Back in 2016, physicist Stephen Hawking and billionaire Yuri Milner revealed a plan to travel to the stars. The so-called Breakthrough Starshot Project is a $ 100 million program to develop and demonstrate the technology needed to visit a nearby star system. Potential targets include Proxima Centauri, a system about four light years away, with several exoplanets, one of which is similar to Earth.
Breakthrough Starshot Project
Hawking and Milner's plan was to build thousands of tiny microchip-sized spaceships and use light to accelerate them to relativistic speeds - that is, close to the speed of light. A large fleet increases the chances that at least one of them will arrive safely. Each "star chip" is attached to a light sail the size of a badminton court and then irradiated with extremely powerful ground-based lasers.
There are many advantages to laser movement. The most important thing is that spaceships do not need any fuel, which means they should not take extra cargo with them. Also, by accelerating the light sail, you can accelerate the boat to 20% the speed of light. In this scenario, the fleet will arrive at Proxima Centauri in less than 30 years.
The fantastically powerful lasers required for such a mission would be particularly difficult and expensive to develop. An obvious question arises: is there another way to achieve relativistic speeds?
Today we have a kind of answer, thanks to the work of David Kipping, an astronomer at Columbia University in New York. Kipping came up with a new form of gravitational slingshot, the same technique that NASA used to send, for example, the Galileo spacecraft to Jupiter. The idea is to accelerate the spacecraft by pointing it near a huge object such as a planet. Thus, the spacecraft will take away part of the planet's speed and accelerate with its help.
Gravitational slingshots work great on massive bodies. In the 1960s, physicist Freeman Dyson calculated that a black hole could accelerate a spacecraft to relativistic speeds. But forces on a spaceship approaching such an object are likely to destroy it.
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So Kipping came up with a smart alternative. His idea is to direct photons around the black hole and then use the extra energy they receive to accelerate the lightsail. "The kinetic energy of the black hole is transferred to the beam of light in the form of blueshift, and upon return, the photons not only accelerate the spacecraft, but also add energy to it," says Kipping.
This process depends on the extremely powerful gravitational field around the black hole. Since photons have a small, but still rest mass, this field is able to trap light in a circular orbit.
Kipping's work is based on a slightly different orbit, directing the photons emitted by the spacecraft around the black hole and back again - a kind of boomerang orbit. While traveling, the photons on the boomerang will receive kinetic energy from the movement of the black hole.
It is this energy that can accelerate a spacecraft equipped with an appropriate light sail. Kipping calls his idea a "halo engine". The halo engine transfers the kinetic energy of a moving black hole to the spacecraft using gravity. Moreover, the spacecraft does not consume any of its own fuel in this process.
Since the halo engine uses the motion of a black hole, it is best applied to binaries in which a black hole orbits another object. The photons then receive energy from the movement of the black hole at the appropriate points in its orbit.
And such an engine must work with any mass that is significantly less than the mass of the black hole. Kipping says planet-sized mechanisms are possible with him. Thus, a sufficiently advanced civilization can travel at relativistic speeds from one part of the galaxy to another, jumping from one binary system of black holes to another. “An advanced civilization could use the light sail concept to achieve relativistic speeds and extremely efficient movement,” he says.
The same mechanism can also slow down the spacecraft. So this advanced civilization is likely to look for pairs of binary systems with black holes that will act as accelerators and moderators.
The Milky Way contains about 10 billion binary black hole systems. But Kipping notes that there will likely only be a limited number of trajectories that tie them together, so these interstellar highways are likely to be very valuable.
Of course, the technology required to exploit this concept is currently beyond the reach of humanity. But astronomers should be able to figure out where the best stellar highways are located, as well as look for technosignatures of civilizations that can exploit them.
Ilya Khel