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In general, the principle of relativity is the requirement that the laws of physics be the same for all observers. Somewhat more particularly, the measurements an observer
makes in his frame of reference of an important event are
related in a particular way to those made by another observer. Several principles of "relativity" have been developed, starting
with the Galilean version.
Galileo and others considered only inertial reference frames
(see the discussion of such frames in frame of
reference) in uniform translation relative to each other. The measurement of time was assumed to be the same for all
observers, but the measurements of distance made by an observer differ from those by another depending on the relative velocity
between the observers' frames of reference. The measurements made by one can be transformed to those by another one. Such a
transformation is called the Galilean
transformation.
Einstein saw, however, that if the vacuum speed of light was really the ultimate velocity in our physical world, a different kind of transformation
was called for. If information about an important event cannot reach us faster than light (or any form of electromagnetic
phenomenon) can, the Galilean assumption of universal time has to be discarded. We can only do physics based on what we can know;
physics based on mere, unsupported suppositions has to be ignored. The transformation of an observer's measurements to another
observer's measurements, now changed, is called the Lorentz
transformation. For details, see theory of
relativity and special relativity.
General relativity extended the principle of relativity to
accelerated frames of reference in a gravity field.
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