Exploring Triality Explicitly: Convenient bases for $\mathrm{SO}(8)$, $\mathrm{Spin}(1, 7)$, and $\mathrm{G}_2$

Abstract

The property of triality only appears in one linear simple Lie algebra: $D_4$, a.k.a. $\mathfrak{so}(8, \mathbb{C})$. Though often explored in abstract, it is desirable to have an explicit realization of the concept since there are no other linear examples to gain intuition from. In this paper several convenient representations and bases are constructed in order to facilitate the exploration of the three fold symmetry known as the triality of representations. In particular the three $8$ dimensional representations for the Euclidean and Lorentzian real forms of $\mathfrak{so}(8,\mathbb{C})$ are constructed, and the maps between representations are given in each case, respectively. It is also seen explicitly how $\mathfrak{g}_2 \subset \mathfrak{so}(8, \mathbb{R})$ arises as the intersection of non-conjugate $\mathfrak{spin}(7,\mathbb{R})$ sub-algebras, and also as the stabilizer of the outer automorphism group $\mathrm{Out}(\mathfrak{so}(8,\mathbb{R}))$. It is argued that $\mathfrak{spin}(1,7)$ is in some sense the more natural stage for triality to play out upon, and it is shown that triality can be seen to be simply the multiplication of bases by third roots of unity, just as dualities are often the application of second roots of unity upon Lie algebra bases. Once these are understood a short discussion is had about obstacles to a theory of triality which attempt to explain the three generations of matter via some form of triality.