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 corresponding to complex numbers and the variations thereof:
This article is about the "ordinary biquaternions" named by William Rowan Hamilton in 1844 (see "Proceedings of the Royal Irish Academy" 1844 & 1850 page 388). Some of the more prominent proponents of these biquaternions include Alexander Macfarlane, Arthur W. Conway, Ludwik Silberstein, and Cornelius Lanczos. As developed below, the unit quasi-sphere of the biquaternions provides a representation of the Lorentz group, which is the foundation of special relativity.
The algebra of biquaternions can be considered as a tensor product formula_1 (taken over the reals) where or formula_2 is the field of complex numbers and or formula_3 is the division algebra of (real) quaternions. In other words, the biquaternions are just the complexification of the quaternions. Viewed as a complex algebra, the biquaternions are isomorphic to the algebra of complex matrices . They are also isomorphic to several Clifford algebras including , the Pauli algebra , and the even part of the spacetime algebra.
Definition.
Let be the basis for the (real) quaternions , and let be complex numbers, then
is a "biquaternion". To distinguish square roots of minus one in the biquaternions, Hamilton and Arthur W. Conway used the convention of representing the square root of minus one in the scalar field C by "h" to avoid confusion with the in the quaternion group. Commutativity of the scalar field with the quaternion group is assumed:
Hamilton introduced the terms bivector, "biconjugate, bitensor", and "biversor" to extend notions used with real quaternions .
Hamilton's primary exposition on biquaternions came in 1853 in his "Lectures on Quaternions". The editions of "Elements of Quaternions", in 1866 by William Edwin Hamilton (son of Rowan), and in 1899, 1901 by Charles Jasper Joly, reduced the biquaternion coverage in favor of the real quaternions.
Considered with the operations of component-wise addition, and multiplication according to the quaternion group, this collection forms a 4-dimensional algebra over the complex numbers C. The algebra of biquaternions is associative, but not commutative. A biquaternion is either a unit or a zero divisor. The algebra of biquaternions forms a composition algebra and can be constructed from bicomplex numbers. See "" below.
Place in ring theory.
Linear representation.
Note the matrix product
Because "h" is the imaginary unit, each of these three arrays has a square equal to the negative of the identity matrix.
When this matrix product is interpreted as i j = k, then one obtains a subgroup of matrices that is isomorphic to the quaternion group. Consequently,
represents biquaternion "q" = "u" 1 + "v" i + "w" j + "x" k.
Given any 2 × 2 complex matrix, there are complex values "u", "v", "w", and "x" to put it in this form so that the matrix ring M(2,C) is isomorphic to the biquaternion ring.
Subalgebras.
Considering the biquaternion algebra over the scalar field of real numbers , the set
forms a basis so the algebra has eight real dimensions. The squares of the elements , and are all positive one, for example, .
The subalgebra given by
is ring isomorphic to the plane of split-complex numbers, which has an algebraic structure built upon the unit hyperbola. The elements and also determine such subalgebras.
Furthermore,
is a subalgebra isomorphic to the tessarines.
A third subalgebra called coquaternions is generated by and . It is seen that , and that the square of this element is . These elements generate the dihedral group of the square. The linear subspace with basis thus is closed under multiplication, and forms the coquaternion algebra.
In the context of quantum mechanics and spinor algebra, the biquaternions , and (or their negatives), viewed in the representation, are called Pauli matrices.
Algebraic properties.
The biquaternions have two "conjugations":
where formula_13 when formula_14
Note that formula_15
Clearly, if formula_16 then is a zero divisor. Otherwise formula_17 is defined over the complex numbers. Further, formula_18 is easily verified. This allows an inverse to be defined by
Relation to Lorentz transformations.
Consider now the linear subspace
 is not a subalgebra since it is not closed under products; for example formula_22. Indeed, cannot form an algebra if it is not even a magma.
Proposition: If is in , then formula_23
Proof: From the definitions,
Definition: Let biquaternion satisfy formula_25 Then the Lorentz transformation associated with is given by
Proposition: If is in , then is also in .
Proof: formula_27
Proposition: formula_28
Proof: Note first that means that the sum of the squares of its four complex components is one. Then the sum of the squares of the "complex conjugates" of these components is also one. Therefore, formula_29 Now
Associated terminology.
As the biquaternions have been a fixture of linear algebra since the beginnings of mathematical physics, there is an array of concepts that are illustrated or represented by biquaternion algebra. The transformation group formula_31 has two parts, formula_32 and formula_33 The first part is characterized by formula_34 ; then the Lorentz transformation corresponding to is given by formula_35 since formula_36 Such a transformation is a rotation by quaternion multiplication, and the collection of them is O(3) formula_37 But this subgroup of is not a normal subgroup, so no quotient group can be formed.
To view formula_38 it is necessary to show some subalgebra structure in the biquaternions. Let represent an element of the sphere of square roots of minus one in the real quaternion subalgebra . Then and the plane of biquaternions given by formula_39 is a commutative subalgebra isomorphic to the plane of split-complex numbers. Just as the ordinary complex plane has a unit circle, formula_40 has a unit hyperbola given by
Just as the unit circle turns by multiplication through one of its elements, so the hyperbola turns because formula_42 Hence these algebraic operators on the hyperbola are called hyperbolic versors. The unit circle in and unit hyperbola in are examples of one-parameter groups. For every square root of minus one in , there is a one-parameter group in the biquaternions given by formula_43
The space of biquaternions has a natural topology through the Euclidean metric on -space. With respect to this topology, is a topological group. Moreover, it has analytic structure making it a six-parameter Lie group. Consider the subspace of bivectors formula_44. Then the exponential map
formula_45 takes the real vectors to formula_32 and the -vectors to formula_33 When equipped with the commutator, forms the Lie algebra of . Thus this study of a six-dimensional space serves to introduce the general concepts of Lie theory. When viewed in the matrix representation, is called the special linear group SL(2,C) in .
Many of the concepts of special relativity are illustrated through the biquaternion structures laid out. The subspace corresponds to Minkowski space, with the four coordinates giving the time and space locations of events in a resting frame of reference. Any hyperbolic versor corresponds to a velocity in direction of speed where is the velocity of light. The inertial frame of reference of this velocity can be made the resting frame by applying the Lorentz boost given by since then formula_48 so that formula_49
Naturally the hyperboloid formula_50 which represents the range of velocities for sub-luminal motion, is of physical interest. There has been considerable work associating this "velocity space" with the hyperboloid model of hyperbolic geometry. In special relativity, the hyperbolic angle parameter of a hyperbolic versor is called rapidity. Thus we see the biquaternion group provides a group representation for the Lorentz group.
After the introduction of spinor theory, particularly in the hands of Wolfgang Pauli and Élie Cartan, the biquaternion representation of the Lorentz group was superseded. The new methods were founded on basis vectors in the set
which is called the "complex light cone". The above representation of the Lorentz group coincides with what physicists refer to as four-vectors. Beyond four-vectors, the standard model of particle physics also includes other Lorentz representations, known as scalars, and the -representation associated with e.g. the electromagnetic field tensor. Furthermore, particle physics makes use of the representations (or projective representations of the Lorentz group) known as left- and right-handed Weyl spinors, Majorana spinors, and Dirac spinors. It is known that each of these seven representations can be constructed as invariant subspaces within the biquaternions.
As a composition algebra.
Although W.R. Hamilton introduced biquaternions in the 19th century, its delineation of its mathematical structure as a special type of algebra over a field was accomplished in the 20th century: the biquaternions may be generated out of the bicomplex numbers in the same way that Adrian Albert generated the real quaternions out of complex numbers in the so-called Cayley–Dickson construction. In this construction, a bicomplex number ("w,z") has conjugate ("w,z")* = ("w", – "z").
The biquaternion is then a pair of bicomplex numbers ("a,b"), where the product with a second biquaternion ("c, d") is
If formula_53 then the "biconjugate" formula_54
When ("a,b")* is written as a 4-vector of ordinary complex numbers,
The biquaternions form an example of a quaternion algebra, and it has norm
Two biquaternions "p" and "q" satisfy formula_57 indicating that "N" is a quadratic form admitting composition, so that the biquaternions form a composition algebra.