In
mathematics
Mathematics is a field of study that discovers and organizes methods, Mathematical theory, theories and theorems that are developed and Mathematical proof, proved for the needs of empirical sciences and mathematics itself. There are many ar ...
, a versor is a
quaternion
In mathematics, the quaternion number system extends the complex numbers. Quaternions were first described by the Irish mathematician William Rowan Hamilton in 1843 and applied to mechanics in three-dimensional space. The algebra of quater ...
of
norm one, also known as a unit quaternion. Each versor has the form
:
where the r
2 = −1 condition means that r is an
imaginary unit
The imaginary unit or unit imaginary number () is a mathematical constant that is a solution to the quadratic equation Although there is no real number with this property, can be used to extend the real numbers to what are called complex num ...
. There is a
sphere of imaginary units in the quaternions. Note that the expression for a versor is just
Euler's formula
Euler's formula, named after Leonhard Euler, is a mathematical formula in complex analysis that establishes the fundamental relationship between the trigonometric functions and the complex exponential function. Euler's formula states that, for ...
for the imaginary unit r. In case (a
right angle
In geometry and trigonometry, a right angle is an angle of exactly 90 Degree (angle), degrees or radians corresponding to a quarter turn (geometry), turn. If a Line (mathematics)#Ray, ray is placed so that its endpoint is on a line and the ad ...
), then
, and it is called a ''
right versor''.
The mapping
corresponds to
3-dimensional rotation, and has the angle 2''a'' about the axis r in
axis–angle representation
In mathematics, the axis–angle representation parameterizes a rotation in a three-dimensional Euclidean space by two quantities: a unit vector indicating the direction of an axis of rotation, and an angle of rotation describing the magnitu ...
.
The collection of versors, with quaternion multiplication, forms a
group
A group is a number of persons or things that are located, gathered, or classed together.
Groups of people
* Cultural group, a group whose members share the same cultural identity
* Ethnic group, a group whose members share the same ethnic iden ...
, and appears as a
3-sphere
In mathematics, a hypersphere or 3-sphere is a 4-dimensional analogue of a sphere, and is the 3-dimensional n-sphere, ''n''-sphere. In 4-dimensional Euclidean space, it is the set of points equidistant from a fixed central point. The interior o ...
in the 4-dimensional quaternion algebra.
Presentation on 3- and 2-spheres
Hamilton denoted the versor of a quaternion ''q'' by the symbol U ''q''. He was then able to display the general quaternion in
polar coordinate form
: ''q'' = T ''q'' U ''q'',
where T ''q'' is the norm of ''q''. The norm of a versor is always equal to one; hence they occupy the unit
3-sphere
In mathematics, a hypersphere or 3-sphere is a 4-dimensional analogue of a sphere, and is the 3-dimensional n-sphere, ''n''-sphere. In 4-dimensional Euclidean space, it is the set of points equidistant from a fixed central point. The interior o ...
in
. Examples of versors include the eight elements of the
quaternion group
In group theory, the quaternion group Q8 (sometimes just denoted by Q) is a nonabelian group, non-abelian group (mathematics), group of Group order, order eight, isomorphic to the eight-element subset
\ of the quaternions under multiplication. ...
. Of particular importance are the
right versors, which have
angle π/2. These versors have zero scalar part, and so are
vectors of length one (unit vectors). The right versors form a
sphere of square roots of −1 in the quaternion algebra. The generators ''i'', ''j'', and ''k'' are examples of right versors, as well as their
additive inverse
In mathematics, the additive inverse of an element , denoted , is the element that when added to , yields the additive identity, 0 (zero). In the most familiar cases, this is the number 0, but it can also refer to a more generalized zero el ...
s. Other versors include the twenty-four
Hurwitz quaternion
In mathematics, a Hurwitz quaternion (or Hurwitz integer) is a quaternion whose components are ''either'' all integers ''or'' all half-integers (halves of odd integers; a mixture of integers and half-integers is excluded). The set of all Hurwitz ...
s that have the norm 1 and form vertices of a
24-cell
In four-dimensional space, four-dimensional geometry, the 24-cell is the convex regular 4-polytope (four-dimensional analogue of a Platonic solid) with Schläfli symbol . It is also called C24, or the icositetrachoron, octaplex (short for "octa ...
polychoron.
Hamilton defined a quaternion as the quotient of two vectors. A versor can be defined as the quotient of two unit vectors. For any fixed
plane the quotient of two unit vectors lying in depends only on the
angle
In Euclidean geometry, an angle can refer to a number of concepts relating to the intersection of two straight Line (geometry), lines at a Point (geometry), point. Formally, an angle is a figure lying in a Euclidean plane, plane formed by two R ...
(directed) between them, the same as in the unit vector–angle representation of a versor explained above. That's why it may be natural to understand corresponding versors as directed
arcs that connect pairs of unit vectors and lie on a
great circle
In mathematics, a great circle or orthodrome is the circular intersection of a sphere and a plane passing through the sphere's center point.
Discussion
Any arc of a great circle is a geodesic of the sphere, so that great circles in spher ...
formed by intersection of Π with the
unit sphere
In mathematics, a unit sphere is a sphere of unit radius: the locus (mathematics), set of points at Euclidean distance 1 from some center (geometry), center point in three-dimensional space. More generally, the ''unit -sphere'' is an n-sphere, -s ...
, where the plane Π passes through the origin. Arcs of the same direction and length (or, the same,
subtended angle
In geometry, an angle subtended (from Latin for "stretched under") by a line segment at an arbitrary vertex is formed by the two rays between the vertex and each endpoint of the segment.
For example, a side of a triangle ''subtends'' the o ...
in
radian
The radian, denoted by the symbol rad, is the unit of angle in the International System of Units (SI) and is the standard unit of angular measure used in many areas of mathematics. It is defined such that one radian is the angle subtended at ...
s) are
equipollent and correspond to the same versor.
Such an arc, although lying in the
three-dimensional space
In geometry, a three-dimensional space (3D space, 3-space or, rarely, tri-dimensional space) is a mathematical space in which three values ('' coordinates'') are required to determine the position of a point. Most commonly, it is the three- ...
, does not represent a path of a point rotating as described with the sandwiched product with the versor. Indeed, it represents the left multiplication action of the versor on quaternions that preserves the plane Π and the corresponding great circle of 3-vectors. The 3-dimensional rotation defined by the versor has the angle two times the arc's subtended angle, and preserves the same plane. It is a rotation about the corresponding vector , that is
perpendicular
In geometry, two geometric objects are perpendicular if they intersect at right angles, i.e. at an angle of 90 degrees or π/2 radians. The condition of perpendicularity may be represented graphically using the '' perpendicular symbol'', � ...
to .
On three unit vectors, Hamilton writes
:
and
:
imply
:
Multiplication of quaternions of norm one corresponds to the (non-commutative) "addition" of great circle arcs on the unit sphere. Any pair of great circles either is the same circle or has two
intersection points. Hence, one can always move the point and the corresponding vector to one of these points such that the beginning of the second arc will be the same as the end of the first arc.
An equation
:
implicitly specifies the unit vector–angle representation for the product of two versors. Its solution is an instance of the general
Campbell–Baker–Hausdorff formula in
Lie group
In mathematics, a Lie group (pronounced ) is a group (mathematics), group that is also a differentiable manifold, such that group multiplication and taking inverses are both differentiable.
A manifold is a space that locally resembles Eucli ...
theory. As the 3-sphere represented by versors in
is a 3-parameter Lie group, practice with versor compositions is a step into
Lie theory
In mathematics, the mathematician Sophus Lie ( ) initiated lines of study involving integration of differential equations, transformation groups, and contact (mathematics), contact of spheres that have come to be called Lie theory. For instance, ...
. Evidently versors are the image of the
exponential map applied to a ball of radius π in the quaternion subspace of vectors.
Versors compose as aforementioned vector arcs, and Hamilton referred to this
group operation
In mathematics, a group is a set with an operation that combines any two elements of the set to produce a third element within the same set and the following conditions must hold: the operation is associative, it has an identity element, and ev ...
as "the sum of arcs", but as quaternions they simply multiply.
The geometry of
elliptic space has been described as the space of versors.
Representation of SO(3)
The
orthogonal group
In mathematics, the orthogonal group in dimension , denoted , is the Group (mathematics), group of isometry, distance-preserving transformations of a Euclidean space of dimension that preserve a fixed point, where the group operation is given by ...
in three dimensions,
rotation group SO(3)
In mechanics and geometry, the 3D rotation group, often denoted SO(3), is the group of all rotations about the origin of three-dimensional Euclidean space \R^3 under the operation of composition.
By definition, a rotation about the origin is a ...
, is frequently interpreted with versors via the
inner automorphism
In abstract algebra, an inner automorphism is an automorphism of a group, ring, or algebra
Algebra is a branch of mathematics that deals with abstract systems, known as algebraic structures, and the manipulation of expressions within thos ...
where is a versor. Indeed, if
:
and vector is perpendicular to ,
then
:
by calculation. The plane
is isomorphic to
and the inner automorphism, by commutativity, reduces to the identity mapping there.
Since quaternions can be interpreted as an algebra of two complex dimensions, the rotation
action can also be viewed through the
special unitary group
In mathematics, the special unitary group of degree , denoted , is the Lie group of unitary matrices with determinant 1.
The matrices of the more general unitary group may have complex determinants with absolute value 1, rather than real 1 ...
SU(2)
In mathematics, the special unitary group of degree , denoted , is the Lie group of unitary matrices with determinant 1.
The matrices of the more general unitary group may have complex determinants with absolute value 1, rather than real 1 ...
.
For a fixed , versors of the form
where
form a
subgroup
In group theory, a branch of mathematics, a subset of a group G is a subgroup of G if the members of that subset form a group with respect to the group operation in G.
Formally, given a group (mathematics), group under a binary operation  ...
isomorphic to the
circle group
In mathematics, the circle group, denoted by \mathbb T or , is the multiplicative group of all complex numbers with absolute value 1, that is, the unit circle in the complex plane or simply the unit complex numbers
\mathbb T = \.
The circle g ...
. Orbits of the left multiplication action of this subgroup are fibers of a
fiber bundle
In mathematics, and particularly topology, a fiber bundle ( ''Commonwealth English'': fibre bundle) is a space that is a product space, but may have a different topological structure. Specifically, the similarity between a space E and a pr ...
over the 2-sphere, known as
Hopf fibration in the case other vectors give isomorphic, but not identical fibrations. gives an elementary introduction to quaternions to elucidate the Hopf fibration as a mapping on unit quaternions. He writes "the fibers of the Hopf map are circles in S
3 ".
Versors have been used to represent rotations of the
Bloch sphere
In quantum mechanics and computing, the Bloch sphere is a geometrical representation of the pure state space of a two-level quantum mechanical system ( qubit), named after the physicist Felix Bloch.
Mathematically each quantum mechanical syst ...
with quaternion multiplication.
Elliptic space
The facility of versors illustrate
elliptic geometry
Elliptic geometry is an example of a geometry in which Euclid's parallel postulate does not hold. Instead, as in spherical geometry, there are no parallel lines since any two lines must intersect. However, unlike in spherical geometry, two lines ...
, in particular
elliptic space, a three-dimensional realm of rotations. The versors are the points of this elliptic space, though they refer to
rotations in 4-dimensional Euclidean space. Given two fixed versors and , the mapping
is an ''elliptic motion''. If one of the fixed versors is 1, then the motion is a ''Clifford translation'' of the elliptic space, named after
William Kingdon Clifford
William Kingdon Clifford (4 May 18453 March 1879) was a British mathematician and philosopher. Building on the work of Hermann Grassmann, he introduced what is now termed geometric algebra, a special case of the Clifford algebra named in his ...
who was a proponent of the space. An elliptic line through versor is
Parallelism in the space is expressed by
Clifford parallels. One of the methods of viewing elliptic space uses the
Cayley transform
In mathematics, the Cayley transform, named after Arthur Cayley, is any of a cluster of related things. As originally described by , the Cayley transform is a mapping between skew-symmetric matrices and special orthogonal matrices. The transform ...
to map the versors to
Subgroups
The set of all versors, with their multiplication as quaternions, forms a
continuous group
In mathematics, topological groups are the combination of groups and topological spaces, i.e. they are groups and topological spaces at the same time, such that the continuity condition for the group operations connects these two structures ...
''G''. For a fixed pair
of right versors,
is a
one-parameter subgroup that is isomorphic to the
circle group
In mathematics, the circle group, denoted by \mathbb T or , is the multiplicative group of all complex numbers with absolute value 1, that is, the unit circle in the complex plane or simply the unit complex numbers
\mathbb T = \.
The circle g ...
.
Next consider the finite subgroups, beyond the
quaternion group
In group theory, the quaternion group Q8 (sometimes just denoted by Q) is a nonabelian group, non-abelian group (mathematics), group of Group order, order eight, isomorphic to the eight-element subset
\ of the quaternions under multiplication. ...
Q
8:
As noted by
Hurwitz, the 16 quaternions
all have norm one, so they are in ''G''. Joined with Q
8, these unit
Hurwitz quaternion
In mathematics, a Hurwitz quaternion (or Hurwitz integer) is a quaternion whose components are ''either'' all integers ''or'' all half-integers (halves of odd integers; a mixture of integers and half-integers is excluded). The set of all Hurwitz ...
s form a group ''G''
2 of order 24 called the
binary tetrahedral group
In mathematics, the binary tetrahedral group, denoted 2T or ,Coxeter&Moser: Generators and Relations for discrete groups: : Rl = Sm = Tn = RST is a certain nonabelian group of order (group theory), order 24. It is an group extension, extension of ...
. The group elements, taken as points on S
3, form a
24-cell
In four-dimensional space, four-dimensional geometry, the 24-cell is the convex regular 4-polytope (four-dimensional analogue of a Platonic solid) with Schläfli symbol . It is also called C24, or the icositetrachoron, octaplex (short for "octa ...
.
By a process of
bitruncation of the 24-cell, the
48-cell on ''G'' is obtained, and these versors multiply as the
binary octahedral group.
Another subgroup is formed by 120
icosian
In mathematics, the icosians are a specific set of Hamiltonian quaternions with the same symmetry as the 600-cell. The term can be used to refer to two related, but distinct, concepts:
* The icosian Group (mathematics), group: a multiplicative g ...
s which multiply in the manner of the
binary icosahedral group
In mathematics, the binary icosahedral group 2''I'' or Coxeter&Moser: Generators and Relations for discrete groups: : Rl = Sm = Tn = RST is a certain nonabelian group of order 120.
It is an extension of the icosahedral group ''I'' or (2,3,5) o ...
.
Hyperbolic versor
A hyperbolic versor is a generalization of quaternionic versors to
indefinite orthogonal group
In mathematics, the indefinite orthogonal group, is the Lie group of all linear transformations of an ''n''-dimension (vector space), dimensional real number, real vector space that leave invariant a nondegenerate form, nondegenerate, symmetric bi ...
s, such as
Lorentz group
In physics and mathematics, the Lorentz group is the group of all Lorentz transformations of Minkowski spacetime, the classical and quantum setting for all (non-gravitational) physical phenomena. The Lorentz group is named for the Dutch physi ...
.
It is defined as a quantity of the form
:
where
Such elements arise in
split algebras, for example
split-complex number
In algebra, a split-complex number (or hyperbolic number, also perplex number, double number) is based on a hyperbolic unit satisfying j^2=1, where j \neq \pm 1. A split-complex number has two real number components and , and is written z=x+y ...
s or
split-quaternions. It was the algebra of
tessarines discovered by
James Cockle in 1848 that first provided hyperbolic versors. In fact, Cockle wrote the above equation (with in place of ) when he found that the tessarines included the new type of imaginary element.
This versor was used by
Homersham Cox (1882/1883) in relation to quaternion multiplication. The primary exponent of hyperbolic versors was
Alexander Macfarlane
Alexander Macfarlane FRSE LLD (21 April 1851 – 28 August 1913) was a Scottish logician, physicist, and mathematician.
Life
Macfarlane was born in Blairgowrie, Scotland, to Daniel MacFarlane (Shoemaker, Blairgowrie) and Ann Small. He s ...
, as he worked to shape quaternion theory to serve physical science. He saw the modelling power of hyperbolic versors operating on the split-complex number plane, and in 1891 he introduced
hyperbolic quaternions to extend the concept to 4-space. Problems in that algebra led to use of
biquaternion
In abstract algebra, the biquaternions are the numbers , where , and are complex numbers, or variants thereof, and the elements of multiply as in the quaternion group and commute with their coefficients. There are three types of biquaternions cor ...
s after 1900. In a widely seen review, Macfarlane wrote:
:... the root of a quadratic equation may be versor in nature or scalar in nature. If it is versor in nature, then the part affected by the radical involves the axis perpendicular to the plane of reference, and this is so, whether the radical involves the square root of minus one or not. In the former case the versor is circular, in the latter hyperbolic.
Today the concept of a
one-parameter group
In mathematics, a one-parameter group or one-parameter subgroup usually means a continuous group homomorphism
:\varphi : \mathbb \rightarrow G
from the real line \mathbb (as an additive group) to some other topological group G.
If \varphi is in ...
subsumes the concepts of versor and hyperbolic versor as the terminology of
Sophus Lie
Marius Sophus Lie ( ; ; 17 December 1842 – 18 February 1899) was a Norwegian mathematician. He largely created the theory of continuous symmetry and applied it to the study of geometry and differential equations. He also made substantial cont ...
has replaced that of Hamilton and Macfarlane.
In particular, for each such that or , the mapping
takes the
real line
A number line is a graphical representation of a straight line that serves as spatial representation of numbers, usually graduated like a ruler with a particular origin (geometry), origin point representing the number zero and evenly spaced mark ...
to a group of hyperbolic or ordinary versors. In the ordinary case, when and − are
antipodes
In geography, the antipode () of any spot on Earth is the point on Earth's surface diametrically opposite to it. A pair of points ''antipodal'' () to each other are situated such that a straight line connecting the two would pass through Ea ...
on a sphere, the one-parameter groups have the same points but are oppositely directed. In physics, this aspect of
rotational symmetry
Rotational symmetry, also known as radial symmetry in geometry, is the property a shape (geometry), shape has when it looks the same after some rotation (mathematics), rotation by a partial turn (angle), turn. An object's degree of rotational s ...
is termed a
doublet.
defined the parameter ''
rapidity
In special relativity, the classical concept of velocity is converted to rapidity to accommodate the limit determined by the speed of light. Velocities must be combined by Einstein's velocity-addition formula. For low speeds, rapidity and velo ...
'', which specifies a change in
frame of reference
In physics and astronomy, a frame of reference (or reference frame) is an abstract coordinate system, whose origin (mathematics), origin, orientation (geometry), orientation, and scale (geometry), scale have been specified in physical space. It ...
. This ''rapidity'' parameter corresponds to the real variable in a one-parameter group of hyperbolic versors. With the further development of
special relativity
In physics, the special theory of relativity, or special relativity for short, is a scientific theory of the relationship between Spacetime, space and time. In Albert Einstein's 1905 paper, Annus Mirabilis papers#Special relativity,
"On the Ele ...
the action of a hyperbolic versor came to be called a
Lorentz boost
In physics, the Lorentz transformations are a six-parameter family of linear transformations from a coordinate frame in spacetime to another frame that moves at a constant velocity relative to the former. The respective inverse transformation ...
.
Lie theory
Sophus Lie
Marius Sophus Lie ( ; ; 17 December 1842 – 18 February 1899) was a Norwegian mathematician. He largely created the theory of continuous symmetry and applied it to the study of geometry and differential equations. He also made substantial cont ...
was less than a year old when Hamilton first described quaternions, but Lie's name has become associated with all groups generated by exponentiation. The set of versors with their multiplication has been denoted Sl(1,q) by .
[ — This text denotes the real, complex, and quaternion division algebras by , , and , respectively, rather than now standard , , and .] Sl(1,q) is the
special linear group
In mathematics, the special linear group \operatorname(n,R) of degree n over a commutative ring R is the set of n\times n Matrix (mathematics), matrices with determinant 1, with the group operations of ordinary matrix multiplication and matrix ...
of one dimension over quaternions, the "special" indicating that all elements are of norm one. The group is isomorphic to SU(2,c), a
special unitary group
In mathematics, the special unitary group of degree , denoted , is the Lie group of unitary matrices with determinant 1.
The matrices of the more general unitary group may have complex determinants with absolute value 1, rather than real 1 ...
, a frequently used designation since quaternions and versors are sometimes considered archaic for group theory. The
special orthogonal group SO(3,r) of rotations in three dimensions is closely related: it is a 2:1 homomorphic image of SU(2,c).
The subspace
is called the
Lie algebra
In mathematics, a Lie algebra (pronounced ) is a vector space \mathfrak g together with an operation called the Lie bracket, an alternating bilinear map \mathfrak g \times \mathfrak g \rightarrow \mathfrak g, that satisfies the Jacobi ident ...
of the group of versors. The commutator product
is just double the
cross product
In mathematics, the cross product or vector product (occasionally directed area product, to emphasize its geometric significance) is a binary operation on two vectors in a three-dimensional oriented Euclidean vector space (named here E), and ...
of two vectors, which forms the multiplication operation in the Lie algebra. The close relation to SU(1,c) and SO(3,r) is evident in the isomorphism of their Lie algebras.
[
Lie groups that contain hyperbolic versors include the group on the unit hyperbola and the special unitary group SU(1,1).
]
Etymology
The word is derived from Latin ''versari'' = "to turn" with the suffix ''-or'' forming a noun from the verb (i.e. ''versor'' = "the turner"). It was introduced by William Rowan Hamilton
Sir William Rowan Hamilton (4 August 1805 – 2 September 1865) was an Irish astronomer, mathematician, and physicist who made numerous major contributions to abstract algebra, classical mechanics, and optics. His theoretical works and mathema ...
in the 1840s in the context of his quaternion
In mathematics, the quaternion number system extends the complex numbers. Quaternions were first described by the Irish mathematician William Rowan Hamilton in 1843 and applied to mechanics in three-dimensional space. The algebra of quater ...
theory.
Versors in geometric algebra
The term "versor" is generalised in geometric algebra
In mathematics, a geometric algebra (also known as a Clifford algebra) is an algebra that can represent and manipulate geometrical objects such as vectors. Geometric algebra is built out of two fundamental operations, addition and the geometric pr ...
to indicate a member of the algebra that can be expressed as the product of invertible vectors, .
Just as a quaternion versor can be used to represent a rotation of a quaternion with mapping , so can a versor in Geometric Algebra be used to represent the result of reflections on a member of the algebra with mapping .
A rotation can be considered the result of two reflections, so it turns out a quaternion versor can be identified as a 2-versor in the geometric algebra of three real dimensions .
In a departure from Hamilton's definition, multivector versors are not required to have unit norm, just to be invertible. Normalisation can still be useful however, so it is convenient to designate versors as ''unit versors'' in a geometric algebra if , where the tilde denotes reversion of the versor.
See also
* cis (mathematics)
is a mathematical notation defined by , where is the cosine function, is the imaginary unit and is the sine function. is the argument of the complex number (angle between line to point and x-axis in polar form). The notation is less commo ...
()
* Quaternions and spatial rotation unit vector, Unit quaternions, known as versor, ''versors'', provide a convenient mathematics, mathematical notation for representing spatial Orientation (geometry), orientations and rotations of elements in three dimensional space. Specifically, th ...
* Rotations in 4-dimensional Euclidean space
* Turn (geometry)
References
Sources
*
*
*
*
:
*
*
*
*
:: Section IV: Versors and unitary vectors in the system of quaternions.
:: Section V: Versor and unitary vectors in vector algebra.
External links
''Versor''
at Encyclopedia of Mathematics
The ''Encyclopedia of Mathematics'' (also ''EOM'' and formerly ''Encyclopaedia of Mathematics'') is a large reference work in mathematics.
Overview
The 2002 version contains more than 8,000 entries covering most areas of mathematics at a graduat ...
.
* Luis Ibáñe
Quaternion tutorial
{{Webarchive, url=https://web.archive.org/web/20120204055438/http://www.itk.org/CourseWare/Training/QuaternionsI.pdf , date=2012-02-04 from National Library of Medicine
The United States National Library of Medicine (NLM), operated by the United States federal government, is the world's largest medical library.
Located in Bethesda, Maryland, the NLM is an institute within the National Institutes of Health. I ...
Spherical trigonometry
Quaternions
Rotation in three dimensions
William Rowan Hamilton