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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 …