Most of the nuggets from the initial singularity had decayed. But some survived.

"And this is why you need to live out here?"

"The first the inner Solar System knows of the presence of a nugget is when it hits the top of an atmosphere, and its energy crystallizes into a shower of exotic particles. Yes, you can learn something from that — but it’s like watching shadows on the wall. I want to study the raw stuff. And that’s why I’ve come so far out. Damn it, there are only about a hundred humans farther from the Sun, and most of them are light-years away, in starships like the Cauchy, crawling at near lightspeed to God knows where. Harry, a quark nugget sets up a bow wave in the interstellar medium. Like a sparkle of high-energy particles, scattered ahead of itself. It’s faint, but my detectors can pick it up, and — maybe one time out of ten — I can send out a probe to pick up the nugget itself."

Harry tugged at the corner of his mouth — a gesture that reminded Michael jarringly of a frail eighty-year-old who had gone forever. "Sounds terrific," Harry said. "So what?"

Michael bit back an angry response. "It’s called basic research," he said. "Something we humans have been doing for a couple of thousand years now—"

"Just tell me," Harry said mildly.

"Because quark nuggets are bundles of matter pushed to the extreme. Some can be moving so close to lightspeed that thanks to time dilation, they reach my sensors barely a million subjective years after leaving the singularity itself."

"I guess I’m impressed." Harry sucked on his brandy, turned and walked easily across the transparent floor, showing no signs of vertigo or distraction. He reached a metal chair, sat on it, and crossed his legs comfortably, ignoring the zero-gravity harness. The illusion was good this time, with barely a thread of space between the Virtual’s thighs and the surface of the chair. "I always was impressed with what you achieved. You, with Miriam Berg, of course. I’m sure you knew that, even if I didn’t say it all that often."

"No, you didn’t."

"Even a century ago you were the authority on exotic matter. Weren’t you? That was why they gave you such responsibility on the Interface project."

"Thanks for the pat on the head." Michael looked into the sky-blue emptiness of his father’s eyes. "Is that what you’ve come to talk about? What did you retain when you had your head cleaned out? Anything?"

Harry shrugged. "What I needed. Mostly stuff about you, if you want to know. Like a scrap-book…"

He sipped his drink, which glowed in the light of the comet, and regarded his son.

Wormholes were flaws in space and time that connected points separated by lightyears — or by centuries — with near-instantaneous passages of curved space. They were useful… but difficult to build.

On the scale of the invisibly small — on Planck length scales, in which the mysterious effects of quantum gravity operate — spacetime is foamlike, riddled with tiny wormholes. Michael Poole and his team, a century earlier, had pulled such a wormhole out of the foam and manipulated its mouths, distorting it to the size and shape they wanted.

Big enough to take a spacecraft.

That was the easy part. Now they had to make it stable.

A wormhole without matter in its throat — a "Schwarzchild" solution to the equations of relativity — is unusable. Lethal tidal forces would bar the wormhole portals, the portals themselves would expand and collapse at lightspeed, and small perturbations caused by any infalling matter would result in instability and collapse.

So Poole’s team had had to thread their wormhole with "exotic" matter.

Space contracted toward the center of the throat and then had to be made to expand again. A repulsive effect in the throat had come from exoticity, the negative energy density of the exotic matter. The wormhole was still intrinsically unstable, even so; but with feedback loops it could be made self-regulating.

At one time negative energy had been thought impossible. Like negative mass, the concept seemed intuitively impossible. But there had been encouraging examples for Michael and his team. Hawking evaporation of a black hole was a kind of mild exoticity… But the negative energy levels Poole had needed were high, equivalent to the pressure at the heart of a neutron star.

It had been a challenging time.

Despite himself Michael found memories of those days filling his head, more vivid than the washed-out lifedome, the imperfect image of his father. Why was it that old memories were so compelling? Michael and his team — including Miriam, his deputy — had spent more than forty years in a slow orbit around Jupiter; the exotic matter process had depended on the manipulation of the energies of the magnetic flux tube that connected Jupiter to its moon, Io. Life had been hard, dangerous — but never dull. As the years had worn away they had watched again and again as the robot probes dipped into Jupiter’s gravity well and returned with another holdful of shining exotic material, ready to be plated over the growing tetrahedra of the portals.

It had been like watching a child grow.

Miriam and he had grown to depend on each other, completely, without question. Sometimes they had debated if this dependence was the content of love. Mostly, though, they had been too busy.

"You were never happier than in those times, were you, Michael?" Harry asked, disconcertingly direct.

Michael bit back a sharp, defensive reply. "It was my life’s work."

"I know it was. But it wasn’t the end of your life."

Michael gripped the whiskey globe harder, feeling its warm smoothness glide under his fingers. "It felt like it, when the Cauchy finally left Jupiter’s orbit towing one of the Interface portals. I’d proved that exotic material was more than just a curiosity; that it could be made available for engineering purposes on the greatest of scales. But it was an experiment that was going to take a century to unfold—"

"Or fifteen centuries, depending on your point of view."

The Cauchy was dispatched on a long, near-lightspeed Jaunt in the direction of Sagittarius — toward the center of the Galaxy. It was to return after a subjective century of flight — but, thanks to time dilation effects, to a Solar System fifteen centuries older.

And that was the purpose of the project.

Michael had sometimes studied Virtuals of the wormhole portal left abandoned in Jovian orbit; it was aging at the same rate as its twin aboard the Cauchy, just as he and Miriam were. But while Miriam and Michael were separated by a growing "distance" in Einsteinian spacetime — a distance soon measured in lightyears and centuries — the wormhole still joined the two portals. After a century of subjective time, for both Michael and Miriam, the Cauchy would complete its circular tour and return to Jovian orbit, lost in Michael’s future.

And then it would be possible, using the wormhole, to step in a few hours across fifteen centuries of time.

The departure of the ship, the waiting for the completion of the circuit, had left a hole in Michael’s life, and in his heart.

"I found I’d become an engineer rather than a scientist… I’d restricted my attention to the single type of material we could fabricate in our Io flux tube accelerators; the rest of exotic physics remained untouched. So I decided—"

"To run away?"

Again Michael was stabbed by anger.

His father leaned forward from the chair, hands folded before him; the gray light from the comet below played over his clear, handsome face. The brandy glass was gone now, Michael noticed, a discarded prop. "Damn it, Michael; you had become a powerful man. It wasn’t just science, or engineering. To establish and complete the Interface project you had to learn how to build with people. Politics. Budgets. Motivation. How to run things; how to manage — how to achieve things in a world of human beings. You could have done it again, and again; you could have built whatever you wanted to, having learned how.