 Solution

An Einstein-Hilbert action contains two derivatives, so its cubic graviton vertex produces only the $k\times k$ part of the product of the two $T$ tensors. The string amplitude also contains cross terms of order ${\alpha }^{\prime }{k}^4$ and a term of order ${\alpha }^{\prime 2}{k}^6$. These cannot arise from pure Einstein gravity. They require higher-curvature corrections, schematicallywith index contractions and coefficients fixed by string amplitudes up to field redefinitions. The full bosonic-string effective action also contains the dilaton and Kalb-Ramond field. Thus Einstein gravity is only its leading low-energy approximation.