# Abstract Algebra/Quaternions

## Contents

The algebra of Quaternions is an structure first studied by the Irish mathematician William Rowan Hamilton which extends the two-dimensional complex numbers to four dimensions. Multiplication is non-commutative in quaternions, a feature which enables its representation of three-dimensional rotation. Hamilton's provocative discovery of quaternions founded the field of hypercomplex numbers. Suggestive methods like dot products and cross products implicit in quaternion products enabled algebraic description of geometry now widely applied in science and engineering.

## Definition

A Quaternion corresponds to an ordered 4-tuple ${\displaystyle q=(a,b,c,d)}$, where ${\displaystyle a,b,c,d\in \mathbb {R} }$. A quaternion is denoted ${\displaystyle q=a+bi+cj+dk}$. The set of all quaternions is denoted by ${\displaystyle \mathbb {H} }$.

It is straightforward to define component-wise addition and scalar multiplication on ${\displaystyle \mathbb {H} }$, making it a real vector space.

The rule for multiplication was a product of Hamilton's ingenuity. He discovered what are known as the Bridge-stone Equations:

${\displaystyle i^{2}=j^{2}=k^{2}=ijk=-1}$

From the above equations alone, it is possible to derive rules for the pairwise multiplication of ${\displaystyle i}$, ${\displaystyle j}$, and ${\displaystyle k}$:

${\displaystyle ij=k,jk=i,ki=j}$ (positive cyclic permutations)
${\displaystyle ji=-k,kj=-i,ik=-j}$ (negative cyclic permutations).

Using these, it is easy to define a general rule for multiplication of quaternions. Because quaternion multiplication is not commutative, ${\displaystyle \mathbb {H} }$ is not a field. However, every nonzero quaternion has a multiplicative inverse (see below), so the quaternions are an example of a non-commutative division ring. It is important to note that the non-commutative nature of quaternion multiplication makes it impossible to define the quotient ${\displaystyle p/q}$ of two quaternions p and q unambiguously, as the quantities ${\displaystyle pq^{-1}}$ and ${\displaystyle q^{-1}p}$ are generally different.

Like the more familiar complex numbers, the quaternions have a conjugation, often denoted by a superscript star: ${\displaystyle q^{*}}$. The conjugate of the quaternion ${\displaystyle q=a+bi+cj+dk}$ is ${\displaystyle q^{*}=a-bi-cj-dk}$. As is the case for the complex numbers, the product ${\displaystyle qq^{*}}$ is always a positive real number equal to the sum of the squares of the quaternion's components. Using this fact, it is fairly easy to show that the multiplicative inverse of a general quaternion ${\displaystyle q}$ is given by

${\displaystyle q^{-1}={\frac {q^{*}}{qq^{*}}}}$

where division is defined since ${\displaystyle qq^{*}}$ is a scalar. Note that, unlike in the complex case, the conjugate ${\displaystyle q^{*}}$ of a quaternion ${\displaystyle q}$ can be written as a polynomial in q:

${\displaystyle q^{*}=-{\frac {1}{2}}(q+iqi+jqj+kqk)}$.

## Versors and elliptic space

William Kingdon Clifford used Hamilton’s quaternions to explicate rotation geometry as an elliptic space with its own variety of lines, parallels, and surfaces. The ideas were reviewed in 1948 by Lemaitre and Coxeter and that sketch has these definitions:

A versor is a quaternion of norm one, thus it lies on a 3-dimensional sphere found in the 4-space of quaternions. The versors are given by Euler's formula for complex numbers where the imaginary unit is taken from the unit sphere in the 3-space of vector quaternions:

${\displaystyle v=\cos c+s\sin c=e^{cs},\ \ s^{2}=-1.}$

The distance between two versors u and v is ${\displaystyle d(u,v)=\arccos(uv^{*}+vu^{*})/2.}$

A right parataxy on elliptic space is effected by multiplying on the right by a versor ${\displaystyle v=e^{cs}.}$ Similarly a left parataxy arises from left multiplication. In recognition of his contribution to elliptic geometry, a parataxy is called a Clifford translation.

The general displacement of elliptic space is a combination of two parataxies, one left, one right:${\displaystyle x\mapsto uxv.}$ Note that if ${\displaystyle u=v^{*},}$ then the real line in the quaternions is fixed and the displacement is a rotation of the 3-space of quaternion vectors.

The term line is appropriated for elliptic geometry. These lines are not straight, but they are parametrized by real numbers. Each line is associated with a right versor like s when c = π/2 in v. Then ${\displaystyle L=\{e^{cs}:c\in R\}}$ is a typical elliptic line. It corresponds to the axis of the rotation

${\displaystyle x\mapsto e^{cs}xe^{-cx}.}$

Now for u not on L, there are two Clifford parallels to L through u:

${\displaystyle \{ue^{cs}:c\in R\},\quad \{e^{cs}u:c\in R\}.}$

For fixed right versors r and s, a Clifford surface can be formed as a union of Clifford parallels or as

${\displaystyle \{e^{cs}e^{dr}:b,c\in R\}.}$

To form elliptic space from versors, two versors u and v are equivalent if u + v = 0. Modulo this equivalence, the versors, their algebra and geometry, represent elliptic space.

## Pauli Spin Matrices

Quaternions are closely related to the Pauli spin matrices of Quantum Mechanics. The Pauli matrices are often denoted as
${\displaystyle \sigma _{1}={\begin{pmatrix}0&1\\1&0\end{pmatrix}}}$ , ${\displaystyle \sigma _{2}={\begin{pmatrix}0&-i\\i&0\end{pmatrix}}}$ , ${\displaystyle \sigma _{3}={\begin{pmatrix}1&0\\0&-1\end{pmatrix}}}$

(Where ${\displaystyle i}$ is the well known quantity ${\displaystyle {\sqrt {-1}}}$ of complex numbers)

The 2×2 identity matrix is sometimes taken as ${\displaystyle \sigma _{0}}$. It can be shown that ${\displaystyle S}$, the real linear span of the matrices ${\displaystyle \sigma _{0}}$, ${\displaystyle i\sigma _{1}}$, ${\displaystyle i\sigma _{2}}$ and ${\displaystyle i\sigma _{3}}$, is isomorphic to the set of all quaternions, ${\displaystyle \mathbb {H} }$. For example, take the matrix product below:

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

Or, equivalently:

${\displaystyle i\sigma _{3}i\sigma _{2}=i\sigma _{1}}$

All three of these matrices square to the negative of the identity matrix. If we take ${\displaystyle 1=\sigma _{0}}$, ${\displaystyle i=i\sigma _{3}}$, ${\displaystyle j=i\sigma _{2}}$, and ${\displaystyle k=i\sigma _{1}}$, it is easy to see that the span of the these four matrices is "the same as" (that is, isomorphic to) the set of quaternions ${\displaystyle \mathbb {H} }$.

## Exercise

1. Using the Bridge-stone equations, explicitly state the rule of multiplication for general quaternions, that is, given ${\displaystyle q_{1}=a_{1}+b_{1}i+c_{1}j+d_{1}k}$ and ${\displaystyle q_{2}=a_{2}+b_{2}i+c_{2}j+d_{2}k}$, give the components of their product ${\displaystyle q=q_{1}q_{2}}$

## References

• E.T. Bell, Men of Mathematics, Simon & Schuster, Inc.
• The Wikipedia article on Pauli Spin Matrices