NewScientist wrote: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 wasn't 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 Proxima Centauri - 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, physicist Jia Liu at New York University outlined his design for a spacecraft powered by dark matter (arxiv.org/abs/0908.1429v1). Soon afterwards, mathematicians Louis Crane and Shawn Westmoreland at Kansas State University in Manhattan proposed plans for a craft powered by an artificial black hole (arxiv.org/abs/0908.1803).
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's more, Crane believes that feasibility studies like his touch on questions in cosmology that other research hasn't considered.
Fuel As You Go
Take 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.
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, 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 Liu.
His plan is to drive the rocket using the energy released when dark matter particles annihilate each other. Here's where Liu's idea depends on more speculative physics. No one knows what dark matter is actually made of, though there are numerous theories of the subatomic world that contain potential dark matter candidates. 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 1017 joules, more than 10 billion times as much energy as a kilogram of dynamite, and plenty to propel the rocket forwards.
Even less certain is the detail of how a dark matter rocket might work. Liu imagines the engine as a "box" with a door that is open in the direction of the rocket's motion (see diagram). As dark matter enters, the door is closed and the box is shrunk to compress the dark matter and boost its annihilation rate. Once the annihilation occurs, another door opens and the products rocket out. The whole cycle is repeated, over and over again.
Liu points out that the faster his dark matter rocket travels, the quicker it will scoop up dark matter and accelerate. Precisely how quickly it can accelerate depends on the density of the surrounding dark matter, the collecting area of the engine and the mass of the rocket. In his calculations, 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.
There is just one small problem, however. To work most efficiently, Liu's rocket would have to fly through dense regions of dark matter. As far as we know, the greatest concentration of dark matter is 26,000 light years away at the centre of the Milky Way. Still, Liu points out that no one has made a detailed map of the dark matter in our galaxy and he hopes that nearer concentrations will be found.
And if that's not a deal-breaker how do you build an engine box that does not leak dark matter? "This is the idea's Achilles' heel," says Crane. Dark matter, by its very nature, interacts extremely weakly with normal matter and may pass right through it. This could well be why experiments on Earth have failed so far to snag any passing dark matter particles.
Crane thinks that fabricating a rocket out of a material we can't yet be sure exists is a leap of faith too far. He prefers dealing with more established physics and technology. Liu is undaunted. He points to theories that contain lots of other particles besides ordinary and dark matter particles, such as ones with extra dimensions. "It is possible there exists a type of matter which interacts strongly enough with both," he says. "This could be used to make a box."
Black hole sweetspot
To get to the stars, you need to squeeze every last joule of energy from your fuel. Chemical rockets are terribly inefficient, converting just 10-8 per cent of their mass into energy. Even fusion converts less than 1 per cent of nuclear fuel into energy. An antimatter rocket would be the gold standard. "Granted you can extract 100 per cent of the energy from matter-antimatter annihilation," says Crane. "However, antimatter is hugely inefficient to make in the first place, and it is dangerous stuff - if it touches your spacecraft, it blows it to kingdom come."
Crane is convinced that the only option is in fact 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 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. Crane has calculated that a black hole weighing about 1 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 'sweetspot'," says Crane.
The first person to propose using a mini black hole for propulsion was science fiction writer Arthur C. Clarke in his novel Imperial Earth. Recently, Hawking has also publicised the idea, advocating hunting down a pre-existing black hole. Crane is unsure this would work. "What are the chances of finding one drifting through the solar system?" he asks.
Instead, we'd have to make our own. To create a black hole, says 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," 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.
According to Crane, his million-tonne black hole starship could accelerate to close to the speed of light in a few decades. If that's too slow for you, there is a way to speed things up. A smaller black hole would give off more Hawking radiation, so it could propel you faster as long as you take along extra matter to feed it. Once you were travelling at this speed in your starship, time would slow down for you so you would age more slowly than your friends and family on Earth. "It might be possible to reach the Andromeda galaxy 2.5 million light years away within a human lifetime," says Crane.
Mind-blowing as all this may seem, Crane reiterates that, as far as he can see, this is the only feasible way to travel to the stars. Which raises an interesting question: could an advanced alien civilisation already be cruising the Milky Way aboard black hole starships?
Crane thinks it is possible. So perhaps looking for black hole starships would a fruitful way to hunt for extraterrestrial intelligence. As the black hole powering the starship emits its Hawking radiation, it would shudder and send a ripple through space-time. We might be able to detect such ripples, otherwise known as gravitational waves, here on Earth.
First, we'd need to build new gravitational wave observatories. Existing facilities, such as the twin LIGO detectors in Hanford, Washington, and Livingston, Louisiana are primed to look for low-frequency gravitational waves emitted by coalescing black holes and neutron stars, which will be quite different from the rapid ripples given off by a black hole starship. "Currently, we're looking for sluggish gravitational waves with frequencies of a few hertz," says Crane. "I think it would be worth scanning the sky for ultra-high-frequency gravitational waves."
Perhaps ET has chosen to build a dark matter starship instead. "If advanced extraterrestrial civilisations are currently using dark matter rockets, the places of high dark matter density might be like big cities where traffic is concentrated," says Liu.
This leads him to speculate on why no extraterrestrials have come our way as far as we know. "Because the dark matter density in our neighbourhood is low relative to the centre of our Milky Way, it is hard to get here," he says. So the same problem that prevents us from exploring the universe in a dark matter starship could be stopping ET from paying a visit.
Baby Universes
Aside from the technological challenges, Crane thinks black hole starships may also have remarkable philosophical implications. Crane first started thinking about artificial black holes 12 years ago when physicist Lee Smolin, now at Canada's Perimeter Institute for Theoretical Physics in Waterloo, Ontario, asked Crane to read the manuscript of his book The Life of the Cosmos.
Nobody knows what happens at the singularity of a black hole, the point where space and time become so warped that the laws of relativity break down. In his book, Smolin suggested that a new universe could be created and bud off. So universes in which black holes are likely to arise will give birth to more and more such universes. This means that our universe could be a baby universe, and is more likely to have come from one that is good at making black holes than one that isn't.
Crane then wondered what would happen if intelligent civilisations could make black holes. This would mean that life in these universes played a key role in the proliferation of baby universes. Smolin felt the idea was too outlandish and left it out of his book. But Crane has been thinking about it on and off for the last decade.
He believes we are seeing Darwinian selection operating on the largest possible scale: only universes that contain life can make black holes and then go on to give birth to other universes, while the lifeless universes are an evolutionary dead end.
His latest calculations made him realise how uncanny it was that there could be a black hole at just the right size for powering a starship. "Why is there such a sweet spot?" he asks. The only reason for an intelligent civilisation to make a black hole, he sees, is so it can travel the universe.
"If this hypothesis is right," he says, "we live in a universe that is optimised for building starships!"
