[video; background/commentary; media: 1, 2] Gravitational slingshots around a neutron star in a compact binary have been proposed as a means of accelerating large masses to potentially relativistic speeds [1963]. Such a slingshot is attractive since fuel is not expended for the acceleration, however it does entail a spacecraft diving into close proximity of the binary, which could be hazardous.
It is proposed here that such a slingshot can be performed remotely using a beam of light which follows a boomerang null geodesic [just above the photonsphere]. Using a moving black hole [moving solo holes as well as binary ones orbiting each other rapidly] as a gravitational mirror, kinetic energy from the black hole is transferred to the beam of light as a blueshift and upon return the recycled photons not only accelerate, but also add energy to, the spacecraft.
It is shown here that this gained energy can be later expended to reach a terminal velocity of ~133% the velocity of the black hole.
A civilization could exploit black holes as galactic way points but would be difficult to detect remotely, except for an elevated binary merger rate and excess binary eccentricity.
…A civilization using a network of binaries may not only accelerate from them but also decelerate upon return, thus potentially undoing the slight distortions made to the binary. Even so, the binary temporarily spends time at closer semi-major axis where gravitational radiation is more effective and thus one still expects elevated merger rates to result.
One-way trips, perhaps from a central hub, would lead to an even higher rate of binary in-spiral on-top of the natural gravitational radiation. If journeys are made isotropically, an eccentric binary may not result but accelerated in-spiral would persist. However, only a discrete set of highways exist between galactic binary black holes and thus the distortions can never be perfectly isotropic meaning that excess eccentricity would likely persist.
…The terminal velocity of the spacecraft is 133% the black hole’s speed, to first-order. Critically, this velocity in not sensitive to the mass of the spacecraft, with the only assumption being that said mass is much less than that of the black hole. Accordingly, a major advantage of the halo drive is that Jupiter-mass spacecraft could be accelerated to relativistic speeds.
…An advanced civilization using such a system would first have to have achieved interstellar flight to journey towards the nearest suitable BH. They could then could use BHs in binary systems as way-points throughout the galaxy, of which there are likely 𝒪[107] in the Milky Way (2013), serving as both acceleration and deceleration stations. Alternatively, they could use the larger population of BHs which do not reside in compact binaries ( et al 2017) via their proper motions, although this would not permit for such high velocities.