The Arks had left long before the discovery of the McAndrew balanced drive, before the discovery of even the Mattin Drive. They were multi-generation ships, bumbling along into the interstellar void with speeds that were only a few percent of light speed.

And who was on-board them when they left? Any fairly homogeneous group of strange people, who shared enough of a common philosophy or delusion to prefer the uncertainties of star travel to the known problems within the Solar System. It took courage to set out like that, to sever all your ties with home except occasional laser and radio communication. Courage, or an overpowering conviction that you were part of a unique and chosen group.

To put that another way, McAndrew was proposing to take us out to meet a community about which we knew little, except that by the usual standards they were descended from madmen.

“Mac, I don’t remember which one was the Ark of Massingham. How long ago did it leave?”

Even mad people can have sane children. Four of the Arks, as I recalled, had turned around and were on their way back to the System.

“About seventy-five years ago. It’s one of the earlier ones, with a final speed a bit less than three percent of light speed.”

“Is it one of the Arks that has turned back?”

He shook his head. “No. They’re still on their way. Target star is Tau Ceti. They won’t get there for another three hundred years.”

“Well, why pick them out? What’s so special about the Ark of Massingham?” I had a sudden thought. “Are they having some problem that we could help with?”

We had saved two of the Arks in the past twenty years. For one of them we had been able to diagnose a recessive genetic element that was appearing in the children, and pass the test information and sperm filter technique over the communications link. The other had needed the use of an unmanned high-acceleration probe, to carry a couple of tons of cadmium out to them. They had been unlucky enough to choose a freak asteroid, one that apparently lacked the element even in the tiniest traces.

“They don’t report any problem,” said Mac. “We’ve never had a response to any messages we’ve sent to them, so far as the records on Triton Station are concerned. But we know that they are doing all right, because every three or four years a message has come in from them. Never anything about the Ark itself, it has always been… scientific information.”

McAndrew had hesitated as he said that last phrase. That was the lure, no doubt about it.

“What kind of information?” I said. “Surely we know everything that they know. We have hundreds of thousands of scientists in the System, they can’t have more than a few hundred of them.”

“I’m sure you’re right on the numbers.” Professor Limperis spoke when McAndrew showed no inclination to do so. “I’m not sure it’s relevant. How many scientists does it require to produce the work of one Einstein, or one McAndrew? You can’t just sit down and count numbers, as though you were dealing with — with bars of soap, or poker chips. You have to deal with individuals.”

“There’s a genius on the Ark of Massingham,” said McAndrew suddenly. His eyes were gleaming. “A man or woman who has been cut off from most of physics for a whole lifetime, working alone. It’s worse than Ramanujan.”

“How do you know that?” I had seldom seen McAndrew so filled with feeling. “Maybe they’ve been getting messages from somebody in the System here.”

McAndrew laughed, a humorless bark. “I’ll tell you why, Jeanie. You flew the Merganser. Tell me how the drive worked.”

“Well, the mass plate at the front balanced the acceleration, so we didn’t get any sensation of fifty gee.” I shrugged. “I didn’t work out the math for myself, but I’m sure I could have if I felt like it.”

I could have, too. I was a bit rusty, but you never lose the basics once you have them planted deep enough in your head.

“I don’t mean the balancing mechanism, that was just common sense.” He shook his head. “I mean the drive. Didn’t it occur to you that we were accelerating a mass of trillions of tons at fifty gee? If you work out the mass conversion rate you will need, you find that even with an ideal photon drive you’ll consume the whole mass in a few days. The Merganser got its drive by accelerating charged particles up to within millimeters a second of light speed. That was the reaction mass. But how did it get the energy to do it?”

I felt like telling him that when I had been on Merganser there had been other details — such as survival — on my mind. I thought for a few moments, then shook my head.

“You can’t get more energy out of matter than the rest mass energy, I know that. But you’re telling me that the drives on Merganser and Hoatzin do it. That Einstein was wrong.”

“No!” McAndrew looked horrified at the thought that he might have been criticizing one of his senior idols. “All I’ve done is build on what Einstein did. Look, you’ve done a fair amount of quantum mechanics. You know that when you calculate the energy for the vacuum state of a system you don’t get zero. You get a positive value.”

I had a hazy recollection of a formula swimming back across the years. What was it? h/4?w, said a distant voice.

“But you can set that to zero!” I was proud at remembering so much. “The zero point of energy is arbitrary.”

“In quantum theory it is. But not in general relativity.” McAndrew was beating back my mental defenses. As usual when I spoke with him on theoretical subjects, I began to feel I would know less at the end of the conversation than I did at the beginning.

“In general relativity,” he went on, “energy implies space-time curvature. If the zero-point energy is not zero, the vacuum self-energy is real. It can be tapped, if you know what you are doing. That’s where Hoatzin draws its energy. The reaction mass it needs is very small. You can get that by scooping up matter as you go along, or if you prefer it you can use a fraction — a very small fraction — of the mass plate.”

“All right.” I knew McAndrew. If I let him get going he would talk all day about physical principles. “But I don’t see how that has anything to do with the Ark of Massingham. It has an old-fashioned drive, surely. You said it was launched seventy-five years ago.”

“It was.” This was Limperis again, gently insistent. “But you see, Captain Roker, nobody outside the Penrose Institute knows how Professor McAndrew has been able to tap the vacuum self-energy. We have been very careful not to broadcast that information until we were ready. The potential for destructive use is enormous. It destroys the old idea that you cannot create more energy at a point than the rest mass of the matter residing there. There was nothing known in the rest of the System about this use until two weeks ago.”

“And then you released the information?” I was beginning to feel dizzy.

“No. The basic equations for accessing the vacuum self-energy were received by laser communication. They were sent, with no other message, from the Ark of Massingham.”

Suddenly it made sense. It wasn’t just McAndrew who was itching to get in and find out what there was on the Ark — it was everyone at the Penrose Institute. I could sense the excitement in Limperis, and he was the most guarded and politically astute of all the Members. If some physicist, working out there alone two light-years from Sol, had managed to parallel McAndrew’s development, that was a momentous event. It implied a level of genius that was difficult to imagine.

I knew Hoatzin would be on the way in a few days, whether I wanted to go or stay. But there was one more key question.

“I can’t believe that the Ark of Massingham was started by a bunch of physicists. What was the original composition of the group that colonized it?”