Baker–Campbell–Hausdorff formula for generators of $SO(3)$

closed-formlie-algebraslie-groupsnoncommutative-algebra

By noting ${e_1 = (1, 0, 0), e_2 = (0, 1, 0), e_3 = (0, 0, 1)}$ and $E_i = [e_i]_\times$ the generators of $SO(3)$, then we have the following commutator properties :

$$
\begin{align}
E_1 &= [E_2, E_3] \\
E_2 &= [E_3, E_1] \\
E_3 &= [E_1, E_2]
\end{align}
$$

Is it possible to find an explicit expansion of the Baker–Campbell–Hausdorff formula for these generators, such as $\log(e^{E_1} e^{E_2}) $ ?

Best Answer

Of course it's possible. Rodrigues solved the problem in 1840.

In your somewhat idiosyncratic notation, $E_1=L_x$ and $E_2=L_y$ physicists learn in Sophomore year. I will, predictably, choose easy angles $\theta= \pi/2$, instead of your freaky $\theta=1$.

You then trivially have a roll and a pitch, $$ \exp \left ({\pi\over 2} E_1 \right ) = \begin{bmatrix} 1 & 0 & 0 \\ 0 & 0 & -1 \\ 0 & 1 & 0 \end{bmatrix} , \qquad \exp \left({\pi\over 2} E_2 \right) = \begin{bmatrix} 0 & 0 & 1 \\ 0 & 1 & 0 \\ -1 & 0 & 0 \end{bmatrix} ~~~\leadsto \\ \exp \left({\pi\over 2} E_1 \right) \exp \left({\pi\over 2} E_2 \right) = \begin{bmatrix} 0 & 0 & 1 \\ 1 & 0 & 0 \\ 0 & 1 & 0 \end{bmatrix} \\ = \exp\left ( {2\pi\over 3} (E_1+E_2+E_3)/\sqrt 3\right ), $$ Where we have used this formulation of the Rodrigues formula, for $\theta=2\pi /3$, $$M\equiv E_1+E_2+E_3\equiv \sqrt{3} K=\begin{bmatrix} 0 & -1 & 1 \\ 1 & 0 & -1 \\ -1 & 1 & 0 \end{bmatrix} , \leadsto\\ \exp (\theta K)= I+\sin\theta ~K+ (1-\cos\theta) K^2 , ~~~\leadsto \\ \exp \left ( {2\pi\over 3} M/\sqrt{3}\right ) =I +{1\over 2} M +{1+1/2\over 3} M^2. $$

For general angle formulas, you should use the first link, the Rodrigues-Gibbs formula. (After you've appreciated its music, the Original Rodrigues paper is also instructive.)

Related Question