Back in 2016, the physicist Stephen Hawking and the billionaire Yuri Milner unveiled a plan to travel to the stars. The so-called Breakthrough Starshot project is a $100 million program to develop and demonstrate the technologies necessary to visit a nearby star system. Potential targets include Proxima Centauri, a system some four light-years away with several exoplanets, including one Earth-like body orbiting in the habitable zone.
Hawking and Milner’s plan was to build thousands of tiny spacecraft the size of microchips and use light to accelerate them to a relativistic speed—one this close to the speed of light. The large number increases the chances that at least one would arrive safely. Each “starchip” would be attached to a light sail the size of a badminton court and then zapped with hugely powerful ground-based lasers.
Laser propulsion has various advantages. The most significant is that the spacecraft need not carry any fuel, vastly reducing their mass. It should also be capable of accelerating the light sails to a velocity of up to 20% the speed of light. At that rate, a starchip would arrive at Proxima Centauri in less than 30 years.
The fantastically powerful lasers required for such a mission will be particularly difficult and expensive to develop. And that raises an obvious question: is there any other way to reach relativistic speeds?
Today, we get an answer of sorts thanks to the work of David Kipping, an astronomer at Columbia University in New York. Kipping has come up with a novel form of gravitational slingshot, the same technique that NASA has used, for example, to send the Galileo spacecraft to Jupiter. The idea is to accelerate a spacecraft by sending it skimming past a massive object such as a planet. In this way, the spacecraft steals some velocity from the movement of the planet, propelling it on its journey.
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