We evaluate the four-closed-string scattering amplitude, using the Polyakov string path integral in the proper-time gauge. By identifying the Fock space representation of the four-closed-string-vertex, we obtain a field theoretic expression of the closed string scattering amplitudes. In the zero-slope limit, the four-closed-string scattering amplitude reduces to the four-graviton-scattering amplitude of Einsteins gravity. However, at a finite slope, the four-graviton scattering amplitude in the proper-time gauge differs not only from that of Einstein gravity, but also significantly differs from the conventional one obtained by using the vertex operator technique in string theory. This discrepancy is mainly due to the presence of closed string tachyon poles in the four-graviton-scattering amplitude, which are missing in previous works. Because the tachyon poles in the scattering amplitude considerably alter the short distance behavior of gravitational interaction, they may be important in understanding problems associated with the perturbative theory of quantum gravity and the dark matter within the framework of string theory.
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