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Poincaré group
The Poincaré group, named after Henri Poincaré,〔 (Wikisource translation: On the Dynamics of the Electron).〕 is the group of Minkowski spacetime isometries.〔 (Wikisource translation: The Fundamental Equations for Electromagnetic Processes in Moving Bodies).〕 It is a ten-generator non-abelian Lie group of fundamental importance in physics.
== Overview ==
A Minkowski spacetime isometry has the property that the interval between events is left invariant. For example, if everything was postponed by two hours including two events and the path you took to go from one to the other, then the time interval between the events recorded by a stop-watch you carried with you would be the same. Or if everything was shifted five miles to the west, or turned 60 degrees to the right, you would also see no change in the interval. It turns out that the proper length of an object is also unaffected by such a shift. A time or space reversal (a reflection) is also an isometry of this group.
In Minkowski space (i.e. ignoring the effects of gravity), there are ten degrees of freedom of the isometries, which may be thought of as translation through time or space (four degrees, one per dimension); reflection through a plane (three degrees, the freedom in orientation of this plane); or a "boost" in any of the three spatial directions (three degrees). Composition of transformations is the operator of the Poincaré group, with proper rotations being produced as the composition of an even number of reflections.
In classical physics, the Galilean group is a comparable ten-parameter group that acts on absolute time and space. Instead of boosts, it features shear mappings to relate co-moving frames of reference.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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