  Since the pioneering work of Bagger-Lambert and Gustavsson, there has been a
proliferation of three-dimensional superconformal Chern-Simons theories whose
main ingredient is a metric 3-algebra. On the other hand, many of these
theories have been shown to allow for a reformulation in terms of standard
gauge theory coupled to matter, where the 3-algebra does not appear explicitly.
In this paper we reconcile these two sets of results by pointing out the
Lie-algebraic origin of some metric 3-algebras, including those which have
already appeared in three-dimensional superconformal Chern-Simons theories.
More precisely, we show that the real 3-algebras of Cherkis-Saemann, which
include the metric Lie 3-algebras as a special case, and the hermitian
3-algebras of Bagger-Lambert can be constructed from pairs consisting of a
metric real Lie algebra and a faithful (real or complex, respectively) unitary
representation. This construction generalises and we will see how to construct
many kinds of metric 3-algebras from pairs consisting of a real metric Lie
algebra and a faithful (real, complex or quaternionic) unitary representation.
In the real case, these 3-algebras are precisely the Cherkis-Saemann algebras,
which are then completely characterised in terms of this data. In the complex
and quaternionic cases, they constitute generalisations of the Bagger-Lambert
hermitian 3-algebras and anti-Lie triple systems, respectively, which underlie
N=6 and N=5 superconformal Chern-Simons theories, respectively. In the process
we rederive the relation between certain types of complex 3-algebras and metric
Lie superalgebras.
