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Illuminating gravity

Sun., Nov. 5, 2006

RICHLAND – The most sensitive instrument in the world lies in a vacuum tube that spans miles of Hanford desert.

It can detect dust-caused electrical disturbances hundreds of miles away. It can sense the effect of the wind on the earth. A footfall on its 30-inch-thick concrete floor would throw the whole thing off.

But the LIGO project still hasn’t found what it’s looking for: minute ripples in the material of the universe – gravity waves hundreds of millions of times smaller than an atom, left over from cataclysmic events such as the collapse of orbiting stars. If the waves are discovered, it would dramatically expand the way scientists gather information about the universe. It may also provide evidence of some of the more fantastic-sounding elements of modern theoretical physics, such as the existence of additional dimensions.

“What we’re trying to do is create a radically new way of doing astronomy,” said Fred Raab, head of operations at the LIGO Hanford Observatory, about 13 miles outside of Richland on the Hanford site. “When the first detections are made, that will be a development as big as the microscope or telescope.”

Through the National Science Foundation, the federal government has spent roughly $500 million on LIGO, which stands for Laser Interferometer Gravitational-Wave Observatory. And it expects to spend hundreds of millions more in upcoming years to improve the system’s sensitivity even further. The project is a collaboration between California Institute of Technology and Massachusetts Institute of Technology, but it also includes more than 500 scientists worldwide.

The high cost and uncertain prospects of the venture have made for some controversy, especially among some who believe the half-billion dollars spent on LIGO could be applied toward more practical or concrete purposes. An article in American Scientist last year said, “Given presently accepted estimates of the strengths and frequencies of events that cause the waves, there is a reasonable chance that it will see nothing.”

Still, another piece in the magazine emphasized that many scientists believe “the next few years should see the emergence of a worldwide network of instruments” to detect gravity waves. If the current LIGO doesn’t find them, a multimillion-dollar upgrade planned to begin in 2011, which will increase the system’s sensitivity tenfold, should improve the chances.

Raab is hopeful that LIGO will detect a gravity wave before the major upgrade begins in five years. He acknowledges that there’s a great deal of uncertainty – the system is essentially surveying a portion of space on the chance that there was the right kind of event within that area to make the ripples detectable. And given the vast computations required to try to read the vibrations, and the number of scientists involved, it will take a while no matter what to examine and try to come to agreement on the data.

“Detecting gravity waves will be a pretty extraordinary event,” Raab said. “We say in science that extraordinary events require extraordinary proof.”

A warp for the apple

In the most well-known – though probably false – story about the discovery of gravity, an apple knocks Sir Isaac Newton on the head and sets loose the notion of the universal force.

But how does it work? Does some force attract the apple?

Ninety years ago, Albert Einstein proposed a rethinking of gravity. In his special theory of relativity, he predicted that rather than operating as a force – such as magnetism – gravity is a warping of space and time itself. Space is another dimension like the three physical dimensions, and massive objects create warps in this space-time that affect the paths of other objects. It’s somewhat similar to a bowling ball set on a trampoline. If you try to roll a tennis ball past it, the warp of the trampoline pulls in the tennis ball.

“Gravity is space,” Raab said. “Gravity is telling us what space is.”

Gravity waves are tiny remnants of such warping of space. If scientists could actually begin to gather information about the universe from them, it would be a new way of “seeing” the universe. It would be the first time humans have gathered information that did not come from some form of electromagnetism – visible light or other light-type waves above and below the visible spectrum, such as ultraviolet light or X-rays.

“Everything we know about the universe over all the time we’ve studied it going back to ancient times has been carried to us by some form of light” or electromagnetism, he said.

If we started picking up gravity waves, “it’s truly like getting a new sense.”

Raab said that systems such as LIGO could eventually be designed to look for specific types of gravity waves – those emanating from the creation of black holes or those remaining from the Big Bang. Such detections could provide information about the time between the Big Bang and the formation of light 400,000 years later – a time about which little is known.

“What about all that time before? One way to look earlier in time is to pick up the space warps given off by the Big Bang,” he said. “It’s a big quest, and it’s a quest that may go on for decades.”

Gravity waves would also open up the possibilities of learning more about some of the more outrageous-sounding predictions of modern physics. One such possibility is the idea of extra dimensions, something that is predicted by modern string theories.

Raab said that it’s possible that light and other forms of electromagnetism – from which we have all the knowledge of our world – may only travel in certain dimensions. But if gravity actually is space itself, it might reveal other dimensions that we can’t even imagine. Right now, such theories are entirely mathematical.

“But once upon a time, general relativity was just mathematics, too,” he said.

In the cave

The LIGO system – known as an interferometer – uses a big laser and a complex series of vacuum tubes and mirrors to detect tiny vibrations. The interferometer includes two 2.5-mile tunnels that meet in an L-shape, and a wide range of buffers and other means to reduce all other “noise” – everything from the rumble of passing trucks to the ripples from construction work miles away.

A flashlight-sized laser beam is split and sent down each arm, where each beam follows an identical pattern of reflections through finely tuned mirrors. The mirrors are essentially marking a position in space, and the laser is measuring those positions. When the beams return to the splitter, they should cancel each other if the mirrors haven’t been disturbed; if they have been, tiny portions of light escape where the beams don’t line up.

Scientists must then analyze and sort those disturbances, monitoring all kinds of noise in the physical environment. If a gravity wave shows up, the warped nature of the wave would make one laser arm longer than the other. LIGO would also confirm its presence at its twin facilities in Louisiana that are taking the measurements.

The waves are so tiny that it takes a rare, cataclysmic event like the creation of a black hole to bend space enough to register. LIGO is hoping that its reach of hundreds of millions of light years is enough to bring in such an event.

Planning and design work for LIGO began a decade ago, and one year ago the project started its longest “science run” to date: an attempt to gather a year’s worth of data, which will actually take about two years of observations since the system doesn’t operate continuously. The same process is happening at LIGO’s Louisiana project, set up for purposes of comparison and confirmation.

But what’s the practical payoff? The research at LIGO is “basic research” – the discovery of knowledge for its own sake and to help build future knowledge. Raab said with a lot of scientific knowledge, the daily-life applications, if there are any, often don’t materialize for years. He noted that GPS systems are a very practical application of Einstein’s theories of relativity that weren’t created for decades after the scientific work began.

“It’s the part of science where you create the new science for the next century,” he said.

Most physicists seem convinced that gravity waves exist. Some astrological discoveries have strongly suggested their existence, but there’s been no direct observation so far. Asking when that might happen “is like asking the question, what’s in the unexplored sections of the cave?” Raab said.

“It will be within 24 hours of the biggest party in my life.”


 

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