Shipstar

I. How we built the books

Gregory Benford’s take—

In science fiction, a Big Dumb Object is any immense mysterious object that generates an intense sense of wonder just by being there. “The Diamond as Big as the Ritz” by F. Scott Fitzgerald is a non-SF example. They don’t have to be inert constructs, so perhaps the “dumb” aspect also expresses the sensation of being struck dumb by the scale of them.

Larry said to me at a party, “Big dumb objects are so much easier. Collapsed civilizations are so much easier. Yeah, let’s bring them up to speed.”

So we wrote Bowl of Heaven, deciding that we needed two volumes to do justice to a Big Smart Object. The Bowl has to be controlled, because it’s not neutrally stable. His Ringworld is a Big Dumb Object since it’s passively stable, as we are when we stand still. (Or the ringworld would be except for nudges that can make it fall into the sun. Those are fairly easy to catch in time. Larry put active stabilizers into the second Ringworld novel.)

A Smart Object is 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 virtues of any Big Object, whether dumb or smart, are 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 twelve thousand tons per watt. If we calculate R for a cometary object with optical concentrators, traveling anywhere in the galaxy where a zero magnitude star is visible, the result is one hundred 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! 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’s 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 also 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.

Larry Niven’s take—

“The Enormous Big Thing” was my friend David Gerrold’s description of a plotline that flowered after the publication of Ringworld. Stories like Orbitsville, Ring, Newton’s Wake, John Varley’s Titan trilogy and Rendezvous with Rama depend on the sense of wonder evoked by huge, ambitious endeavors. Ringworld wasn’t the first; there had been stories that built, and destroyed, whole universes. These objects often become icons of larger issues implying unknowable reaches and perspectives. Their governing question is usually, “Who built this thing? And why?” They had fallen out of favor.

I wasn’t the first to notice that a fallen civilization is easier to describe than a working one. Your characters can sort through the artifacts without hindrance until they’ve built a picture of the whole vast structure. Conan the Barbarian, and countless barbarians to follow, found fallen civilizations everywhere. I took this route quite deliberately with Ringworld. I was young and untrained, and I knew it.

A fully working civilization, doomed if they ever lose their grasp on their tools, is quite another thing. I wouldn’t have tried it alone. Jerry Pournelle and I have described working civilizations several times, in Footfall, Lucifer’s Hammer, and The Burning City.

With Greg Benford, I was willing to take a whack at a Dyson-level civilization. Greg shaped the Bowl in its first design. It had a gaudy simplicity that grabbed me from the start. It was easy to work with: essentially a Ringworld with a lid, and a star for a motor. We got Don Davis involved in working some dynamite paintings.

Greg kept seeing implications. The Bowl’s history grew more and more elaborate. Ultimately I knew we’d need at least two volumes to cover everything we’d need to show. That gave us time and room.

II. Fun with high tech

Warning: some plot spoilers lurk here.

Our first book, Bowl of Heaven, set up reader expectations and introduced the Folk who ran the place—or thought so. That let us wrap up storylines in the sequel, Shipstar, in part by undermining the expectations built up in Bowl of Heaven. We chose to write all this in two volumes because it took time to figure out. The longer time also let us process what many readers thought of Bowl of Heaven, its problems and processes.

Much of this comes from the intricacies of how the Bowl came to be built. Plus its origins.

We supposed the founders made its understory frame with something like scrith—a Ringworld term, grayish 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 The Astrophysical Journal and 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 substructures, stick it into bigger masses, use molecular glues. Then stabilize 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 life spans. 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. So 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 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.

III. Bowl design

As the book says, the Bowl star is

K2 STAR. SIMILAR TO EPSILON ERIDANI (K2 V). INTERMEDIATE IN SIZE BETWEEN RED M-TYPE MAIN-SEQUENCE STARS AND YELLOW G-TYPE MAIN-SEQUENCE STARS.

So its light is reddish and a tad less bright than Sol. There is a broad, cylindrical segment of the Bowl at its outer edge, the Great Plain. This is huge, roughly the scale of Ringworld, with centrifugal gravity Earth normal times 0.8, so humans can walk easily there. Beyond that is the bowl curve, a hemisphere that arcs inward toward the Knothole. On the hemisphere, the Wok, the centrifugal gravity varies with latitude, and is not perpendicular to the local ground. To make a level walking surface, the Bowl has to have many platforms that are parallel to the jet axis, so gravity points straight down.

The local apparent centrifugal gravity has two vector components:

A: Centrifugal gravity that is perpendicular to the local level surface on the bowl, vs angle Ψ (in radians). Here Ψ is measured away from the polar bowl axis—that is, the jet axis. The curve peaks at 90 degrees, where the Great Plain has a local g of 1 in this plot. (It’s 0.8 of Earth’s.)

B: Below shows the magnitude of centrifugal gravity that is parallel to the local level surface on the bowl, vs angle Ψ—thus, it’s the felt force pushing away from the pole where the Knothole lies, along the local level.

So the pushing-away force is largest at the mid-latitudes, then falls away because the total force is small at the poles. This component also vanishes on the Great Plain.

The Builders designed it this way so that some of the lands are hard to walk upon in the direction of the Knothole. This discourages others from simply traveling to the Knothole by a slog across the entire Bowl; it takes a lot of work, working against a slanted local “gravity”—especially near the Mirror Zones, which are in the mid-latitudes. Remember also that you must pump fluids around, since local forces drive rivers to flow and either they return through clouds and rain, or you must pump them when the weather doesn’t perform well. There’s a tendency for fluids to wind up in the lower gravity regions, too.

We did other such calculations, and many such didn’t get into the final book. But they lurked in our minds. This may be an example of Ernest Hemingway’s dictum that the more you know about a story’s background, the more you can then leave out, and the detail will still make the story stronger because of the confident way you write it.

This odd centrifugal gravity also presents the Builders with a big stress problem. Holding together this whirling, forward-driving system demands nuclear-force levels of strength.

The atmosphere is quite deep, more than two hundred kilometers. This soaks up solar wind and cosmic rays. Also, the pressure is higher than Earth normal by about 50 percent, 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 thirty feet deep. Everything we’re dumping into our air goes into just thirty feet of water. 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 that distant history that isn’t necessarily what we ended up implying and partially describing:

Long before 65 million years ago, there were dinosaurs who maintained internal temperatures through feathers, in a largely warmer world. But they ventured out with rockets into a solar system chilly and hostile. Still, they needed metals and did not want to destroy their biosphere with the pollutants from smelting, fast energy use, excess carbon dioxide, and the like. So the Bird Folk split into two factions:

• the Gobacks who wanted to return to simple habits compatible with the world they once knew, using only minimal technology, and

• the Forwards, who wanted to re-create around the Minor Star (which became the Bowl’s) a fresh paradise that fulfilled the warm, comfortable paradise the Folk had once known. That could send the Forwards out into the galaxy that beckoned, full of living worlds ripe for the spread of the evolving Folk and all they stood for.

Some of the Forwards were impatient to see what worlds lay millennia away. Many had themselves put in stasis to await a planetary rendezvous. Some faiths arose, hoping to commune somehow with the Godminds whose SETI signals told of great feats of engineering … but these turned out to be funeral pyre signals, of greatness departed long before. By that time, Earth was far behind the Bowl and shrouded in nostalgic legend.

So came the Separation, with the warmth-loving Forwards leaving and the Gobacks remaining on Earth. There they returned to the free life available in the ancestral lands. They kept their numbers low and gradually came to dislike the technologies they had inherited from the Forwards and the earlier civilizations. They reverted to a quiet, calm, agricultural culture. And they prospered, until a bright, flaring tail appeared in their skies.…

After all, by then, the dinosaurs didn’t have a space program.

—April 2013