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The term observer has a number of non-equivalent uses in science.
2 General relativity
3 Quantum mechanics
4 Thermodynamics and information theory
5 See also
Main article: Observer (special relativity)
The term observer in special relativity refers most commonly to an
inertial reference frame. 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
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
Main article: Observer (quantum physics)
See also: Schrödinger's cat
In quantum mechanics, "observation" is synonymous with quantum
measurement and "observer" with a measurement apparatus and observable
with what can be measured.
Thermodynamics and information theory
See for example Maxwell's demon.