GW170817: Propagation Delay as a Field Effect in Tensor Field Dynamics

The multi-messenger event GW170817 exhibits an observed arrival-time difference of approximately 1.7 s between the gravitational-wave and electromagnetic signals. In this work it is shown that, within Tensor Field Dynamics (TFD), this delay is not interpreted as a source effect but as a pure propagation effect. Based on TFD Rules 5 and 7 and the refined Rule 8, the delay is calculated entirely from field-induced refractive indices in the galactic halos of the source and observer systems. The intergalactic medium provides no measurable contribution. The derivation is parameter-free, dimensionally consistent, and reproduces the observed delay quantitatively. A clear and falsifiable prediction for future multi-messenger events follows.