Bivector (complex)

In mathematics, a bivector is the vector part of a biquaternion. For biquaternion q = w + xi + yj + zk, w is called the biscalar and xi + yj + zk is its bivector part. The coordinates w, x, y, z are complex numbers with imaginary unit h:

$x=x_{1}+\mathrm {h} x_{2},\ y=y_{1}+\mathrm {h} y_{2},\ z=z_{1}+\mathrm {h} z_{2},\quad \mathrm {h} ^{2}=-1=\mathrm {i} ^{2}=\mathrm {j} ^{2}=\mathrm {k} ^{2}.$

A bivector may be written as the sum of real and imaginary parts:

$(x_{1}\mathrm {i} +y_{1}\mathrm {j} +z_{1}\mathrm {k} )+\mathrm {h} (x_{2}\mathrm {i} +y_{2}\mathrm {j} +z_{2}\mathrm {k} )$

where $r_{1}=x_{1}\mathrm {i} +y_{1}\mathrm {j} +z_{1}\mathrm {k}$ and $r_{2}=x_{2}\mathrm {i} +y_{2}\mathrm {j} +z_{2}\mathrm {k}$ are vectors. Thus the bivector $q=x\mathrm {i} +y\mathrm {j} +z\mathrm {k} =r_{1}+\mathrm {h} r_{2}.$

The Lie algebra of the Lorentz group is expressed by bivectors. In particular, if r₁ and r₂ are right versors so that $r_{1}^{2}=-1=r_{2}^{2}$, then the biquaternion curve {exp θr₁ : θ ∈ R} traces over and over the unit circle in the plane {x + yr₁ : x, y ∈ R}. Such a circle corresponds to the space rotation parameters of the Lorentz group.

Now (hr₂)² = (−1)(−1) = +1, and the biquaternion curve {exp θ(hr₂) : θ ∈ R} is a unit hyperbola in the plane {x + yr₂ : x, y ∈ R}. The spacetime transformations in the Lorentz group that lead to FitzGerald contractions and time dilation depend on a hyperbolic angle parameter. In the words of Ronald Shaw, "Bivectors are logarithms of Lorentz transformations."

The commutator product of this Lie algebra is just twice the cross product on R³, for instance, [i,j] = ij − ji = 2k, which is twice i × j. As Shaw wrote in 1970:

Now it is well known that the Lie algebra of the homogeneous Lorentz group can be considered to be that of bivectors under commutation. [...] The Lie algebra of bivectors is essentially that of complex 3-vectors, with the Lie product being defined to be the familiar cross product in (complex) 3-dimensional space.

William Rowan Hamilton coined both the terms vector and bivector. The first term was named with quaternions, and the second about a decade later, as in Lectures on Quaternions (1853). The popular text Vector Analysis (1901) used the term.

Given a bivector r = r₁ + hr₂, the ellipse for which r₁ and r₂ are a pair of conjugate semi-diameters is called the directional ellipse of the bivector r.

In the standard linear representation of biquaternions as 2 × 2 complex matrices acting on the complex plane with basis {1, h},

${\begin{pmatrix}hv&w+hx\\-w+hx&-hv\end{pmatrix}}$ represents bivector q = vi + wj + xk.

The conjugate transpose of this matrix corresponds to −q, so the representation of bivector q is a skew-Hermitian matrix.

Ludwik Silberstein studied a complexified electromagnetic field E + hB, where there are three components, each a complex number, known as the Riemann–Silberstein vector.

"Bivectors [...] help describe elliptically polarized homogeneous and inhomogeneous plane waves – one vector for direction of propagation, one for amplitude."

References

• Boulanger, Ph.; Hayes, M.A. (1993). Bivectors and Waves in Mechanics and Optics. CRC Press. ISBN 978-0-412-46460-7.
• Boulanger, P.H.; Hayes, M. (1991). "Bivectors and inhomogeneous plane waves in anisotropic elastic bodies". In Wu, Julian J.; Ting, Thomas Chi-tsai; Barnett, David M. (eds.). Modern theory of anisotropic elasticity and applications. Society for Industrial and Applied Mathematics. p. 280 et seq. ISBN 0-89871-289-0.
• Hamilton, William Rowan (1853). Lectures on Quaternions. Royal Irish Academy. Link from Cornell University Historical Mathematics Collection.
• Hamilton, William Edwin, ed. (1866). Elements of Quaternions. University of Dublin Press. p. 219.