Recent fluorescence spectroscopy experiments on single wall carbon nanotubes reveal substantial deviations of observed absorption and emission energies from predictions of noninteracting models of the electronic structure. Nonetheless, the data for nearly armchair nanotubes obey a nonlinear scaling relation as a function of the tube radius R. We show that these effects can be understood in a theory of large radius tubes, derived from the theory of two dimensional graphene where the Coulomb interaction leads to a logarithmic correction to the electronic self-energy and marginal Fermi liquid behavior. Interactions on length scales larger than the tube circumference lead to strong self-energy and excitonic effects that compete and nearly cancel so that the observed optical transitions are dominated by the graphene self-energy effects.