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.