I stumbled across a paper the other morning on arXiv which, at first glance, looked so bizarre I nearly forgot about my coffee. I’ll freely admit, I do rather like audacious concepts, and this one is about as audacious as they come! Given the immense fuel requirements for interstellar travel, collecting your fuel en route would be economically very favourable. So what about dark matter…?
Resting a little tenuously upon the ideas in supersymmetry (that if dark matter is composed of supersymmetric particles, they could annihilate and release energy) the concept is fairly simple. What if we could harness that annihilation and use it as propulsion? The immediate idea is something not unlike the Bussard ramjets popularised by Carl Sagan and Larry Niven. In a way directly analogous to the atmospheric ramjets utilised by some aircraft, a “dark matter engine” would take in dark matter, pressurise it to cause annihilation and use the products as propulsion.
Of course, the immediate problem (acknowledged in the paper’s conclusion) is that it may be difficult to construct such an engine, at least using baryonic matter. As any self-respecting astrophysicist will tell you, dark matter interacts only weakly with regular matter, making its presence felt through gravity and gravity alone. It’s nice, but fanciful to suggest building this engine out of material from the “dark sector”.
But just suppose, for a moment, that such a thing was indeed possible. Suppose that you could create some kind of container which dark matter couldn’t readily escape from. Assuming, for simplicity, a static distribution of dark matter, the stuff is everywhere. Theoretically, there should be some in the same room as you while you’re reading this. The mass density of dark matter in the universe is around five times that of regular matter. If some kind of dark matter ramjet of this ilk could be constructed, it could hypothetically reach relativistic speeds in as short a distance as 20.6 AU (a little over the orbital radius of Uranus). Increased dark matter density, or increased surface area of the engine would thus increase the acceleration rate. Acceleration itself would be proportional to velocity, theoretically causing an exponential increase in overall speed.
To cut a long story short (and skimp somewhat on the calculus), with the estimated dark matter density in a subhalo, it would still be impossible to reach relativistic speeds — although you could still theoretically achieve up to 300 kilometres per second, comparable to the average velocity of the solar wind. Using the vastly increased density around a black hole, however, it would theoretically be possible to accelerate to around 90% the speed of light!
I must admit, I’m not sure about this (although I’m by no means an expert on dark matter). In truth, I suspect this concept may be destined to remain, simply, a concept. We still don’t really know enough about dark matter, if it even exists, to be able to test such an incredible hypothesis. If it’s even remotely possible, however, it’s certainly a fascinating idea.
Mind you, this paper does forget to account for one thing. If you could accelerate to 90% of the speed of light, there’s no suggestion as to how precisely you might stop!
Source — arXiv:0908.1429