> a curious proposition, but why? I think the initial part of this snippet answers the question: this is mostly an effort by someone to type up a fun idea. Sai

> a curious proposition, but why? I think the initial part of this snippet answers the question: this is mostly an effort by someone to type up a fun idea. Said someone is most likely a theorist, who doesn't let small details like slewing speed or exposure time prevent them from writing up a fun idea. Honestly it seems like David Kipping could be a pretty fun guy to sit down and dream up weird stuff with, witness arxiv:1903.03423, "The Halo Drive: Fuel-Free Relativistic Propulsion of Large Masses via Recycled Boomerang Photons" "Abstract: Gravitational slingshots around a neutron star in a compact binary have been proposed as a means of accelerating large masses to potentially relativistic speeds. 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. Using a moving black hole 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 approximately 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"