Humans have figured out how to send spacecraft into the deep reaches of the solar system, but it will take major advances in spaceflight before we can hop over to other star systems or traverse the Milky Way. In the meantime, though, it doesn’t hurt to think about cool ways we might one day be able to accomplish that dream.
Enter: the “halo drive,” a concept that proposes leveraging the power of black holes and other gravitationally powerful phenomena to accelerate future spacecraft to near-light speeds.
Conceived by David Kipping, an astronomer at Columbia University, the halo drive involves shooting lasers at objects such as black holes or neutron stars in order to get a speed boost when the light beam boomerangs back to its starting point.
Kipping outlines the concept, named for the laser halo it would form around a black hole or star, in a forthcoming study in The Journal of the British Interplanetary Society.
The idea hinges on the intense gravitational pull of neutron stars and black holes, which could draw laser beams into a trajectory around them. If shot at just the right angle, these “boomerang photons,” as Kipping calls them, would theoretically spit back at the spacecraft with added energy leeched off the black hole or neutron star.
Binary black hole or neutron star systems, in which two massive objects orbit close together, would be even more ideal because they could provide more energy.
A spacecraft could use these objects to juice up a laser beam that could then accelerate it to relativistic speeds (meaning close to the speed of light). The same laser blasts could also be used to decelerate a spacecraft traveling at this swift clip.
Volià: An intragalactic light rail system.
Kipping got to thinking about the halo drive after working on a paper for the Breakthrough Starshot project, another laser-based interstellar mission concept involving relativistic speeds.
The sheer horsepower it would take to accelerate a spacecraft to such a fast pace prompted Kipping to look to nature for inspiration, since the universe is much better at creating wildly energetic systems than humans.
“If we can’t build something capable of delivering astronomical levels of energy, perhaps we could instead steal the energy from an astronomical object,” Kipping told Motherboard in an email. “The overall philosophy is to try to find loopholes in nature, exploits that allow us to accomplish feats otherwise unimaginable.”
Similar versions of this idea have popped up in past research. Physicist Freeman Dyson developed a concept called the Dyson slingshot, which envisions a spacecraft that enters a neutron star system and gets catapulted out at relativistic speed. Physicist Mark Stuckey also established that a photon could theoretically circle the outer edge of a black hole and exit from the same spot.
Kipping expanded on those ideas, and threw in some useful information for alien hunters as well. Imagine there are already extraterrestrial civilizations that use this propulsion technique. If so, astronomers might be able to detect evidence of the halo drive. Kipping predicts that this form of propulsion could alter orbits of binary black holes slightly, and increase the speed at which these huge objects merge. Astronomers looking for alien “technosignatures,” meaning observational signals of advanced technologies, could search the skies for this type of pattern.
The halo drive could also be used as a kind of “cosmic battery,” Kipping said. Instead of shooting lasers at angles that allow them to exit the system, the light could be aimed so that it was perpetually trapped in rotation like a flywheel.
“This is definitely more speculative,” Kipping said. “If, through whatever mechanism, a civilization needed to store vast amounts of energy somewhere, these cosmic flywheels would be natural candidates. They’re not perfect batteries, because like all batteries they slowly leak energy away via gravitational radiation, but they offer a means to deposit, store, and recover energy simply through the use of lasers.”
It’s a dazzling vision for future space travel, but don’t go looking to buy a halo drive multipass just yet. For one thing, the idea requires that we get close to a neutron star or black hole, which means traveling hundreds or thousands of light years to the nearest known ones. The halo drive would also require a very sturdy spacecraft and crew that could handle the sudden kick to relativistic speeds.
That said, the idea of blasting laser beams at black holes so that crews can ride the backdraft around the Milky Way is too awesome to be weighed down by the limitations of current technologies. It almost certainly won’t happen in our lifetimes, but hopefully we’ll at least get some cool science-fiction stories about it.
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