the street (an "observer" in the familiar sense) is said to perceive Lorentz contraction or relativity of simultaneity (which are phenomena specific to inertial reference frames). Many textbooks address this ambiguity by using the verb "see" to refer to local measurements and "observe" to refer to special relativistic "observations", i.e. coordinates in an inertial reference frame.