An Einstein theory still tantalizes

Scientists hope to prove existence of gravity waves

Science

April 22, 2005|By Michael Stroh | Michael Stroh,SUN STAFF

As the world marks the 50th anniversary of Albert Einstein's death this week, a team of physicists is madly chasing the ghost of one of his last great unproven ideas: gravity waves.

In his 1916 theory of general relativity, Einstein predicted that collapsing stars, colliding black holes and other cosmic train wrecks would unleash ripples of gravitational radiation through space at light speed.

Nine decades later, scientists are still trying to find them. Even Einstein wondered whether the subatomic flutters he predicted could ever be detected. But after three years of fine tuning and trial runs, a $365 million instrument called LIGO may soon prove him wrong.

Big enough to be seen from space, the Laser Interferometer Gravitational Wave Observatory consists of a pair of 2.5-mile-long vacuum tunnels radiating outward from a central laboratory in the shape of an "L."

Inside, an infrared laser is split, beamed down each arm and reflected back toward the beam splitter by a set of polished quartz mirrors. Without gravity waves to interfere, the returning beams merge and cancel each other out. A photodetector stationed nearby sees only darkness.

But if a gravity wave passes, Einstein's theory predicts it should stretch one tunnel and squeeze the other, throwing the returning beams slightly out of sync. The result: a telltale flash of light leaking into the waiting photodetector.

Sounds straightforward - or as straightforward as these things get. But Barry Barish, the Cal Tech physicist leading the massive international project, says there's a reason why gravity waves have proven so elusive. "This is a very tricky business," he says.

First, consider that gravity waves would stretch or squeeze the LIGO lasers by less than the diameter of an atomic nucleus. That alone requires the detector to be one of the most precise scientific instruments ever constructed, says project scientist Bruce Allen. "Anytime you're measuring something to 20 decimal places, you're pretty cutting-edge."

To ensure accuracy, scientists have ordered two almost-identical instruments built 2,000 miles apart - one in Hanford, Wash., the other in Livingston, La. If a reading is real, it should appear in the data pile of both detectors.

Twin detectors should also help rule out false alarms. Since the instruments went online in 2002, scientists have spent countless hours ferreting out and documenting vibrations and other Earthly sources of interference that can throw off readings.

In Hanford, for example, whipping winds and rumbling traffic on a nearby highway can jar the sensitive lasers. Even surf pounding the Pacific Coast 300 miles to the west has shown up in the readings. Scientists in Louisiana, meanwhile, have been bedeviled by passing trains and falling timber in the heavily logged forests surrounding the detector.

Some project scientists have even been warned not to slam car doors or accelerate too quickly when they leave the LIGO sites. "If you go in and jump up and down near the detector, it can cause problems," says Barish. "So we don't do that."

The LIGO scientists have other reasons to be cautious. "The field," says Allen, "has a very checkered history."

Most gravity wave hunters know the story of Joseph Weber, the respected University of Maryland physicist who embarked on the first attempt to find them 40 years ago, using a set of suspended aluminum bars as detectors. In 1969, Weber claimed success.

But excitement quickly gave way to skepticism when other scientists couldn't duplicate his results. Over the years, others also have claimed to have found gravity waves, only to have their hopes dashed.

"The worst thing is to lose credibility," says Barish. "So we're being extremely cautious."

Still, most scientists believe the waves are waiting to be found. One reason for optimism: researchers already have indirect evidence of their existence.

In the mid-1970s astronomers Joseph Taylor and Russell Hulse discovered a pair of neutron stars spiraling toward collision. The stars' behavior neatly matched what Einstein predicted should happen if they were radiating gravity waves. The pair earned a 1993 Nobel Prize for their work.

This fall, LIGO scientists hope to conduct their first serious, around-the-clock search for gravity waves. But getting accurate measurements isn't the only challenge scientists face. The other will be sifting through oceans of data on the project's 2,500 computers to spot a wave's subtle, fleeting signature.

In February, the LIGO team launched a distributed computing project called Einstein@Home to enlist the help of outsiders. Modeled after SETI@Home, the popular search for extraterrestrial life, Einstein@Home parcels out detector data to volunteers who have downloaded a special screen saver on their home or office computer.

The software is designed to sift data for gravity waves when a machine is idle. Already, more than 50,000 people have signed up. (Visit http://einstein.phys.uwm.edu for information.) After a preliminary run, LIGO scientists estimate Einstein@Home has slashed crunching time from years to months.

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