  Starting from a nonequilibrium configuration we analyse the essential role of
the direct and the inverse binary and triple interactions in reaching an
asymptotic thermal equilibrium in a homogeneous isotropic
electron-positron-photon plasma. We focus on energies in the range 0.1--10 MeV.
We numerically integrate the integro-partial differential relativistic
Boltzmann equation with the exact QED collisional integrals taking into account
all binary and triple interactions in the plasma. We show that first, when
detailed balance is reached for all binary interactions on a timescale
$t_{k}\lesssim10^{-14}$sec, photons and electron-positron pairs establish
kinetic equilibrium. Successively, when triple interactions fulfill the
detailed balance on a timescale $t_{eq}\lesssim10^{-12}$sec, the plasma reaches
thermal equilibrium. It is shown that neglecting the inverse triple
interactions prevents reaching thermal equilibrium. Our results obtained in the
theoretical physics domain also find application in astrophysics and cosmology.
