|
Generally, an object with 'rotational symmetry' also known in biological contexts as 'radial symmetry', is an object that looks the same after a certain amount of rotation. An object may have more than one rotational symmetry; for instance, if reflections or turning it over are not counted. The degree of rotational symmetry is how many degrees the shape has to be turned to look the same on a different side or vertex. It cannot be the same side or vertex. == Formal treatment == Formally the rotational symmetry is symmetry with respect to some or all rotations in ''m''-dimensional Euclidean space. Rotations are direct isometries, i.e., isometries preserving orientation. Therefore a symmetry group of rotational symmetry is a subgroup of ''E''+(''m'') (see Euclidean group). Symmetry with respect to all rotations about all points implies translational symmetry with respect to all translations, so space is homogeneous, and the symmetry group is the whole ''E''(''m''). With the modified notion of symmetry for vector fields the symmetry group can also be ''E''+(''m''). For symmetry with respect to rotations about a point we can take that point as origin. These rotations form the special orthogonal group SO(''m''), the group of ''m''×''m'' orthogonal matrices with determinant 1. For this is the rotation group SO(3). In another meaning of the word, the rotation group ''of an object'' is the symmetry group within ''E''+(''n''), the group of direct isometries; in other words, the intersection of the full symmetry group and the group of direct isometries. For chiral objects it is the same as the full symmetry group. Laws of physics are SO(3)-invariant if they do not distinguish different directions in space. Because of Noether's theorem, rotational symmetry of a physical system is equivalent to the angular momentum conservation law. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Rotational symmetry」の詳細全文を読む スポンサード リンク
|