closer.”
“Let us first attempt to contact them,” said the puppeteer. “Are we at rest?”
Speaker consulted the ship’s brain. “We are approaching the primary at perhaps thirty miles per second. Is that slow enough?”
“Yes. Begin transmissions.”
No laser light was falling on the Liar.
Testing for electromagnetic radiation was more difficult. Radio, infrared, ultraviolet, X-rays—the whole spectrum had to be investigated, from the room-temperature heat given off by the dark side of the Ringworld, up to light quanta energetic enough to split into matter-antimatter pairs. The twenty-one centimeter band was empty; and so were its easy multiples and divisors, which might have been used merely because the hydrogen absorption band was so obvious. Beyond that point Speaker-To-Animals was playing blind man’s bluff with his receivers.
The great pods of communications equipment on the Liar’s wing had opened. The Liar was sending radio messages on the hydrogen absorption frequency and others, bathing successive portions of the ring’s inner surface with laser light of ten different frequencies, and sending Interworld-Morse in alternate blasts of the fusion motors.
“Our autopilot would eventually translate any possible message,” said Nessus. “We must assume that their ground-based computers are at least as capable.”
Speaker’s reply was venomous. “Can your leucotomized computers translate total silence?”
“Concentrate your sendings at the rim. If they have spaceports, the spaceports must be at the rim. To land a spacecraft anywhere else would be horribly dangerous.”
In the Hero’s Tongue Speaker-To-Animals snarled something horribly insulting. Effectively it ended the conversation; but Nessus stayed where he had been for hours now, with his heads poised alertly above the kzin’s shoulders.
The Ringworld waited beyond the hull, a checkered blue ribbon trailing across the sky.
“You tried to tell me about Dyson spheres,” said Teela.
“And you told me to go pick lice out of my hair.” Louis had found a description of Dyson spheres in the ship’s library. Excited by the idea, he had made the mistake of interrupting Teela’s game of solitaire to tell her about it.
“Tell me now,” she coaxed.
“Go pick lice out of your hair.”
She waited.
“You win,” said Louis. For the past hour he had been staring broodingly out at the ring. He was as bored as she was.
“I tried to tell you that the Ringworld is a compromise, an engineering compromise between a Dyson sphere and a normal planet.
“Dyson was one of the ancient natural philosophers, pre-Belt, almost pre-atomic. He pointed out that a civilization is limited by the energy available to it. The way for the human race to use all the energy within its reach, he said, is to build a spherical shell around the sun and trap every ray of sunlight.
“Now if you’ll quit giggling for just a minute, you’ll see the idea. The Earth traps only about half a billionth of the sun’s output. If we could use all that energy…
“Well, it wasn’t crazy then. There wasn’t even a theoretical basis for faster-than-light travel. We never did invent hyperdrive, if you’ll recall. We’d never have discovered it by accident, either, because we’d never have thought to do our experiments out beyond the singularity.
“Suppose an Outsider ship hadn’t stumbled across a United Nations ramrobot? Suppose the Fertility Laws hadn’t worked out? With a trillion human beings standing on each other’s shoulders, and the ramships the fastest thing around, how long could we get along on fusion power? We’d use up all the hydrogen in Earth’s oceans in a hundred years.
“But there’s more to a Dyson sphere than collecting solar power.
“Say you make the sphere one astronomical unit in radius. You’ve got to clear out the solar system anyway, so you use all the solar planets in the construction. That gives you a shell of, say, chrome steel a few yards thick. Now you put gravity generators all over the shell. You’d have a surface area a billion times as big as the Earth’s surface. A trillion people could wander all their lives without ever meeting one another.”
Teela finally got a full sentence in edgewise. “You’re using the gravity generators to hold everything down?”
“Yah, against the inside. We cover the inside with soil.”
“What if one of the gravity generators broke down?”
“Picky, picky, picky. Well…you’d get a billion people drifting up into the sun. All the air swarming up after them. A tornado big enough to swallow the Earth. Not a prayer of getting a repair crew in, not through that kind of a storm…”
“I don’t like it,” Teela said decisively.
“Let’s not be hasty. There may be ways to make a gravity generator foolproof.”
“Not that. You couldn’t see