This is for the firemen of California and neighboring states who fought the fires of October 2003, with particular thanks to those who saved our house and others in Indian Falls, Chatsworth, Los Angeles County.

The Ringworld is about the same mass as Jupiter. Its shape is that of a ribbon a million miles across and six hundred million miles long, which makes it a bit larger than the Earths orbit, and a few miles thick. It circles a yellow dwarf star. Its spin, at 770 miles/second, is enough to give it about Earths gravity of centrifugal force. Walls along both rims, standing a thousand miles high, are enough to hold an atmosphere for millions of years.

Much else derives from these basic assumptions.

The inner surface is a habitat three million times the area of planet Earth. The topography is literally a work of art, carved in by whoever built the thing, so that from underneath the Ringworld resembles the back of a mask.

An inner ring of shadow squares block the sun, giving periods of night; else it would always be noon. A system of pipes leads from the bottoms of oceans, under the Ringworld floor, up the back of the rim wall and over the edge, to recycle seabottom ooze (or flup) into spill mountains. Huge attitude jets stand atop the rim wall, Bussard ramjets using the solar wind of protons for their fuel, to hold the Ringworld against its inherent instability. There are spaceport ledges outside the walls. Two vast salt oceans serve as preserves for seagoing life, and as something more: maps of several worlds at one-to-one scale. The Ringworld floor is of unnaturally strong material, dubbed scrith, with other unusual properties.

The sun itself is involved in the Ringworlds meteor defense. A superconducting network embedded in the Ringworld floor generates a superthermal laser effect in a solar flare. The drawback: it cant fire through the Ringworld itself. Thus any meteorite that strikes the Ringworld, such as the one that made Fist-of-God Mountain, generally rams upward from underneath.

Some details become clues to the nature of the Builders.

The plethora of harbors and fjords, plus the shallow oceans (most of them), suggest a race that uses only the top of an ocean.

The nastier life forms — mosquitoes, flies, jackals, sharks, vampire bats — dont exist. Hominids have moved into some of those ecological slots. The Engineers werent ecologists, they were gardeners.

The inhabitants are hominids in bewildering variety, some intelligent, some not. They fill ecological niches which on Earth are held by almost any mammal, but particularly the nastier life forms, jackals and wolves and vampire bats… as if mankinds ancestor, Homo habilis, had been protected until they numbered hundreds of billions, then abandoned to mutate endlessly.

You dont know the Ringworld until youve grasped its size.

After the book came out, a friend was going to build a scale model for an upcoming convention. He had a marble, a blue immy, to serve as the Earth, for scale. Turns out hed need a ribbon five feet tall and half a mile long. The hotel wasnt big enough.

One guy who tried to map the Ringworld told me he ran out of computer space very rapidly. He ran into too many powers of ten.

David Gerrold speaks of a class of novel called "the Enormous Big Thing." Today you could fill a fair-sized shelf with them. Arthur C. Clarkes Rendezvous with Rama and Bob Shaws Orbitsville are in that class, and so is my own Rainbow Mars.

But Ringworld came first, published in 1970.

It might have been laughed at. Too big, too improbable. Any normal structural material would be torn apart by its spin. I waited for the reviews in some fear.

James Blish wrote that he thought it would win the Hugo Award, but it shouldnt.

The readers gave it a Hugo Award anyway.

The writers gave it a Nebula.

I didnt have a sequel planned. I was not expecting a flood of redesigns.

During one of my speeches, a man pointed out that the Ring-worlds mathematics are simple: its a suspension bridge with no endpoints.

An academic in England pointed out that the tensile strength of the Ringworld frame must be approximately the force that holds an atomic nucleus together. (Hence, scrith.)

A grade school class in Florida spent a semester on the Ringworld. Their conclusion: the worst problem is that, without tectonic activity, all the topsoil would flow into the oceans in a few thousand years. (Hence, flup and the spillpipes.)

At the 1970 World Science Fiction Convention there were MIT students in the halls chanting, "The Ringworld is unstable! The Ringworld is unstable!" (Did the best that I was able… hence, attitude jets.)

Somebody decided that the shadow squares shed too much twilight. Whats needed is five long shadow squares orbiting retrograde.

Ultimately there was too much opportunity for redesign. I had to write The Ringworld Engineers.

All of these readers had found something worth knowing. The Ringworld is a great, gaudy, intellectual toy, a playground with the gates left wide open.

Some readers just read a book and stop.

Others play with the characters, or the assumptions, or the environment. They make up their own homework. We readers have been doing that for unguessable thousands of years: demanding more data on Atlantis from Plato, inventing Purgatory to put between Hell and Heaven, redesigning Dantes Inferno, writing new Odysseys. An amazing subculture has sprung up around Star Trek.

The Internet opens a whole new metaplayground for such people. A number of Web sites have sprung up (well, at least two) whose topic is Larry Nivens fiction.

In September 1999, tipped off by my lovely agent, Eleanor Wood, I logged onto [email protected]. They were arguing about whether you can clone a protector, and whether Seeker and Teela Brown might have left a child behind. If theyd been right I wouldnt have seen a story, but they were off on the wrong foot, and I could fix it. After a few months of following these discussions, rarely interrupting, I had enough material for Ringworlds Children.

This is a playground for the mind. Its a puzzle too, a maze. Question every turn or youll get lost. When youve finished the book, remember not to lock the gate.