This is something I've only recently heard about and only begun reading, on the surface it seems to me it would be a limited type of propulsion given the type of conditions necessary to use this kind of technology, what do you think?... The Halo Drive: Fuel-Free Relativistic Propulsion of Large Masses via Recycled Boomerang Photons Intelligent alien races could travel through interstellar space unseen by firing LASERS into orbiting black hole pairs to create high-speed 'halo drive' propulsion system, study claims One Ivy League Astronomer says humans could achieve fuel-free interstellar travel by harvesting energy from binary black holes in a brand new method dubbed the 'Halo Drive.' Theoretically, said David Kipping, an astronomer at Columbia University, aliens could already be using that technology without our knowledge. The Halo Drive method of propelling a spacecraft would work by utilizing 'gravitational mirrors' – a region of a black holes that swallows up matter and shoots it back out in exactly the same direction – to absorb the holes' energy. The Halo Drive theory uses black hole energy to propel space crafts beyond the speed of light. Artist's impression of a black hole is pictured 'The search for intelligence amongst the cosmos is often guided by considering the possible activities of hypothetical advanced civilizations and the associated technosignatures that would result,' said Kipping in a study published in the journal AirXiv. 'Along these lines, this work has considered how an advanced civilization might utilize the light sailing concept to conduct relativistic and extremely efficient propulsion.' Scientists have observed that when photons enter a gravitational mirror and are subsequently shot back out, they not only gain velocity from their journey, they also retain some of the kinetic energy from the visit. Kipping says using those returned photons, which are called 'boomerang photons,' interstellar travelers could theoretically use the harvested energy to travel at a velocity 133 percent greater than the black hole. This method of harvesting energy from black holes would not only avoid having to tread dangerously close to the gravitational void – astronauts have tapped black holes as a type of slingshot to help propel objects into space for ages – they would also be extremely hard to detect. '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,' said Kipping. As a result, Kipping said, a civilization that has the means to travel close enough to a black hole could theoretically be deploying this type of method of interstellar transit without our knowledge. Another major benefit of the Halo Drive over other proposed forms of interstellar travel, posits Kipping, would be shedding the need for a massive fuel source. While other theoretical forms of travel require a spacecraft to accelerate to what is called 'relativistic speed' – a speed comparable to the speed of light – the Halo Drive would subvert the need for any prior energy other than the light harvested from a black hole. .