Gravitational slingshots work best around hugely massive bodies. In the 1960s, the physicist Freeman Dyson calculated that a black hole could accelerate a spacecraft to relativistic speeds. But the forces on the spacecraft as it approached such an object would be likely to destroy it.

So Kipping has come up with a clever alternative. His idea is to send photons around a black hole and then use the extra energy they gain to accelerate a light sail. “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,” says Kipping.

The process depends on the hugely powerful gravitational field around a black hole. Because photons have a small but measurable rest mass, this field can trap light in a circular orbit.

Kipping’s work is based on a slightly different orbit that steers a photon emitted from a spacecraft around the black hole and back to the spacecraft—a kind of boomerang orbit. During this journey, the boomerang photons gain kinetic energy from the motion of the black hole.

It is this energy that can accelerate a spacecraft fitted with an appropriate light sail. Kipping calls this a “halo drive.” “The halo drive transfers kinetic energy from the moving black hole to the spacecraft by way of a gravitational assist,” says Kipping, pointing out that the spacecraft does not use up any fuel of its own in the process.

Since the halo drive exploits the movement of a black hole, it is best applied to binary systems in which a black hole is orbiting another object. The photons then gain energy from the movement of the black hole at appropriate points in its orbit.

And the drive should work for any mass that is significantly smaller than the black hole. Kipping says this could allow planet-size vehicles. So a sufficiently advanced civilization could travel at relativistic speeds from one part of the galaxy to another by hopping from one black-hole binary system to another. “An advanced civilization might utilize the light sailing concept to conduct relativistic and extremely efficient propulsion,” he says.

The same mechanism can also decelerate spacecraft. So this advanced civilization would probably look for pairs of binary black-hole systems to act as accelerators and decelerators.

The Milky Way contains around 10 billion binary black-hole systems. But Kipping points out that there are likely to be just a limited number of trajectories that link them together, so these interstellar highways are likely to be valuable regions.

Of course, the technology to exploit this concept is well beyond humanity’s capability at the moment. But astronomers ought to be able to work out where the best interstellar highways lie and then look for the techno-signatures of civilizations that might be exploiting them.

All that sounds like good fun, and critics might argue that it is little more than fodder for science fiction fans. Perhaps.

But the starchip concept has been discussed for decades, usually on the fringes of science. In the wake of Hawking and Milner’s announcement, the project has suddenly gained legs. Indeed, the first starchip technologies have already been tested in low Earth orbit and the first mission penciled in for around 2036, at a cost of $5 to $10 billion.

That’s an ambitious goal, but even allowing for various delays, interstellar travel is likely to be possible within a hundred years of humanity’s first forays into space. That’s rapid progress. And it suggests that any civilization with even a small head start on us could have made significantly larger strides.

Ref: arxiv.org/abs/1903.03423 : The Halo Drive: Fuel-Free Relativistic Propulsion of Large Masses Via Recycled Boomerang Photons