Literature values for the energy gap of long one-dimensional carbon chains vary from as little as 0.2 eV to more than 4 eV. To resolve this discrepancy, we use the GW many-body approach to calculate the band gap E-g of an infinite carbon chain. We also compute the energy dependence of the attenuation coefficient beta governing the decay with chain length of the electrical conductance of long chains and compare this with recent experimental measurements of the single-molecule conductance of end-capped carbon chains. For long chains, we find E-g = 2.16 eV and an upper bound for beta of 0.21 angstrom(-1). (C) 2014 Author(s).