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November 25, 2009

Dark Matter Rocket

Here is speculation on top of speculation as we do not yet know what Dark Matter is or even have full agreement that it exists. The dark matter starship assumes that there is dark matter around and that the dark matter can be scooped up for fuel like Bussard Ramjet could scoop of hydrogen. They speculate that one of the leading candidates of what dark matter is (netralinos) can release energy as its own matter and antimatter.

His plan is to drive the rocket using the energy released when dark matter particles annihilate each other. Here's where Liu's idea depends on more speculative physics. No one knows what dark matter is actually made of, though there are numerous theories of the subatomic world that contain potential dark matter candidates. One of the frontrunners posits that dark matter is made of neutralinos, particles which have no electric charge. Neutralinos are curious in that they are their own antiparticles: two neutralinos colliding under the right circumstances will annihilate each other.

If dark matter particles do annihilate in this way, they will convert all their mass into energy. A kilogram of the stuff will give out about 10^17 joules, more than 10 billion times as much energy as a kilogram of dynamite, and plenty to propel the rocket forwards.

Even less certain is the detail of how a dark matter rocket might work. Liu imagines the engine as a "box" with a door that is open in the direction of the rocket's motion (see diagram). As dark matter enters, the door is closed and the box is shrunk to compress the dark matter and boost its annihilation rate. Once the annihilation occurs, another door opens and the products rocket out. The whole cycle is repeated, over and over again.


The New Scientist looks at the Dark Matter rocket and the Blackhole starship. The Blackhole starship has already been reviewed on this site.

An interesting new point was raised about blackhole starships and that there appears to be a blackhole sweetspot for the right sized black hole to make fast spaceship propulsion.

Is the Universe Optimized for Blackhole Travel ?

Crane then wondered what would happen if intelligent civilisations could make black holes. This would mean that life in these universes played a key role in the proliferation of baby universes. Smolin felt the idea was too outlandish and left it out of his book. But Crane has been thinking about it on and off for the last decade.

He believes we are seeing Darwinian selection operating on the largest possible scale: only universes that contain life can make black holes and then go on to give birth to other universes, while the lifeless universes are an evolutionary dead end.

His latest calculations made him realise how uncanny it was that there could be a black hole at just the right size for powering a starship. "Why is there such a sweet spot?" he asks. The only reason for an intelligent civilisation to make a black hole, he sees, is so it can travel the universe.

"If this hypothesis is right," he says, "we live in a universe that is optimised for building starships!"


Dark Matter Rocket
Dark Matter as a Possible New Energy Source for Future Rocket Technology

Current rocket technology can not send the spaceship very far, because the amount of the chemical fuel it can take is limited. We try to use dark matter (DM) as fuel to solve this problem. In this work, we give an example of DM engine using dark matter annihilation products as propulsion. The acceleration is proportional to the velocity, which makes the velocity increase exponentially with time in non-relativistic region. The important points for the acceleration are how dense is the DM density and how large is the saturation region. The parameters of the spaceship may also have great influence on the results. We show that the (sub)halos can accelerate the spaceship to velocity 10^−5c  10^−3c. Moreover, in case there is a central black hole in the halo, like the galactic center, the radius of the dense spike can be large enough to accelerate the spaceship close to the speed of light.


The dark matter spaceship could reach the relativistic speed in about 2 days and the length needed for acceleration is about 10^−4pc.



We have used two assumptions on DM in this work. First, we have assumed static DM for simplicity. But the DM particle may have velocity as large as O(10−3c). Once we know the velocity distribution of DM, it can be solved by programming the direction of the spaceship. Second, we have assumed the DM particle and the annihilation products can not pass through the wall of the engine. For the annihilation products, they may be SM fermions which have electric charges. Thus we can make them go into certain direction by the electromagnetic force. 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.

Sometimes, when looking at the N-body simulation pictures of DM, I think it may describe the future human transportation in some sense. 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. 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.


FURTHER READING
Nextbigfuture highlights from week 39-45 has 13 space related highlights including several advanced propulsion articles.

* Mach Effect Propulsion
* antimatter rockets
* Gravitational field propulsion
* Winterberg's advanced deuterium fusion rocket design
* Vasimr, EMDrive and more
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