“And I’m telling you that’s impossible,” Louise growled. She felt her irritation rise. “How could there be cosmic string in the middle of a void like this?”
Uvarov raised his skull-like face and cackled, relishing the conflict.
The three of them were suspended just below the forest skydome. Louise was on a zero-gee scooter, and Uvarov had been strapped into a stripped-down life support chair attached to three of the flexible little scooters.
Mark, irritatingly, was choosing to manifest himself as a disembodied head, twice life-size, hovering in the air. “How’s Spinner-of-Rope?” he asked Louise.
She grunted. “Bearing up. We’re thirty-three days into the mission, now thirty-three days for Spinner in that couch. And the last ten of them inside this damn hole in the sky.”
“Well, this is really a pretty exciting part of the journey,” Mark said. “We’re crossing the edge of the greatest cosmological void ever detected: more than two hundred million light-years across. As far as we can tell, we’re the only scrap of baryonic matter in all that immensity. That’s an exciting thought even without my evidence of cosmic string…”
“Not exciting for Spinner,” Louise said drily. “For her this void is nothing but sensory deprivation.”
“Hmmm,” said Uvarov. “The Universe as an immense sensory deprivation tank… maybe that’s a good image to sum up the photino birds’ cosmic handiwork.”
Now schematic graphics of remote galaxies — sheets of them, at the boundary of the huge void — peppered the dome with splashes of false color; here and there fragments of text and supplementary images were interspersed amid the insect like galactic swarms.
Mark’s head swiveled around toward Louise. “Look, I’m sorry you don’t think it’s appropriate for me to have dragged you up here. Maybe I should have waited for proof of the string’s existence. Well, I didn’t realize we were out here to do science. I thought we were trying to find ways to stay alive — to anticipate what we’re up against. And that means reacting — and thinking, Louise — as quickly and as flexibly as possible. All right, maybe I’m guessing. But — what if it is cosmic string out there? Have you thought about that?”
Louise turned her face, uncertainly, up to the dome. “If it is string — here — then, perhaps, we’re heading into something even more extraordinary than we’ve anticipated.”
Uvarov chuckled. “Perhaps we should stick to the facts, my dear Mark.”
“There are no facts,” Louise said. “Only a handful of observations. And — across distances measured in hundreds of millions of light-years, and taken from a platform moving through a hyperdrive journey — they’re damned imprecise observations at that.”
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.”