"My garage door was open," she said.

"That seems like an awful chance. If we're so dangerous."

"As individuals, you aren't dangerous to me. Examine your own feelings. Aren't you surprised not to be a little afraid?" I thought that was due to my science-fictional objectivity. "No, I don't have control over your mind, and I can't 'read' it. You trust me because you can sense my intentions directly. It's not a well-developed talent in humans, though, and I doubt that it would work in a crowd, or over television. It's in groups that you are dangerous."

I'd noticed that myself. "You flew a flying saucer through Westchester and parked it in her garage?"

"No lights," Seven said. "Four in the morning."

"It's not a flying saucer," Lydia added. "It's a big black sphere, like a huge bowling ball."

"And it's broken down," I said. "You can orbit Earth, slip down, and tuck it into someone's garage, but you can't go from star to star. Is that it?

"He could go to other stars," Lydia said, "but it would take a long time."

"I could reach the star nearest here in about twelve years. But it would take nearly a hundred centuries for me to get home that way. Most of my friends would be dead."

"It's like if you drove to California," Lydia said, "and your car broke down there and you only had first gear. You could drive back to New York, but it makes more sense to look for a mechanic."

"But there are not mechanics in this part of California," Seven said. "I have to find some intelligent-what was that word?"

"Blacksmith."

"-blacksmith, and see whether he can fix it under my guidance. But I'm not a mechanic either. I know a little about the basic principles involved, but that's all." He rested his chin on one bony knee. "I'm not even sure how to take it apart safely."

"What's its power source?"

"Simple fusion of hydrogen atoms."

"That could be dangerous, all right."

"No, that's not what bothers me. It's the part that makes distances smaller. You're not supposed to use that near a planet."

"Makes distances smaller?"

"Yes. If you used it near a planet, it would make part of the planet very small. I think the rest of it would come apart, stretching."

"How does it work?"

"It makes distances smaller, so you don't have to travel as long."

I rubbed my eyes. When I opened them he was still there. "I understand that part.

What I mean is, do you know how it makes distances smaller?"

"The process?"

"That's right."

"This is why I need a scientist." He daintily took a sugar cube from the bowl on the tray and rubbed it between his palms. It disappeared. "All I know is that you tell the ship where you want to go, and it tells you how long it will take. You can stay awake or sleep. When you are ready, it goes."

"You must know some scientists," Lydia said.

"Yeah. A magician, too."

"I don't want many people to know I'm here. Not until I can leave quickly."

I had to admit that made sense. "Why don't you do this," Lydia said. "Pretend it's for a story.... Ask some scientists whether there's some rationale for a drive like this thing you made up. You must do that sort of thing all the time."

"Yeah." Like the physicist who told me my antigravity device violated the laws of conservation of energy, momentum, and natural resources. Sort of condescending.

"Worth a try, I guess."

"I could show you the vehicle," Seven said. Now that did sound interesting. I helped him to his feet, and Lydia took us around to the back door of the garage.

It really wasn't too helpful. The s.p.a.ceship looked like a prop for a low-budget TV movie. A featureless flat black sphere about eight feet in diameter. Seven said something to it and it clam-sh.e.l.led open. It looked pretty low-rent inside, too. Just a comfy-looking settee in a small round room wallpapered with shabby red satin. There were three gray boxes under the settee that he identified as the fusion drive, the "shrinker," and a life-support center. He didn't know how to get the boxes open.

I couldn't really fault him for that. I've been riding the subway all my life, but if one stopped dead I wouldn't have the faintest idea of how to get it started again.

That a.n.a.logy stuck in my mind as I rattled home in the last train back to the city.

Suppose the subway broke down and when I got out there was no one around but a bunch of Stone Age savages. Or even colonial Americans, say. Well, it's run by an electrical motor. You know-electricity? Ben Franklin?

Maybe somewhere in the city there was someone fiddling with the equivalent of Leyden jars and kites and keys. Even if I could find him, could we turn him into a metaphorical subway repairman?

The next morning I called Lydia and she confirmed that it hadn't been a dream, hallucination, or joke. So I took a jigger's worth out of my own private life-support center and splashed my way through the freezing rain to the public library.

There's a section there that has all the publications of the New York Academy of Sciences. I scanned t.i.tles and skimmed a few articles, looking for people who had an interest in exotic propulsion systems. I discarded a few names as being too prom- inent, figuring they'd have had too much experience with screwb.a.l.l.s. By afternoon I had three names to call. One turned out to be on sabbatical, one was openly contemptuous, and one was Lazlo Crane.

Dr. Crane is an a.s.sistant professor in the aeros.p.a.ce engineering department of NYU. He had written a paper with a new angle on using black holes for interstellar flight. I couldn't even understand the one-paragraph summary, but the t.i.tle was clear enough. His office wasn't too far, and I was out of phone change, so I slogged on over.

He looked sort of like a Lazlo. Tall and skinny, with a wisp of beard; prematurely bald, squinting through thick gla.s.ses. He was working on a crossword puzzle, standing up with the newspaper folded on top of a filing cabinet. Like Thomas Wolfe used to do, writing, though Wolfe was beefier and not quite Lazlo's seven feet.

"What's a five-letter word meaning 'sanctuary'?" he asked without looking up.

"The middle letter's a k, I'm sure of it."

"Sekos," I said, and spelled it.

"Fits." He scribbled it in, using a pencil. Amateur. "Do I know you?" He peered at me over his gla.s.ses.

I introduced myself and he gave the rare response: "The science fiction writer?"

I replied modestly in the affirmative. He shook hands and said, "Used to read your stuff," without elaborating. Too busy, I supposed, what with all the s.p.a.ce drives and crossword puzzles.

He sat down and nodded in the direction of the only other chair. "Can I do something for you?"

"I read your paper in the last Academy Proceedings. I thought maybe you could help me with a problem."

"A science problem? I thought you stayed away from that. Sort of made it up as you went along."

Pleasant fellow. "Trying to clean up my act," I said, and outlined what I knew of Seven's drive. It didn't take long, of course.

He pulled on his lower lip a couple of times and rubbed his beard out of shape.

"Why do you need an explanation? Why not just say there's this black box that makes distances shorter?"

Gray box.. "That would be kind of absurd, wouldn't it?"

"Not really." He shrugged, a quick spasm. "Like the way you handled time travel in Time and the Chinaman-"

"Time and/or the Chinaman."

"Whatever. You just presented it as an established fact. If they actually had this distance-shrinker, they wouldn't stand around talking about it. They'd just use it, wouldn't they?"

"Well, that's one way to handle it. A good way, usually, if you do it convincingly.

But even if I don't actually describe it in detail, I'd like to know how it works, what it looks like."

"A black box, probably." He leaned back and thought for a minute. "There is an angle. You know how to make artificial gravity?

"Sure, you spin the thing around-"

"No, that's not gravity. It's just imposing a rotating frame of reference. If you drop something it doesn't fall in a straight line. It doesn't even drop, really. It only seems to."

"Okay." I think.

"The only way we know how to make artificial gravity is to put a ma.s.s under the thing. You put a scale on a table and put something on it that weighs a pound. Roll a ten-ton lead weight under the table, and it weighs a tiny fraction of an ounce more."

"That's not really artificial gravity, though," I said. "That's natural, organic gravity.

"Semantics. Don't think of that block of lead in Newtonian terms-more ma.s.s, therefore a greater attractive force. Don't think in terms of force at all. Think of it as a device that changes the shape of s.p.a.ce." He stood up quickly. "Let's go to the undergraduate lab."

I followed him through the door and down the hall. "You know about the rubber sheet model?"

"I've seen pictures."

"We have one here. Here." He pushed open a door and we went into a large room full of long tables cluttered with electronic gear. In one corner was a round table a couple of yards in diameter, a taut rubber sheet nailed to a wooden frame with a wooden lip around it. Lazio reached into a jar and took out a marble-sized ball bearing and rolled it across the sheet.

"Straight lines, see?" The ball bearing bounced from the opposite side and came back. Lazio picked it up. "Now we put a planet in there, or a sun." He filled his hand with a metal sphere about half the size of a bowling ball and set it in the center of the sheet, turning it into a kind of elastic bowl.

"We use this thing to demonstrate different kinds of orbits." He rolled the ball bearing out and it dipped down in a graceful curve, came out banking to the left, rolled back in, and began looping around in a series of ellipses. He scooped up a handful of the ball bearings and rolled them in at various angles and speeds. "See there, there, that one's almost a circle, like the earth's...o...b..t."

"Kind of a miniature solar system," I said.

"Except that it runs down. Friction with the air and the rubber surface."

It was a hypnotic sight. We watched them whispering around for a minute.

"Now the important thing is that these things are still moving in a kind of straight line, though it doesn't look like it from our point of view."

"Path of least resistance?" I said.

"Something like that. The path they follow is called a geodesic. How much it deviates from a simple straight line, obviously, depends on how ma.s.sive the central object is and how far it is from the orbiting object."

"The closer they get, the faster they roll," I said. "Then they go out again and slow down.

"Right. Now what we're actually talking about, at any given moment, is the angle the rubber sheet makes, from the horizontal, at the spot right under the ball bearing.

The greater the angle is, the more the ball's influenced."

"Sure." Pretty sure, anyhow.

"That angle corresponds to what in four-dimensional s.p.a.ce-time we call the gravitational gradient."

"If you say so."

"Come on, now, it's not that hard. You read the black hole paper, didn't you?"

Moment of truth.

Look. Dr. Crane, twenty years ago I flunked calculus and switched to English. The part of your paper that was in English, I read."

He twisted his beard. "Not much, eh?"

"Enough to see that you might have what I want. Go ahead. The gravitational gradient.

"Well, what's interesting in terms of your s.p.a.ce drive is what happens to the gravitational gradient very close to a tremendously ma.s.sive, very small object. Like a black hole."

He pulled the weight out of the middle of the sheet and the ball bearings sort of relaxed, rolling off and clicking against the sides of the table and each other.

"Now look." He pushed his finger down into the sheet and one ball bearing, the closest, rolled into the dimple he made. None of the others was affected. "If you think about the push being the same, here, but the scale much reduced-the ball is smaller than a BB and my finger is narrower than a hypodermic needle-you can see you're approaching a condition where the sides of the gravity well, the rubber sheet, are almost vertical. As it approaches the point of the needle, the BB falls faster and faster."

"But not for long."

"That's the point. It's like rolling the lead weight under the scale. I think I have a way to fool s.p.a.ce-time. Make it seem as if there were a small black hole just a tiny distance away, constantly retreating in the direction you want to go. It's only the gradient that makes a difference, not the overall situation." He poked the rubber sheet again. "See? The other ball bearings don't even know I'm here. The gradient becomes infinitesimal, out where they are."