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Relativistic combination of non-collinear 3-velocities using quaternions

Quaternions have an (over a century-old) extensive and quite complicated interaction with special relativity. Since quaternions are intrinsically 4-dimensional, and do such a good job of handling 3-dimensional rotations, the hope has always been that the use of quaternions would simplify some of the algebra of the Lorentz transformations. Herein we report a relatively nice result for the relativistic combination of non-collinear 3-velocities. If we work with the relativistic half-velocities $w$ defined by $v={2w\over1+w^2}$, and promote them to quaternions using $\mathbf{w} = w \; \mathbf{\hat n}$, where $\mathbf{\hat n}$ is a unit quaternion, then we shall show \[ \mathbf{w}_{1\oplus2} = \mathbf{w}_1 \oplus \mathbf{w}_2 =(1-\mathbf{w}_1\mathbf{w}_2)^{-1} (\mathbf{w}_1 +\mathbf{w}_2) = (\mathbf{w}_1 +\mathbf{w}_2)(1-\mathbf{w}_2\mathbf{w}_1)^{-1}. \] All of the complicated angular dependence for relativistic combination of non-collinear 3-velocities is now encoded in the quaternion multiplication of $\mathbf{w}_1$ with $\mathbf{w}_2$. This result can furthermore be extended to obtain an elegant and compact formula for the associated Wigner angle: \[ \mathrm{e}^{\mathbfΩ} = \mathrm{e}^{Ω\; \mathbf{\hatΩ} } = (1-\mathbf{w}_1\mathbf{w}_2)^{-1} (1-\mathbf{w}_2\mathbf{w}_1), \] in terms of which \[ {\mathbf{\hat{n}}}_{1\oplus2} = \mathrm{e}^{\mathbfΩ/2} \;\; {\mathbf{w}_1+\mathbf{w}_2\over |\mathbf{w}_1+\mathbf{w}_2|}; \qquad\qquad {\mathbf{\hat{n}}}_{2\oplus1} = \mathrm{e}^{-\mathbfΩ/2} \;\; {\mathbf{w}_1+\mathbf{w}_2\over |\mathbf{w}_1+\mathbf{w}_2|}. \] Thus, we would argue, many key results that are ultimately due to the non-commutativity of non-collinear boosts can be easily rephrased in terms of the algebra of quaternions.