Diaspora

As well as recording any sudden catastrophes, TERAGO was constantly monitoring a few hundred periodic sources. It took an event of rare violence to produce a burst of gravitational radiation sufficiently intense to be picked up halfway across the universe, but even routine orbital motion created a weak but dependable stream of gravitational waves. If the objects involved were as massive as stars, orbited each other rapidly, and weren't too remote, TERAGO could tune into their motion like a hydrophone eavesdropping on a churning propeller.

Lacerta G-1 was a pair of neutron stars, a mere hundred light years away. Though neutron stars were far too small to be observed directly—about twenty kilometers wide, at most—they packed the magnetic and gravitational fields of a full-sized star into that tiny volume, and the effects on any surrounding matter could be spectacular. Most were discovered as pulsars, their spinning magnetic fields creating a rotating beam of radio waves by dragging charged particles around in circles at close to lightspeed, or as X-ray sources, siphoning material from a gas cloud or a normal companion star and heating it millions of degrees by compression and shock waves on its way down their tight, steep gravity well. Lac G-1 was billions of years old, though; any local reservoir of gas or dust which might have been used to make X-rays was long gone, and any radio emissions had either grown too weak to detect, or were being beamed in unfavorable directions. So the system was quiet across the electromagnetic spectrum, and it was only the gravitational radiation from the dead stars' slowly decaying orbit that betrayed their existence.

This tranquillity wouldn't last forever. G-1a and G-1b were separated by just half a million kilometers, and over the next seven million years gravitational waves would carry away all the angular momentum that kept them apart. When they finally collided, most of their kinetic energy would be converted into an intense flash of neutrinos, faintly tinged with gamma rays, before they merged to form a black hole. At a distance, the neutrinos would be relatively harmless and the "tinge" would carry a far greater sting; even a hundred light years would he uncomfortably close, for organic life. Whether or not the fleshers were still around when it happened, Karpal liked to think that someone would take on the daunting engineering challenge of protecting the Earth's biosphere, by placing a sufficiently large and opaque shield in the path of the gamma ray burst. Now there was a good use for Jupiter. It wouldn't he an easy task, though; Lac G-1 was too far above the ecliptic to be masked by merely nudging either planet into a convenient point on its current orbit.

Lac G-1's fate seemed unavoidable, and the signal reaching TERAGO certainly confirmed the orbit's gradual decay. One small puzzle remained, though: from the first observations, G-la and G-1b had intermittently spiraled together slightly faster than they should have. The discrepancies had never exceeded one part in a thousand—the waves quickening by an extra nanosecond over a couple of days, every now and then—but when most binary pulsars had orbital decay curves perfect down to the limits of measurement, even nanosecond glitches couldn't be written off as experimental error or meaningless noise.

Karpal had imagined that this mystery would be among the first to yield to his solitude and dedication, but a plausible explanation had eluded him, year after year. Any sufficiently massive third body, occasionally perturbing the orbit, should have added its own unmistakable signature to the gravitational radiation. Small gas clouds drifting into the system, giving the neutron stars something they could pump into energy-wasting jets, should have caused Lac G-1 to blaze with X-rays. His models had grown wilder and more daring, but all of them had come unstuck from a lack of corroborative evidence, or from sheer implausibility. Energy and momentum couldn't just be disappearing into the vacuum, but by now he was almost ready to give up trying to balance the books from a hundred light years away.

Almost. With a martyr's sigh, Karpal touched the highlighted name on the screen, and a plot of the waves from Lacerta for the preceding month appeared.