In quantum field theory, the dimension of the coupling plays an important role in the renormalizability property of the theory, and therefore on the applicability of perturbation theory. If the coupling is dimensionless in the natural units system (i.e. c = 1 , ℏ ℏ = 1 ), like in QED, QCD, and the weak interaction, the theory is renormalizable and all the terms of the expansion series are finite (after renormalization). If the coupling is dimensionful, as e.g. in gravity ( [ G N ] = energy − − 2 ), the Fermi theory ( [ G F ] = energy − − 2 ) or the chiral perturbation theory of the strong force ( [ F ] = energy ), then the theory is usually not renormalizable. Perturbation expansions in the coupling might still be feasible, albeit within limitations, as most of the higher order terms of the series will be infinite.