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Powering Up the Dark Matter Starship

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.


Comments on this entry are closed.

  • T_U_T November 30, 2009, 13:39

    What about dark energy ? Dark energy can be used to construct various FLT drives like the alcubierre drive. What about a dark energy ramjet, that starts slow ( only a few m/s ), but as it encounters more dark energy, the stuff becomes trapped in the warp bubble and increases the space-time curvature making the whole contraption faster till it moves at speeds exceeding the speed of light by orders of magnitude ?

  • george scaglione November 30, 2009, 17:27

    T U T,just stopped by and read your comment above so far you where the only one though.made me want to reply in so far as what you say visavie the use of dark matter for space craft propulsion! your comments where a breath of fresh air as opposed to the ones i have just commented on again as regards the use of black holes as space craft propulsion.the one idea did not seem clear at first but when i saw your concepts i saw a arena that i had been reading about and discussing for quite a while now.in short,yes black holes or dark energy both seem viable candidates for some really ADVANCED propulsion. thank you very much your friend george

  • Lucy November 30, 2009, 20:56

    Just to clarify: dark matter, dark energy, negative matter, and negative energy are not the same things. Dark matter is the unknown matter that might explain anamolous galactic movements; dark energy is the “vacuum energy,” or cosmological constant believed to be driving the expansion of the universe. Negative matter, or better yet, negative mass-energy is hypothetical exotic stuff that would interact with gravity in a way opposite to regular matter. It’s this stuff that’s used in wormholes and warp drives.

  • Tobias Holbrook December 1, 2009, 5:36

    Actually, Dark matter may make starship easier, if it’s composed of MACHOs like brown darfs, white dwarfs, and black holes.

  • Duncan Ivry December 1, 2009, 12:25

    It is said above “speculative concepts, even when they’re so far out on the edge that they’ve attained a kind of intellectual redshift”. Yes, propulsion using dark matter does not have much to do with science, it is funny speculation, far away from any science.

    The author of the paper: Jia Liu. He is at the Center for Cosmology and Particle Physics, Department of Physics, New York University, and at the Institute of Theoretical Physics, School of Physics, Peking University, Beijing. Generally these should be estimable institutions. Important: I do *not* want to pick especially on Jia Liu; there are a lot of authors of speculative papers; I take him just as an example.

    The paper “Dark Matter as a Possible New Energy Source for Future Rocket Technology”. Jia Liu assumes a box (the spaceship) moving and gathering not moving dark matter (DM) particles. It is assumed, that these can be kept (somehow) by the walls of the box. After having compressed the box in order to have more DM particles per unit, it is assumed, that the DM particles annihilate each other, and that the annihilation products are Standard Model particles, i.e. “normal” matter. It is further assumed, that these normal particles move into the direction opposite to the one the box moves (no problem for normal matter electrically charged). The result is a propulsion of the box.

    After having described this DM propulsion engine, Jia Liu proceeds with a discussion and calculation of how such an engine accelerates depending on certain — again — assumptions about DM distribution and density. The mathematics are not difficult to understand for a professional mathematician or physicist.

    There are serious and questionable assumptions. Jia Liu discusses these assumptions — a little bit. A statement like this is typical here: “It may be difficult using matter to build the containers for the DM … the dark sector may be as complex as our baryon world …”. There is a lot of “may be” in this paper. The calculations and diagrams are built on 99 percent speculation and one percent fact.

    My summary regarding science: The paper “Dark Matter as a Possible New Energy Source for Future Rocket Technology” should not be put aside lightly, it should be thrown away with great force. Regarding science fiction, fun, and nonsense: A hesitant okay.

    The science of physics needs speculation (and other sciences need it too). Several times in the past I have been the one fighting against members of the ancien regime who don’t want anything new.

    But …

    I am more and more disconcerted (a) by so many people, working at academic institutions, publishing papers far away from real live, from real physics, from anything possible in the near and not so near furture, based on unfounded assumptions, (b) by peers reviewing these papers as if they contained valuable science, and (c) by academic institutions providing an estimable framework for all this.

  • T_U_T December 1, 2009, 13:26

    dark energy is the “vacuum energy,” or cosmological constant believed to be driving the expansion of the universe.

    Anyway, it violates the energy conditions that normally prohibit things like wormholes or FTL. That is enough.

  • george scaglione December 1, 2009, 13:52

    lucy thank you for your comments on these subjects they ring true to me. and tobias dark matter,might, be able to do as you suggest (if ! ) as always it can be harnessed! thank you both for your interesting comments. respectfully your friend george

  • Cooper December 1, 2009, 15:04

    Hi Duncan,
    I think you’re right that these papers shouldn’t be made to seem like they’re actual science, or that they’re possible even when enormous technological and scientific problems stand between us and such a conclusion, but I don’t agree that these papers should be so strongly rejected as you are arguing. The whole point of this blog is to speculate on how we’re going to be able to achieve interstellar flight, and while it’s all “ad astra incrementis,” there must be a goal in mind for us to get anywhere. This paper should be described as being highly speculative, but that doesn’t mean it isn’t important. If we were all so pragmatic about things, we wouldn’t have gotten to where we are now.

  • Adam December 1, 2009, 15:15

    If Dark Matter is actually ‘Shadow matter’ then we may find it rather reluctant to annihilate, even if it’s potentially easier to handle.

  • James M. Essig December 1, 2009, 16:58

    Hi Duncan Ivry;

    Part of what it means to do science is to push the limits of current paradymns and to introduce new ones. Just as prophets are said to be generally unaccepted in their own times, developers of revolutionary concepts are often viewed as flakes by their contemporary narrow minded colleagues. Einstein is a great case in point.

    I suppose if the Higgs Boson(s) is(are) discovered at the LHC, Peter Higgs will be famous for not caving into the narrow minded notion that F= ma can be explained as a product of inertia in terms of its ultimate genesis. No doubt, just as their are folks who deny the existence of the well established quarks, there will be people who deny the existence of the Higgs Boson. Personally, I will side with the visionaries who concieved of the LHC and actually sucessfully persued the cause of its approximately 10 billion Euro design and construction.

    We must not stifle scientific speculations and dreams because they are alledged fantasy, or for other lame exuses.

    I say again to all of those folks who support the ideals of Tau Zero and Centauri Dreams, Ad Astra Incrementis!

  • Duncan Ivry December 1, 2009, 17:44

    Cooper: ” This paper should be described as being highly speculative, but that doesn’t mean it isn’t important.”

    Trying to translate this for me (correct me if I’m wrong), you say that being highly speculative does not imply being unimportant. That’s my opinion too.

    I gave reasons for my rejection of the paper. It’s one of the patterns of unfounded speculation:
    (1) Start from something which is not known (okay, here 99 percent unkown).
    (2) Perform conclusions in a logically correct way; try to impress the readers, especially lay-people, by applying special feats, e.g. vocabulary, mathematics, diagrams.
    (3) Lead to something which is, consequently, not known too.
    (More eval would be starting from something false; we have those cases in Centauri Dream comments very well).

    So tell me, what should we do with Jia Liu’s paper, which is, I think, constructed like this? If your opinion is, that it is important, please, tell me with regard to what aspects and why (preferably scientific aspects but others as well if you like). To the best of my knowledge, I have no reasons favoring the paper’s importance.

  • Duncan Ivry December 1, 2009, 18:02

    James M. Essig

    Famous people, okay. Let me quote Richard Feynman, physicist, Nobel Laureate, author of the legendary Feynman Lectures on Physics (which I used at university), that scientists have to “Keep an open mind, but not so open that your brain falls out.”

  • Ron S December 1, 2009, 20:05

    “…are often viewed as flakes by their contemporary narrow minded colleagues. Einstein is a great case in point. …if the Higgs Boson(s) is(are) discovered at the LHC, Peter Higgs will be famous for not caving into the narrow minded notion that…”

    James, these are gross distortions of the historical record.

    Regarding Duncan’s position on speculation, for the most part I am of the same mind. However there is room for speculation if the core premise is not outrageous. I am reminded of Kip Thorne’s speculations into wormholes and time machines, where he feared damaging his reputation by publishing. When he did so he chose some terribly obscure technical terms rather than the common sci-fi-ish ones to avoid immediate censure. As he further related, he was surprised by how well his work got accepted by his colleagues in the physics community.

    Perhaps he got the balance right between good science and speculative concepts that are allowed by the mathematics, although not (yet?) supported by the data.

    Did Liu get the balance right? I don’t know. Instead it was that paper on black hole propulsion that irritated me with its overt crossing the line into multiple areas of speculation and outright nonsense. But I steered clear of it since I only saw the posting after 40+ comments were already made.

    What generally worries me about excessive speculation by supposed scientists is that less-discriminating enthusiasts among the public treat is as akin to fact, while those with an anti-scientific axe to grind exploit it to lobby against the entire scientific enterprise.

    I am ok with Paul bringing attention to each of these topics since it helps all of us to learn how to better discriminate between sense and nonsense.

  • James M. Essig December 2, 2009, 12:11

    Hi Duncan Ivry and Ron S;

    For the record, I see the concept of black hole based propulsion as rationally informed speculation on the provisional basis that black holes of suitable mass can be fabricated, and held in a spaced relationship either gravatically or electrodynamically to a gamma ray reflector composed of suitable materials.

    Sorry, I am not going to change my mind regarding the possibility of at some point developing such a craft. I am somewhat hopefull that their are more ways to skin the proverbial cat in regards to the black hole propulsion concept in the event that the specific kinematical arrangements discussed in these two related threads on the subject do not pan out. Personally, I think the concepts discussed by Paul in the two associated articles are worthy of further consideration and cannot be dismissed as nonsense.

    I will admit that our ability to produce black holes needs to be developed and radiation resistant gamma ray reflective materials need to be developed before hand and I hold out hope that such can occur one day even if the black hole powered craft idea is never inplemented in lieu of better more capable space transport systems.

    I have thus stated my opinions regarding the black hole drive concepts and will now move on to other newer threads.

  • ljk December 10, 2009, 2:00


    Wednesday, December 09, 2009

    A Blueprint for a Quantum Propulsion Machine

    Push on the electromagnetic fields in the quantum vacuum and you should get an equal and opposite force.

    The quantum vacuum has fascinated physicists ever since Hendrik Casimir and Dirk Polder suggested in 1948 that it would exert a force on a pair of narrowly separated conducting plates. Their idea was eventually confirmed when the force was measured in 1997. Just how to exploit this force is still not clear, however.

    In recent years, a new way of thinking about the quantum vacuum has emerged which has vastly more potential. And today, one physicist describes how it could be used to create propulsion.

    Before we discuss that, let’s track back a little. According to quantum mechanics, any vacuum will be filled with electromagnetic waves leaping in and out of existence. It turns out that these waves can have various measurable effects, such as the Casimir-Polder force.

    The new approach focuses on the momentum associated with these electromagnetic fields rather than the force they exert. The question is whether it is possible to modify this momentum because, if you can, you should receive an equal and opposite kick. That’s what rocket scientists call propulsion.

    Today, Alex Feigel at the Soreq Nuclear Research Center, a government lab in Yavne Israel, suggests an entirely new way to modify the momentum of the quantum vacuum and how this can be exploited to generate propulsion.

    Feigel’s approach combines two well-established ideas. The first is the Lorentz force experienced by a charged particle in electric and magnetic fields that are crossed. The second is the magnetoelectric effect–the phenomenon in which an external magnetic field induces a polarised internal electric field in certain materials and vice versa.

    The question that Feigel asks is in what circumstances the electromagnetic fields in a quantum vacuum can exert a Lorentz force. The answer is that the quantum vacuum constantly interacts with magnetoelectric materials generating Lorentz forces. Most of the time, however, these forces sum to zero.

    However, Feigel says there are four cases in which the forces do not sum to zero. Two of these are already known, for example confining the quantum field between two plates, which excludes longer wavelength waves.

    But Feigel says the two others offer entirely new ways to exploit the quantum vacuum using magnetoelectric nanoparticles to interact with the electromagnetic fields it contains.

    The first method is to rapidly aggregate a number of magnetoelectric nanoparticles, a process which influences the boundary conditions for higher frequency electromagnetic waves, generating a force.

    The second is simply to rotate a group of magnetoelectric nanoparticles, which also generates a Lorentz force.

    Either way, the result is a change in velocity. As Feigel puts it: “mechanical action of quantum vacuum on magneto-electric objects may be observable and have a significant value.”

    The beauty of Feigel’s idea is that it can be easily tested. He suggests building an addressable array of magnetoelectric nanoparticles, perhaps made of a material such as FeGaO3 which has a magnetoelectric constant of 10^-4 in a weak magnetic field.

    These nanoparticles simply have to be rotated in the required way to generate a force. Feigel calls it a magnetoelectric quantum wheel.

    Of course, nobody is getting a free lunch here. “Although the proposed engine will consume energy for manipulation of the particles, the propulsion will occur without any loss of mass,” says Feigel. He even suggests, with masterful understatement, that this might have practical implications.

    So here is a high-risk idea with a huge potential payoff. The question is: who has the balls to try it?

    Ref: http://arxiv.org/abs/0912.1031: A Magneto-Electric Quantum Wheel