Rob moved around to the side of the desk, next to Regulo, and the two men stared at the listing in silence for a few minutes.

“It’s clear enough what it’s saying,” said Rob at last. “If the author is accurate, he can make doped silicon whiskers, dislocation-free, that are twenty times as strong as the toughest that we’ve been making from graphite. He only quotes the strength under tension, so my first question would be to ask him about the strength under compression and shear.”

“I did ask him. The shear strength is not bad, the compressive strength is lousy. Very much the same as with graphite whiskers.”

Rob shrugged. “So you could make a load-bearing cable out of doped silicon, instead of graphite. I don’t see why that would be especially valuable. We don’t need anything stronger for any of the bridges I know about, not even the ones on the design cards — and that includes the Tasmanian Bridge, with a planned main span of three hundred and forty kilometers.”

“Quite right.” Regulo leaned over his desk and ran his fingers across one part of the top. Under the pressure of his hand a glowing legend appeared, set in block letters in the pink surface: “THINK BIG.”

“That’s what you have to learn to do, Merlin. Think big, not small. I’m interested in something that’s orders of magnitude beyond any piffling bridges. If you had no limit on funds, do you think you could modify the Spider so that it could fabricate and extrude doped silicon cable, instead of graphite?”

Rob hesitated. He was still looking curiously at the top surface of Regulo’s desk. He leaned across and rubbed the place where Regulo had touched it. Again, the glowing red sign, THINK BIG, appeared.

“Piezoelectric effects?”

Regulo laughed harshly. “Not quite that. You’ll have time to figure out the details if we work together. Press the surface a few other places, see what you get.”

Each part of the desk top responded to slight pressures from Rob’s hand: “WIN SMALL”; “IDEAS-THINGS-PEOPLE”; “ROCKETS ARE WRONG” — Rob stared hard at that one. It was exactly in line with what Corrie had said about Regulo. The older man was watching with undisguised pleasure as the red signs glowed from the desk top, then faded after a few seconds to the usual smooth pink.

“I’ve got my working philosophy built into that desk,” he said. “You should take a half hour and go over the whole thing — but not right now. I still want your answer: can you modify the Spider?”

Rob nodded. “It would take me maybe a month’s work, but I could do it. I designed the Spider with a lot of flexibility of operation.”

“And it could still extrude any shape of taper, same as it did for your work on the bridges?”

Rob nodded again, this time without comment. Regulo sat up straighter in his chair, grunting as he came upright from his stooped posture.

“All right, then.” He placed both hands flat on the desk. “I have one more question, then I’ll answer some of the ones I’m sure you have. If I made the money available, could you speed up the Spider? Could you increase the maximum production rate of extruded cable from ten kilometers a day up to something like two hundred a day?”

Rob frowned, biting his lip in concentration. “That’s a tougher one,” he said at last. “I’ll have to have time to think about it before I can give you a definite answer. I don’t know of any specific reason why I couldn’t, off-hand, but that’s not the sort of answer you need. Why would you ever want to do it, though? When I designed the Spider, I made it so that it would work faster than every other component of the bridge-building operation. I don’t see any point in speeding it up — nothing else would be able to keep pace with it.”

“I’ll tell you why.” Regulo held out his hand. “Look at that. Look at the rest of me. I’m an old man, right — and that means I’ve not got the time to wait about that you have. Don’t let anyone try and tell you that it’s the young men who are in a hurry. It’s the old ones, who have learned how precious time can be. I don’t know about you, but I’m not willing to sit about for ten years, waiting for a supporting cable to be extruded. One year, maybe — we’ll need that much time to arrange everything else. But no longer than one year. I want this fast.”

Rob sat down again in the chair facing Regulo. “You know there’s an old saying about engineering projects. Fast — cheap — good. You can only have two out of three.”

Regulo waved a hand. “Oh, I know, I know. I’ve already made my pick. You give me fast and good, and let me worry about the costs.”

Rob stared hard at the ruined face, trying to read the feelings behind the deformed mask. It was impossible. Only the eyes were human, and they glittered with an intense intellectual interest. “All right. Fast and good. It’s still your ball. You realize that an extrusion rate of two hundred kilometers a day could spin a supporting cable out of the Spider in one year to go twice around the Earth? At ten kilometers a day we’d have thousands of kilometers of cable — more than we’d ever need. What are you playing at, designing bridges to put on Jupiter?”

“No. Something a lot more interesting and a lot more useful.” Regulo leaned across to the control panel at the side of the desk and pressed a sequence of keys. The big display screen on the right-hand wall came alive with the stylized image of the Earth-Moon system, roughly to scale. “You already know my view of rockets, from the motto in the top of the desk. I’m responsible for hauling more material up from Earth than anyone else, and we use rockets for all of that; but I happen to believe that I’m working with an obsolete piece of technology. Even with the best nuclear propulsion systems, it still takes an awful lot of energy to hoist a payload up from the surface of Earth into orbit. And it takes just as much energy and reaction mass to get the damned stuff back down again.

“Now, Rob, you’re trained in physics as well as engineering. I checked that much of your background, before I ever asked Cornelia to try and bring you up here. So you know very well that a Newtonian gravitational field is conservative. A potential function exists for it. What does that mean? I’ll tell you: it means that in principle you should be able to take a mass from one point of the field — let’s say the surface of the Earth — out to some other point — let’s say geosynchronous orbit — using a certain amount of energy. Then you should be able to take it back down to Earth — and you should recover all the energy you expended to get it up in the first place. That’s the whole point of a conservative field, what you used going up, you should recover when you come back down again.”

Rob shrugged. “I understand the ideas behind potential fields. They don’t help at all in practice. The Earth’s gravitational field is very close to conservative, true enough, but you still have to use energy to get the rockets up into space from the surface. And you still need reaction mass and energy to stop them falling too fast when you want to go back down.”

“We do. Isn’t that a terrible situation, from the point of view of engineering efficiency? So there’s where we have to begin.” Regulo pressed another key on the control console and the wall display became animated, showing the Earth and Moon rotating together about their common center of mass, with the Earth also rotating on its axis.

“Suppose we don’t use rockets at all,” he said. “Rockets are like ferry-boats, taking materials and people up and down. Suppose that instead of ferry-boats we were to build a bridge to space. The idea is simple enough: we take a cable, tethered to a point of the Earth’s surface, somewhere on the equator. It extends vertically upwards, all the way up to synchronous orbit, where we are now, and on beyond it. At the far end, we have some kind of ballast weight. See the picture? The whole thing hangs there in equilibrium, with the downward forces from all the length of cable below geosynchronous altitude just balancing the outward forces from centrifugal acceleration. The ballast weight wants to fly outward, but the cable prevents that, and the outward tension on the cable is just balanced by the force on the tether point, down on the surface. The whole assembly rotates, at exactly the same speed as the Earth. Like this.”