This is one of those fascinating physics tricks. NewScientist (16 March 2019, paywall) has the story on laser light and black holes:
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.
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.
And don’t forget the initial boost you’d get just by shooting the laser. I wonder how many SF writers ran right to their tripewriters word processors and came up with a story incorporating that phenomenon when they read this?
And now I’m thinking that’s just what Larry Niven did in The Borderland of Sol. It won the Hugo Award for Novelette in ’76, so don’t sneer just yet.
