A Stellar Engine, Literally

Often, in science-fiction, it’s not easy to tell what came before: the science or the fiction. Many inventions appeared in the ink before they did in reality. The submarine, for example, surfaced in the 1870 Jules Verne classic, “Twenty Thousand Leagues Under the Sea,” much before it made it to our naval fleets.

In this instance, the fiction came before.

Now, take a book of our age. In “Singularity Sky,” published in 2005, Charles Stross describes the Lord Vanek—the flagship of the imperial space-navy of a future civilization called the New Republic—as a 90,000 tonne-warship, powered by a spinning black hole that’s the size of an electron, and the weight of a mountain range.

That a spacecraft can have for its power source, a black hole, is so outlandish that it’s easy to dismiss it as a brainwave of the loony Hatter. Sure, it does have an aura of the absurd. But it turns out that it’s firmly rooted in esoteric equations that predate the novel by a good 50 years.

Here, the science came before.

In his 1955 paper, “Geons,” John Archibald Wheeler—the man who popularized the term, “black hole”—suggested that if a column of energy so pure and so intense could be concentrated into an area of space, it could warp space-time in that region enough for it to become a black hole. Such a synthetic black hole, formed of energy, not mass, is called a “kugelblitz.”

20 years or so later, Stephen Hawking made a startling discovery. He found out that black holes are not as impermeable as they’re believed to be in classical general relativity.

Due to the seemingly bizarre effects of “quantum tunneling” near a black hole’s “event horizon,” matter, trapped inside, can tunnel its way through that impenetrable barrier into the outside, in the form of radiation. In the process, the star, already dead, slowly evaporates away.

Splicing the two postulates provides the groundwork for a stellar propulsion mechanism that converts the energy of a kugelblitz into raw momentum. Two scientists at the University of Kansas, Louis Crane and Shawn Westmoreland, had a look at whether it could be done. In a paper, published in 2009, they said it could, at least, in theory.

The smaller the radius of a black hole, the smaller is its mass. That also makes it staggeringly powerful. But on the downside, its puissance comes at the price of a shorter lifespan.

To be a viable engine for a long-haul spaceflight, such a black hole would have to be tiny enough to yield a lot of juice, but not be so minute as to whittle away before it can deliver its starship to its destination.

The fastest space voyage that humans can endure is a one-way trip to Proxima Centauri, our neighboring star, 4.3 light-years away. But due to time dilation, the trip would take 3.5 years (from the perspective of the travelers.)

A journey of this duration can be powered by a black hole with a radius of 0.9 attometers (one attometer is 10-18 of a meter), and a mass of 606,000 tonnes. It’d generate 160 petawatts (or 160 billion megawatts.)

(New York’s record-breaking power consumption during the scorcher of July, 2013, by comparison, is a tiny fraction—a mere 33,955 megawatts.) A vessel with such a black hole at its core would be capable of achieving 10 percent of the speed of light in just 20 days.

But building one calls for energy of biblical proportions, and engineering might of the sort not seen since the construction of the ancient pyramids.

The only fountain of such vim and vigor is the Sun. Capturing its blaze would require placing an enormous, square solar panel, each side of which measures about 230 miles, in a circular orbit around the Sun, at a distance of 620,000 miles. In a year, it’d absorb a formidable cache of photons to fire up a massive gamma laser, set up not too far away, which, in turn, would be deployed to generate the black hole.

But all this is presently out of our technological grasp. And until it is, it’ll remain a mad, tantalizing science-fictional construct.


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