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
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.
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, ?, and the outer angle, ?. 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, ? minus ?, then defines how much the star heats up. We set ? = 30 degrees, and ? = 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