In his indispensable 1994 book Black Holes and Time Warps, physicist Kip Thorne wrote of the tantalizing discoveries that awaited in the coming century. In particular, the existence of gravitational waves—ripples in the very fabric of space and time caused by the motion, and especially the collision, of extremely massive objects—might soon graduate from theoretical prediction to known fact. And those waves could carry all-important hints about their origins. “Gravitational-wave detectors will soon bring us observational maps of black holes, and the symphonic sounds of black holes colliding—symphonies filled with rich, new information about how warped spacetime behaves when wildly vibrating,” Thorne wrote.
That time is nearly upon us, he now believes. The California Institute of Technology theorist writes in the August 3 issue of Science that in five years’ time, ongoing upgrades to the world’s leading gravitational-wave detectors will make those instruments sensitive enough to detect gravitational waves from colliding and merging black holes, which would provide yet another major experimental confirmation of Albert Einstein’s theory of general relativity. The detection would also open up a new regime for studying black holes, those cosmic gluttons whose gravitational pull is so strong that it forms a one-way funnel into the black hole’s maw. Even light cannot escape once it has crossed the event horizon, a black hole’s point of no return.
As of now, astrophysicists can only infer the presence of a black hole by monitoring the environs around the putative object. In the case of Sagittarius A*, in the center of our own Milky Way galaxy, for instance, astronomers can see flares of radiation emanating from the black hole’s location, caused by infalling material heating up and radiating outside the event horizon. Stars at the galactic center betray the presence of Sagittarius A* as well—their orbits point to the existence of a nearby compact object with the mass of four million suns.
The strong gravitational wave signature expected from merging black holes, in comparison, would carry a wealth of information both about the objects involved and about their cataclysmic interaction—the geometrodynamics, or “stormy behavior,” as Thorne puts it, of black holes and other extremely curved regions of spacetime. “The gravitational waveforms from a merging black-hole binary carry detailed information about the initial black holes (their masses, spins, and orbit), the final merged black hole (its mass and spin), and the geometrodynamics of the merger,” he writes in Science.
Two major gravitational-wave detector projects have been on the lookout for these spacetime ripples, but so far the search has not produced any results. Both the LIGO and Virgo observatories are L-shaped instruments with extremely long arms—four kilometers for the two LIGO facilities in Washington and Louisiana, and three kilometers for Italy’s Virgo. They rely on long-baseline interferometry, firing lasers down the perpendicular arms to see if one direction has been stretched or compressed relative to the other by a passing gravitational wave.
The fact that neither has detected gravitational waves is no great cause for concern, writes Thorne, who helped spearhead the development of LIGO in the 1980s. “The initial LIGO and Virgo interferometers (with sensitivities at which it is plausible but not likely to see waves) were designed to give the experimental teams enough experience to perfect the techniques and design for advanced interferometers—that will have sensitivities at which they are likely to see lots of waves,” he writes. “The advanced LIGO and advanced Virgo interferometers are now being installed and by 2017 should reach sensitivities at which black-hole mergers are observed.” Sounds like the race is on to detect gravitational waves, one of the biggest prizes in physics.