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Plane of rotation

In physics and geometry, the plane of rotation is the two-dimensional subspace perpendicular to the axis of rotation, within which points of a rotating body trace circular paths. This plane is defined by the direction of the angular velocity vector \vec{\omega}, which points along the axis according to the right-hand rule, with the rotation occurring counterclockwise when viewed along \vec{\omega}. For rigid body motion about a fixed axis, all points lie in parallel planes perpendicular to the axis, but the primary plane of rotation refers to the specific plane containing the relevant circular trajectories. In three-dimensional , rotations are orientation-preserving transformations that fix the while rotating vectors within the by an angle \theta, preserving lengths and angles. The \phi, \omega = d\phi/dt, and \alpha = d\omega/dt are measured in this , with \omega and \alpha as vectors along the . \vec{L} = \vec{r} \times \vec{p} for a particle also points to this , along the , highlighting its role in conserving rotational . In higher-dimensional geometry, rotations in \mathbb{R}^n (for n > 3) require specifying the plane of rotation explicitly, as multiple axes can be perpendicular to it, forming a fundamental 2D invariant under the special orthogonal group SO(n). This concept extends to applications in , , and , where rotations about arbitrary planes model complex transformations without altering the .

Definitions and Fundamentals

Plane in Geometry

In , a is defined as a flat, two-dimensional surface that extends infinitely in all directions, possessing length and width but no thickness. This surface lies evenly with the straight lines drawn upon it, forming the foundational structure for planar figures such as lines, circles, and polygons. One key property of a plane is its characterization as the locus of all points in space that are equidistant from two fixed points, known as the perpendicular bisector plane of the segment joining those points. Alternatively, a plane can be described as the affine span of two linearly independent vectors originating from a fixed point, generating all points reachable by linear combinations of those directions plus the origin point. These properties ensure that planes are invariant under translations and maintain parallelism with other planes in . In three-dimensional Euclidean space, the general equation of a plane is given by ax + by + cz = d, where a, b, and c are constants not all zero, representing the components of the plane's normal vector \mathbf{n} = (a, b, c), which is perpendicular to every line lying within the plane, and d is a constant determining the plane's position relative to the origin. For instance, the coordinate planes include the xy-plane defined by z = 0 (with normal vector (0, 0, 1)), the xz-plane by y = 0, and the yz-plane by x = 0, serving as standard references in Cartesian coordinates.

Plane of Rotation

In n-dimensional , a is a linear with that preserves and lengths. The plane of refers to the unique two-dimensional invariant subspace in which this acts as a proper by an θ, while leaving all vectors outside this subspace unchanged. This plane is spanned by two orthogonal unit vectors, say \mathbf{u} and \mathbf{v}, such that any vector in the plane is rotated within it, and the is the identity on the . The ambient n-dimensional decomposes orthogonally into the of this two-dimensional and its (n-2)-dimensional . Vectors in the complement are pointwise fixed, meaning the has eigenvalue 1 with multiplicity n-2 on this , ensuring no distortion or movement occurs there. This highlights the localized nature of the , confining its effect to the specified while maintaining the overall structure of the . In an adapted to this decomposition—where the first two basis vectors span the —the matrix representation of the transformation takes a block-diagonal form. On the , it is the standard two-dimensional : \begin{pmatrix} \cos \theta & -\sin \theta \\ \sin \theta & \cos \theta \end{pmatrix}, extended by the (n-2) × (n-2) on the . This form ensures the matrix is orthogonal with 1, confirming it represents a proper . Vectors lying in the rotation plane are mapped to other vectors in the same plane, rotated by θ, while vectors orthogonal to the plane remain . This invariance property underscores the plane's role as the sole locus of non-trivial action, distinguishing rotations from more general orthogonal transformations.

Rotations in Low Dimensions

In Two Dimensions

In two dimensions, rotations are orientation-preserving isometries of the that fix a single point, typically the origin. These rotations are parameterized by a single angle \theta \in [0, 2\pi), representing the counterclockwise rotation amount, and are represented by the rotation matrix R(\theta) = \begin{pmatrix} \cos \theta & -\sin \theta \\ \sin \theta & \cos \theta \end{pmatrix}, which transforms a point (x, y)$ to (x \cos \theta - y \sin \theta, x \sin \theta + y \cos \theta)$. Geometrically, each point under rotation traces a centered at the , preserving its from the center. The collection of all such rotations forms the special orthogonal group SO(2), isomorphic to the circle group U(1), which is abelian—since rotations about the same point commute—and compact as a closed bounded subset of the plane. In applications, 2D rotations underpin basic computer graphics transformations for orienting shapes and objects, while in polar coordinates, they simplify to adding \theta to the angular coordinate (r, \phi) \mapsto (r, \phi + \theta).

In Three Dimensions

In three-dimensional , any orientation-preserving with one, known as a , can be expressed as a single rotation by an angle \theta around a fixed passing through the origin, as stated by . This theorem, originally proved by Leonhard Euler in 1775 using , establishes that the rotation group SO(3) consists of such single-axis rotations. The is defined by a \mathbf{u} = (u_x, u_y, u_z) with \|\mathbf{u}\| = 1, and the angle \theta determines the magnitude of the rotation, typically taken in the range [0, \pi] to avoid redundancy. The plane of rotation is the two-dimensional orthogonal to the \mathbf{u}, comprising all \mathbf{v} such that \mathbf{u} \cdot \mathbf{v} = 0. In this , the behaves analogously to a two-dimensional , mapping within the plane by the angle \theta while leaving the invariant. along the satisfy \mathbf{u}' = \mathbf{u}, serving as the eigenvector with eigenvalue 1. To compute the image of a general \mathbf{v} under this , provides an explicit vector expression, originally derived by Euler in 1775 and later reformulated by in 1840: \mathbf{v}' = \mathbf{v} \cos \theta + (\mathbf{u} \times \mathbf{v}) \sin \theta + \mathbf{u} (\mathbf{u} \cdot \mathbf{v}) (1 - \cos \theta) This formula decomposes \mathbf{v} into components parallel and perpendicular to \mathbf{u}, rotating only the perpendicular part within the plane. In rigid body dynamics, the axis-angle representation and associated plane of rotation are fundamental for modeling physical motions. For instance, the Earth's daily rotation occurs by an angle of $2\pi radians around its polar axis, with the equatorial plane—perpendicular to this axis and intersecting the surface at the equator—serving as the plane of rotation. This setup produces the observed day-night cycle and Coriolis effects, illustrating how the invariant axis constrains the rotation to a specific plane.

Rotations in Higher Dimensions

In Four Dimensions

In four-dimensional , rotations are elements of the special SO(4), which consists of all orientation-preserving linear transformations that preserve lengths and angles. Unlike in three dimensions, where rotations occur around a single axis ( to a unique plane of rotation), four-dimensional rotations can involve one or two independent planes of action, allowing for greater complexity in their geometric interpretation. These rotations fix the origin and preserve the overall structure of the space, acting on coordinates (x, y, z, w). Simple rotations in are the direct analogue of rotations in lower dimensions, acting within a single plane while leaving the orthogonal complement fixed . For instance, a simple rotation by an \theta in the xy- would leave the zw- unchanged, similar to a extended into higher by on the extra . The corresponding is block-diagonal, with a block for the active and an block for the fixed : \begin{pmatrix} \cos \theta & -\sin \theta & 0 & 0 \\ \sin \theta & \cos \theta & 0 & 0 \\ 0 & 0 & 1 & 0 \\ 0 & 0 & 0 & 1 \end{pmatrix} This form ensures that vectors in the fixed plane remain invariant. Simple rotations represent a subset of SO(4) where one of the possible rotation angles is zero. Double rotations extend this by involving two independent rotations in mutually orthogonal 2D planes, parameterized by distinct angles \theta_1 and \theta_2. Here, the space decomposes into two invariant 2D planes, with no fixed subspace beyond the origin; points in each plane rotate independently by their respective angles. The matrix representation is block-diagonal, combining two 2D rotation blocks: \begin{pmatrix} \cos \theta_1 & -\sin \theta_1 & 0 & 0 \\ \sin \theta_1 & \cos \theta_1 & 0 & 0 \\ 0 & 0 & \cos \theta_2 & -\sin \theta_2 \\ 0 & 0 & \sin \theta_2 & \cos \theta_2 \end{pmatrix} Such rotations capture the general case for most elements of SO(4), excluding simple rotations. Isoclinic rotations form a special subclass of double rotations where the two angles are equal in magnitude (\theta_1 = \theta_2 = \theta) or opposite (\theta_1 = -\theta_2), but the planes are not aligned in the standard orthogonal basis—instead, they are related by a 45-degree twist in the geometric algebra sense. Known as Clifford's "screw rotations," these act with the same angular speed in infinitely many pairs of orthogonal invariant planes, leading to a more intertwined action across the space. For a left-isoclinic rotation, the matrix takes the form: R_L = \begin{pmatrix} l_0 & -l_3 & l_2 & l_1 \\ l_3 & l_0 & -l_1 & -l_2 \\ -l_2 & l_1 & l_0 & -l_3 \\ -l_1 & l_2 & l_3 & l_0 \end{pmatrix}, where the coefficients l_i satisfy l_0 = \cos \theta and the l_i (for i=1,2,3) derive from \sin \theta scaled by unit bivectors defining the twisted planes, such as \frac{1}{\sqrt{2}}(e_{23} + e_{41}). Right-isoclinic rotations follow a similar structure with opposite signing. This twisted configuration arises from the commutativity of the defining bivectors in the Clifford algebra Cl(4). All rotations in SO(4), whether , , or isoclinic, preserve volumes in space, as their matrices have 1, ensuring they are orientation-preserving transformations. This property follows from the structure of SO(4), where the from the skew-symmetric so(4) yields matrices with \det R = e^{\operatorname{tr}(A)} = e^0 = 1 for any skew-symmetric generator A. In applications, parameterizations of the SO(3,1) in can be related to this framework, particularly through early work by Rosen (1930) on general Lorentz transformations using rotation decompositions.

In Dimensions Greater Than Four

In dimensions greater than four, rotations in the special orthogonal group generalize the concept of planes of rotation by decomposing into a of rotations within multiple orthogonal 2-dimensional planes, potentially accompanied by fixed 1-dimensional directions when n is . This structure arises because every element of can be simultaneously block-diagonalized in an , where the blocks consist of rotation matrices in the chosen planes and 1×1 blocks for any fixed directions. The maximum number of such independent planes is \lfloor n/2 \rfloor, as each plane accounts for two dimensions, leaving at most one dimension fixed if n is odd. The canonical form of the rotation matrix is thus block-diagonal, with each 2×2 block of the form \begin{pmatrix} \cos \theta_i & -\sin \theta_i \\ \sin \theta_i & \cos \theta_i \end{pmatrix} for rotation angles \theta_i in the respective planes, and 1×1 blocks of value 1 for fixed directions. Rotations within disjoint planes commute with each other, and the overall rotation is their , preserving the orthogonality of the planes. For example, in five dimensions (n=5), a general decomposes into two orthogonal of plus one fixed line, allowing independent angular adjustments in each plane while the fixed remains . In six dimensions (n=6), up to three orthogonal can be involved, enabling more complex configurations such as isoclinic rotations where all three angles are equal. These decompositions facilitate computational representations and analysis of higher-dimensional rotations. The Lie algebra \mathfrak{so}(n) consists of n \times n skew-symmetric matrices, which generate infinitesimal rotations; each basis element corresponds to an infinitesimal rotation in a specific coordinate plane ij, with the (i,j)-entry equal to 1, (j,i)-entry equal to -1, and zeros elsewhere. The dimension of \mathfrak{so}(n) is n(n-1)/2, matching the number of independent planes.

Mathematical Properties

Connection to Reflections

In two-dimensional Euclidean space, a rotation by an angle θ within a plane can be expressed as the composition of two reflections over lines lying in that plane, where the lines are separated by an angle of θ/2. Specifically, reflecting a point first over a line at angle φ and then over a line at angle φ + θ/2 results in a counterclockwise rotation by θ around their intersection point. The plane of rotation is the span of these two lines. This decomposition extends to higher dimensions through the Cartan–Dieudonné theorem, which states that every in the of dimension n (n ≥ 2) is the composition of at most n reflections over hyperplanes. Proper rotations, which preserve orientation and form the special SO(n), are precisely those that arise as products of an even number of such reflections. For instance, in three dimensions, any is the product of two reflections over planes that both contain the of . In contrast, products of an odd number of reflections yield improper isometries, such as reflections or rotary inversions, which reverse and belong to the O(n) \ SO(n). This even-odd distinction underscores the fundamental role of reflections in generating the full while isolating rotations as orientation-preserving elements.

Representation via Bivectors

In , a simple B = \mathbf{u} \wedge \mathbf{v} represents an oriented two-dimensional , or , spanned by the linearly independent vectors \mathbf{u} and \mathbf{v}. This encodes both the and the of the intrinsically, without reference to a coordinate basis. The magnitude of the is given by \|B\| = \|\mathbf{u}\| \|\mathbf{v}\| \sin \phi, where \phi is the between \mathbf{u} and \mathbf{v}; for rotations, this magnitude is scaled such that the component in the corresponding relates directly to \sin(\theta/2), with \theta denoting the . The operator itself arises via the applied to the . Specifically, for a B corresponding to a skew-symmetric generator in the of the , the finite R is R = \exp(B), where the is defined through its expansion. In the context of Clifford (, this takes the form of a R = \exp\left( -\frac{\theta}{2} \frac{\mathbf{u} \wedge \mathbf{v}}{\|\mathbf{u} \wedge \mathbf{v}\|} \right), where \frac{\mathbf{u} \wedge \mathbf{v}}{\|\mathbf{u} \wedge \mathbf{v}\|} is the unit defining the plane's orientation. This applies the to a \mathbf{x} through the product \mathbf{x}' = R \mathbf{x} \tilde{R}, where \tilde{R} is the reverse of R (equivalent to its Clifford conjugate), ensuring the transformation preserves lengths and angles within the plane while leaving perpendicular components unchanged. A general rotation in higher dimensions can be decomposed as a product of exponentials of simple bivectors that commute, reflecting the fact that rotations act independently in orthogonal planes. For instance, in three dimensions, any rotation is a single simple bivector exponential, but in four or more dimensions, it factors into commuting plane rotations, such as R = \exp(B_1) \exp(B_2) where B_1 \wedge B_2 = 0. This representation offers key advantages: it is coordinate-free and intrinsic to the of the , facilitating computations in arbitrary dimensions without indices or basis-dependent components. Widely adopted in frameworks, it unifies vector, , and tensor operations, enabling efficient handling of in physics and .

Eigenvalues and Eigenplanes

For a simple by an angle \theta in a specific within n-dimensional , the corresponding R \in \mathrm{SO}(n) has eigenvalues consisting of the pair e^{i\theta} and e^{-i\theta} associated with that , along with the eigenvalue 1 having multiplicity n-2 for the . These complex eigenvalues lie on the unit circle in the , reflecting the property of rotations that preserves lengths. When \theta = 0, all eigenvalues are real and equal to +1 with multiplicity n (the transformation). When \theta = \pi, the eigenvalues are -1 with multiplicity 2 and +1 with multiplicity n-2. The of such a is (\lambda^2 - 2\cos\theta \, \lambda + 1)(\lambda - 1)^{n-2}, where the quadratic factor arises from the plane of rotation and the remaining factor accounts for the fixed directions. This determines the spectral properties, with roots matching the eigenvalues described above. The plane of rotation corresponds to a 2-dimensional real , which is the realification of the eigenspace spanned by the eigenvectors for e^{i\theta} and e^{-i\theta}. Within this , vectors are rotated by \theta, while the remains fixed pointwise (eigenvalue 1). For the special case \theta = \pi, the plane of rotation becomes a 2-dimensional eigenspace with eigenvalue -1 (multiplicity 2), while the remains fixed pointwise (eigenvalue 1 with multiplicity n-2). In higher dimensions, a simple rotation matrix is diagonalizable over the numbers, yielding a block-diagonal form with 1×1 blocks for the real eigenvalues 1 and separate 1×1 blocks for e^{i\theta}, e^{-i\theta}. Over the reals, the (via real decomposition, which for s consists of blocks rather than non-trivial blocks due to diagonalizability over \mathbb{C}) reveals 2×2 submatrices for the and 1×1 blocks elsewhere, directly highlighting the eigenplane structure. The trace of the matrix provides a key spectral invariant: \operatorname{Tr}(R) = n - 2 + 2\cos\theta for a simple rotation, linking the rotation angle directly to the sum of eigenvalues. For a general rotation composed of multiple orthogonal plane rotations with angles \theta_k (say, m planes), this generalizes to \operatorname{Tr}(R) = n - 2m + \sum_{k=1}^m 2\cos\theta_k, summing the contributions from each eigenplane.

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