Astrophysicists have discovered the first potentially habitable planet outside our solar system. It's called Gliese 581 g, and it's about 20 light-years from Earth. If we got tired of living here, how would we get to Gliese 581 g?
By bending the laws of physics. Unlike crossing the Atlantic or putting a man on the moon, interstellar travel isn't an engineering problem; it's a physics problem. Scientists have given up on the idea of traveling to distant solar systems through anything resembling traditional means. For one thing, it would take far too long. Current space shuttles don't go much faster than 17,500 miles per hour. At that pace, it would take 766,000 years to get to Gliese 581 g—that's more than three times longer than homo sapiens have been around. Even if we had the technology to go faster, the energy considerations are daunting. If you wanted to get to Proxima Centauri—the second-closest star to Earth at a mere 4.22 light-years away—within a century on conventional propellant, your fuel tank would have to be larger than the visible universe. Consequently, astrophysicists are focusing on technology that would permit shuttles to travel faster than light, without fuel, or both. Current ideas for pulling off this trick include laser sails, warp drives, and wormholes.
Laser sails are the most technically feasible option. When a beam of light strikes an object, it exerts a small amount of force. In a vacuum environment, like space, the force can drive an object forward. If we gave a space ship enough jet fuel to put it into outer space, we could shine a laser beam on its reflective sail to move it farther and farther away. In theory, we could push it all the way to Gliese 581 g. But this concept has serious limitations. First, even a laser beam loses focus over great distances. To keep the beam trained on a sail more than two light years away, both the lens and the sail would have to be hundreds of miles across. Second, once the ship got to Gliese 581 g, we'd have no way to slow it down. Third, if the ship drifted out of the laser's path, the crew would be lost, because it would be years before we even knew there was a problem. Plus, no one really knows how long the trip would take.
Warp drive is a clever way of skirting the principle that nothing can travel faster than light. The idea is that, if you can move space-time itself opposite the direction you're traveling, your speed will increase. (It's like running on a moving walkway.) Physicists believe this is possible—indeed, many think space-time expanded faster than the speed of light shortly after the big bang. The problem is making it happen. To accelerate space-time, you'd probably have to wrap your spaceship in something called negative energy, which may or may not exist.
A wormhole is a sort of space fistula—a passageway that provides a shortcut between two points in space that aren't otherwise adjacent. Physicists speculate that we may be able to make one by building two enormous rings—each about the size of Earth's orbit around the sun—out of really dense matter, and then spinning the rings around really fast. Wormholes would be a great way to commute back and forth between Earth and Gliese 581 g, but they don't work as an exploration strategy because you need to get to your destination before you can build the hole. In addition, we'd probably need to build the rings out of matter from a neutron star, and the nearest one of those is 250 light-years away.
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Explainer thanks Bernard Haisch of the Digital Universe Foundation.