# Biquaternion

In abstract algebra, the biquaternions are the numbers w + x i + y j + z k, where w, x, y, and z are complex numbers and the elements of {1, i, j, k} multiply as in the quaternion group. As there are three types of complex number, so there are three types of biquaternion:

This article is about the ordinary biquaternions named by William Rowan Hamilton in 1844 (see Proceedings of 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 presentation of the Lorentz group, which is the foundation of special relativity.

The algebra of biquaternions can be considered as a tensor product CH (taken over the reals) where C is the field of complex numbers and H is the algebra of (real) quaternions. In other words, the biquaternions are just the complexification of the (real) quaternions. Viewed as a complex algebra, the biquaternions are isomorphic to the algebra of 2×2 complex matrices M2(C). They can be classified as the Clifford algebra Cℓ2(C) = Cℓ03(C). This is also isomorphic to the Pauli algebra Cℓ3,0(R), and the even part of the spacetime algebra Cℓ01,3(R).

## Definition

Let {1, i, j, k} be the basis for the (real) quaternions, and let u, v, w, x be complex numbers, then

q = u 1 + v i + w j + x k

is a biquaternion.[1] To distinguish square roots of minus one in the biquaternions, Hamilton[2][3] and Arthur W. Conway used the convention of representing the square root of minus one in the scalar field C by h since there is an i in the quaternion group. Then

h i = i h, h j = j h, and h k = k h since h is a scalar.

Hamilton's primary exposition on biquaternions came in 1853 in his Lectures on Quaternions, now available in the Historical Mathematical Monographs of Cornell University. The two editions of Elements of Quaternions (1866 & 1899) reduced the biquaternion coverage in favor of the real quaternions. He introduced the terms bivector, biconjugate, bitensor, and biversor.

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. The algebra of biquaternions is associative, but not commutative. A biquaternion is either a unit or a zero divisor.

## Place in ring theory

### Linear representation

Note the matrix product

$\begin{pmatrix}h & 0\\0 & -h\end{pmatrix}\begin{pmatrix}0 & 1\\-1 & 0\end{pmatrix} = \begin{pmatrix}0 & h\\h & 0\end{pmatrix}$

where 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 the matrix group that is isomorphic to the quaternion group. Consequently

$\begin{pmatrix}u+hv & w+hx\\-w+hx & u-hv\end{pmatrix}$

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 is isomorphic[4] to the biquaternion ring.

### Subalgebras

Considering the biquaternion algebra over the scalar field of real numbers R, the set {1, h, i, hi, j, hj, k, hk } forms a basis so the algebra has eight real dimensions. Note the squares of the elements hi, hj, and hk are all plus one, for example,

$(hi)^2 \ = \ h^2 i^2 \ = \ (-1) (-1) \ =\ +1 .$

Then the subalgebra given by $\lbrace x + y(hi) : x, y \in R \rbrace$ is ring isomorphic to the plane of split-complex numbers, which has an algebraic structure built upon the unit hyperbola. The elements hj and hk also determine such subalgebras. Furthermore, $\lbrace x + yj : x,y \in C \rbrace$ is a subalgebra isomorphic to the tessarines.

A third subalgebra called coquaternions is generated by hj and hk. First note that (hj)(hk) = (−1) i, and that the square of this element is −1. These elements generate the dihedral group of the square. The linear subspace with basis {1, i, hj, hk} thus is closed under multiplication, and forms the coquaternion algebra.

In the context of quantum mechanics and spinor algebra, the biquaternions hi, hj, and hk (or their negatives), viewed in the M(2,C) representation, are called Pauli matrices.

## Algebraic properties

The biquaternions have two conjugations:

• the biconjugate or biscalar minus bivector is $q^* = w - xi - yj - zk \!\ ,$ and
• the complex conjugation of biquaternion coefficients $q^{\star} = w^{\star} + x^{\star} i + y^{\star} j + z^{\star} k \!$

where $z^{\star} = a - bh$ when $z = a + bh,\quad a,b \in R,\quad h^2 = -1.$

Note that $(pq)^* = q^* p^*, \quad (pq)^{\star} = p^{\star} q^{\star} , \quad (q^*)^{\star} = (q^{\star})^*.$

Clearly, if $q q^* = 0 \!$ then q is a zero divisor. Otherwise $\lbrace q q^* \rbrace^{-1} \!$ is defined over the complex numbers. Further, $q q^* = q^* q \!$ is easily verified. This allows an inverse to be defined as follows:

• $q^{-1} = q^* \lbrace q q^* \rbrace^{-1}\!$, iff $qq^* \neq 0.$

### Relation to Lorentz transformations

Consider now the linear subspace [5]

$M = \lbrace q : q^* = q^{\star} \rbrace = \lbrace t + x(hi) + y(hj) + z(hk) : t, x, y, z \in R \rbrace .$

M is not a subalgebra since it is not closed under products; for example $(hi)(hj) = h^2 ij = -k \notin M.$. Indeed, M cannot form an algebra if it is not even a magma.

Proposition: If q is in M, then $q q^* = t^2 - x^2 - y^2 - z^2 \!$.

proof: $q q^* = (t+xhi+yhj+zhk)(t-xhi-yhj-zhk) \!$

$= t^2 - x^2(hi)^2 - y^2(hj)^2 - z^2(hk)^2 = t^2 - x^2 - y^2 - z^2. \!$

Definition: Let biquaternion g satisfy g g * = 1. Then the Lorentz transformation associated with g is given by

$T(q) = g^* q g^{\star}.$

Proposition: If q is in M, then T(q) is also in M.

proof: $(g^* q g^{\star})^* = (g^{\star})^* q^* g = (g^*)^{\star} q^{\star} g = (g^* q g^{\star})^{\star}.$

Proposition: $\quad T(q) (T(q))^* = q q^* \!$

proof: Note first that g g * = 1 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, $g^{\star} (g^{\star})^* = 1.$ Now

$(g^* q g^{\star})(g^* q g^{\star})^* = g^* q g^{\star} (g^{\star})^* q^* g = g^* q q^* g = q q^*.$

## 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 $G = \lbrace g : g g^* = 1 \rbrace \!$ has two parts, $G \cap H$ and $G \cap M.$ The first part is characterized by $g = g^{\star}$ ; then the Lorentz transformation corresponding to g is given by $T(q) = g^{-1} q g \!$ since $g^* = g^{-1}. \!$ Such a transformation is a rotation by quaternion multiplication, and the collection of them is O(3) $\cong G \cap H .$ But this subgroup of G is not a normal subgroup, so no quotient group can be formed.

To view $G \cap M$ it is necessary to show some subalgebra structure in the biquaternions. Let r represent an element of the sphere of square roots of minus one in the real quaternion subalgebra H. Then (hr)2 = +1 and the plane of biquaternions given by $D_r = \lbrace z = x + yhr : x, y \in R \rbrace$ is a commutative subalgebra isomorphic to the plane of split-complex numbers. Just as the ordinary complex plane has a unit circle, $D_r \!$ has a unit hyperbola given by

$exp(ahr) = \cosh(a) + hr\ \sinh(a),\quad a \in R. \!$

Just as the unit circle turns by multiplication through one of its elements, so the hyperbola turns because $\exp(ahr) \exp(bhr) = \exp((a+b)hr). \!$ Hence these algebraic operators on the hyperbola are called hyperbolic versors. The unit circle in C and unit hyperbola in Dr are examples of one-parameter groups. For every square root r of minus one in H, there is a one-parameter group in the biquaternions given by $G \cap D_r.$

The space of biquaternions has a natural topology through the Euclidean metric on 8-space. With respect to this topology, G is a topological group. Moreover, it has analytic structure making it a six-parameter Lie group. Consider the subspace of bivectors $A = \lbrace q : q^* = -q \rbrace \!$. Then the exponential map $\exp:A \to G$ takes the real vectors to $G \cap H$ and the h-vectors to $G \cap M.$ When equipped with the commutator, A forms the Lie algebra of G. Thus this study of a six-dimensional space serves to introduce the general concepts of Lie theory. When viewed in the matrix representation, G is called the special linear group SL(2,C) in M(2,C).

Many of the concepts of special relativity are illustrated through the biquaternion structures laid out. The subspace M 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 exp(ahr) corresponds to a velocity in direction r of speed c tanh a where c is the velocity of light. The inertial frame of reference of this velocity can be made the resting frame by applying the Lorentz boost T given by g = exp(0.5ahr) since then $g^{\star} = \exp(-0.5ahr) = g^*$ so that $T(\exp(ahr)) = 1 .$ Naturally the hyperboloid $G \cap M,$ 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 G 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

$\lbrace q \ :\ q q^* = 0 \rbrace = \lbrace w + xi + yj + zk \ :\ w^2 + x^2 + y^2 + z^2 = 0 \rbrace$

which is called the "complex light cone".