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.