Louise laughed. “A knot. Knot-making is a skill, up there in the forest, isn’t it, Spinner? I’ll bet you’d have been proud to come up with a structure like that.”
“Actually,” Mark said, “and I hate to be pedantic, but that isn’t a knot, topologically speaking. If you could somehow stretch it out — straighten up the cusps and curves — you’d find it would deform into a simple loop. A circle.”
Spinner heard Garry Uvarov’s rasp. “And I hate to be a pedant, in my turn, but in fact a simple closed loop is a knot — called the trivial knot by topologists.”
“Thank you, Doctor,” Louise said drily.
Spinner frowned, peering at the detailed image of the string loop; in the false colors of her faceplate it was a tracery of blue, frozen against the remote background of the galaxy core. She realized now that she was looking at one projection of a complex three-dimensional object. Subvocally she called for a depth enhancement and change in perspective.
The loop seemed to loom toward her, lifting away from the starry background, and the string was thickened into a three-dimensional tubing, so that she could see shadows where one strand overlaid another.
The image rotated. It was like a sculpture of hosepipe, rolling over on itself.
Mark commented, “But the string isn’t stationary, of course. I mean, the whole loop is cutting through this galaxy at more than half lightspeed — but in addition the structure is in constant, complex motion. Cosmic string is under enormous tension — a tension that increases with curvature — and so those loops and cusps you see are struggling to straighten themselves out, all the time. Most of the length of the string is moving at close to lightspeed — indeed, the cusps are moving at lightspeed.”
“Absurd,” Spinner heard Uvarov growl. “Nothing material can reach lightspeed.”
“True,” Mark said patiently, “but cosmic string isn’t truly material, in that sense, Uvarov. Remember, it’s a defect in spacetime… a flaw.”
Spinner watched the beautiful, sparkling construct turn over and over. It was like some intricate piece of jewelry, a filigree of glass, perhaps. How could something as complex, as real as this, be made of nothing but spacetime?
“I can’t see it move,” she said slowly.
“What was that, Spinner?”
“Mark, if the string is moving at close to lightspeed — how come I can’t see it? The thing should be writhing like some immense snake…”
“You’re forgetting the scale, Spinner-of-Rope,” Mark said gently. “That loop is over a thousand light-years across. It would take a millennium for a strand of string to move across the diameter of the loop. Spinner, it is writhing through space, just as you say, but on timescales far beyond yours or mine…
“But watch this.”
Suddenly the three-dimensional image of the string came to life. It twisted, its curves straightening or bunching into cusps, lengths of the string twisting over and around each other.
Mark said, “This is the true motion of the string, projected from the velocity distribution along its length. The motion is actually periodic… It resumes the same form every twenty thousand years or so. This graphic is running at billions of times true speed, of course — the twenty millennia period is being covered in around five minutes.
“But the graphic is enough to show you an important feature of this motion. It’s non-intersecting… The string is not cutting itself at any point in the periodic trajectory. If it did, it would bud off smaller sub-loops, which would oscillate and cut themselves up further, and so on… the string would rapidly decay, shrivelling through a thousand cuts, and leaking away its energy through gravitational radiation.”
Spinner wished, suddenly, that she wasn’t human: that she could watch the motion of this loop unfold, without having to rely on Mark’s gaudy projections. How wonderful it would be to be able to step out of time!
…Close your eyes, Spinner.
“What?”
You can step out of time, just as you desire. Close your eyes, and imagine you are a god.
…And here, in her mind’s eye — so much more dramatic than any Virtual! — came the knot of string, sailing out of space. The knot wriggled like some huge worm, closed on itself as if swallowing its own tail.
The knot struck the rim of this defenseless galaxy and scythed toward the core, battering stars aside like blades of grass.
It was a disturbing, astonishing image. She snapped open her eyes, dispelling the vision; fear flooded her, prickling over her flesh.
She wasn’t normally quite so imaginative, she thought drily. Maybe her companion had had something to do with that brief, vivid vision…
She returned her attention to the harmless-looking Virtual display. Now Mark showed Spinner the loop’s induced magnetic field, a yellow glow of energy which sleeved the fake blue of the string itself.
“As it hauls through the galaxy’s magnetic field, that string is radiating a lot of electromagnetic energy,” Mark said. “I see a flood of high-energy photons…”
Cosmic string wasn’t actually one-dimensional; it was a Planck length across, a fine tube containing charged particles: quarks, electrons and their antiparticles, gathered into super-heavy clusters. As a result, string acted as a superconducting wire.
The string knot was cutting through this galaxy’s magnetic field. As it did so immense electrical currents — of a hundred billion billion amps or more — were induced in the string. These currents generated strong magnetic fields around the string.
The string’s induced field was stronger than a neutron star’s, and dominated space for tens of light-years around the knot.
Mark said, “The string has a maximum current capacity. If it’s overloaded, the string starts to shed energy. It glows with gamma radiation. And the lost energy crystallizes into matter: ions and electrons, whispering into existence all along the length of the string.” Spinner saw representations of particles — out of scale, of course — popping into existence around the string image. “So the string is glowing as brightly as a star.”
“Yes,” Louise put in. “But the distribution of the radiation is odd, Mark. Look at this. The radiation is beamed forward of the loop’s motion — parallel to that forward spike of gravitational radiation.”
“Like a searchlight,” Morrow said.
Or a spear…
She heard Morrow saying, “Mark, what is driving the string? What is impelling it through space, and into this galaxy?”
“Gravitational radiation,” Mark said simply.
Louise said, “Morrow, gravity waves are emitted whenever large masses are moved through space. Because the loop is asymmetrical it’s pushing out its gravitational radiation in particular directions — in spikes, ahead of and behind it. It is pushing out momentum… It is a gravitational rocket, using its radiation to drive through space.”
Mark said, “Of course the gravitational radiation is carrying away energy — the string is shrinking, slowly. In the end it will collapse to nothing.”
“But not fast enough to save this galaxy,” Uvarov growled.
“No,” Louise said. “Before it has time to decay away, the string is going to reach the core — and devastate the galaxy.”
Close your eyes.
Spinner-of-Rope shivered. Once again the voice had come from her left — from somewhere outside her suit. She stared at the Virtual image in her faceplate, not daring to look around.