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Catch a Wave

Sensing ripples in the space-time sea from gravity's juggernauts

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2:26pm, June 4, 2002
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The starship suddenly shudders violently. "What was that?" the alarmed captain asks the ship's computer.

The computer deduces that a burst of gravitational waves has rolled through the spacecraft.

These waves, it reports, have originated from a pair of gargantuan black holes, as near to the craft as the moon is to Earth.

The captain knew that the black holes—dense objects that exert such strong gravity that even light cannot escape—had been swirling around each other. But when they slammed together, merging into one, the collision sent a fast-moving ripple through the very fabric of space (or more accurately, of space-time) that was stronger than he expected. It alternately stretched and compressed everything and everyone in its path.

"You are accustomed to gravitational waves so weak that only very delicate instruments can detect [their force]," the computer tells the captain. "Here, close to the coalescing holes, they were enormously strong."

Physicist Kip S. Thorne of the California Institute of Technology in Pasadena describes this far-future scenario at greater length in his book Black Holes and Time Warps (Norton, 1994). In it, he fulfills a dream of many contemporary scientists who specialize in the study of gravity and Albert Einstein's theory of general relativity—to somehow encounter gravitational waves.

Yet unlike the starship captain, today's scientists can't even claim familiarity with gravitational waves detected with sensitive instruments. For more than 30 years, using delicately balanced metal bars, researchers have tried unsuccessfully to discern the subtle stretching and shrinking that passing gravitational waves would cause (SN: 3/18/78, p. 169). The task is so challenging that detectors must measure changes in length that are less than a thousandth of the diameter of a proton.

Although detectors that rely on metal bars are still being improved, another technology has emerged that many researchers believe is more likely to capture the long-sought prize. In November, Caltech and the Massachusetts Institute of Technology completed the first phase of construction of a $300 million, two-site observatory called the Laser Interferometer Gravitational-Wave Observatory, or LIGO (SN: 2/29/92, p. 134), funded by the National Science Foundation.

Between now and 2002, LIGO scientists plan to install and fine-tune detector equipment. The observatory, flagship of a fleet of new laser-based gravitational-wave observatories being developed globally, is then expected to begin searching for waves.

If any of the instruments finally catch a gravitational wave, an important prediction of Einstein's 84-year-old theory of general relativity would be validated. Scientists could then begin subjecting the theory to rigorous tests at the tremendous gravity typical of extremely dense matter, such as black holes.

The feat would also mark the beginning of a fundamentally different way of probing the universe, particularly its dark and violent side. It might even provide a means of witnessing the universe at the merest instant after the Big Bang.

"LIGO is really the first step in what I see as the greatest challenge of the 21st century: Opening the gravitational-wave window to the universe," says cosmologist Michael S. Turner of the University of Chicago.

Controversial predictions

Soon after Einstein unveiled his general theory of relativity in 1916, scientists began to ponder its controversial predictions of gravitational waves.

General relativity merges space and time into a seamless four-dimensional entity. The presence of mass or energy curves space-time. That curvature manifests itself as an attractive force between objects—gravity.

Think of a ball resting on a rubber sheet. It dimples the sheet and any object on the curved surface rolls toward the ball, as if impelled by a force. If the ball rolls or jiggles, or if two balls spiral around each other, the sheet quivers with waves that rush outward.

Likewise, the motion of massive objects can generate waves of curvature—gravitational waves—that ripple through the fabric of space-time. Not all massive objects would be expected to trigger gravitational waves, according to theorists. Objects that move with perfect spherical symmetry, a spinning ball, for instance, provide such an exception.

No one has ever detected an actual gravitational wave. Nonetheless, the behavior of an unusual astrophysical object discovered in 1974 convinced most scientists that gravitational waves are real.

In that year, Russell A. Hulse and Joseph H. Taylor Jr., then both at the University of Massachusetts at Amherst, found a pair of extremely dense stellar cinders, known as neutron stars, rapidly orbiting each other. One of them, a pulsar, emits periodic bursts of radio waves as it orbits.

Gradual changes in the pulsation rate indicated that the stars were slowly spiraling toward one another. The slowdown perfectly matched the predictions of general relativity if the pair was shedding energy in the form of gravitational waves (SN: 10/23/93, p. 262). For this insight, Hulse and Taylor won the Nobel Prize in Physics in 1993.

Scientists have identified a variety of objects that appear capable of generating detectable gravitational waves and have calculated, at least roughly, what kind of waveform each source would produce.

Along with black holes or neutron stars spiraling into each other, the bestiary includes mighty explosions, known as supernovas, that occur when aging stars collapse. Even a lone, spinning neutron star in our galaxy, if its surface bears the slightest deviation from perfect smoothness, can generate detectable gravitational waves, researchers say.

Some gravitational waves in the universe may be leftovers from the Big Bang. "Almost every model of the early universe that people have looked at produces a background of gravitational wave radiation," says Bruce Allen of the University of Wisconsin-Milwaukee. Researchers intend to analyze LIGO's data for this static, which would portray the universe at an age of 10-22 seconds.

Here's another tantalizing prospect: Entirely unknown objects may reveal themselves in gravitational waves. "This is opening a new window on the universe that is more radically different than any other window we've ever opened," says Thorne. "There are just bound to be surprises."

Distorting space-time

Gravitational waves may give themselves away by the strange way they distort space-time. Tug on a piece of woven fabric, and the threads running parallel to the direction of the stretch will squeeze together while the perpendicular threads pull apart. Gravitational waves have a similar effect, called a strain, on space-time, scientists say.

At each of LIGO's two sites, one in Washington State and the other in Louisiana, a right angle made of steel tubes, each 4 kilometers long, reaches out toward the horizon. The beam of a laser is split into two parts that shoot simultaneously down the arms from the intersection.

Heavy mirrors, reflecting the beams, hang inside the ends of each tube, which contains a vacuum.

Using a time-tested technique known as interferometry, LIGO's designers have the returning beams come together at a detector. By inverting the crests and troughs of the light wave returning along one path, they cause the combined beams to interfere in such a way that they cancel each other out.

So, when there is no gravitational wave, the detector sees darkness.

If gravitational waves pass through LIGO's perpendicular arms, however, they will stretch space-time along one arm while shrinking it along the other, creating a tiny difference in the path lengths. Typically the offset is much less than one wavelength of the laser light. When the beams from the different paths coincide, they will no longer line up perfectly, upsetting the absolute cancellation. The detector will sense some light.

By using an intense laser and an exquisitely sensitive detector, LIGO scientists expect to discern mirror displacements as small as one trillionth of the wavelength of the laser light, equivalent to a strain of one part in 1021 for 4-km arms.

Caltech researchers have already shown that they can measure such small displacements on a prototype interferometer with shorter arms, notes LIGO director Barry C. Barish of Caltech. "It's phenomenal. It's impossible, almost," says Barish. "There's real magic in interferometry."

A rich signal

Gravitational-wave observatories, such as LIGO, tune into a single, richly varying, oscillating signal. It combines signals of many frequencies coming simultaneously from all directions. In that sense, the observatories act like ears, just as telescopes act like eyes.

The frequencies that LIGO can detect, 10 to 10,000 Hertz, lie squarely in the human-hearing band. Played through speakers, they even translate into a sort of celestial music.

That's as far as the analogy goes, however, scientists emphasize. Gravitational waves are not sound. Whereas sound is a wave of compression and expansion of a substance in space, gravitational waves are a warping of space itself.

Nonetheless, gravitational-wave scientists find it handy to think of their instrument's signals as sounds. Inward-spiraling binary stars will give off a "chirp," say Barish and Rainer Weiss of MIT. The emanations of a black hole after it chows down a star are "burplike."

Some researchers, such as Peter Saulson of Syracuse (N.Y.) University, find listening to the signals from their detectors useful. "To debug some of the prototype instruments we build, people do listen to them," he says. "It's a very good trick because your ear is a very good signal-processing device."

Scientists must pluck the minuscule patterns caused by gravitational waves from a sea of other, spurious signals. This unwanted noise can come, for instance, from ground vibrations, variations in laser intensity in the equipment, and random jiggling of atoms.

Observatory builders seismically isolate their detectors and incorporate many types of damping and corrections to eliminate noise. Scientists are also seeking ways to quiet random atom jiggles (SN: 10/23/99, p. 263).

By having two identical detectors separated by 3,000 km, LIGO scientists can distinguish local noise at each site from more far-reaching signals, such as gravitational waves. Being part of an even larger network of detectors would also help the instruments separate wheat from chaff and triangulate on celestial sources.

In the next few years, such a network is expected to take shape with the start-up of VIRGO, an Italian interferometer similar to a single LIGO detector, and two smaller interferometers in Germany and Japan. Five detectors using metal bars are also listening for signals at 900-Hz resonant frequency.

Finally, LIGO observers will also make use of simulated waveforms from known sources to help them in distinguishing signals from noise. Generating those waveforms has proven very difficult, even with supercomputers, because the equations of general relativity are so challenging (SN: 6/26/93, p. 408). Buoyed by recent successes with modeling a glancing collision between black holes, researchers say they hope to master the complete sequence of an inward-spiraling pair of black holes by the time LIGO begins collecting data.

Black hole mergers

At its planned sensitivity, LIGO may detect a handful of black hole mergers—the events it has the best chance of catching—during its first 3-year scientific run, its designers say. On the other hand, if it makes no detection during its inaugural period, they won't be dismayed.

"Our plan has always been to turn it on at a sensitivity where it is likely to see something and then upgrade to where it would be surprising if it does not," says Thorne, who was one of the original proponents of LIGO in the 1960s.

Researchers are already working on the improved technology to be installed in 2005. At that time, LIGO is scheduled to switch, for instance, from quartz mirrors to synthetic sapphire ones, which are denser and shed the laser beam's heat more efficiently.

The overhaul should boost the observatory's sensitivity approximately 15-fold, Thorne says, increasing its detection range for a black hole merger from 600 million light-years away to nearly 10 billion light-years. By eavesdropping on so much more space, LIGO should raise its rate of detecting sources by a factor of more than 3,000.

Looking beyond even the upgraded LIGO, space agency officials in the United States and Europe are considering building an orbiting gravitational wave observatory called the Laser Interferometer Space Antenna, or LISA. It would extend gravitational wave observations into a much lower frequency range—from a cycle every 10 seconds to one every 3 hours. Scientists could then tune in to slowly orbiting binary stars and colliding galaxies with huge black holes at their cores.

Consisting of three spacecraft in a triangular array, 5 million km on a side, LISA is "the wave of the future for this field," says Weiss, who 30 years ago came up with the concept of using interferometers for gravitational wave detection. Because signals in LISA's band are expected to be stronger and more abundant than those that would trigger ground-based detectors, the space observatory would be able to detect several sources every week, he says.

Maybe then gravitational waves will seem as commonplace as scientists say they really are.

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