  A direct measurement of the decay width of the excited $0^+_1$ state of
$^6$Li using the relative self-absorption technique is reported. Our value of
$\Gamma_{\gamma, 0^+_1 \to 1^+_1} = 8.17(14)_\mathrm{stat.}(11)_\mathrm{syst.}
\mathrm{eV}$ provides sufficiently low experimental uncertainties to test
modern theories of nuclear forces. The corresponding transition rate is
compared to the results of ab initio calculations based on chiral effective
field theory that take into account contributions to the magnetic dipole
operator beyond leading order. This enables a precision test of the impact of
two-body currents that enter at next-to-leading order.
