Catching Einstein's Waves
Astronomers have been observing the skies in essentially the same way for nearly 400 years. Since Galileo turned his telescopes on the moon in 1609, they’ve used ever more sophisticated means of detecting light emitted by distant objects, gathering up not just visible light but radio waves, x-rays, and other forms of electromagnetic radiation. But some of the most exciting things in space don’t send light our way.
The ability to observe the universe using something other than light could usher in a new era of astronomy–an era that MIT physicists believe will soon be upon us. Their hopes hinge on detecting gravitational waves, a kind of fundamental radiation predicted by Einstein in 1916 but not yet directly observed. Analyzing gravitational waves, they believe, will provide an unprecedented way to study the activity of spiraling neutron stars, black holes, and the cores of collapsing stars.
Not only are gravitational waves predicted by Einstein’s theory of relativity, but the predictions are supported by indirect empirical evidence, such as changes in the orbits of binary neutron stars that scientists have observed for decades. Before astronomers can analyze gravitational waves from distant objects, however, experimental physicists must detect the waves directly.
A team of researchers from MIT and Caltech can’t resist the challenge. They jointly operate the Laser Interferometer Gravitational-Wave Observatory (LIGO), which is funded by the National Science Foundation at sites in Washington state and Louisiana. Built in the 1990s, LIGO is based on designs developed in the 1970s by Rainer Weiss ‘55, PhD ‘62, who is now professor emeritus of physics at MIT. By detecting changes in distance between finely calibrated mirrors, LIGO’s instruments measure tiny distortions in the fabric of space-time. With the first generation of detectors, which cost $230 million, the probability of detecting a gravitational wave was low. But LIGO physicists think they now have the technology to turn the corner. With upgrades in the works, they hope to achieve a tenfold increase in sensitivity–enough, they believe, to detect gravitational waves a few times a month by 2014.
These physicists relish tackling one of the most difficult precision engineering problems in the world: measuring distances smaller than the smallest atomic nuclei. “It’s really hard. And that has a huge attraction,” says Nergis Mavalvala, PhD ‘97, an associate professor of physics who’s designing upgraded instruments for LIGO. But even more exciting than the challenge is the potential payoff. “Gravity-wave physics is going to change the way we see the universe,” says associate professor of physics Erik Katsavounidis, who’s analyzing LIGO data. “There are few things in that class.”
From Newton to Einstein
Newton described gravity as a force of attraction between masses. But even Newton was dissatisfied with his inability to explain what causes gravity. That was left to Einstein, who explained how gravity arises by describing space itself in a new way.
Before Einstein, space was thought to be absolute, existing outside the influence of the masses moving within it. Einstein conceived of space as malleable and proposed that time and space were part of a four-dimensional system. Masses like the sun distort the fabric of space-time, causing what physicists call space-time curvature. This curvature affects the motion of other masses: in other words, it is what we experience as gravity.
As masses accelerate, Einstein proposed, they cause ripples in space-time like the waves that travel through water in the wake of a boat. These space-time wakes are what he called gravitational waves. Like light and sound waves, they’re described in terms of frequency and wavelength. In Newton’s equations, the force of gravity is exerted instantaneously, an approximation that works beautifully for most of what we can observe. But gravitational waves propagate through space-time at the speed of light. So any change in the gravitational pull of a distant object takes time to reach Earth, just as its light does. All this “takes a while to wrap your head around,” admits Scott Hughes, an assistant professor of physics.
“As gravitational waves travel, they stretch and squeeze space-time,” says Mavalvala. If one passed through you right now, you might get a little taller, then a little shorter; a little wider, then a little thinner. Similarly, if a gravitational wave passes through two objects, the distance between them varies. LIGO is designed to measure this effect.
It so happens that the things LIGO will be good at detecting, such as black holes colliding with neutron stars, can’t be seen well with telescopes. But while such phenomena are interesting, the real value of studying them, says Hughes, is that they will let us test the laws of physics in areas of the universe very unlike our own.
Newton’s laws work well in our solar system, where gravity is weak. But near black holes, says Rainer Weiss, “space is so strongly gravitating that it’s not flat anymore; it’s curled up on itself in horrible ways.” With LIGO, he says, “we’re going to be seeing things from regions in the universe where Einstein is the whole story. Newton you can forget about.” LIGO, physicists hope, will open up what Hughes calls an extreme laboratory. “Measuring gravitational waves will give us insight into the deepest nature of space and time,” says Edward Bertschinger, head of MIT’s physics department. “Until we thoroughly study them, we haven’t understood gravity.”
It took a long time, however, for the physics establishment to believe that technologies designed to measure gravitational waves were worth investing in. Weiss, who has spent his entire career at MIT, played an important role in turning the tide.
The origins of LIGO
Even Einstein recognized that gravitational waves would be difficult, if not impossible, to measure. Although he believed they really existed, in the 1930s physicists began to think of gravitational waves as mathematical curiosities. And with no way to test his ideas about them, Einstein himself backed down from his earlier claims.
But in 1960, a man whom Weiss calls “courageous” and “an imaginative nut” decided to try to measure gravitational waves. Joseph Weber, a professor at the University of Maryland, built a detector that worked something like a metal xylophone bar; but instead of vibrating when struck by a mallet, it would vibrate when struck by gravitational waves. Weiss says that Weber “saw all sorts of wonderful things” and claimed he’d detected what Einstein predicted.
The problem was, no one could duplicate his results, though Weber, who died in 2000, stuck by them. Weiss says that a more careful physicist would have been more skeptical of his own conclusions; he speculates that Weber’s machines may have detected such things as lightning strikes or problematic phone lines, but Weber didn’t investigate other possible explanations for his data. Without independent confirmation of Weber’s waves, “the field went into a terrible state,” Weiss says.
Weiss, no less imaginative than Weber, was also interested in gravitation from an experimental perspective. He had flunked out of MIT in the 1950s but was given another chance by the legendary physics professor Jerrold Zacharias, who hired him to work in his lab. After earning his PhD at MIT, Weiss spent time at Princeton University in the lab of Robert Dicke, a leading expert in gravitation.
Soon after he came back to MIT as a professor, Weiss was assigned to teach a graduate-level class on relativity. It was the spring of 1966, and Weber’s detector was up and running. “I couldn’t understand what Weber was doing,” Weiss recalls, so he decided to explain gravitational waves to his students by “devising the most simple-minded explanation of how you could detect one that I could imagine.” His idea was to use an interferometer, an L-shaped configuration of equally spaced mirrors that uses laser light to precisely measure distance. As gravitational waves pass objects, they stretch and compress space-time in such a way that the distances between those objects change. The greater the initial distance between two objects, the greater the change. The greater the change, the easier it is to measure.
Over the next few years, Weiss developed prototypes for what would become LIGO. Researchers at Caltech, as well as funders at the National Science Foundation, got behind his plans before the MIT administration did; the Institute was reluctant to dedicate money to what seemed like a risky proposition. But once Weiss’s collaboration with Caltech’s Kip Thorne and Ronald Drever was in place and funding was secured, MIT got on board.
Today’s LIGO is a set of huge interferometers, two of them with four-kilometer-long arms and a third with two-kilometer arms. More like an ear than an eye, LIGO will catch whatever waves it can “hear,” no matter what direction they come from. But Weiss hopes the technology will come to serve as a new kind of telescope.
At the corner of each L is a mirror that splits a beam of laser light in two and sends one beam down each arm, through vacuum-sealed stainless-steel tubing, to a mirror at the end. The mirrors reflect the beams back to the corner, where they recombine to create an interference pattern of bright and dark spots. This pattern stays the same as long as nothing’s moving. But if a gravitational wave passes through the interferometer, it subtly squeezes and stretches space, jostling the mirrors so that the pattern changes.
This sounds simple enough. But getting the noise out of the system is incredibly complex. “Everything else on the planet can move the mirrors more than gravitational waves can,” Mavalvala says, with very little exaggeration. She ticks off a short list of phenomena that can disturb LIGO: tectonic-plate movements, ocean waves, road traffic, subways, “even just the activity of people moving around.” Not even in the fabrication of microprocessors with very tiny features do things need to be held nearly as still as they do in LIGO’s detectors. “Only a visionary could look at the technical requirements of LIGO and be undaunted,” Bertschinger says.
LIGO scientists are analyzing data from the detectors’ first full-scale run, from November 2005 through October 2007. The sheer volume of data–the detectors generated about a gigabyte a day–presents a huge computational challenge. Even more difficult is deciding how to determine when a gravitational wave has been detected beyond any reasonable doubt. Given Weber’s legacy, Erik Katsavounidis says, “we want to be dead sure.”
Researchers think LIGO’s current technology should be able to detect violent cosmic events such as supernovas, as long as they aren’t too far. “For our galaxy, we have good sensitivity,” says Katsavounidis. Supernovas are rare, however; they’re thought to occur within LIGO’s range about once every 30 years. Until LIGO can see farther, he says, the best potential sources of detectable gravitational waves might be objects astrophysicists don’t even know about yet.
The Next Generation
Using the largest ultrahigh-vacuum system on Earth, LIGO can detect mirror displacements as tiny as 10-18 meters–a thousandth as big as the nucleus of the smallest atom. That sensitivity will double within about a year, thanks to improvements that Mavalvala likens to putting a turbo engine in a car. And the LIGO team is planning to replace the initial detectors altogether. Built for a project called Advanced LIGO, the new set of detectors will be 10 times as sensitive, increasing the volume of observable space a thousandfold.
In the LIGO team’s brightly lit lab of hangarlike proportions on the west side of MIT’s campus, Mavalvala ducks her baseball-hatted head under one of two 15-meter stainless-steel tubes. The tubes connect in an L shape to form a smaller-scale replica of the Louisiana and Washington interferometers. Nearby, an apparatus that appears to have been constructed from a giant Erector set sits on an elevated steel platform encased in plastic sheeting. Mavalvala explains how Advanced LIGO will make sure the platforms from which the mirrors are suspended stay very, very still.
When the apparatus is running, accelerometers on the platforms detect motion, and motors correct for it by moving the platform in the opposite direction. Each mirror will hang from its platform on a wire holding metal and glass weights. The resulting pendulum has a natural frequency lower than that of the gravitational waves. When the platform is shaken rapidly by seismic noise, the pendulums will buffer the motion of the mirrors, ensuring that the tiny motions due to gravitational waves will not be masked.
These enhancements will improve LIGO’s sensitivity to low-frequency gravitational waves. But when measuring displacements far smaller than an atom, researchers must also contend with different sources of noise that limit sensitivity in other ranges. In the intermediate range, LIGO is limited by thermal motion: atoms at temperatures above absolute zero jostle around. Thus, the metal atoms in the wires suspending LIGO’s mirrors introduce noise into the system. Advanced LIGO will use specially made fibers of bare glass, a less “lossy” material: the atoms move less, and less of their motion is transferred to the mirror.
At higher frequencies, light’s quantum properties are the problem. “When you make a measurement with light,” Mavalvala says, “you have to deal with the noise properties of the light itself.” Greater laser power means a better signal-to-noise ratio: Advanced LIGO’s laser will have 20 times the power of the current one.
With these improvements, “we should see something once a week,” says David Shoemaker, SM ‘80, the MIT senior research scientist who leads Advanced LIGO. “If we see nothing, there’s something wrong with general relativity.”
When a gravitational wave is first detected, “everyone’s going to have a raucous party,” says Scott Hughes. “Then after the hangovers are over, we’re going to say, ‘Okay, now what do we do?’”
By creating computer models of objects like black holes, Hughes is trying to figure out how to use gravitational waves to do astronomy. Since no light escapes from black holes, physicists have seen them only indirectly–say, by detecting x-rays that stars emit when they’re pulled inside one. But when black holes “eat” something, says Weiss, “they let out a very satisfied burp”–a gravitational wave. “Given the way that general relativity describes things, given the detectors as we’ve designed them,” says Hughes, “how precisely can I do things like measure the mass and spin of a black hole?”
The physicists asking such questions are stepping out into the unknown, and they can’t predict everything they might learn. But, says Bertschinger, “I want MIT to be part of that era–to participate in the feast of science to come.”
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