Uvarov turned his head to the Virtual. “Tell me about your observations, then. Why are these double images so all-fired important?”

“I’ve been taking observations of the far side of the void,” the Virtual said. “I’ve been looking for evidence of gravitational tensing… The distortion of light from distant objects by the gravitational field of some huge, interposed mass. I wasn’t looking for strings specifically. I was trying to see if I could detect any structure within the void — any concentrations of density.”

“Are the strings so massive, then, that they can distort light so far?”

Louise said, “It isn’t really as simple as that, Uvarov. Yes, strings are massive: their width is only the Planck length, but their density is enormous — a one-inch length would have a mass of around ten million billion tons… a string stretching from Sol to Saturn, say, would have around one Solar mass. We expect strings to be found either in loops thousands of light-years across, or else they will be endless — stretched right across the Universe by the expansion from the singularity.”

Uvarov nodded. “Therefore, if they are so massive, their gravitational fields are correspondingly huge.”

“Not quite,” Louise said. “Strings are very exotic objects. They aren’t like stars, or planets, or even galaxies. They simply aren’t Newtonian objects, Uvarov. The relativistic gravitational fields around them are different.”

Uvarov turned to her. “Are you telling me the strings are antigravitational, like the domain walls of the nightfighter’s discontinuity-drive wings?”

“No…”

Far enough from a loop — a finite length of string — the mass of the string would attract other bodies, just as would any other massive object. But an observer close to a string, either a loop or part of an infinite string, would not experience the gravitational effects to be expected from such massive concentrations of matter.

Louise said, “Uvarov, gravitational attraction works by distorting spacetime. Spacetime is flat if no heavy objects are present; an object will sail across it in a straight line, like a marble across a tabletop. But the spacetime close to a Newtonian object, like a star, is distorted into a well, into which other objects fall. But close to a string, spacetime is locally flat — it’s what’s called a Minkowski spacetime. Objects close by aren’t attracted to the string, despite the huge mass…”

“But,” Mark said, “the spacetime around a string is distorted. It is conical.”

Uvarov frowned. “Conical?”

“Imagine spacetime as a flat sheet. The presence of the string removes a slice from that sheet — like a slice of a pie, cut out of spacetime. What’s left of the spacetime is joined up — the hole left by the missing slice is closed up so that the spacetime is like a cone. Still flat, but with a missing piece.

“If you were to draw a circle around a string, you would find its circumference shorter than you would expect from its radius — it’s just like drawing a circle around the apex of a cone.”

“And this small spacetime defect is sufficient to cause the double images you speak of?”

“Yes,” Mark said.

A cosmic string wasn’t visible directly. But its path could be made visible, by a track of double images of remote objects, separated by about six arc seconds, along the length of the string.

Louise said, “Uvarov, imagine two photons setting off toward us from a remote galaxy, beyond a string. One of them comes to us directly. The second, passing on the far side of the string, travels through the conical defect. The second photon actually has less distance to travel to reach us, thanks to the defect; its journey time is less than the first’s by around ten thousand years. Hence, the double images.”

Uvarov grunted. “Louise, you have explained to me how the network of strings was the web around which the galaxies coalesced. I do not understand how this can be, if the gravitational effects of these strings are so slight.”

Louise sighed. “The strings are primeval objects: they were formed within an invisible fraction of a second after the Big Bang itself, during the symmetry loss caused by the decomposition of the unified superforce. Since then, the expansion of the Universe has stretched the strings. So the strings are under great tension — a tension caused by the expansion of the Universe itself… The strings whip through space, at close to the speed of light.

“Where the strings pass, their conical defects cause them to leave a wake. Matter falls in toward the two-dimensional, sheet-like path swept out by the string. And it’s this infalling that caused the formation of the baryonic matter structures we observe now: clusters of galaxies, in threads and sheets.”

“In fact,” Mark said, “the wake is itself observable. Or should be. It imposes a slight Doppler shift on the microwave background radiation. I should be able to see a slightly brighter sky on one side of the invisible string than on the other…”

“And have you seen this?” Uvarov snapped.

“No,” Mark admitted. “Damn it. The Northern couldn’t be a much worse platform for this kind of measurement; the microwave Doppler is below my level of resolution.”

“But do you think you’ve found some image pairs,” Uvarov persisted.

“Yes,” Mark said, sounding excited again. “Two pairs so far, and a few other candidates. The two pairs are aligned, just as you’d expect them to be if a string is the cause…”

“Enough,” Uvarov snapped. He raised his chair into the air above them and prowled across the underside of the sky-dome, his ravaged profile silhouetted against the false colors of the galaxies. “Now tell me what this means. Let us accept, Louise, that your Virtual lover has found a fragment of this — string. So what? Why should we care?”

“We’re in a void, Uvarov,” Louise said patiently. “We’d expect to find string at the heart of huge baryonic structures — like the Great Wall, for instance, a sheet of clusters half a billion light-years long, which — ”

“But we are not at the heart of such a huge baryonic structure. Is that your point, Louise?”

“Yes. That’s the point. There’s no reason why we should find string here, in this void, away from any concentrations of matter.”

“I see. There is nothing out there but dark matter,” Uvarov growled quietly. “Nothing but the photino birds, and their even more exotic cousins — and whatever they’ve chosen to build, here at the heart of their dark empire, far from any baryonic structure.”

Uvarov wheeled to face Louise, his scooters spurting puffs of reaction gas. “If it exists, will the string have any effect on the photino birds?”

“Possibly,” Mark said. “Strings are gravitational defects. Dark matter is influenced by gravity…”

Uvarov nodded. “So perhaps the string is here to do damage to the photino birds. Is that possible? Perhaps the string has been moved here deliberately.”

“I hadn’t thought of that, but I guess it’s possible.” Mark peered up into the dome, his eerie, disembodied head looking bizarre. “Yes. If someone is waging war on the photino birds, then maybe they are using lengths of cosmic string as weapons. Think of that. And more: who in this Universe is capable of such an act, but the Xeelee themselves?

“Lethe — fighting wars with bits of cosmic string. How have they the audacity to even imagine such weapons?”

Louise looked up into the dome’s sketchy, gaudy rendering of the Universe. Suddenly these scraps of data seemed pathetic, their understanding hopelessly limited. Were the final wars for the destiny of the Universe being played out between Xeelee and photino birds, somewhere in this huge void, even now, as she stared up in her blindness and ignorance?

“Keep gathering your data, Mark,” she said. “In another few days we’ll be out of this damn void.”