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.

Local Gravity versus Angle from the Jet Axis

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.

Sideways “Gravity” versus Angle from the Jet Axis

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

BOOKS BY GREGORY BENFORD AND LARRY NIVEN

Bowl of Heaven

TOR BOOKS BY GREGORY BENFORD

Jupiter Project

The Stars in Shroud

Shiva Descending

Artifact

In Alien Flesh

Far Futures

Beyond Human

TOR BOOKS BY LARRY NIVEN

N-Space

Destiny’s Road

Rainbow Mars

Scatterbrain

Ringworld’s Children

The Draco Tavern

Stars and Gods

Playgrounds of the Mind

ABOUT THE AUTHORS

GREGORY BENFORD is a professor of physics at the University of California, Irvine, and lives in Irvine. He is a winner of the United Nations Medal for Literature, and the Nebula Award for his classic novel Timescape. Visit him at www.gregorybenford.com.

LARRY NIVEN is the author of the Ringworld series and many other science fiction masterpieces. His Beowulf’s Children, coauthored with Jerry Pournelle and Steven Barnes, was a New York Times bestseller. He lives in Chatsworth, California. Visit him at www.larryniven.net.

This is a work of fiction. All of the characters, organizations, and events portrayed in this novel are either products of the authors’ imaginations or are used fictitiously.

SHIPSTAR

Copyright © 2014 by Gregory Benford and Larry Niven

All rights reserved.

Cover photograph of Galaxy 4499 by NASA, ESA, A. Aloisi (STSCI/ESA), and The Hubble Heritage (STSCI/AURA)—ESA/Hubble Collaboration

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The Library of Congress Cataloging-in-Publication Data is available upon request.

ISBN 978-0-7653-2870-0 (hardcover)

ISBN 978-1-4299-4968-2 (e-book)

e-ISBN 9781429949682

First Edition: April 2014