旋轉平面

旋轉平面，是一個用於描述空間旋轉的抽像概念。九維以下的旋轉平面數量如下表所示：已隱藏部分未翻譯內容，歡迎參與[ 翻譯]。In it is an alternative to the , but unlike the axis of rotation it can be used in other dimensions, such as , or more dimensions.數學上，旋轉平面可用多種方式描述。可用平面和旋轉角度來描述，可用克利福德代數的二重向量來描述。旋轉平面又與旋轉矩陣的特徵值和特徵向量有關。And in particular s they are related to other algebraic and geometric properties, which can then be generalised to other dimensions.Planes of rotation are not used much in two and three dimensions, as in two dimensions there is only one plane so identifying the plane of rotation is trivial and rarely done, while in three dimensions the axis of rotation serves the same purpose and is the more established approach. The main use for them is in describing more complex rotations in , where they can be used to break down the rotations into simpler parts. This can be done using , with the planes of rotations associated with in the algebra.定義.平面.本文所有平面都透過原點For this article, all "s" are planes through the , that is they contain the . Such a plane in is a two-dimensional of the space. It is completely specified by any two non-zero and non-parallel vectors that lie in the plane, that is by any two vectors b, such thatformula_1where φ is the angle between the vectors; hence the requirement that the vectors be nonzero and nonparallel.If the bivector B is simplyformula_2This is true in all dimensions, and can be taken as the definition on the plane. In particular, from the properties of the exterior product it is satisfied by both b, and so by any vector of the formformula_3with c ranges over the whole plane, so this can be taken as another definition of the plane.旋轉平面.A "plane of rotation" for a particular is a plane that is to itself by the rotation. The plane is not fixed, but all vectors in the plane are mapped to other vectors in the same plane by the rotation. This transformation of the plane to itself is always a rotation about the origin, through an angle which is the for the plane.Every rotation except for the rotation (with matrix the ) has at least one plane of rotation, and up toformula_4planes of rotation, where n is the dimension.九維以下的旋轉平面數量如下表所示：When a rotation has multiple planes of rotation they are always to each other, with only the origin in common. This is a stronger condition than to say the planes are at s; it instead means that the planes have no nonzero vectors in common, and that every vector in one plane is orthogonal to every vector in the other plane. This can only happen in four or more dimensions. In two dimensions there is only one plane, while in three dimensions all planes have at least one nonzero vector in common, along their .In more than three dimensions planes of rotation are not always unique. For example the negative of the in four dimensions (the ),formula_5describes a rotation in four dimensions in which every plane through the origin is a plane of rotation through an angle π, so any pair of orthogonal planes generates the rotation. But for a general rotation it is at least theoretically possible to identify a unique set of orthogonal planes, in each of which points are rotated through an angle, so the set of planes and angles fully characterise the rotation.二維.在二維空間只有一個旋轉平面，即空間本身的平面。在笛卡爾坐標系是笛卡爾平面，在複數是複平面。因此，任何旋轉都是整個平面，只保持原點固定。完全由帶符號的旋轉角度指定，例如在 -π 到 π 的範圍內。因此若角度為θ，復平面的旋轉則由以下尤拉公式給出：formula_6笛卡爾平面的旋轉則由以下旋轉矩陣給出：formula_7三維.三維空間可以有無數個旋轉平面，但當旋轉平面有了一個，就不能第二個旋轉平面。三維任何的旋轉平面，都總是有一個固定的軸，即旋轉軸。這可以用如下矩陣來描述：formula_8已隱藏部分未翻譯內容，歡迎參與[ 翻譯]。In there are an infinite number of planes of rotation, only one of which is involved in any given rotation. That is, for a general rotation there is precisely one plane which is associated with it or which the rotation takes place in. The only exception is the trivial rotation, corresponding to the identity matrix, in which no rotation takes place.In any rotation in three dimensions there is always a fixed axis, the axis of rotation. The rotation can be described by giving this axis, with the angle through which the rotation turns about it; this is the representation of a rotation. The plane of rotation is the plane orthogonal to this axis, so the axis is a of the plane. The rotation then rotates this plane through the same angle as it rotates around the axis, that is everything in the plane rotates by the same angle about the origin.One example is shown in the diagram, where the rotation takes place about the θ (about the axis or in the plane):formula_8Another example is the 地球自轉. The axis of rotation is the line joining the North Pole and 南極 and the plane of rotation is the plane through the between the and Hemispheres. Other examples include mechanical devices like a or which store in mass usually along the plane of rotation.In any three dimensional rotation the plane of rotation is uniquely defined. Together with the angle of rotation it fully describes the rotation. Or in a continuously rotating object the rotational properties such as the rate of rotation can be described in terms of the plane of rotation. It is perpendicular to, and so is defined by and defines, an axis of rotation, so any description of a rotation in terms of a plane of rotation can be described in terms of an axis of rotation, and vice versa. But unlike the axis of rotation the plane generalises into other, in particular higher, dimensions.四維空間中的一般旋轉只有一個固定點，即原點。因此，旋轉軸不能用於四個維度。但是可以使用旋轉平面，並且在四個維度中的每個非平凡旋轉都有一個或兩個旋轉平面。A general rotation in has only one fixed point, the origin. Therefore an axis of rotation cannot be used in four dimensions. But planes of rotation can be used, and each non-trivial rotation in four dimensions has one or two planes of rotation.簡單旋轉.A rotation with only one plane of rotation is a . In a simple rotation there is a fixed plane, and rotation can be said to take place about this plane, so points as they rotate do not change their distance from this plane. The plane of rotation is orthogonal to this plane, and the rotation can be said to take place in this plane.For example the following matrix fixes the w coordinates.formula_10In two and three dimensions all rotations are simple, in that they have only one plane of rotation. Only in four and more dimensions are there rotations that are not simple rotations. In particular in four dimensions there are also double and isoclinic rotations.雙重旋轉.In a there are two planes of rotation, no fixed planes, and the only fixed point is the origin. The rotation can be said to take place in both planes of rotation, as points in them are rotated within the planes. These planes are orthogonal, that is they have no vectors in common so every vector in one plane is at right angles to every vector in the other plane. The two rotation planes span four-dimensional space, so every point in the space can be specified by two points, one on each of the planes.A double rotation has two angles of rotation, one for each plane of rotation. The rotation is specified by giving the two planes and two non-zero angles, zw-plane is given by the matrixformula_11等斜旋轉.A special case of the double rotation is when the angles are equal, that is if "α""β" ≠ 0. This is called an , and it differs from a general double rotation in a number of ways. For example in an isoclinic rotation, all non-zero points rotate through the same angle, α. Most importantly the planes of rotation are not uniquely identified. There are instead an infinite number of pairs of orthogonal planes that can be treated as planes of rotation. For example any point can be taken, and the plane it rotates in together with the plane orthogonal to it can be used as two planes of rotation.高維.As already noted the maximum number of planes of rotation in n dimensions isformula_12so the complexity quickly increases with more than four dimensions and categorising rotations as above becomes too complex to be practical, but some observations can be made.Simple rotations can be identified in all dimensions, as rotations with just one plane of rotation. A simple rotation in ("n" − 2)-dimensional subspace orthogonal to the plane of rotation.A general rotation is not simple, and has the maximum number of planes of rotation as given above. In the general case the angles of rotations in these planes are distinct and the planes are uniquely defined. If any of the angles are the same then the planes are not unique, as in four dimensions with an isoclinic rotation.In even dimensions ("n"2, 4, 6...) there are up to "n"3, 5, 7, ...) there are  planes and angles of rotation, the same as the even dimension one lower. These do not span the space, but leave a line which does not rotate – like the in three dimensions, except rotations do not take place about this line but in multiple planes orthogonal to it.數學性質.The examples given above were chosen to be clear and simple examples of rotations, with planes generally parallel to the coordinate axes in three and four dimensions. But this is not generally the case: planes are not usually parallel to the axes, and the matrices cannot simply be written down. In all dimensions the rotations are fully described by the planes of rotation and their associated angles, so it is useful to be able to determine them, or at least find ways to describe them mathematically.反射.Every simple rotation can be generated by two reflections. Reflections can be specified in ("n" − 1)-dimensional subspace to reflect in, so a two-dimensional reflection is in a line, a three-dimensional reflection is in a plane, and so on. But this becomes increasingly difficult to apply in higher dimensions, so it is better to use vectors instead, as follows.A reflection in ("n" − 1)-dimensional subspace. To generate simple rotations only reflections that fix the origin are needed, so the vector does not have a position, just direction. It does also not matter which way it is facing: it can be replaced with its negative without changing the result. Similarly s can be used to simplify the calculations.So the reflection in a m, thus:formula_13where the product is the geometric product from .If n perpendicular to it, the result isformula_14This is a simple rotation in n. It can be checked using geometric algebra that this is a rotation, and that it rotates all vectors as expected.The quantity nm is its inverse asformula_15So the rotation can be writtenformula_16where "R"mn is the rotor.The plane of rotation is the plane containing m.Conversely all simple rotations can be generated this way, with two reflections, by two unit vectors in the plane of rotation separated by half the desired angle of rotation. These can be composed to produce more general rotations, using up to n is odd, by choosing pairs of reflections given by two vectors in each plane of rotation.二重向量.二重向量s are quantities from , and the , which generalise the idea of vectors into two dimensions. As vectors are to lines, so are bivectors to planes. So every plane (in any dimension) can be associated with a bivector, and every is associated with a plane. This makes them a good fit for describing planes of rotation.Every rotation plane in a rotation has a simple bivector associated with it. This is parallel to the plane and has magnitude equal to the angle of rotation in the plane. These bivectors are summed to produce a single, generally non-simple, bivector for the whole rotation. This can generate a through the , which can be used to rotate an object.Bivectors are related to rotors through the exponential map (which applied to bivectors generates rotors and rotations using 棣莫弗公式). In particular given any bivector B the rotor associated with it isformula_17This is a simple rotation if the bivector is simple, a more general rotation otherwise. When squared,formula_18it gives a rotor that rotates through twice the angle. If mn gives a rotation through twice the angle between the vectors. These can be equated,formula_19from which it follows that the bivector associated with the plane of rotation containing n isformula_20This is a simple bivector, associated with the simple rotation described. More general rotations in four or more dimensions are associated with sums of simple bivectors, one for each plane of rotation, calculated as above.Examples include the two rotations in four dimensions given above. The simple rotation in the e34"β" which are parallel to the two planes of rotation and have magnitudes equal to the angles of rotation.Given a rotor the bivector associated with it can be recovered by taking the logarithm of the rotor, which can then be split into simple bivectors to determine the planes of rotation, although in practice for all but the simplest of cases this may be impractical. But given the simple bivectors geometric algebra is a useful tool for studying planes of rotation using algebra like the above.特徵值和特徵平面.The planes of rotations for a particular rotation using the s. Given a general rotation matrix in n dimensions its has either one (in odd dimensions) or zero (in even dimensions) real roots. The other roots are in complex conjugate pairs, exactlyformula_12such pairs. These correspond to the planes of rotation, the s of the matrix, which can be calculated using algebraic techniques. In addition s of the complex roots are the magnitudes of the bivectors associated with the planes of rotations. The form of the characteristic equation is related to the planes, making it possible to relate its algebraic properties like repeated roots to the bivectors, where repeated bivector magnitudes have particular geometric interpretations.