Gravity’s Field of Dreams
Across the tapestry of the night sky, hundreds or perhaps thousands of stars are doing frantic dances of death, spinning wildly around each other and shooting off waves of invisible gravitational energy like interstellar beacons.
In one of the most exotic observatories in the world, Fred Raab is waiting for those waves to wash up on the shoreline of Earth. When they do, they could change our understanding of the universe.
“We’ve spent 400 years since the invention of the telescope looking at a small portion of what exists,” said Raab, head of the LIGO laboratory in the high desert of southeastern Washington.
LIGO -- the Laser Interferometer Gravitational-Wave Observatory -- could reveal the rest.
“This gives us an observational tool to probe the dark, strong-gravity part of the universe, which we’ve never really done,” said Kip S. Thorne, a Caltech physicist who is one of the world’s foremost experts on relativity.
Like the first bathysphere diving into deep-sea trenches, the $300-million LIGO project, conceived more than 25 years ago, is expected to uncover exotic creatures, such as dancing neutron stars and binary black holes, circling each other like heavyweight fighters. Physicists also may uncover the mysterious “dark matter” that is believed to be all around us but has never been measured. Some think they might find gateways into extra dimensions.
What makes LIGO different from other observatories is that it doesn’t “see” the cosmos by detecting electromagnetic energy in the form of light, radio waves or X-rays. It feels it, measuring waves of gravity that wrinkle space-time like ripples on a lake.
One advantage to gravity-wave science over light-wave science is that whereas light bounces off solid objects, gravity waves go through everything -- planets, stars, people’s bodies.
Raab, Thorne and about 500 other scientists around the world caught up in the race to measure the first gravity waves are essentially giving birth to a new science.
It has been gestating 90 years, since Einstein theorized that large bodies moving through space would give off waves of gravity, traveling at light speed, that would shrink and expand space-time itself.
The problem with gravity waves is that they are so difficult to detect that many physicists long doubted they would ever be found. In November, however, LIGO reached a level of sensitivity at which Thorne and other experts believe they might detect waves.
Now excitement has gripped the scientific community as it awaits word.
It can be felt inside the LIGO control room, where Raab studies a series of constantly changing graphs flashed up on the wall. Like a man translating a foreign language, Raab points to one squiggly line that he says is traffic passing on the main road a dozen miles away. Another is construction in the nearby cities of Richland and Kennewick.
If you know what to look for, Raab said, you can pick out the seismic signature of ocean waves hitting the shoreline of western Washington -- 200 miles away.
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Imagine you are an astronaut and you’ve volunteered, against all sensible medical advice, to expose yourself close-up to gravity waves. What would happen to you?
Presuming that you could somehow survive the violent cataclysms that spawn such waves, you would first be stretched to twice your size, then shrunk to half your size, before you came back to your ordinary height.
A spaceship or planet close enough would be similarly stretched and squeezed by the passing wave.
These effects were predicted by Einstein’s theory of relativity, which gave the world an entirely new view of gravity. To Isaac Newton, gravity was a mysterious sucking force between planets and stars.
Einstein said gravity was nothing more than a warping or bending of space and time by large objects. Space to Einstein was not a nothingness but a “fabric” that could be stretched and shaped.
Just as a bowling ball causes an indentation on a trampoline, a planet or star dents the space around it. As the body moves through space, it gives off waves of gravity, as a spoon stirring milk gives off ripples.
The reason we walk around without being constantly stretched out of shape like rubber-band people is that most gravity waves are far too weak. By the time gravity waves reach Earth, even those thrown off by supernovas produce only tiny wrinkles in our world.
So tiny, in fact, that two neutron stars whirling around each other in some distant part of the universe would stretch our world out of shape by a bit more than 10-15 meters -- about the size of an atomic nucleus.
Even so, cosmologists think there is some pretty convincing evidence of the waves’ existence.
It came from a University of Massachusetts graduate student named Russell A. Hulse, who was hired in 1973 to scan the heavens for a then-new type of object known as a pulsar. Pulsars, it turned out, were neutron stars, collapsed stars that had exploded and blown off most of their mass, leaving a solid mass of neutrons the size of a large city. The stars pulse with radiation emitted at the poles as they spin at tremendous speed, creating an unmistakable signature of electromagnetic radiation.
Their pulses are so regular that the first one was labeled LGM-1, meaning Little Green Men, because they were thought to be transmissions from an alien civilization.
One of the pulsars Hulse found was unusual. PSR 1913 + 16, which is 16,000 light-years away, seemed to be changing its pulse rate. At first, Hulse was irritated. Then, with a flash of insight, he realized the changing pulses were the result of two neutron stars circling each other. Each year, their orbit got shorter by 75-millionths of a second. In 240 million years, they would collide.
That shrinking orbit was due to a small loss of energy, which it turned out was exactly what was predicted by general relativity if the loss was caused by the emission of gravity waves.
It was a eureka moment that earned Nobel prizes for Hulse and his colleague, Joseph H. Taylor Jr., a University of Massachusetts astronomer.
Einstein believed gravity waves would never be directly measured. He couldn’t envision a detector that could tease out something so tiny from all the other things that pound Earth, from ocean waves to miners.
“To think about measuring something that small -- people said it would be crazy,” said Stan Whitcomb, a Caltech physicist who helped design LIGO.
For years, nobody could see the point of it, either. This was before anyone guessed the universe was filled with things like black holes, dark matter and other strange beasts that could not be seen with a telescope.
Now, Thorne said, it’s common knowledge that much, maybe most, of the universe lies hidden from us.
But not, maybe, from a gravity-wave detector.
“We could see things made from warped space-time,” he said. “Things we’ve never really explored.”
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In the dun-colored desert-scape of southeastern Washington sits the Hanford nuclear site. Plutonium for the atomic bomb dropped on Nagasaki was made here. Now, the signs of decay and rust are everywhere. The site has become a relic of the Cold War.
Down a twisting side road, LIGO appears out of the Russian cheatgrass and mustard plants, a bulky apparition with two tubes extending at right angles into the desert.
The 2.4-mile-long tentacles are the heart of LIGO. They are at right angles so that incoming gravity waves will shrink one arm while lengthening the other. An identical facility sits in a forest in southern Louisiana, so that the readings made at one observatory can be cross-checked almost 2,000 miles away.
The National Science Foundation has provided the funding. LIGO is its biggest science project ever.
“The NSF has taken a huge gamble with this,” Whitcomb said.
Inside the arms is a laser interferometer, which works by splitting a laser beam and sending one of the two resulting beams down each arm. The beams then bounce around 100 times on a set of mirrors before being sent back to a photodetector.
The two beams should recombine at exactly the same time since they travel an identical distance.
But if a gravity wave passes by, the beams will be thrown off as the arms are alternately stretched and squeezed.
Detecting such a minute signal has required extraordinary steps.
Because the site had to be as flat as possible, satellites were used to survey the land, which was eventually graded to within three-eighths of an inch over five miles.
To get around the problem of air molecules shaking the mirrors, workers sucked the air out of the tubes down to a billionth of an atmosphere. But that still wasn’t good enough to make sure the speed of light would be constant throughout the tubes. So the team had to get the tubes down to a trillionth of an atmosphere.
The surface of the four 10-inch mirrors in the arms is so smooth it doesn’t vary by more than 30-billionths of an inch. Thirty control systems keep the lasers and mirrors in alignment. The vibration isolation system is so sophisticated, the only thing approaching it is the mechanics used by semiconductor chip makers to etch circuits on the chips.
Even though ground was broken for the LIGO project more than a decade ago, it was only in November that the facility was ready to hunt seriously for gravity waves.
“We’re operating right now where we can see changes a thousandth the size of a proton,” Raab said.
Some vibrations still manage to get through.
“A bulldozer 10 miles away knocks us offline,” he said.
One recent problem was caused by a stunt pilot practicing loops.
Since the November data run began, LIGO has managed to get 10 weeks of clean data.
The hunt is on.
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On the wall outside Thorne’s cluttered office at Caltech are framed letters containing the bets he has made with other prominent scientists, including two with physicist Stephen Hawking. Thorne won both.
In fact, Thorne has lost only two bets, and both were over gravity waves. In 1978, he bet a dinner that gravity waves would be found within a decade. It didn’t happen.
The second time, he bet a case of good California wine that the first gravity wave would be detected by Jan. 1, 2000. Once again, he had to pay up.
Thorne is no longer taking bets on when gravity waves will be found. But found they will be, he said.
It just might not be with this version of LIGO. Even though LIGO is operating within the range where gravity waves are thought to exist, it’s just barely there.
“We’re at a level now where we could see one every 30 years to every three years,” said Jay Marx, executive director of the LIGO program.
Those aren’t great odds. The solution is Advanced LIGO, a $200-million upgrade that will increase the sensitivity by a factor of 10. Among the improvements are a more powerful laser and more sophisticated vibration isolation hardware. Work is expected to begin sometime after 2008.
After the improvements, a gravity wave could be detected every three weeks, Marx said.
Thorne said: “We are at a level where we could see waves now. After the upgrade we will be operating in a domain where we are likely to see waves.”
And if they don’t find waves?
“That would show something is wrong with our understanding of the universe,” he said.
LIGO is operating so smoothly it has exceeded expectations. “We’re pulling in 16,384 samples a second,” Raab said. “We produce a terabyte of information every day.”
But even at that rate, it could take years for success.
Some gravity-wave scientists have by now spent their working lives waiting for the moment of discovery. Raab has been at it 18 years. He admits to some anxiety over the fact that his long-sought quarry is finally in sight.
“The older you get, the more you want to see a result while you’re around to see it,” said Raab.
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(BEGIN TEXT OF INFOBOX)
Hunting for gravitational waves
Scientists from around the world are racing to detect the mysterious gravitational waves hypothesized by Albert Einstein 90 years ago.
Warped space: According to Einstein’s General Theory of Relativity, great cataclysms in the universe should create measureable gravitational waves. The image below is a computer simulation of gravitational waves created by the merging of two black holes.
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Laser Interferometer Gravitational-Wave Observatory (LIGO)
The interferometer is made up of two arms at right angles to one another, each measuring 2.4 miles in length. When a gravitational wave impacts the interferometer, one arm’s light beam will be stretched and the other’s shortened.
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How it works:
1. Laser emits a beam of light into the beam splitter, sending a beam into each light storage arm.
2. Light beam travels the length of the arm in 13 microseconds. The beam bounces between the end and near mirrors 100 times to increase the distance traveled, making it easier to measure any change in length.
3. Light beams recombine at the photodetector. Any change in length will cause the light beams to arrive at slightly different times, indicating the presence of a gravitational wave.
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Sources: Laser Interferometer Gravitational-Wave Observatory, American Museum of Natural History, Jay Marx, CalTech
