  We calculate the optical (cutoff >> frequency >> temperature) conductivity in
clean graphene in the ultimate low-energy regime, when retardation effects of
the electromagnetic interaction become important and when the full Lorentz
symmetry emerges. In contrast to what happens with the short range or with the
Coulomb long-range instantaneous interactions, the optical conductivity is now
no longer equal to its non interacting value, but acquires universal
corrections in powers of the fine structure constant. The coefficient of the
first order correction is computed, and found to be of order one. We also
present the result for the conductivity in the large-N limit, with $N$ as the
number of Dirac fermions species, to the order $1/N^2$.
