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:
A bivector may be written as the sum of real and imaginary parts:
where and are vectors. Thus the bivector [1]
The Lie algebra of the Lorentz group is expressed by bivectors. In particular, if r1 and r2 are right versors so that , 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
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.[3]
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).[1]: 665 The popular text Vector Analysis (1901) used the term.[4]: 249
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.[4]: 436
In the standard linear representation of biquaternions as 2 × 2 complex matrices acting on the complex plane with basis {1, h},
- 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.
"Bivectors [...] help describe elliptically polarized homogeneous and inhomogeneous plane waves – one vector for direction of propagation, one for amplitude."[7]
References
- ^ Trinity College, Dublin
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- ^ a b Edwin Bidwell Wilson (1901) Vector Analysis
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- ^ Silberstein, Ludwik (1907). "Nachtrag zur Abhandlung über 'Elektromagnetische Grundgleichungen in bivectorieller Behandlung'" (PDF). .
- .
- 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. 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.