Editing 16-cell (section)

=== Rotations ===
{| class="wikitable" width=480
|- align=center valign=top
|rowspan=2|[[File:16-cell.gif]]<br />A 3D projection of a 16-cell performing a [[SO(4)#Simple rotations|simple rotation]]
|[[File:16-cell-orig.gif]]<br />A 3D projection of a 16-cell performing a [[SO(4)#Double rotations|double rotation]]
|}
[[Rotations in 4-dimensional Euclidean space]] can be seen as the composition of two 2-dimensional rotations in [[completely orthogonal]] planes.{{Sfn|Kim|Rote|2016|p=6|loc=§&nbsp;5. Four-Dimensional Rotations}} The 16-cell is a simple frame in which to observe 4-dimensional rotations, because each of the 16-cell's 6 great squares has another completely orthogonal great square (there are 3 pairs of completely orthogonal squares).{{Efn|name=six orthogonal planes of the Cartesian basis}} Many rotations of the 16-cell can be characterized by the angle of rotation in one of its great square planes (e.g. the ''xy'' plane) and another angle of rotation in the completely orthogonal great square plane (the ''wz'' plane).{{Efn|Each great square vertex is {{radic|2}} distant from two of the square's other vertices, and {{radic|4}} distant from its opposite vertex. The other four vertices of the 16-cell (also {{radic|2}} distant) are the vertices of the square's completely orthogonal square.{{Efn|name=Clifford parallel great squares}} Each 16-cell vertex is a vertex of ''three'' orthogonal great squares which intersect there. Each of them has a different ''completely'' orthogonal square. Thus there are three great squares completely orthogonal to each vertex: squares that the vertex is not part of.{{Efn|The three ''incompletely'' orthogonal great squares which intersect at each vertex of the 16-cell form the vertex's octahedral [[vertex figure]].{{Efn|name=octahedral pyramid}} Any two of them, together with the completely orthogonal square of the third, also form an octahedron: a central octahedral hyperplane.{{Efn|Three great squares meet at each vertex (and at its opposite vertex) in the 16-cell. Each of them has a different completely orthogonal square. Thus there are three great squares completely orthogonal to each vertex and its opposite vertex (each axis). They form an octahedron (a central hyperplane). Every axis line in the 16-cell is completely orthogonal to a central octahedron hyperplane, as every great square plane is completely orthogonal to another great square plane.{{Efn|name=six orthogonal planes of the Cartesian basis}} The axis and the octahedron intersect only at one point (the center of the 16-cell), as each pair of completely orthogonal great squares intersects only at one point (the center of the 16-cell). Each central octahedron is also the octahedral vertex figure of two of the eight vertices: the two on its completely orthogonal axis.|name=octahedral hyperplanes}} In the 16-cell, each octahedral vertex figure is also a central octahedral hyperplane.|name=completely orthogonal great squares}}|name=vertex and central octahedra}} Completely orthogonal great squares have disjoint vertices: 4 of the 16-cell's 8 vertices rotate in one plane, and the other 4 rotate independently in the completely orthogonal plane.{{Efn|Completely orthogonal great squares are non-intersecting and rotate independently because the great circles on which their vertices lie are [[Clifford parallel]].{{Efn|[[Clifford parallel]]s are non-intersecting curved lines that are parallel in the sense that the perpendicular (shortest) distance between them is the same at each point.{{Sfn|Tyrrell|Semple|1971|loc=§&nbsp;3. Clifford's original definition of parallelism|pp=5-6}} A double helix is an example of Clifford parallelism in ordinary 3-dimensional Euclidean space. In 4-space Clifford parallels occur as geodesic great circles on the [[3-sphere]].{{Sfn|Kim|Rote|2016|pp=7-10|loc=§&nbsp;6. Angles between two Planes in 4-Space}} In the 16-cell the corresponding vertices of completely orthogonal great circle squares are all {{radic|2}} apart, so these squares are Clifford parallel polygons.{{Efn|name=completely orthogonal Clifford parallels are special}} Note that only the vertices of the great squares (the points on the great circle) are {{radic|2}} apart; points on the edges of the squares (on chords of the circle) are closer together.|name=Clifford parallels}} They are {{radic|2}} apart at each pair of nearest vertices (and in the 16-cell ''all'' the pairs except antipodal pairs are nearest). The two squares cannot intersect at all because they lie in planes which intersect at only one point: the center of the 16-cell.{{Efn|name=six orthogonal planes of the Cartesian basis}} Because they are perpendicular and share a common center, the two squares are obviously not parallel and separate in the usual way of parallel squares in 3 dimensions; rather they are connected like adjacent square links in a chain, each passing through the other without intersecting at any points, forming a [[Hopf link]].|name=Clifford parallel great squares}}

In 2 or 3 dimensions a rotation is characterized by a single plane of rotation; this kind of rotation taking place in 4-space is called a [[Rotations in 4-dimensional Euclidean space#Simple rotations|simple rotation]], in which only one of the two completely orthogonal planes rotates (the angle of rotation in the other plane is 0). In the 16-cell, a simple rotation in one of the 6 orthogonal planes moves only 4 of the 8 vertices; the other 4 remain fixed. (In the simple rotation animation above, all 8 vertices move because the plane of rotation is not one of the 6 orthogonal basis planes.)

In a [[Rotations in 4-dimensional Euclidean space#Double rotations|double rotation]] both sets of 4 vertices move, but independently: the angles of rotation may be different in the 2 completely orthogonal planes. If the two angles happen to be the same, a maximally symmetric [[Rotations in 4-dimensional Euclidean space#Isoclinic rotations|isoclinic rotation]] takes place.{{Efn|In an isoclinic rotation, all 6 orthogonal planes are displaced in two orthogonal directions at once: they are rotated by the same angle, and at the same time they are tilted ''sideways'' by that same angle. An isoclinic displacement (also known as a [[William Kingdon Clifford|Clifford]] displacement) is 4-dimensionally diagonal. Points are displaced an equal distance in four orthogonal directions at once, and displaced a total [[Pythagorean distance#Higher dimensions|Pythagorean distance]] equal to the square root of four times the square of that distance. All vertices of a regular 4-polytope are displaced to a vertex at least two edge lengths away. For example, when the unit-radius 16-cell rotates isoclinically 90° in a great square invariant plane, it also rotates 90° in the completely orthogonal great square invariant plane.{{Efn||name=six orthogonal planes of the Cartesian basis}} The great square plane also tilts sideways 90° to occupy its completely orthogonal plane. (By isoclinic symmetry, ''every'' great square rotates 90° ''and'' tilts sideways 90° into its completely orthogonal plane.) Each vertex (in every great square) is displaced to its antipodal vertex, at a distance of {{radic|1}} in each of four orthogonal directions, a total distance of {{radic|4}}.{{Efn|Opposite vertices in a unit-radius 4-polytope correspond to the opposite vertices of an 8-cell hypercube (tesseract). The long diagonal of this [[Tesseract#Radial equilateral symmetry|radially equilateral 4-cube]] is {{radic|4}}. In a 90° isoclinic rotation each vertex of the 16-cell is displaced to its antipodal vertex, traveling along a helical geodesic arc of length 𝝅 (180°), to a vertex {{radic|4}} away along the long diameter of the unit-radius 4-polytope (16-cell or tesseract), the same total displacement as if it had been displaced {{radic|1}} four times by traveling along a path of four successive orthogonal edges of the tesseract.|name=long diagonal of the 4-cube}} The original and displaced vertex are two edge lengths apart by three{{Efn|There are six different two-edge paths connecting a pair of antipodal vertices along the edges of a great square. The left isoclinic rotation runs diagonally between three of them, and the right isoclinic rotation runs diagonally between the other three. These diagonals are the straight lines (geodesics) connecting opposite vertices of face-bonded tetrahedral cells in the left-handed [[#Helical construction|eight-cell ring]] and the right-handed eight-cell ring, respectively.}} different paths along two edges of a great square. But the [[#Helical construction|isocline]] (the helical arc the vertex follows during the isoclinic rotation) does not run along edges: it runs ''between'' these different edge-paths diagonally, on a geodesic (shortest arc) between the original and displaced vertices.{{Efn|name=isocline}} This isoclinic geodesic arc is not a segment of an ordinary great circle; it does not lie in the plane of any great square. It is a helical 180° arc that bends in a circle in two completely orthogonal planes at once. This [[Möbius loop|Möbius circle]] does not lie in any plane or intersect any vertices between the original and the displaced vertex.{{Efn|name=Möbius circle}}|name=isoclinic rotation}} In the 16-cell an isoclinic rotation by 90 degrees of any pair of completely orthogonal square planes takes every square plane to its completely orthogonal square plane.{{Efn|The 90 degree isoclinic rotation of two completely orthogonal planes takes them to each other. In such a rotation of a rigid 16-cell, all 6 orthogonal planes rotate by 90 degrees, and also tilt sideways by 90 degrees to their completely orthogonal (Clifford parallel){{Efn|name=Clifford parallels}} plane.{{Sfn|Kim|Rote|2016|pp=8-10|loc=Relations to Clifford Parallelism}} The corresponding vertices of the two completely orthogonal great squares are {{radic|4}} (180°) apart; the great squares (Clifford parallel polytopes) are {{radic|4}} (180°) apart; but the two completely orthogonal ''planes'' are 90° apart, in the ''two'' orthogonal angles that separate them. If the isoclinic rotation is continued through another 90°, each vertex completes a 360° rotation and each great square returns to its original plane, but in a different orientation (axes swapped): it has been turned "upside down" on the surface of the 16-cell (which is now "inside out"). Continuing through a second 360° isoclinic rotation (through four 90° by 90° isoclinic steps, a 720° rotation) returns everything to its original place and orientation.|name=exchange of completely orthogonal planes}}