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Halo Drive: lasers and black holes could allow spacecraft to reach a speed close to the speed of light

Future spaceships could use black holes as powerful stepping stones to explore the stars.

A new study plans to trigger laser beams bending around a black hole and coming back with extra energy to help propel a spacecraft at a speed close to that of light. Astronomers could look for signs that extraterrestrial civilizations use such a "halo reader," as the study calls it, by checking if pairs of black holes merge more often than expected.

The author of the study, David Kipping, astrophysicist at Columbia University in New York, proposed the idea of ​​the halo in what he calls "the mentality of the player."

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"Sometimes, in a video game, you find an" exploit ", a hack that allows you to do something too powerful that would otherwise be outlawed by the rules of the game," Kipping told Space.com. "In this case, the game is the physical world, and I've tried to think of feats that would allow a civilization to achieve a relativistic flight across the galaxy without the huge energy expenditure that one could to assume naively. "

The use of rockets to fly in space poses a major challenge: the propeller that they carry with them to the mass. Long journeys need a lot of propellant, which makes the rockets heavier, which requires more thrusters, which makes the rockets heavier, and so on. This problem worsens exponentially as the rocket expands.

However, instead of carrying a thruster for propulsion, a spacecraft equipped with mirror-shaped sails could rely on lasers to push them outward. The $ 100 million Starshot Revolutionary Initiative, announced in 2016, plans to use powerful lasers to propel swarms of spacecraft to Alpha Centauri, the star system closest to ours, at speeds up to 20% that of light.

The spacecraft that Starshot Breakthrough aims to launch are each just the size of a microchip. In order to accelerate larger spaceships at relativistic speeds – up to a significant fraction of the speed of light – Kipping resorted to gravity.

Spacecraft now routinely use "slingers maneuvers", in which the gravity of a body, such as a planet or a moon, propels vessels through space and increases their speed. In 1963, the famous physicist Freeman Dyson suggested that spaceships of any size could rely on slingers maneuvers around compact pairs of white dwarfs or neutron stars to fly at relativistic speeds. (Dyson came up with the notion of what became known as a Sphere of Dyson, a megastructure that encapsulates a star to capture the maximum of its energy and fuel an advanced civilization.)

However, a "Dyson Slingshot" may damage a spacecraft due to extreme gravitational forces and dangerous radiation emitted by these pairs of dead stars. Instead, Kipping suggests that gravity could help spaceships by increasing the energy of the laser beams pulled at the edge of the black holes.

Black holes have such powerful gravitational fields that nothing can escape them once they get close enough, not even light. Their gravitational fields can also distort light photon paths that do not fall into holes.

In 1993, physicist Mark Stuckey suggested that a black hole could, in principle, act as a "gravitational mirror," in that the gravity of the black hole could launch a photon around it for that purpose. he returns to his source. Kipping calculated that if a black hole was moving toward the source of a photon, the "photon boomerang" would siphon some of the energy from the black hole.

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Using what he calls a "halo player" – named for the ring of light that he would create around a black hole – Kipping discovered that even spacecraft with the mass of Jupiter could reach relativistic speeds. "A civilization could exploit black holes as galactic crossing points," he writes in a study accepted by the Journal of the British Interplanetary Society and detailed online February 28 on the pre-print server arXiv.

The faster a black hole moves, the more energy a halo can get from it. As such, Kipping 's is mainly focused on using pairs of black holes spiraling to each other before melting.

Astronomers could look for signs that extraterrestrial civilizations are exploiting pairs of black holes to move with such an engine. For example, ear discs actually steal energy at such black hole binaries, by increasing the melting rate of black hole pairs compared to what one would expect to see naturally, Kipping said.

His discoveries were based on boosters of pairs of black holes orbiting their speed at relativistic speeds. Although there are an estimated 10 million pairs of black holes in the Milky Way, Kipping pointed out that few of these were probably in orbit at relativistic speeds as they would merge rather quickly.

Nevertheless, he noted that isolated rotating black holes could also launch halo readers at relativistic speeds, "and we already know many examples of relativistic supermassive rotating black holes".

The major disadvantage of a halo drive would be that one "has to get to the nearest black hole," Kipping said. "That's like paying a one-time toll to get into the road system, you have to pay a little bit of energy to get to the nearest access point, but then you can use it for free as much as you want. ".

The halo reader only works near a black hole, at a distance of about five to 50 times its diameter. "That's why you first have to go to the nearest black hole and [why you] We simply can not do this through light-years of space, "said Mr. Kipping. We always need a way to move to nearby stars to borrow the road system.

"If we want to achieve a relativistic flight, we need huge levels of energy, whatever the propulsion system you use," he added. "One way around this problem is to use astronomical objects as a power source because they literally possess astronomical energy levels in them. In this case, the black hole binary is essentially a giant battery that is waiting to be able to exploit it. The idea is to work with nature and not against it. "

Kipping is currently studying ways to exploit other astronomical systems for relativistic flight. Such techniques "may not be as efficient or fast as the halo-drive approach, but these systems possess the deep energy reserves necessary for these journeys," Kipping said.

Follow Charles Q. Choi on Twitter @cqchoi. Follow us on twitter @Spacedotcom and on Facebook.

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