The Dark Matter Engine
Dark matter rockets, or rather reciprocating ramjets, are proposed by Jia Liu in http://arxiv.org/abs/0908.1429. Dark matter particles annihilate on collision. The spacecraft opens its engine to sweep in dark matter, closes the opening, and compresses the dark matter until it annihilates to produce energy. This is as efficient as it gets. The other end of the engine is then opened to function as a jet. This work cycle repeats to accelerate the spacecraft. According to Jia, speeds vary on dark matter density and distribution, but relativistic speeds seem feasible — small fractions of c for the most part, but much higher near kernels of dark matter distribution. It does not take long, or far, for the spacecraft to accelerate to cruising velocity. Braking is just firing in reverse.
The best thing about this scheme is that, if it works, we are not going to run out of fuel anytime soon. Since the demise of the Bussard ramjet, which it resembles except in fuel, there has not been a plausible “hard science fiction” scenario for interstellar propulsion on a large scale (antimatter could in principle be manufactured near stars and used for interstellar hops, but not for longer journeys, and not perhaps on a large scale).
I imagine very small “cylinders”, or perhaps a wheel or turbine of some kind.
Granting the dark matter engine, what else is required for a plausible human expansion into interstellar colonies?
First, journeys are still waaay too long, so either some safe form of hibernation must be developed, or else the human life span must be extended at least to several centuries.
Second, it must become more economic to mine, manufacture, farm, and indeed live in outer space than on Earth, and it must be possible to do this from a base of one or a few starships.
About the economics, there can be no doubt. There is just so obviously oodles more of what we want and need floating around free for the taking out there, than there is buried somewhere on someone else’s property around here.
About the base scale, that is problematic on several counts. One can imagine some sort of partly automatic, self-regulating machine ecology. But what is the minimum size? If it is too big, then cultural factors kick in — the scale of initial investment might turn out to be daunting: politically imprudent, culturally unimaginable.
Of course, we haven’t actually detected any dark matter just yet, but we shouldn’t let a little thing like that cramp our imaginations. After all, as Vera Rubin proved, something is definitely making the galaxies spin faster.