Let's face it. Someday our planet is going to get too crowded and too used-up to hold all of humanity. At this point, or hopefully before that, we are going to need to get off this rock and find another one to call home. Writers and visionaries and scientists have been coming up with ways to fly since Icarus and the Ramayana*. Slightly more recently, we've gotten Verne and Wells, who actually broke people away from their planets' gravity, followed the Golden Age of science fiction and all the cheesy movies of the 1950s. This was soon followed, of course, by government bodies such as the National Aeronautics and Space Administration (NASA), the Russian Federal Space Agency (RFSA), and the European Space Agency (ESA), all of whom have been limiting themselves almost completely to the standard but inefficient rocket engine.
At the present moment, we have several popular, theoretical ways of actually achieving meaningful space travel (and by meaningful, I mean, further than the moon, because really? Been there, done that.) My favourites are the ion thruster and the solar sail, because they promise fairly cheap, green travel over long periods of time. Oh, and the antimatter drive, but that's only because I find the whole idea of antimatter cool. There are actually going to be solar sail tests**!
So far, this is probably old news for most of you, right? Sorry. Just wanted to make sure we were all up to speed. Because there are other ways.
First up is the gravitational space corridor***, in which we position spacecraft in naturally-occurring channels of low energy and let them "fall" between moons or planets. We wouldn't need much fuel, because the gravity or lack thereof would provide most of the motion.
Second in line is the air gun****. This is not something to be used on humans, because the g-forces are enough to kill, but for supplies, it's plausible. And yes, it's basically what it sounds like. You stick whatever you want to leave Earth into the barrel of a huge hydrogen-powered gun, and fire. (We could then use plasma rockets for course correction and other forms of steering once the material's left atmo.*****)
Here, have a video of said rocket in testing:
Third and for the moment lastly is the black hole engine.****** Yes, you read that right. Some black holes emit Hawking radiation, and that radiation could be used as a power source. You'd just need the technology to create a black hole, and a way to keep it stable, and then you're set. A civilization using this method would probably be at least a Type II on the Kardashev Scale, I'd imagine, so it'll be a while.
Moral of the post: next time you're getting ready to create a space-faring civilization, don't go with the boring old ion thrusters and nuclear rockets and anti-gravity. Pick something from the list I just gave! You'll stand out.******* (I bet there are all kinds of things that could go wrong if you were powering a ship with a black hole. Nudge nudge, wink wink.)
* I'm serious.
** via Boing Boing
*** via io9
**** via Futurismic
***** not my idea. Credit goes to the folks at Futurismic (see above)
****** via io9
******* This is simply my opinion. It's not a promise, so please don't sue if you listen to me and don't stand out.