Causal Gravity Assists: Relativistic Energy Harvesting and Interstellar Propulsion via the Liénard-Wiechert Vacuum Wake

Standard gravitational assists (slingshots) are limited by the orbital velocity of planetary bodies, making them insufficient for relativistic interstellar travel. We propose a novel propulsion mechanism, the Causal Drive, based on Causal Latency Theory (CLT). Due to the finite speed of information updates ($c$), massive bodies moving through the vacuum create a "Causal Wake"—an anisotropic deformation of the gravitational potential characterized by a steep "Bow Shock" and a rarefied "Wake." We demonstrate via numerical simulation that a spacecraft approaching a relativistic source from the direction of motion can extract linear momentum from the source via a Gravitational Fermi Acceleration mechanism. Unlike standard assists, the coupling efficiency increases with the source velocity; as $\beta \to 1$, the wake "stiffens" into an effective potential wall, allowing for near-elastic reflection ($\Delta v \approx 2v_{source}$). We validate this model against astrophysical anomalies, showing that the trajectory of 'Oumuamua and the recently discovered 3I/ATLAS (C/2025 N1) match the kinematic signature of "Wake Surfing." Finally, we simulate the scattering of stars by a relativistic black hole, predicting a distinct anisotropy in Hypervelocity Star distributions that serves as a smoking gun for vacuum wake dynamics.