“WE'RE NOT ASTRONOMERS, YET. We’re still instrument builders. We’ve built a telescope, but we haven’t yet seen a star.”
So Warren W. Johnson, Louisiana State University physics professor, summed up the current state of gravitational wave astronomy—a science with maybe a billion dollars invested in equipment worldwide and more than three decades of effort, but no direct detection of gravitational waves…yet. This fall could see the first, and scores of astronomers, physicists, and mathematicians around the world are watching detectors—indeed, have staked their careers on the belief that gravitational waves will be detected very soon.
What are gravitational waves? What would they tell us about the universe? And who cares whether they’re detected?
Gravitational waves are ripples in space-time. That’s the usual explanation offered to a lay audience, likening gravitational waves to the expanding circular waves seen when you throw a pebble into a pond. Not a bad analogy for invoking an image of crests and troughs radiating out from a disturbance. But that vivid comparison, of course, begs the question: What is space-time?
Space-time is what we live in—the three dimensions of space in which we all go to school or work, mow the lawn, and watch TV, plus the fourth dimension of time, in which we measure how long we take to do these things or note when we start and stop. Every one of us is constantly traveling through space-time, as Earth carries us around the sun and as the arrow of time inexorably carries us away from our births and toward our deaths.
Perhaps you have to be a physicist to truly visualize space-time in full four-dimensional splendor being warped by a passing gravitational wave, but even children can grasp the concept surprisingly accurately by imagining a red-and-white-checked tablecloth spread for a summer picnic. The tablecloth itself represents space-time, compressed from four dimensions into two; indeed, the checks can be used to specify locations in good old-fashioned X/Y coordinates (“the potato salad is 20 checks east and 15 checks north of the edge of the picnic table”). Juice splatters that stained three white checks in one area are embedded in the fabric of the tablecloth.
Now watch carefully. If the tablecloth around the juice stains is pulled on the fabric’s bias, the square checks will elongate into diamond shapes, stretched in one direction and compressed in another. One pair of juice stains will move farther apart, and another pair will move closer together. The X/Y coordinates of all three juice stains on the tablecloth remain the same because the stains are firmly embedded in their checks; yet the distances between the three stains have changed because the tablecloth itself has been deformed.
That’s exactly what passing gravitational waves are believed to do: They do not disturb the placement of objects in four-dimensional space-time, but they change the distance between them by stretching space-time itself in one direction while compressing it in the perpendicular direction. That’s what Albert Einstein mathematically predicted in 1916 in his general theory of relativity.
Einstein’s theory grappled with changing gravitational fields, such as that of a massive star when it explodes and throws off most of its mass. Centuries earlier, Galileo, Kepler, and Newton had all derived equations that accurately described the behavior of gravity between ordinary objects and Earth or the mutual interaction among suns and planets in space—the equivalent of ants crawling across the checked tablecloth. But Einstein wondered exactly how objects could “sense” changes in other objects’ positions or masses across the vacuum of space. So he invented a new concept of gravity. He realized that gravity could be explained as a curvature of space-time.
Mathematic calculations show that a single object drifting in a straight line at an unchanging velocity would remain embedded in space-time, sitting at the bottom of its gravitational well, its gravitational field a static force. But an object accelerating—exploding or rotating asymmetrically—or two objects revolving around each other would cause disturbances in space-time, or gravitational waves, which would propagate outward in all directions. The more massive the object(s) and the faster the motions, the greater the deformation of space-time, and the stronger the disturbances. And if the right kinds of instruments could be built, those gravitational waves should be detectable.
Physicists do. An unambiguous sign of gravitational waves would confirm the speed and characteristics of such waves as predicted by Einstein’s general theory of relativity, which undergirds all modern physics of the very massive and the very fast.
And astronomers do. Gravitational waves carry information about extreme astronomical processes now unknowable any other way.
“All the light we see from an [exploding] star is just from individual atoms in its outer layers,” explains Lee Samuel Finn, director of the Center for Gravitational Wave Physics at Pennsylvania State University in State College. “We can’t peer into its thermonuclear engine. But gravitational waves come from its bulk matter, traveling through the outer layers without scattering, extinction, or reddening, letting us directly see the collapse of the stellar core.”
Gravitational disturbances, like light and sound, move in waves with characteristics like frequency, wavelength, and strength that can vary over time. In fact, one type of detector is trying to convert gravitational vibrations into ordinary sound.
Hum a Few Bars
“We’re like deaf people, watching other people’s lips move and trees fall. We suspect there is sound, but we have never heard it, and can only guess how to build something that can detect its vibrations,” explains Michael E. Zucker, a gravitational wave physicist who splits his time between the Massachusetts Institute of Technology and a gravitational wave detector outside Baton Rouge, Louisiana.
Among gravitational wave scientists, auditory analogies abound, and the analogies are apt. Gravitational waves are expected to arrive here at all frequencies and from all directions in space. Just as following a single conversation in a large cocktail party requires a listener to reject the background chatter and the clinking of glass and silver, the challenge of locating an individual source of a gravitational wave requires determining direction with two “ears,” plus filtering out noise, including noise at the same frequency as the desired signal.
The oldest way of trying to detect gravitational waves is literally listening for them, using solid aluminum bars that function essentially as giant tuning forks. As you can make a tuning fork held near your mouth hum faintly if you sing at its resonant frequency, acoustic bar detectors are expected to hum when struck by a gravitational wave of the bar’s resonant frequency. One of the largest is ALLEGRO, which stands for A Louisiana Low-temperature Experiment and GRavitational wave Observatory. Designed and built by Louisiana State University’s William O. Hamilton (professor emeritus of physics) and Warren Johnson, it is a huge aluminum tank on the ground floor of the Physics and Astronomy Building on the campus in Baton Rouge. The tank, its vacuum pump periodically emitting a stream of ticking burbles, is surrounded with all manner of pipes, hoses, and other structures. Its external cylinder is basically a giant Thermos bottle, insulating a smaller chamber cooled to the temperature of liquid nitrogen (77 degrees Kelvin); that first chamber in turn insulates a second, chilled to the temperature of liquid helium, a mere 4.2 degrees K above absolute zero (the lowest temperature possible in nature).
Deep inside the frigid, dark inner chamber, in as rarefied a vacuum as is possible to produce in the atmosphere, is a two-and-a-half-ton solid cylinder of aluminum alloy suspended by a single titanium-alloy wire so that it hangs in exact balance. The bar’s size, the properties of its aluminum, and the precision with which it was manufactured all contribute to its sensitivity and bandwidth: It is able to “hear” in two narrow bands near the resonant frequency of 900 Hertz. Should a gravitational wave of those frequencies pass through the bar, the wave should set the bar to ringing.
Theorists have predicted that certain classes of supernovae could produce gravitational waves at a frequency within ALLEGRO’s bandwidth. Because the tone would be so extraordinarily faint (and inaudible anyway in a vacuum), delicate accelerometers are affixed to the bar’s ends to sense minute accelerations of the bar produced by the vibration. The vacuum eliminates air molecules, whose bouncing off the metal surface might otherwise damp the faint vibration, and the extreme cold quiets the thermal jiggling of the bar’s own molecules.
ALLEGRO has been listening for gravitational waves almost continuously since 1991. Because no scientist would believe any pulse to be a real gravitational wave unless it were registered nearly simultaneously by another detector of at least equal sensitivity, ALLEGRO has been collaborating with four other acoustic bar detectors in the United States and Europe.
So far, no pulse has been definitively proven to be due to a gravitational wave, but neither Hamilton nor Johnson is discouraged, primarily because astronomers now realize that the higher frequencies are likely to come from comparatively low-mass and infrequent astronomical events within our own galaxy. One hoped-for signal is a crescendoing and rising-pitch glissando from pairs of nearby neutron stars locked in an inward death spiral until they abruptly coalesce into a stellar-mass black hole, giving off one urgent accelerating chirp. That final death chirp is calculated to be brief, lasting maybe two minutes at most as it rises through ALLEGRO’s narrow range of resonant frequencies. Says Johnson: “The gravitational chirp of this in-spiral event, if it were converted to sound waves, would sound like a big, low-pitched bird.” The waves could be quite weak, depending on distance, but statistical calculations show that each year about a dozen pairs of neutron stars coalesce into black holes within “shouting” distance of Earth. Rarer still—maybe only three times a century in our galaxy—would be the scream of a massive star ending its life in a catastrophic supernova explosion. So, counting on luck as much as attention to detail, Hamilton and Johnson and ALLEGRO keep a patient vigil.
The Light Fantastic
Mirrors and lasers are the heart of a wholly different type of gravitational wave detector, which this fall will begin to record data at full sensitivity. This is the Laser Interferometer Gravitational-wave Observatory, or LIGO (pronounced LYE-go), its twin L-shaped detectors separated by more than 1,800 miles: one in the forests of Livingston, Louisiana, and the other in the desert of Hanford, Washington.
“LIGO is the biggest hole in the atmosphere ever built,” quips LIGO-Livingston director Mike Zucker. Each LIGO facility consists of a pair of vacuum chambers, their ends meeting at right angles. Each chamber is monumental, measuring four feet in diameter and two and a half miles long.
LIGO does not listen for gravitational waves in the same way the acoustic bar detectors do. Its purpose is to directly measure the degree to which passing gravitational waves momentarily deform space-time itself. “General relativity predicts [a passing wave] will lengthen one arm and compress the other,” says Rainer Weiss, emeritus physics professor at MIT. So if the two distant LIGO sites independently detected a tell-tale pattern of deflections nearly simultaneously, scientists would feel confident that they had observed a gravitational wave pass through Earth—and that, moreover, its measured behavior matched Einstein’s prediction.
But what a measurement! The deflection of space-time is so minuscule that over the 2.5-mile lengths of LIGO’s perpendicular arms—the arms at each site usefully if unoriginally dubbed X and Y—the scientists are preparing to measure deflections amounting to 10-16 centimeter, a thousandth the diameter of a sub-atomic neutron or proton.
Such a precise measurement presses science and engineering to the ragged edge of the possible. “Half of our technology is devoted to being able to detect a signal. The other half is devoted to identifying and eliminating sources of noise,” Zucker says.
To detect a signal, LIGO operates with elegant simplicity: At the junction of the arms, the input beam of an infrared laser strikes a beam-splitter—essentially a half-reflective mirror—which directs half the beam down the length of vacuum in each arm. At the end of each arm, a mirror reflects the laser light back to the apex, where (after some 100 reflections back and forth) both split beams are recombined. Now here’s the clever trick. The lengths of the arms are very slightly different, so the recombining laser beams will interfere destructively: The crests of the light waves in the laser beam returning from its trip down the X axis will cancel the troughs of the light waves returning from the Y axis. Thus, in the absence of gravitational waves, no light should reach the ultimate photo-detector. But should a passing gravitational wave distort space-time as Einstein predicted—and thus alter the relative lengths of LIGO’s perpendicular X and Y arms—the recombining beams should interfere constructively: Light wave crests should fall on crests, troughs on troughs, light should shine on the ultimate photo-detector, and physicists the world over should dance.
Problem is, the living world is replete with sources of noise, most of which could distort the lengths of LIGO’s arms by degrees far greater than the anticipated signal.
Daytime-warming expansions and nighttime-cooling contractions cause tiny but measurable differences in the detector, as do the pounding of ocean waves on distant beaches, the hum from 60-Hertz power lines, and the thumping from tree farms right around the Livingston LIGO site, where mighty growling machines chop soft pines for paper. Thus the mirrors within the LIGO arms are suspended as pendulums from a heroic arrangement of springs and masses that damp seismic vibrations; recently, hydraulic actuators and electronic controls were added to actively counter seismic disturbances.
The twin LIGO detectors are sensitive to a wide range of frequencies, bracketing those detectable by the highly tuned ALLEGRO and other acoustic bar detectors: “from about 50 Hertz—an octave above the lowest note on a piano—to 10,000 Hertz, about that of the squeak of a mouse,” says Weiss. And the LIGO detectors are not alone. Somewhat smaller versions are operating in Germany, Italy, and Japan. In addition to searching for signals from supernovae, astronomers hope they can capture the entire glissando accelerating up to the death chirp of binary neutron stars coalescing into black holes. LIGO is so sensitive, in fact, that eventually it should detect supernova explosions, in-spiraling neutron stars, and black holes swallowing gases (and burping) all the way out to the Virgo Cluster, some 45 million light-years away. “We’re already within tasting distance of this!” Weiss exclaims.
Gravitational astronomers’ dearest hopes, however, lie on drawing boards. NASA and the European Space Agency are planning the Laser Interferometer Space Antenna, a constellation of three spacecraft that will orbit the sun in formation, 20 degrees behind Earth. When completed and launched in 2014, LISA will be the largest spaceborne instrument ever built.
Like LIGO, LISA would operate as an interferometer, but instead of being L-shaped with split beams recombined at the apex, LISA’s three spacecraft will form an equilateral triangle, all three spacecraft sending beams that travel in both directions along each side and are reflected back by small free-floating test masses. Instead of being two and a half miles long, each of LISA’s arms will be a little more than three million miles long—so long that the laser beam will need more than 16 seconds to travel its length.
Why such long arms? Signal. LISA is being designed to be sensitive to frequencies from below 0.1 Hertz down to 0.0001 Hertz—frequencies with wavelengths so long that the detector must be extremely large in order to sense them. “In that frequency range, the universe is doing a lot of big, exciting, violent stuff,” says Robin “Tuck” Stebbins, the U.S. LISA project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. The number of sources giving off gravitational radiation at such long wavelengths is expected to be so huge that investigators worry about a “confusion limit,” where only the loudest sources can be separated from the combined din. LISA’s size will make it so much more sensitive than ALLEGRO and LIGO that Stebbins says, “If LISA doesn’t see thousands of signals at turn-on, it’s broken.”
Astronomers have seen indirect evidence of gravitational waves, most recently in May, when NASA’s Chandra X-ray Observatory measured the orbital period of two white dwarf stars circling each other. Einstein’s theory predicts that massive in-spiraling stars will shed energy as gravitational waves and that, as the system loses energy, the two stars will move closer together. Although the Chandra observations confirm the prediction—the orbital period of the stars is decreasing, so they are drawing closer to each other—the cause of that behavior remains unproved. And it will remain so until LISA, operating in space, senses a movement in its tiny test masses of a half-billionth of an inch, the subtle shiver of a gravitational wave passing by.