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Building the Bowl of Heaven

Because his new novel Shipstar had just reached the top of my reading stack, and because I had been writing about Shkadov Thrusters last week, I asked Gregory Benford if he could provide a deeper explanation of how these enormous structures might work. Greg had already noted in an email to me that a Shkadov Thruster is inherently unstable, and earlier discussions of the idea on Centauri Dreams had raised doubts about the acceleration possible from such a device. However, I’ve referred to what Benford and Larry Niven have created as a ‘modified’ Shkadov Thruster, and I was anxious to hear their thinking on what might be possible. Greg, an award-winning science fiction author and physicist, here offers his insights into — and reservations about — a propulsion scheme capable of moving stars.

by Gregory Benford


Physicist Leonid Shkadov first described in 1987 a stellar propulsion system made by putting an enormous mirror in a static, fixed position near a star. To stay there it had to balance gravitational attraction towards and light pressure away from the star, exactly—or else it would either fall into or away from the star. Since the radiation pressure of the star would be asymmetrical, i.e. more radiation is being emitted in one direction as compared to another, the excess radiation pressure acts as net thrust, so tiny that the Sun would, after a million years, have speed of 20 m/s, and have moved 0.03 light years—far less than its orbital speed around the galaxy, ~100 km/sec.

Surely we can do better, I thought back in the early 2000s. So I mentioned some ideas to Larry Niven, and eventually we wrote two novels about a different sort of stellar thruster — Bowl of Heaven and Shipstar. Here’s an explanation from the Afterword to Shipstar:

We think of such engines as Smart Objects–statically unstable but dynamically stable, as we are when we walk. We fall forward on one leg, then catch ourselves with the other. That takes a lot of fast signal processing and coordination. (We’re the only large animal without a tail that’s mastered this. Two legs are dangerous without a big brain or a stabilizing tail.) There’ve been several Big Dumb Objects in sf, but as far as I know, no smart ones. Our Big Smart Object is larger than Ringworld and is going somewhere, using an entire star as its engine.

Our Bowl is a shell more than a hundred million miles across, held to a star by gravity and some electrodynamic forces. The star produces a long jet of hot gas, which is magnetically confined so well it spears through a hole at the crown of the cup-shaped shell. This jet propels the entire system forward – literally, a star turned into the engine of a “ship” that is the shell, the Bowl. On the shell’s inner face, a sprawling civilization dwells. The novel’s structure doesn’t resemble Larry’s Ringworld much because the big problem is dealing with the natives.

The virtue of any Big Object, whether Dumb or Smart, is energy and space. The collected solar energy is immense, and the living space lies beyond comprehension except in numerical terms. While we were planning this, my friend Freeman Dyson remarked, “I like to use a figure of demerit for habitats, namely the ratio R of total mass to the supply of available energy. The bigger R is, the poorer the habitat. If we calculate R for the Earth, using total incident sunlight as the available energy, the result is about 12 000 tons per Watt. If we calculate R for a cometary object with optical concentrators, travelling anywhere in the galaxy where a 0 magnitude star is visible, the result is 100 tons per Watt. A cometary object, almost anywhere in the galaxy, is 120 times better than planet Earth as a home for life. The basic problem with planets is that they have too little area and too much mass. Life needs area, not only to collect incident energy but also to dispose of waste heat. In the long run, life will spread to the places where mass can be used most efficiently, far away from planets, to comet clouds or to dust clouds not too far from a friendly star. If the friendly star happens to be our Sun, we have a chance to detect any wandering life-form that may have settled here.”

This insight helped me think through the Bowl, which has an R of about 10-10!


Image: Artwork by Don Davis, as are all the images in this essay.


Shdakov thrusters aren’t stable. They are not statites, Bob Forward’s invention, because they’re not in orbit. Push them, as the actual photon thrust will do, and they’ll fall outward, doomed. So how to build something that harvests a star’s energy to move it and can be stabilized?

I worried this subject, and thought back to the work my brother Jim and I had done on speeding up sails by desorption of a “paint” we could put onto a sail surface, to be blown off by a beam of microwave power striking it. This worked in experiments we did at JPL under a NASA grant, with high efficiency. Basically, throwing mass overboard is better than reflecting sunlight, because photons have very little momentum. The ratio of a photon’s momentum to that of a particle moving at speed V is

(V/c)(2Ep )/EM

where Ep is the photon energy and EM the kinetic energy of the mass M. So if those two energies are the same, the photon has a small fraction of the mass’s momentum, V/c.

So don’t use photons. Use a jet of the mass brought out from the star by forcing it to eject a jet—straight through the center of the Bowl. Jets must be confined by magnetic fields, or else they spray outward like a firehose. Get the magnetic fields from where the reflecting band of mirrors on the Bowl focuses it—on the nearest part of the star. Create a jet from that reflected energy. Make the jet push the star away. Use the jet’s magnetic fields to entwine with fields built into the Bowl itself. Let the jet hug the Bowl toward the star. Only by shaping the magnetic fields of star and jet can we move the Bowl, with constant attention to momentum and stability.

Stable, if you manage it. Who does that? How?


Larry Niven and I started building the Bowl in our minds:

The local centrifugal gravity avoids entirely the piling up of mass to get a grip on objects, and just uses rotary mechanics. So of course, that shifts the engineering problem to the Bowl structural demands.

Big human built objects, whether pyramids, cathedrals, or skyscrapers, can always be criticized as criminal wastes of a civilization’s resources, particularly when they seem tacky or tasteless. But not if they extend living spaces and semi-natural habitat. This idea goes back to Olaf Stapledon’s Star Maker: “Not only was every solar system now surrounded by a gauze of light traps, which focused the escaping solar energy for intelligent use, so that the whole galaxy was dimmed, but many stars that were not suited to be suns were disintegrated, and rifled of their prodigious stores of sub-atomic energy.”

Our smart Bowl craft is also going somewhere, not just sitting around, waiting for visitors like Ringworld–and its tenders live aboard.

We started with the obvious: Where are they going, and why?
Answering that question generated the entire frame of the two novels. That’s the fun of smart objects – they don’t just awe, they intrigue.

My grandfather used to say, as we headed out into the Gulf of Mexico on a shrimping run, A boat is just looking for a place to sink.

So heading out to design a new, shiny Big Smart Object, I said, An artificial world is just looking for a seam to pop.

You’re living just meters away from a high vacuum that’s moving fast, because of the Bowl’s spin (to supply centrifugal gravity). That makes it easy to launch ships, since they have the rotational velocity with respect to the Bowl or Ringworld… but that also means high seam-popping stresses have to be compensated. Living creatures on the sunny side will want to tinker, try new things…

“Y’know Fred, I think I can fix this plumbing problem with just a drill-through right here. Uh—oops!”

The vacuum can suck you right through. Suddenly you’re moving off on a tangent at a thousand kilometers a second—far larger than the 50 km/sec needed to escape the star. This makes exploring passing nearby stars on flyby missions easy.

But that easy exit is a hazard, indeed. To live on a Big Smart Object, you’d better be pretty smart yourself.


Mechanical Engineering

Very smart, it turns out.

As we explained in Shipstar:

We supposed the founders made its understory frame with something like scrith–a Ringworld term, greyish translucent material with strength on the order of the nuclear binding energy, stuff from the same level of physics as held Ringworld from flying apart. This stuff is the only outright physical miracle needed to make Ringworld or the Bowl work mechanically. Rendering Ringworld stable is a simple problem—just counteract small sidewise nudges. Making the Bowl work in dynamic terms is far harder; the big problem is the jet and its magnetic fields. This was Benford’s department, since he published many research papers in Astrophysical Journal in the like on jets from the accretion disks around black holes, some of which are far bigger than galaxies. But who manages the jet? And how, since it’s larger than worlds? This is how you get plot moves from the underlying physics.

One way to think of the strength needed to hold the Bowl together is by envisioning what would hold up a tower a hundred thousand kilometers high on Earth. The tallest building we now have is the 829.8 m (2,722 ft) tall Burj Khalifa in Dubai, United Arab Emirates. So for Ringworld or for the Bowl we’re imagining a scrith-like substance 100,000 times stronger than the best steel and carbon composites can do now. Even under static conditions, though, buildings have a tendency to buckle under varying stresses. Really bad weather can blow over very strong buildings. So this is mega-engineering by master engineers indeed. Neutron stars can cope with such stresses, we know, and smart aliens or even ordinary humans might do well too. So: let engineers at Caltech (where Larry was an undergraduate) or Georgia Tech (where Benford nearly went) or MIT (where Benford did a sabbatical) take a crack at it, then wait a century or two—who knows what they might invent? This is a premise and still better, a promise—the essence of modern science fiction.

Our own inner solar system contains enough usable material for a classic Dyson sphere. The planets and vast cold swarms of ice and rock, like our Kuiper Belt and Oort Clouds—all that, orbiting around another star, can plausibly give enough mass to build the Bowl. For alien minds, this could be a beckoning temptation. Put it together from freely orbiting sub-structures, stuck it into bigger masses, use molecular glues. Then stabilizes such sheet masses into plates that can get nudged inward. This lets the builders lock them together into a shell–for example, from spherical triangles. The work of generations, even for beings with very long lifespans. We humans have done such, as seen in Chartres cathedral, the Great Wall, and much else.



Still: Who did this? Maybe the Bowl was first made for just living beneath constant sunshine. Think of it as an interstellar Florida, warm and mild, with a fantastic night sky. Which keeps moving, over time.

At first the builders may have basked in the glow of their smaller sun, developing and colonizing the Bowl with ambitions to have a huge surface area with room for immense natural expanses. But then the Bowl natives began dreaming of colonizing the galaxy. They hit on the jet idea, and already had the Knothole as an exit for it. Building the mirror zone took a while, but then the jet allowed them to voyage. It didn’t work as well as they thought, and demanded control, which they did by using large magnetic fields.

The system had virtues for space flight, too. Once in space, you’re in free fall; the Bowl mass is fairly large but you exit on the outer hull at high velocity, so the faint attraction of the Bowl is no issue. Anyone can scoot around the solar system, and it’s cleared of all large masses. (The Bowl atmosphere serves to burn any meteorites that punch through the monolayer.)

The key idea is that a big fraction of the Bowl is mirrored, directing reflected sunlight onto a small spot on the star, the foot of the jet line. From this spot the enhanced sunlight excites a standing “flare” that makes a jet. This jet drives the star forward, pulling the Bowl with it through gravitation.

The jet passes through a Knothole at the “bottom” of the Bowl, out into space, as exhaust. Magnetic fields, entrained on the star surface, wrap around the outgoing jet plasma and confine it, so it does not flare out and paint the interior face of the Bowl — where a whole living ecology thrives, immensely larger than Earth’s area. So it’s a huge moving object, the largest we could envision, since we wanted to write a novel about something beyond Niven’s Ringworld.

For plausible stellar parameters, the jet can drive the system roughly a light year in a few centuries. Slow but inexorable, with steering a delicate problem, the Bowl glides through the interstellar reaches. The star acts as a shield, stopping random iceteroids that may lie in the Bowl’s path. There is friction from the interstellar plasma and dust density acting against the huge solar magnetosphere of the star, essentially a sphere 100 Astronomical Units in radius.

So the jet can be managed to adjust acceleration, if needed. If the jet becomes unstable, the most plausible destructive mode is the kink – a snarling knot in the flow that moves outward. This could lash sideways and hammer the zones near the Knothole with virulent plasma, a dense solar wind. The first mode of defense, if the jet seems to be developing a kink, would be to turn the mirrors aside, not illuminating the jet foot. But that might not be enough to prevent a destructive kink. This has happened in the past, we decided, and lives in Bowl legend.

The reflecting zone of mirrors is defined by an inner angle, Θ, and the outer angle, Ω. Reflecting sunlight back onto the star, focused to a point, then generates a jet which blows off. This carries most of what would be the star’s solar wind, trapped in magnetic fields and heading straight along the system axis. The incoming reflected sunlight also heats the star, which struggles to find an equilibrium. The net opening angle, Ω minus Θ, then defines how much the star heats up. We set Ω = 30 degrees, and Θ = 5 degrees, so the mirrors subtend that 25 degree band in the Bowl. The Bowl rim can be 45 degrees, or larger.

The K2 star we gave the Bowl is now running in a warmer regime, heated by the mirrors, thus making its spectrum nearer that of Sol. This explains how the star can have a spectral class somewhat different from that predicted by its mass. It looks oddly colored, more yellow than its mass would indicate.

For that matter, that little sun used to be a little bigger. It’s been blowing off a jet for many millions of years. Still, it should last a long time. The Bowl could circle the galaxy itself several times.

The atmosphere is quite deep, more than 200 km. This soaks up solar wind and cosmic rays and makes the Bowl toasty through greenhouse effect. Also, the pressure is higher than Earth normal by about 50%, depending on location in the Bowl. It is also a reservoir to absorb the occasional big, unintended hit to the ecology. Compress Earth’s entire atmosphere down to the density of water and it would only be 30 feet deep. Everything we’re dumping into our air goes into just 30 feet of compressed nitrogen and oxygen, then. The Bowl has much more, over a hundred yards deep in equivalent water. Too much carbon dioxide? It gets more diluted.

This deeper atmosphere explains why in low-grav areas surprisingly large things can fly–big aliens and even humans. We humans Earthside enjoy a partial pressure of 0.21 bars of oxygen, and we can do quite nicely in a two-bar atmosphere of almost pure oxygen (but be careful about fire). The Bowl has a bit less than we like: 0.18 bar, but the higher pressure compensates. This depresses fire risk, someone figures out later.

Starting out, we wrote a background history of where the Builders came from, which we didn’t insert into the novel. It lays out a version of what made the Builders do all this.


Is this plausible?

Not really. It demands the scrith, for example, which nobody knows how to make.

And the Bowl is a vast accident waiting to happen. You can’t just say Don’t blame me, it’s nonlinear. Somebody has to manage that jet forever. The natives get to take part in slow-motion starflight, but they’re always in danger. Their society must keep this from being obvious, or they’d all go crazy.

Our goal in writing the two novels, and perhaps stories to follow, was to show how strange an alien mindset could be, by giving it a real, physical presence, in the Bowl. Also, we wanted to see what it felt like to think of where humanity itself might go, given time, purpose, and the true essential, imagination.

© 2014 by Gregory Benford


Comments on this entry are closed.

  • Alex Tolley June 30, 2014, 11:03

    Instead of a single, rotating bowl requiring scrith, could the structure be a set of connected rotating objects on a non-rotating frame? These might be cones or O’Neill type cylinders, anchored to a frame. Traversing from one unit to another might be interesting, or possibly no worse that stepping across lanes in a moving walkway (e.g. Asimov’s “Caves of Steel, or Heinlein’s “The Roads Must Roll). The atmospheric containment is different, but the structure isn’t impossible. It doesn’t have the grandeur of a single space, but it would offer lots of room, and it’s cellular nature would offer a lot of redundancy.

    How is steering accomplished? Can the main jet be deflected like gimbaling rocket engines, or does it require the equivalent of vernier thrusters – small focusing mirrors sent out to generate small puffs of gas from the star to push it “sideways”?

  • James D. Stilwell June 30, 2014, 11:08

    Building the Bowl of Heaven
    by Paul Gilster on June 30, 2014
    An amazing post…the Bowl…
    Wondering if this might explain how the galactic civilization in Clarke’s novel “City and the Stars” finally escaped the rampaging Mad Mind…humanity leaving for unknown regions of the universe…The novel is set a billion years in the future…One thing humanity is always stumbling over…unintended consequences…Need to talk to the AI guys how this Mad Mind went awry ripping half the known stars to shreds…

  • RobFlores June 30, 2014, 12:50

    Scrith is not the sand in the gears here.
    A society that has to create generation after generation of engineering crew
    to manage and pilot this project without interruption is a bigger one.
    A few ways this could be done:
    Unethically: Engineering Crew is grown and given artificial patterning.
    and is disposed of (via genetic faults). These crew are isolated from the
    bowl’s inhabitants. The patterning should include a safety feature to
    keep anyone from the escaping the E-crew section from returning. so
    why not use robots: Hmm shades of R98’s

    More Ethical: 100 Engineering crew out of 100,000,000 are awaken
    every million years for 1 year. Some contact allowed w/ rest of crew.

    Bound to fail: The methods we use on Earth to transfer
    knowledge to the next generation. Engineering crew chosen and trained by
    the rest of the inhabitants. There is now way this would work over even
    a few hundred thousand years.

  • Michael June 30, 2014, 13:07

    The structure might not do to well from the constant stream of gas and dust flying about!

    Now if the bowl is pulled in by gravity and pushed away by sunlight what counteracts the outward push of the plasma, more gravity? but then the system is stable and there is no thrust. Or am I missing something.

  • Alex Tolley June 30, 2014, 14:39

    RobFlores – what about making the engineering a religion? The manual becomes part of the holy book, and all the answers lay within? Or just use AI and robots?

  • Rob Henry June 30, 2014, 18:58

    Alex Tolley, that static frame could replace scrith on Ringworld but I can’t see how it could do so here, the forces being oblique rather than normal.

    I am a bit concerned that the greenhouse, light absorption, and temperature rise due to adiabatic depth are being underrated in this very deep atmosphere. Interesting comment here that higher total pressure of 1.5 bars making up for a partial O2 of 0.18 bar. Let us see… Human lung water vapour pressure is always 0.062 bar now 0.18 x (1/(1-.062))/(1.5/(1.5 – .062)) is 0.184 which is still significantly less than 0.21… so no it doesn’t

  • Eniac June 30, 2014, 22:14

    Reflecting sunlight back onto the star, focused to a point…

    I am afraid this is not optically possible. Since object and image distance are necessarily the same here, image size is the same as object size, which means the smallest spot you can concentrate the sunlight onto is the size of the sun. You will need some sort of solar powered laser to do better than that, mirrors just won’t do.

  • Al Jackson June 30, 2014, 22:39

    I have a Kindle ,of, Bowl sitting on my hard drive, but am up to my ears in a JBIS technical paper… so.. someday.

    So you all just did away with all the planets?
    They went into building the ‘artifact’?
    This is a good thing.

    I was always bothered by the Shkadov ‘drive’ became it seemed to take a planetary system with it.
    Say it was the solar system. The problem there is the current Solar System is , well kinda stable, boy! an old question , dating back to Newton but formulated by Lagrange. Progress was made , in out of the box thinking by Poincare ( who discovered Chaos theory, first person to recognize and recoil from it).

    A lot of work has been done on that since Poincare , but really only since the numerical integration work of Wisdom and Laskar in the 1980’s , Laskar still peruses it , with surprising results.

    The important thing here are the mean motion resonances between the planets of the Solar System.
    (For instance Pluto crosses Neptune’s orbit, if it were not for their 3:2 resonance , there would be no argument about Pluto being a planet, it wouldn’t be there!)
    A small ,over all long time, perturbation to the sun’s orbit would like break all those resonances and it’s World’s Collide!

    Don’t need no stinking planets!

    One thing, I suppose you all have it, a star moving in the Galaxy like that would be on a perturbed Galactic orbit , still subject to the Galactic Tide.
    Would want to probably steer clear of Giant Molecular Clouds.

  • norman Wells July 1, 2014, 13:14

    Many of your contributors look to the imagined ‘Far future’ for material to write about more and more far fetched scenarios employing spectacular if improbable technological developments, But there exists a real problem for those who greatly daring are planning manned voyages to Mars and other Solar system destinations, using high risk untried technical solutions ,in the near future .In these circumstances the ability to communicate with planet Earth for help advice and assurance. is of considerable importance but as distances from Earth increase, the difficulty of holding normal phone conversations or digital information exchanges rapidly increases also , due to the increasing time delays resulting from the finite transmission times of e/m propagation[ the light speed limit ] If FTL communications are forever impossible then all the fanciful Centauri Dreams are severely handicapped from becoming reality . I would have thought that FTL solutions would have excited far more interest and speculation . But pretty well all sci-fi stories I have ever read pay little attention to how we talk to one another over the immensity of Space let alone the need for the ability to make material transit times practicable for humans!

  • Mark July 1, 2014, 22:44

    As long as we’re requiring utterly imaginary materials (scrith) for this invention, why not simplify the whole thing?

    Construct a classical medieval catapult of a size arbitrarily much larger than the star. Make it out of a material far far stronger than anything known to modern physics, and of course the material chosen should be utterly heat resistant up to several tens of millions of degrees. Fasten the base of the catapult to the back of a very large turtle (the one that holds up the universe will do nicely), and then simply place the star to be propelled into the sling. Wind up the catapult and then release. The star will be propelled away from the catapult at a high speed, and no further engineering input would be required during the journey.

    Now I know what you’re thinking… what about passengers? Got that covered. Immediately after launching the star, you launch a habitable planet after it, on a slightly different trajectory calculated to place it into a nice stable orbit around the star. Then you just relax and enjoy the ride.

  • Michael July 2, 2014, 10:17

    @Mark July 1, 2014 at 22:44

    Lets try to be sensible Mark!

  • RobFlores July 2, 2014, 11:04

    Just remember Mark, just 200 years ago a television set would have been
    a magicians trick to even a ‘natural philosopher’.

    Alex, the engineering crew problem, is a fine knife edge. You need minds
    who are nimble enough to overcome the unexpected, yet constrained to the
    task at hand (world ship mechanics) and remove all other interests/diversions.

  • Alex Tolley July 2, 2014, 11:59


    smallest spot you can concentrate the sunlight onto is the size of the sun

    However, you could, with wasted energy, reflect more energy onto an area smaller than the sun. Consider that at the edges of the bowl, the mirrors could focus the light to a place between the sun and the mirror, allowing grazing of the star. The energy absorbed by the star could therefore be concentrated at a relatively small point of the star, albeit with the consequence of a loss of much of the focused energy. If that was enough however, you could get the local mass escape that is needed.

    My question is whether the sun’s emission spectrum is ideal for this job. Would a different spectrum be better, and if so, could the “mirrors” change the incoming light to this new spectrum?

  • Gregory Benford July 2, 2014, 13:00

    Thanks for comments!
    Norman: try STARSHIP CENTURY for close study of our near term starship-directed problems.
    Al: Yep, used the planets in the Bowl. Thrifty! (Going to Loncon I hope?)
    Eniac : Yes, the mirrors are smart–phased arrays, not just reflectors, as you say. We don’t have such but it’s within the possible. Not lasers, but coherent reflection from smart surfaces. There are more complex technologies, as you’ll see in SHIPSTAR.
    Rob: A lower O2 isn’t much problem. I’m sitting right now at 32% less O at 8000 feet in the high Sierras, and millions live in such high spots. In fact it’s good stress for the body, improves fitness on Earth.
    Alex: Yes, wish we could avoid the scrith problem with some ingenious geometry. Keep thinking on it!
    All on engineering: as SHIPSTAR makes clear, strong social control is essential.
    Pretty tough, too.

  • Wojciech J July 2, 2014, 15:31

    I have my objections as to the need of a civilization capable of creating such an object to have a living space. With such technologies it seems likely that they either could easily become post-biological or capable of creating smaller, easier to manage habitats, that would nevertheless provide more than enough place to exist and grow.
    I am curious as to the mention that authors did sketch the history of the Builders and reasons for them constructing his object? Is it explained in the second novel?

  • Wojciech J July 2, 2014, 15:50

    norman Wells-lack of FTL doesn’t forbid interstellar travel. And personally I believe it makes it more interesting, imagine all the stars with all the independent cultures evolving for thousands of years separately, but still in knowledge of each other.

  • Gregory Benford July 2, 2014, 16:09

    Wojciech J: Yep, explained in SHIPSTAR & its Afterword, too.
    All: On scrith: maybe there’s a stable way to make material with the shear strength of neutron stars, without huge gravity. No fundamental law precludes it. So scrith may be possible.
    Mostly Larry & I wanted to look at really long term/big scale thinking, ion a novelistic way–one novel in two volumes.

  • Rob Henry July 3, 2014, 0:51

    If fires are hard to get going there, I suspect that the low gravity will prove more important than the lower O2 or than the heat carried away by diluting gas. On Earth, fires only tend to become raging infernos after hot air is rising sufficiently fast to drag in an endless supply of fresh oxidant. Note that a cubic metre of anthracite coal must drag down a minimum of 20,000 cubic metres of bowl air before it can burn to completion. A very tall order, and surely only possible within a sensible time frame with high gravity!

  • Michael July 3, 2014, 9:44

    There is bound to be a lot of clouds about as the hydrogen from the star will interact with the oxygen in the bowl to create a lot of water.

  • Alex Tolley July 3, 2014, 9:53

    An Alternative Structure – The Solar Pulse Jet

    Here is my thought on avoiding the need for scrith.
    The bowl structure becomes a Dyson swarm, each orbiting the star. Some of these objects are habitats, most are solar energy collectors and a few are particle beam generators. The job of the collectors is to accumulate the solar energy needed for the beam pulse. One can think of this as being like the LHC or the NIF laser. When the particle accelerators are in the correct position, the collectors send their energy to them to fire a pulse at the star, creating the mass ejection for thrust. Perhaps those particles could be muons.
    Each pulse only happens when the beamers are aligned so that the solar jet is generated when the direction of thrust is correct. Because of the time delay, there should be a gap in the swarm behind the accelerators. All the objects in the swarm must track their distance from the star and use some energy to redirect and accelerate the solar wind to adjust their orbits to track the movement of the star.

    Another advantage of the swarm is that the timing of the firing can be adjusted, allowing steering in the orbital plane. For a Dyson swarm, with objects orbiting in different planes, the steering can be in 3 dimensions.

    Detecting such objects should be relatively easy. Their spectral signatures will be pushed into the IR, and if their direction of travel is away from us, there should be regular transit intensity peaks (swarm gaps) and even stronger peaks as the star flares. The pattern should be regular over a number of years. and would be expected be more frequent than a nova to maximize thrust.

    Such an approach might be less efficient than the bowl, but I think it offers a plausible way to move a sun without invoking massive structures. It will require a technical civilization to allow travel between the habs and to ensure that the many objects in the swarm are kept running correctly.

  • Gregory Benford July 3, 2014, 15:13

    Yes, the F0lk of the Bowl suppress fire whenever possible, since recycling CO2 is a chore without plate techtonics. Of course, there’s no coal to burn; just wood.

  • Eniac July 3, 2014, 15:43

    Alex, I also like the idea of having a swarm instead of a bowl, and particle beams for creating the fusion spot on the sun’s surface. I have two concerns, though: 1) I am not very confident that shooting beams of any kind into the sun would have the desired effect, and 2) It seems likely that more efficient and compact ways of producing fusion energy would exist and that dragging the sun
    along might not be the best or fastest way to go travel, if you are living in artificial habitats, anyway.

  • coolstar July 3, 2014, 18:37

    I’m afraid this idea always left me cold (no pun intended). I appreciate the explanations by Dr. Benford but still can’t see a way around the fact that
    a) you can’t steer, at all as far as I can see and
    b) you can’t really, at least for “long” explore any star systems you pass by bc it won’t be “long” until you’re going to fast to get back.
    I leave aside the fact that “Bowl of Heaven” was the worst edited sf novel I can remember (and I’m an astrophysicist like Benford but a bit younger).

  • Rob Henry July 3, 2014, 20:48

    Alex Tolley, if we are going down that route (Induced coronal mass ejections), why even bother to provide the power for it. Since these happen naturally, you would be looking at trying to employ the butterfly effect to induce them and put them in the place you want. Though that looks like it has no minimum energy requirement, chaos theory and the quantum nature of our universe would give it one. The trouble is I have no idea how to give value that a ballpark figure – does anyone here have any ideas of how?

    Also, I have never understood why this activity reduces with age. Are they in part driven by a primordial store of energy that reduces with time? Do CMEs act as if they are a finite resource?

  • Alex Tolley July 4, 2014, 10:45

    @Eniac, I would go even farther and ask why we want to expend such massive energies even propelling a large habitat (worldship) around.

    But primarily, I was just interested in thinking about how to make the “big, smart object”, that Benford and Niven wanted to explore for their fiction, possibly more plausible. The swarm and pulsing model would make a great intro scene for an SF movie. Starting from a view of the star field, a tiny dot is moving and pulsing. As we zoom in, we see these titanic objects and forces at play, with the star erupting. Then zoom in further to a huge habitat, noting its scale and entering it…

    But I would be concerned at the thinking of the builders. A star moving at 1000 km/s is 1/300 c. It would cross the galaxy in a mere 30 million years, half the time that primates evolved the diverse hominids. While I would not expect it to collide with another star or its planetary system, I would worry about its gravitational perturbation of the Oort clouds, possibly causing unforeseen effects (good or bad) millions of years later on planetary systems. And what of Oort objects plowing into the swarm – how damaging might that be, with what consequences?

    When the time comes, I hope we use small scale technologies, with low energy requirements to travel between the stars and galaxies.

  • Gregory Benford July 4, 2014, 12:25

    The Bowl can be steered. See SHIPSTAR to see how–basically, shaping the jet to deflect it in small angles, yet keep it within the Knothole.
    The Bowl velocity is controlled to that needed. The huge magnetosphere of the star has a fricitional drag from interstellar plasma, so the system doesn’t just keep accelerating. To explore a nearby star, drop off exploring parties, who can later catch up.
    I gather Coronal Mass Ejections may be larger when stars are young, mostly because the stellar rotation slows from angular momentum loss through CMEs over billions of years.
    I like Alex’s swarm idea, and Eniac’s right that there may be better ways of inducing fusion on the star surface. I’ve worked on relativistic beam dynamics and fusion effects, theory & experiment, and there’s continuing research into how to do that. Not easy, no. Without giving away a plot point, the jet is very intricately run, but not by the Folk of the Bowl. Read SHIPSTAR to see how.
    Must admit I envisioned the Bowl and chose it over the swarm idea, early in the 2000s, because the Bowl is a striking image of vast potential. A swarm is far more likely, yes. That’s on the of tradeoffs hard sf makes! Fiction is not a grant proposal. (Though both are forms of fantastic thinking.)

  • Michael July 4, 2014, 16:01

    @Gregory Benford July 4, 2014 at 12:25

    ‘The huge magnetosphere of the star has a fricitional drag from interstellar plasma, so the system doesn’t just keep accelerating…’

    There should be little or no drag as the solar wind/light, which moves with the star, clears a path, something like this,


    ‘I gather Coronal Mass Ejections may be larger when stars are young, mostly because the stellar rotation slows from angular momentum loss through CMEs over billions of years.’

    Stars rotation slowing is mainly due to the interaction between the magnetic field of the star and the stars solar wind as a whole not just CME’s (via torque). Causing fusion on a magnetically active body may be better achieved at the poles where the magnetic field is concentrated most, it will tend to funnel it to the surface.

    Alex Tolley July 4, 2014 at 10:45

    ‘While I would not expect it to collide with another star or its planetary system, I would worry about its gravitational perturbation of the Oort clouds, possibly causing unforeseen effects (good or bad) millions of years later on planetary systems…’

    At a thousand km/s very messy indeed!

    ‘When the time comes, I hope we use small scale technologies, with low energy requirements to travel between the stars and galaxies.’

    More likely

  • ljk July 7, 2014, 10:00

    My question is, where would one be moving an entire solar system TO and why?

  • Gregory Benford July 7, 2014, 13:03

    Michael: These bow shocks are the CAUSE of braking a star’s movement–see http://arxiv.org/abs/astro-ph/0503031
    The rotating star delivers torque to the surrounding interstellar medium, through solar wind outflow, slowing it over time.

    ljk: They’re touring the whole galaxy, for many millions of years. Building it isdescribed in BOWL, motivations in SHIPSTAR.

  • Michael July 7, 2014, 13:17

    @ljk July 7, 2014 at 10:00

    ‘My question is, where would one be moving an entire solar system TO and why?’

    They could see their Sun as their God. Seems a complete waste of good energy resources to do such an enterprise, to me anyway.

  • Rob Henry July 8, 2014, 18:41

    The penny has just dropped for me. Depending on what these smart array mirrors can do, this might have massive implication for the future of our Sol system. Does anyone know where I can find out about the limit of their potential.

    Say, for example, mirrors can reflect light from 0.1 AU to Europa with only a 99% loss, then just 8 million square kilometres of mirror would make it as bright as Earth, and if blue light is focused better, it would be more biologically productive.

  • Ron S July 8, 2014, 19:33

    I think it would be simpler either to move Europa closer to the Sun or to orbit an artificial sun around Europa.

  • Alex Tolley July 9, 2014, 10:47

    @Rob Henry – It may be easier to have more mirror in cis-Jupiter space focusing the weaker sunlight onto Europa. Either way, the next question is – how long might Earth intensity sunlight need to melt the surface ice of Europa? Without an enclosing membrane, would the “steam” from the exposed water create enough surface pressure for humans to operate without pressure suits?

    It might make sense to use local mirrors or fresnel lenses to focus the beam onto a small spot, melting the ice and reducing the albedo, much like the sink holes that are now appearing in the Greenland ice sheet. This could presage full hemispheric insolation or be useful as a pilot experiment with smaller mirrors.

    No need to stop at Europa, as Ganymede and Callisto might make very suitable abodes.

    And let;s not forget that those beams could also provide power for human settlements and spacecraft.

  • Gregory Benford July 9, 2014, 12:26

    Rob Henry, Ron S: Interesting idea! Phased array at visible wavelengths is a truly advanced tech, of which I know little. Easier surely than moving a moon, tho, out of a deep grav well.

  • Michael July 9, 2014, 13:05

    Why not cover the moons in a transparent aerogel insulation that lets the sunlight in (but not the heat out) together with Jupiter’s tidal heating they will warm it up. But even then as mentioned earlier it would take hundreds of thousands or millions of years to melt the ice and then there is the problem of low gravity. Building large world ships is the best use of resources in my opinion.

  • Rob Henry July 9, 2014, 18:03

    Ron S, I can see how it would be easy to move Pluto by celestial ping pong (and with no minimum energy requirement by using a cascade effect starting on much smaller bodies and using heaps of computing power), but Europa has far more angular momentum, is deep in Jupiter’s well and must shuffle past three other giant moons. As for the second idea – how big is Europa’s Hill Sphere?

  • Ron S July 10, 2014, 11:31

    Rob, either way we need “magical” but physically-tractable technology. On such questions I think it is unproductive to be overly judgmental. That is, looking far into the future it can be foolhardy to be rigid about probabilities of which technologies become doable. Then there’s Clarke’s solution, which is to convert Jupiter into a star.

  • Michael July 10, 2014, 14:21

    @Ron S July 10, 2014 at 11:31

    ‘Then there’s Clarke’s solution, which is to convert Jupiter into a star.’

    This requires a huge increase in mass to bring about, I think the min is 13 Jup mass for deuterium. However if you had a large supply of fusion fuel you could heat the clouds of Jupiter to incandescence but it would be such a waste.

  • Rob Henry July 10, 2014, 20:27

    No Michael that was not his solution. In his story alien (deuterion-deuteron?) fusion (catalyzing?) machines surfaced and activated. Now let’s see…

    Jupiter has a hundredth the surface area of the Sun and 220 times closer than the Earth is to the sun, so we will need one five hundredth the luminosity per surface area from it. Heat radiated is the fourth power of temperature, so we can reduce the temperature 4.7 fold 5778/4.7 = 1230K and there are heaps of materials that could withstand that temperature is it is possible.

  • ljk August 4, 2014, 14:00

    I chose to put these articles here because they both involve interstellar propulsion methods that seem to be defying the laws of physics, or so we are being led to believe….




  • Gregg Eshelman September 5, 2014, 4:40

    Ringworlds (Engineers was the first Niven book I read, with the Whelan cover where the floating city can be used as a map to follow the action) spheres, discs and bowls are all well and good, but what I’d really like to read is a good dystopic story set on a planet with a high enough population to justify covering it with a city.

    Problem is most SF authors have completely undershot the mark, too few inhabitants, too much building. Right now the entire living human population could fit inside the borders of Texas with around 1,500 square feet per person. Round that up to 2,000 and evenly parceling out Earth’s land nets around 81 billion lots.

    If you want moderately crowded, 200 square feet per person, the world wrapping city only needs four levels. Bottom level is where all the major mechanics go. Power plants, waste treatment, mining, all the dirty nasty bits of supporting life. Top level and the roof are where the food gets grown and raised.

    Give over 50% of both the two middle levels to hallways, walls, HVAC, electric, plumbing, light shafts, public spaces etc and with 200 square feet of living space per human that’s a capacity of roughly 270 TRILLION – and that’s just covering the land, not the oceans, and there are parts of some cities right now with a higher density than that.

    The most recent book in Jasper T. Scott’s “Dark Space” series gets into this level of population on one planet. But annoyingly it only mentions a population of “trillions” and the city depth is far far more than 4 levels.

    Other worlds like Trantor, Coruscant, Helior, and many others simply have far too much building for the population and density depicted. Randolph Lalonde started off his “Spinward Fringe” series on a giant space station with a walkable surface area half that of old Earth, yet his protagonist lives in a literally closet sized room. Do the executives live in billion square foot apartments? 90% of the station volume is spacedocks? Population of 100 trillion and life support takes up the rest of the room? No explanation as to why.

    If Coruscant (capital planet in the Star Wars universe) is Earth sized and all covered in such buildings as seen in the movies… either every inhabitant has an entire floor all their own or there’s 1,000 trillion people and a massive fleet of ships hauling supplies in and crap out.

    There was a book some years ago that involved several buildings, each a mile square at the base and somewhere around 30 stories – and way too low of a population. That’s all I remember about it, didn’t read because of the massive math fail. ISTR that the number of people could all have fit in just one of the buildings and still rattled around like two dry peas in a large coffee can.

    If you are going to write about a civilization encysted planet, do the math or have a darn good explanation for why you don’t have enough people or why so few (even if it is a couple of trillion) are all packed into a small area the size of Texas.

  • Ken September 16, 2014, 18:52

    I’m no rocket scientist, and this may have been answered already, but it seems to me that if the star is propelling itself through space and the bowl is being pulled along ( essentially falling towards the star, but the star keeps accelerating away ) then the whole system has to be under constant acceleration. If it’s been accelerating for millions of years, shouldn’t it be going at an enormous speed by now? Perhaps a good portion of the speed of light?