I had intended to start the week with a look at Charlie Stross’ ideas on the ‘starship’ metaphor, but I’ll hold the Stross essay until tomorrow because I want to finish up Marcus Chown’s piece in New Scientist. On Friday we talked about the idea of using Hawking radiation from a man-made black hole to propel a starship. That’s outrageous idea number one, but Chown actually began the article with a look at Jia Liu’s ideas on using dark matter to propel a ship, something along the lines of a Bussard ramjet without the hydrogen.
The Uses of Speculation
I enjoy looking at speculative concepts, even when they’re so far out on the edge that they’ve attained a kind of intellectual redshift, but New Scientist‘s squib for the story surprised me: “We could reach the stars if we built a black hole starship or a dark matter rocket – we’ve got the physics to do it.”
Er, no, we don’t have the physics to build a dark matter rocket. We don’t know what dark matter is. The case for its existence seems strong, based on anomalous galactic rotations and galactic cluster gravitational lensing, but we’re nowhere close to understanding the stuff. The rhetorical stance that should be de rigeur for discussing ideas on the edge is a simple acknowledgment that speculation is worth doing for its own sake and does not necessarily imply fast or feasible implementation. No further justification needed as long as we don’t play to the grandstand.
Robert Bussard is well known among propulsion theorists, having proposed as far back as 1960 that a ramjet scooping up interstellar hydrogen with magnetic fields could sustain a fusion reaction and thus work its way up close to the speed of light. Indeed, the Tau Zero Foundation was named after Poul Anderson’s wonderful novel Tau Zero (1970), which posited a runaway Bussard ramjet whose crew couldn’t shut down the engine, to the point where the ship was punching through entire galaxies in mere minutes of time as experienced by the crew. No spoiler here — you’ll need to read the book (a classic) to see how it ends.
An Alternative to the Bussard Ramjet
Jia Liu (New York University) has the notion of modifying the Bussard ramjet to use dark matter, and this is precisely where we do not have the physics to proceed (at least, not yet). If dark matter is made of neutralinos, just one of the dark matter candidates, then the curious thing is that neutralios are their own antiparticles. As Chown writes: “…two neutralinos colliding under the right circumstances will annihilate each other.” The benefit is obvious — 100 percent of the mass of the dark matter is converted into energy.
As with the Bussard concept, the faster the dark matter starship goes, the better. From the article:
Liu points out that the faster his dark matter rocket travels, the quicker it will scoop up dark matter and accelerate. Precisely how quickly it can accelerate depends on the density of the surrounding dark matter, the collecting area of the engine and the mass of the rocket. In his calculations, Liu assumes the starship weighs a mere 100 tonnes and has a collecting area of 100 square metres. “Such a rocket might be able to reach close to the speed of light within a few days,” he says. So the journey time to Proxima Centauri would be slashed from tens of thousands of years to just a few.
A Dark Matter Transportation Hub
But a rocket like this would be at its best when encountering dense regions of dark matter. It’s optimized, in other words, for the galactic core and any areas where dark matter should be concentrated. In his recent paper on the subject, Liu says that fact makes him think of current human transportation, especially when he examines N-body simulation pictures of dark matter:
In the picture, there are bright big points which stand for large dense halos, and the dim small points for small sparse halos. Interestingly, these halos have some common features with the cities on the Earth. The dense halos can accelerate the spaceship to higher speed which make it the important nodes for the transportation. However, the sparse halos can not accelerate the spaceship to very high speed, so the spaceship there would better go to the nearby dense halo to get higher speed if its destination is quite far from the sparse halos.
Remind you of anything? Well, I’m just back from Austin, which I had to reach by going through Dallas. To get to Cleveland from here, I have to go to Chicago.
Similarly, if we want to take international flight, we should go to the nearby big cities. The small cities usually only have flights to the nearby big cities, but no international flights. Thus we can understand the dense halos may be very important nodes in the future transportation, like the big cities on the Earth.
Too Weak an Interaction?
I must say there are airline hubs that remind me more of black holes than dark matter concentrations, but let that pass. The ‘hub’ comparison is familiar, but everything else about this has us shooting in the dark. Just as we have not nailed down our model for dark matter, so we have no accurate idea of where all the dark matter concentrations are located in our own galaxy, other than the apparent clustering at the core. We will presumably develop maps of the stuff one day. We also run into the problem that some models of dark matter assume extremely weak interactions with normal matter. Assuming dark matter exists (and I believe it does), this would account for the extreme difficulty thus far in identifying dark matter particles through ongoing experiments here on Earth.
Liu is upfront about this, positing in the second paragraph of his paper that the particles he’s dealing with do interact with normal matter: “Here we assume the DM particle and the annihilation products can not pass through the wall of the box.” The ‘box’ is the spacecraft engine, which he sees as having an open door that accepts incoming dark matter, after which “…we compress the box and raise the number density of the DM for annihilation, where we assume the annihilation process happens immediately.”
Later in the paper, Liu focuses again on the problem of weak interactions in various dark matter models:
The most serious problem comes from DM which are weakly interacting with matter. Current direct searches of DM have given stringent bound on cross-section of DM and matter. It may be difficult using matter to build the containers for the DM, because the cross-section is very small. However, the dark sector may be as complex as our baryon world, for example the mirror world. Thus the material from dark sector may build the container, since the interactions between particles in dark sector can be large.
A New Reaction Mass
So we don’t know what dark matter is yet, and we may be dealing with something that can’t be housed in any propulsion system made of normal matter because it fails to interact with it. But let’s not give up on dark matter as reaction mass just yet. Given that it is thought to outweigh visible matter by about six to one, finding a way to exploit it for propulsion would be a major breakthrough, and one we cannot rule out for the far future. If the neutralinos needed for Liu’s starship are available, exploiting their energy may be well beyond our current technology, but until we’ve got a handle on what dark matter actually is and how it interacts, continuing speculation seems advisable.
The paper is Jia Liu, “Dark Matter as a Possible New Energy Source for Future Rocket Technology,” available online.