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| {{redirect|SO(3)|its definition over an arbitrary field|special orthogonal group}}
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| In [[classical mechanics|mechanics]] and [[geometry]], the '''3D rotation group''' is the [[group (mathematics)|group]] of all [[rotation]]s about the [[origin (mathematics)|origin]] of [[three-dimensional space|three-dimensional]] [[Euclidean space]] '''R'''<sup>3</sup> under the operation of [[function composition|composition]].<ref>Jacobson (2009), p. 34, Ex. 14.</ref> By definition, a rotation about the origin is a transformation that preserves the origin, [[Euclidean distance]] (so it is an [[isometry]]), and [[orientation (geometry)|orientation]] (i.e. ''handedness'' of space). A distance-preserving transformation which reverses orientation is an [[improper rotation]], that is a [[reflection (mathematics)|reflection]] or, in the [[general position]], a [[rotoreflection]]. The origin in Euclidean space establishes a one-to-one correspondence between [[point (geometry)|points]] and their [[coordinate vector]]s. Rotations about the origin can be thought of as magnitude-preserving [[linear transformation]]s of [[Euclidean vector|Euclidean 3-dimensional vectors]] (whose [[vector space]] is also denoted as '''R'''<sup>3</sup>).
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| Composing two rotations results in another rotation; every rotation has a unique [[inverse function|inverse]] rotation; and the [[identity map]] satisfies the definition of a rotation. Owing to the above properties (along with the [[associative property]], which rotations obey), the set of all rotations is a [[group (mathematics)|group]] under composition. Moreover, the rotation group has a natural [[manifold]] structure for which the group operations are [[smooth function|smooth]]; so it is in fact a [[Lie group]]. The rotation group is often denoted '''SO(3)''' (or, less ambiguously, SO(3, '''[[real numbers|R]]''')) for reasons explained [[#Orthogonal and rotation matrices|below]].
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| ==Length and angle==
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| Besides just preserving length, rotations also preserve the [[angle]]s between vectors. This follows from the fact that the standard [[dot product]] between two vectors '''u''' and '''v''' can be written purely in terms of length:
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| :<math>\mathbf{u}\cdot\mathbf{v} = \tfrac{1}{2}\left(\|\mathbf{u}+\mathbf{v}\|^2 - \|\mathbf{u}\|^2 - \|\mathbf{v}\|^2\right).</math>
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| It follows that any length-preserving transformation in '''R'''<sup>3</sup> preserves the dot product, and thus the angle between vectors. Rotations are often defined as linear transformations that preserve the inner product on '''R'''<sup>3</sup>. This is equivalent to requiring them to preserve length.
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| ==Orthogonal and rotation matrices==
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| {{Main|Orthogonal matrix|Rotation matrix}}
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| Every rotation maps an [[orthonormal basis]] of '''R'''<sup>3</sup> to another orthonormal basis. Like any linear transformation of [[finite-dimensional]] vector spaces, a rotation can always be represented by a [[matrix (mathematics)|matrix]]. Let ''R'' be a given rotation. With respect to the [[standard basis]] '''e'''<sub>1</sub>, '''e'''<sub>2</sub>, '''e'''<sub>3</sub> of '''R'''<sup>3</sup> the columns of ''R'' are given by (''R'''''e'''<sub>1</sub>, ''R'''''e'''<sub>2</sub>, ''R'''''e'''<sub>3</sub>). Since the standard basis is orthonormal, the columns of ''R'' form another orthonormal basis. This orthonormality condition can be expressed in the form
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| :<math>R^\mathsf{T}R = I\,</math>
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| where ''R''<sup>T</sup> denotes the [[transpose]] of ''R'' and {{mvar|I}} is the 3 × 3 [[identity matrix]]. Matrices for which this property holds are called [[orthogonal matrix|orthogonal matrices]]. The group of all 3 × 3 orthogonal matrices is denoted O(3), and consists of all proper and improper rotations.
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| In addition to preserving length, proper rotations must also preserve orientation. A matrix will preserve or reverse orientation according to whether the [[determinant]] of the matrix is positive or negative. For an orthogonal matrix ''R'', note that det ''R''<sup>''T''</sup> = det ''R''<sup>'' -1''</sup> implies (det ''R'')<sup>2</sup> = 1 so that det ''R'' = ±1. The [[subgroup]] of orthogonal matrices with determinant +1 is called the ''special [[orthogonal group]]'', denoted SO(3).
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| Thus every rotation can be represented uniquely by an orthogonal matrix with unit determinant. Moreover, since composition of rotations corresponds to [[matrix multiplication]], the rotation group is [[isomorphic]] to the special orthogonal group SO(3).
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| Improper rotations correspond to orthogonal matrices with determinant −1, and they do not form a group because the product of two improper rotations is a proper rotation.
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| ==Group structure==
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| The rotation group is a [[group (mathematics)|group]] under [[function composition]] (or equivalently the [[matrix product|product of linear transformations]]). It is a [[subgroup]] of the [[general linear group]] consisting of all [[invertible matrix|invertible]] linear transformations of the [[real coordinate space|real 3-space]] '''R'''<sup>3</sup>.<ref>''n'' × ''n'' real matrices are identical to linear transformations of '''R'''<sup>''n''</sup> expressed in its [[standard basis]].</ref>
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| Furthermore, the rotation group is [[nonabelian group|nonabelian]]. That is, the order in which rotations are composed makes a difference. For example, a quarter turn around the positive ''x''-axis followed by a quarter turn around the positive ''y''-axis is a different rotation than the one obtained by first rotating around ''y'' and then ''x''.
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| The orthogonal group, consisting of all proper and improper rotations, is generated by reflections. Every proper rotation is the composition of two reflections, a special case of the [[Cartan–Dieudonné theorem]].
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| ==Axis of rotation==
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| {{main|Axis–angle representation}}
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| Every nontrivial proper rotation in 3 dimensions fixes a unique 1-dimensional [[linear subspace]] of '''R'''<sup>3</sup> which is called the ''axis of rotation'' (this is [[Euler's rotation theorem]]). Each such rotation acts as an ordinary 2-dimensional rotation in the plane [[orthogonal]] to this axis. Since every 2-dimensional rotation can be represented by an angle φ, an arbitrary 3-dimensional rotation can be specified by an axis of rotation together with an [[angle of rotation]] about this axis. (Technically, one needs to specify an orientation for the axis and whether the rotation is taken to be [[Clockwise and counterclockwise|clockwise]] or [[counterclockwise]] with respect to this orientation).
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| For example, counterclockwise rotation about the positive ''z''-axis by angle φ is given by
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| :<math>R_z(\varphi) = \begin{bmatrix}\cos\varphi & -\sin\varphi & 0 \\ \sin\varphi & \cos\varphi & 0 \\ 0 & 0 & 1\end{bmatrix}.</math>
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| Given a [[unit vector]] '''n''' in '''R'''<sup>3</sup> and an angle φ, let ''R''(φ, '''n''') represent a counterclockwise rotation about the axis through '''n''' (with orientation determined by '''n'''). Then
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| * ''R''(0, '''n''') is the identity transformation for any '''n'''
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| * ''R''(φ, '''n''') = ''R''(−φ, −'''n''')
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| * ''R''(π + φ, '''n''') = ''R''(π − φ, −'''n''').
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| Using these properties one can show that any rotation can be represented by a unique angle φ in the range 0 ≤ φ ≤ π and a unit vector '''n''' such that
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| * '''n''' is arbitrary if φ = 0
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| * '''n''' is unique if 0 < φ < π
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| * '''n''' is unique up to a [[sign (mathematics)|sign]] if φ = π (that is, the rotations ''R''(π, ±'''n''') are identical).
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| ==Topology==
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| {{Main|Hypersphere of rotations}}
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| The Lie group SO(3) is [[diffeomorphism|diffeomorphic]] to the [[real projective space]] '''RP'''<sup>3</sup>.
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| Consider the solid ball in '''R'''<sup>3</sup> of radius π (that is, all points of '''R'''<sup>3</sup> of distance π or less from the origin). Given the above, for every point in this ball there is a rotation, with axis through the point and the origin, and rotation angle equal to the distance of the point from the origin. The identity rotation corresponds to the point at the center of the ball. Rotation through angles between 0 and −π correspond to the point on the same axis and distance from the origin but on the opposite side of the origin. The one remaining issue is that the two rotations through π and through −π are the same. So we [[quotient space|identify]] (or "glue together") [[antipodal point]]s on the surface of the ball. After this identification, we arrive at a [[topological space]] [[homeomorphic]] to the rotation group.
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| Indeed, the ball with antipodal surface points identified is a [[smooth manifold]], and this manifold is [[diffeomorphism|diffeomorphic]] to the rotation group. It is also diffeomorphic to the [[real projective space|real 3-dimensional projective space]] '''RP'''<sup>3</sup>, so the latter can also serve as a topological model for the rotation group.
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| These identifications illustrate that SO(3) is [[connectedness|connected]] but not [[simply connected]]. As to the latter, in the ball with antipodal surface points identified, consider the path running from the "north pole" straight through the interior down to the south pole. This is a closed loop, since the north pole and the south pole are identified. This loop cannot be shrunk to a point, since no matter how you deform the loop, the start and end point have to remain antipodal, or else the loop will "break open". In terms of rotations, this loop represents a continuous sequence of rotations about the ''z''-axis starting and ending at the identity rotation (i.e. a series of rotation through an angle φ where φ runs from 0 to [[turn (geometry)|2π]]).
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| Surprisingly, if you run through the path twice, i.e., run from north pole down to south pole, jump back to the north pole (using the fact that north and south poles are identified), and then again run from north pole down to south pole, so that φ runs from 0 to 4π, you get a closed loop which ''can'' be shrunk to a single point: first move the paths continuously to the ball's surface, still connecting north pole to south pole twice. The second half of the path can then be mirrored over to the antipodal side without changing the path at all. Now we have an ordinary closed loop on the surface of the ball, connecting the north pole to itself along a great circle. This circle can be shrunk to the north pole without problems. The [[Plate trick|Balinese plate trick]] and similar tricks demonstrate this practically.
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| The same argument can be performed in general, and it shows that the [[fundamental group]] of SO(3) is [[cyclic group]] of order 2. In [[physics]] applications, the non-triviality of the fundamental group allows for the existence of objects known as [[spinor]]s, and is an important tool in the development of the [[spin-statistics theorem]].
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| The [[universal cover]] of SO(3) is a [[Lie group]] called [[spinor group|Spin(3)]]. The group Spin(3) is isomorphic to the [[special unitary group]] SU(2); it is also diffeomorphic to the unit [[3-sphere]] ''S''<sup>3</sup> and can be understood as the group of [[versor]]s ([[quaternion]]s with [[absolute value]] 1). The connection between quaternions and rotations, commonly exploited in [[computer graphics]], is explained in [[quaternions and spatial rotation]]s. The map from ''S''<sup>3</sup> onto SO(3) that identifies antipodal points of ''S''<sup>3</sup> is a [[surjective]] [[homomorphism]] of Lie groups, with [[Kernel (algebra)|kernel]] {±1}. Topologically, this map is a two-to-one [[covering map]].
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| ==Lie algebra==
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| {{merge section from|Rotation matrix#Lie theory|date=September 2013}}
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| {{See also|Infinitesimal rotation}}
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| Since '''SO'''(3) is a [[Lie subgroup]] of the [[general linear group]] '''GL'''(3), its [[Lie algebra]] can be identified with a Lie subalgebra of '''gl'''(3), the algebra of 3 × 3 matrices with the commutator given by
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| :<math>[A,B] = AB - BA. \, </math> | |
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| The condition that a matrix ''A'' belong to '''SO'''(3) is that
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| :(*){{Quad}}<math>AA^\mathsf{T} = I. \,</math>
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| If ''A''(''t'') is a one-parameter subgroup of '''SO'''(3) parametrised by ''t'', then differentiating (*) with respect to ''t'' gives
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| :<math>A'(0) + A'(0)^\mathsf{T} = 0 \, </math>
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| and so the Lie algebra '''SO'''(3) consists of all [[skew-symmetric matrix|skew-symmetric]] 3 × 3 matrices.
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| ==Representations of rotations==
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| {{Main|Rotation formalisms in three dimensions}}
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| We have seen that there are a variety of ways to represent rotations:
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| * as orthogonal matrices with determinant 1,
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| * by axis and rotation angle
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| * in [[quaternion]] algebra with [[versor]]s and the map [[3-sphere]] ''S''<sup>3</sup> → SO(3) (see [[quaternions and spatial rotation]]s).
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| Another method is to specify an arbitrary rotation by a sequence of rotations about some fixed axes. See:
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| * [[Euler angle]]s
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| See [[charts on SO(3)]] for further discussion.
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| ==Generalizations==
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| The rotation group generalizes quite naturally to ''n''-dimensional [[Euclidean space]], '''R'''<sup>''n''</sup> with its standard Euclidean structure. The group of all proper and improper rotations in ''n'' dimensions is called the [[orthogonal group]] O(''n''), and the subgroup of proper rotations is called the [[special orthogonal group]] SO(''n''), which is a [[Lie group]] of dimension n(n-1)/2.
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| In [[special relativity]], one works in a 4-dimensional vector space, known as [[Minkowski space]] rather than 3-dimensional Euclidean space. Unlike Euclidean space, Minkowski space has an inner product with an indefinite [[metric signature|signature]]. However, one can still define ''generalized rotations'' which preserve this inner product. Such generalized rotations are known as [[Lorentz transformation]]s and the group of all such transformations is called the [[Lorentz group]].
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| The rotation group SO(3) can be described as a subgroup of [[SE(3)|''E''<sup>+</sup>(3)]], the [[Euclidean group]] of [[Euclidean group#Direct and indirect isometries|direct isometries]] of Euclidean '''R'''<sup>3</sup>. This larger group is the group of all motions of a [[rigid body]]: each of these is a combination of a rotation about an arbitrary axis and a translation along the axis, or put differently, a combination of an element of SO(3) and an arbitrary translation.
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| In general, the rotation group of an object is the [[symmetry group]] within the group of direct isometries; in other words, the intersection of the full symmetry group and the group of direct isometries. For [[chirality (mathematics)|chiral]] objects it is the same as the full symmetry group.
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| ==See also==
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| <div style="-moz-column-count:2; column-count:2;">
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| *[[Orthogonal group]]
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| *[[Angular momentum]]
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| *[[Coordinate rotation]]s
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| *[[Charts on SO(3)]]
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| *[[Euler angles]]
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| *[[Rodrigues' rotation formula]]
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| *[[Infinitesimal rotation]]
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| *[[Pin group]]
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| *[[Quaternions and spatial rotation]]s
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| *[[Rigid body]]
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| *[[Spherical harmonics]]
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| *[[Plane of rotation]]
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| *[[Lie group]]
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| </div>
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| == Notes ==
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| <references/>
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| ==References==
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| *A. W. Joshi ''Elements of Group Theory for Physicists'' (2007 New Age International) pp. 111ff.
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| *{{MathWorld|title=Rotation Group|urlname=RotationGroup}}
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| *[[Mathematical Methods in the Physical Sciences]] by [[Mary L. Boas|Mary L Boas]] pp. 120,127,129,155ff and 535
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| * {{Citation| last=Jacobson| first=Nathan| author-link=Nathan Jacobson| year=2009| title=Basic algebra| edition=2nd| volume = 1 | series= | publisher=Dover| isbn = 978-0-486-47189-1}}
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| {{DEFAULTSORT:Rotation Group}}
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| [[Category:Lie groups]]
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| [[Category:Rotational symmetry]]
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| [[Category:Rotation in three dimensions]]
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| [[Category:Euclidean solid geometry]]
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