x Abu Dhabi, UAEFriday 28 July 2017

Riding a black hole to reach the stars

Interstellar travel has been the subject of many ambitious proposals. Now a spacecraft that could draw on dark matter might be the answer.

The galaxy Centaurus A is the nearest to Earth that contains a supermassive black hole that could possibly power a spacecraft.
The galaxy Centaurus A is the nearest to Earth that contains a supermassive black hole that could possibly power a spacecraft.

"Space is big," wrote Douglas Adams in his book The Hitchhiker's Guide to the Galaxy. "You just won't believe how vastly, hugely, mind-bogglingly big it is." He was not exaggerating. Even our nearest star, Proxima Centauri, is a staggering 4.2 light years away - more than 200,000 times the distance from the Earth to the Sun. Or, if you like, the equivalent of 50 million trips to the Moon and back.

Such vast distances would seem to put the stars far beyond the reach of human explorers. Suppose we had been able to hitch a ride on Nasa's Voyager 1, the fastest interstellar space probe built to date. Voyager 1 is now heading out of the solar system at about 17 kilometres per second. At this rate it would take 74,000 years to reach Promixa Centauri. It's safe to say we wouldn't be around to enjoy the view.

So what would it take for humans to reach the stars within a lifetime? For a start, we would need a spacecraft that can rush through the cosmos at close to the speed of light. There has been no shortage of proposals: vehicles propelled by repeated blasts from hydrogen bombs, or from the annihilation of matter and antimatter. Others resemble vast sailing ships with giant reflective sails, pushed along by laser beams.

All these ambitious schemes have their shortcomings and it is doubtful they could really go the distance. Now there are two radical new possibilities on the table that might just enable us - or rather our distant descendants - to reach the stars. In August, the physicist Jia Liu at New York University outlined his design for a spacecraft powered by dark matter. Soon afterwards, the mathematicians Louis Crane and Shawn Westmoreland at Kansas State University in the United States proposed plans for a craft powered by an artificial black hole.

No one disputes that building a ship powered by black holes or dark matter would be a formidable task. Yet remarkably, there seems to be nothing in our present understanding of physics to prevent us from making either of them. What is more, Prof Crane believes that feasibility studies like his touch on questions in cosmology that other research has not considered. Take Prof Liu's dark matter starship. Most astronomers are convinced of the existence of dark matter because of the way its gravity tugs on the stars and galaxies we see with our telescopes. Such observations suggest that dark matter outweighs the universe's visible matter by a factor of about six - so a dark matter starship could have a plentiful supply of fuel.

Prof Liu was inspired by an audacious spacecraft proposed by the American physicist Robert Bussard in 1960. Bussard's "ramjet" design used magnetic fields generated by the craft to scoop up the tenuous gas of interstellar space. Instead of using conventional rockets, the craft would be propelled by forcing the hydrogen gas it collected to undergo nuclear fusion and ejecting the energetic by-products to provide thrust.

Because dark matter is so abundant throughout the universe, Dr Liu envisages a rocket that need not carry its own fuel. This immediately overcomes one of the drawbacks of many other proposed starships, whose huge fuel supply greatly adds to their weight and hampers their ability to accelerate. "A dark matter rocket would pick up its fuel en route," says Prof Liu. His plan is to drive the rocket using the energy released when dark matter particles annihilate each other. One of the frontrunners posits that dark matter is made of neutralinos, particles which have no electric charge. Neutralinos are curious in that they are their own antiparticles: two neutralinos colliding under the right circumstances will annihilate each other.

If dark matter particles do annihilate in this way, they will convert all their mass into energy. A kilogram of the stuff will give out about 1,017 joules, more than 10 billion times as much energy as a kilogram of dynamite, and plenty to propel the rocket forwards. In his calculations, Prof Liu assumes the starship weighs a mere 100 tonnes and has a collecting area of 100 square metres. "Such a rocket might be able to reach close to the speed of light within a few days," he says. So the journey time to Proxima Centauri would be slashed from tens of thousands of years to just a few.

But how do you build an engine box that does not leak dark matter? "This is the idea's Achilles' heel," says Prof Crane. Dark matter, by its very nature, interacts extremely weakly with normal matter and may pass right through it. Prof Crane is convinced that the only option is to exploit what is called Hawking radiation. In the 1970s, Stephen Hawking showed that black holes are not completely black: they can "evaporate", when all of their mass converts into a ferocious sleet of subatomic particles. It is this radiation that Prof Crane believes could be used to propel a starship across the galaxy.

Very small black holes emit far more Hawking radiation than large, stellar-mass holes, according to the equations describing black holes. Prof Crane has calculated that a black hole weighing about one million tonnes would make a perfect energy source: it is small enough to generate enough Hawking radiation to power the starship, yet large enough to survive without radiating away all its mass during a typical interstellar journey about 100 years long. "To my amazement, there is a 'sweet spot'," says Prof Crane.

To create a black hole, says Prof Crane, you need to concentrate a tremendous amount of energy into a tiny volume. He envisages a giant gamma ray laser "charged up" by solar energy. The energy would be collected by solar panels 250 kilometres across, orbiting just a few million kilometres away from the sun and soaking up sunlight for about a year. "It would be a huge, industrial effort," Prof Crane admits.

The resulting million-tonne black hole would be about the size of an atomic nucleus. The next step would be to manoeuvre it into the focal range of a parabolic mirror attached to the back of the crew quarters of a starship. Hawking radiation consists of all sorts of species of subatomic particles, but the most common will be gamma ray photons. Collimated into a parallel beam by the parabolic mirror, these would be the starship's exhaust and would push it forward.

"It might be possible to reach the Andromeda galaxy 2.5 million light years away within a human lifetime," Prof Crane says. * New Scientist Marcus Chown is the author of We Need To Talk About Kelvin