“Halo Drive” can achieve relativistic speeds by shining light around a black hole

If we ever want to explore the galaxy (let alone the rest of the universe) we have a speed problem. In late 2023, NASA’s Parker Solar Probe reached the fastest speed ever achieved by a man-made object, reaching 635,266 kilometers (394,736 miles) per hour.

While impressive, this is only 0.059 percent of the speed of light. Visiting our nearest neighbor Proxima Centauri, 4.2 light-years away, at these speeds would take about 7,700 years, making generations of spacecraft (or robotic probes) necessary to explore it or any other more interesting star. away.

To achieve these speeds, and on other missions to visit objects at the far reaches of the Solar System, NASA has regularly used “gravity assists”. As the spacecraft approaches a large body (planets or a star), momentum is transferred from the planet to the spacecraft, slowing the object’s orbit by a small amount in exchange for a significant increase in speed. Basically, you steal some kinetic energy from the planet or star.

“Several robotic spacecraft have used the ‘gravity assist’ technique to reach their targets ‘up’ in the Sun’s gravity well. Voyager 2 was launched in August 1977 and flew by Jupiter for reconnaissance and a trajectory boost to Saturn ,” NASA explains. .

“Voyager 1 launched the following month and did the same (reaching Jupiter before Voyager 2 did). Voyager 2 then received an assist from Saturn and another later from Uranus, going up to Neptune and beyond wide. Galileo received one push from Venus and two from Earth, while orbiting the Sun en route to its destination, Jupiter received two pushes from Venus, one from Earth and one from Jupiter to gain enough momentum to reach Saturn. .

It has been proposed that we could send spacecraft to relativistic speeds using a gravitational boost around a neutron star in a compact binary system. However, such a mission would be quite dangerous, and in 2019, David Kipping, assistant professor of astronomy at Columbia University, suggested another way we could use this neat trick safely, by shooting protons. around a black hole.

Black holes are a source of too much gravity to help, formed by massive stars (or perhaps through outright collapse) that have collapsed under their sheer mass, not allowing even light to escape. But trying to fly a spacecraft around one is the behavior of someone who wants to become spaghetti.

But as light passes through gravitational wells we know that it also gains energy. As light is traveling at the speed limit of the universe—the speed at which all massless particles must travel—it cannot gain or lose speed by falling into or out of a gravity well. Instead, as light falls into a gravity well, its frequency becomes higher and blue-shifted, while light leaving a gravity well becomes red-shifted. It is this that is exploited by the “Halo Drive”.

The basic idea is to send a beam of light around a pair of black holes orbiting each other before a merger, or a single rapidly rotating black hole, and use the higher-energy blue-shifted light to accelerate your spaceship.

“Using a moving black hole as a gravitational mirror, the kinetic energy from the black hole is transferred to the light beam as a blue shift, and upon return the recycled photons not only accelerate but also add energy to the spacecraft,” he writes. Kipping on. paper. “It is shown here that this gained energy can later be spent to reach a final velocity of approximately 133% of the black hole velocity.”

As the light travels around the black hole, it would form a halo, giving the machine its name.

“The proposed system is for a spacecraft to fire a beam of energy aimed at a black hole at a carefully chosen angle so that the beam bounces back at the spacecraft – a so-called geodesic boomerang,” Kipping continued. “If the black hole is moving toward the spacecraft, as can easily be accomplished using a compact binary, this halo of particles will return with a higher energy (and momentum). This energy is then transferred to the spacecraft allowing acceleration . In general then, the halo motion transfers kinetic energy from the moving black hole to the spacecraft by means of a gravitational assist.”

Using the machine, an interstellar civilization could jump between the tracks of black holes without fuel, using them to slow down as they approach. According to the paper, the mass of the spacecraft is quite insignificant as long as it is much lower than that of the black hole system, meaning it can propel Jupiter-sized craft up to relativistic speeds.

Using a halo disk would have only minimal detectable effects on binary black holes, since using them to slow down would effectively undo the effect of using them to accelerate (thanks, Newton’s third law).

“However, the finite time differences between departure and arrival would cause the binary to spend time on a narrower semi-major axis than it would naturally, during which time it would experience faster gravitational radiation in spiral,” Kipping added. “Consequently, a possible halo machine technique would be an improved scale of spiral binary black holes versus their neutron star counterparts.”

The study was published by the British Interplanetary Society and is available on arXiv.