A BLACK hole’s gravitational pull is so strong that it bends light around it like a strange cosmic mirror. Interstellar spacecraft could make use of this effect to steal energy from a black hole and get a speed boost without needing extra fuel.
We already use a version of this energy-stealing. Spacecraft heading from Earth to the outer solar system slingshot around Jupiter for a gravity assist, speeding up by ever so slightly slowing the planet in its orbit.
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Spacecraft could theoretically perform the same trick with a black hole for an even greater boost in speed. It is a risky manoeuvre, however, as the craft risks falling in to the eternal clutches of the black hole.
Luckily, David Kipping at Columbia University has found that you don’t need to use the spaceship itself for the slingshot: you can use light as a sort of proxy.
If you fire a laser at just the right angle to travel around a black hole that is moving towards you, the light will return with more energy than it started with. Catch the beam as it hurtles back and this extra energy could be used to power your ship.
It is similar to a tennis ball hitting a racquet. If the racquet is still, the ball will lose some energy and fall to the ground. But if it is moving towards the ball, it imparts some extra energy.
This trick works best with a black hole that is part of a pair, because their orbital dance around each other means they can travel close to the speed of light. Kipping calculated that the method could accelerate a spacecraft to about 1.3 times the velocity of one of the black holes, though still less than the speed of light. He calls it the because of the circle of light it would produce around the black hole.
Of course, to be able to use a halo drive, a civilisation would already have to be capable of interstellar travel to the nearest pair of black holes. It would also need an extremely powerful laser. “The engineering parts are really difficult to deal with in any realistic way,” says Avi Loeb at Harvard University. “I wouldn’t say it’s practical.”
Get past that, and you have a free source of propulsion to travel between black hole pairs in a kind of transport network throughout the galaxy.
“It’s kind of like a highway system where you have to pay a one-time toll,” says Kipping. “Once you pay the fee, you can ride the highway system as long as you want.”
Article amended on 14 March 2019
We corrected where the energy for a slingshot comes from
