-Caveat Lector-
http://seattlepi.nwsource.com/local/72187_gravity28.shtml
Catching a cosmic wave of gravity
Measuring teensy tremor could make big bang in physics
Tuesday, May 28, 2002
By TOM PAULSON
SEATTLE POST-INTELLIGENCER REPORTER
RICHLAND -- For the two dozen or so scientists and engineers gathered in the
shrub-steppe desert east of Rattlesnake Mountain, south of the Hanford
nuclear works, the task has been like trying to build a telescope from
scratch on the wildly pitching deck of a ship in rough seas.
Except that it's not really a telescope so much as a unique device created
to observe the cosmic signature of something scientists long considered
impossible to detect -- if they believed in the phenomena at all.
Gravity waves.
Albert Einstein predicted their discovery in 1916 as part of his general
theory of relativity, but even he had occasional doubts. It wasn't until
recently, with the development of several new technologies, that it was
considered remotely possible to look for evidence of these space tremors.
"It's an entirely new kind of exploratory enterprise," said Fred Raab, an
atomic physicist and displaced New Yorker running the Hanford arm of a
project known as the Laser Interferometer Gravitational Wave Observatory, or
LIGO ("lie-go"). The other half of this wave antenna is 1,900 miles away in
Livingston, La.
Given Einstein's ambivalence about the atomic bomb, which he had a part in
developing but later condemned, it's perhaps fitting that one of his major
predictions may some day be confirmed on the Hanford Nuclear Reservation.
LIGO gets switched on, sort of, in July.
"We expect it will be a long struggle to perfect the instrument," Raab
cautioned. "Hundreds of things need to be just right to get the sensitivity
we need. These are very, very sensitive measurements we're trying to
achieve."
That's an understatement.
A gravity wave is best thought of as a space-quake, easiest to detect when
produced by something big like a supernova (a star's explosive collapse), a
black hole or the collision of two neutron stars.
A supernova at the center of the Milky Way would produce a massive gravity
wave that despite its size would send only the tiniest of shudders through
the page of a newspaper -- a flutter 10,000 times smaller than the diameter
of an atomic nucleus.
Given that, consider the problem LIGO scientists have had in trying to build
an instrument that can detect this unbelievably small signal among all the
other "noise" on Earth -- from earthquakes to a truck rumbling by a mile
away to the waves beating on the Pacific Coast some 300 miles away.
"The ocean waves on Earth create what's known as a microseism," said Raab.
This microseismic tremor passes through the planet about every eight
seconds, he said, and also physically distorts the Earth. The scientists
have had to build a computerized support system for LIGO that compensates
for this by physically adjusting the instruments every eight seconds.
They also had to build in a compensating adjustment for the gravitational
pull of the sun every day. Raab, an expert on precision measurements,
explained that the surface of the Earth heaves up when the sun is overhead,
so they also have to adjust for that -- but on the scale of millionths to
trillionths of a meter, an unprecedented level of control.
And those are just a few of the more predictable problems.
The $300 million project is sponsored by the National Science Foundation and
run jointly by the Massachusetts Institute of Technology and California
Institute of Technology.
Similar projects are getting under way around the world -- in Germany,
Japan, Italy and Australia. The reason these projects are being done now is
sort of a twist on the mountaineer's rationale for climbing: because the
technology is finally there to make detecting a gravity wave possible.
Catching a wave
Why should anyone care?
For scientists, they know that whoever first detects a gravity wave is
likely to win a Nobel Prize. For the general public, it may be harder to
appreciate why such an uncertain venture is worth so much money. What good
is relativity anyway, and why do we need to try to get a bead on gravity?
The theory of relativity says space and time and matter are all different
manifestations of the same basic stuff, often referred to as "the fabric of
space-time." Einstein's mathematical equation stating that energy and mass
are different sides of the same coin likely sounded esoteric and impractical
at first. But that was before E = mc2 led to such developments as nuclear
power.
Gravity is the force produced by the clumps, distortions or movement within
the fabric of space-time, usually depicted visually showing a massive object
like the sun as a heavy ball dimpling the fabric of space-time.
There's no longer any question whether the theory of relativity is correct.
Anyone who's ever fallen off a ladder certainly knows that gravity exists.
But scientists still don't really know what gravity is made of or exactly
how it works. After nearly a century of debate about the existence of
gravity waves, several decades of attempts at detecting them and years of
painstaking preparation in the design and construction of LIGO, the search
will soon be on.
"This is one of the few (unsolved) problems in physics that's been around
for almost a century," said Barry Barish, a physicist at Caltech and
director of LIGO. "The prediction of gravitational waves is one of the most
important features of Einstein's theory."
It's a fundamental new inquiry into the nature of the universe, said Barish,
which means it is impossible to predict the course of discovery or its
practical implications. But the history of such endeavors shows that these
discoveries always significantly alter our lives and our knowledge.
"There's a whole side to the universe we don't have any other way to observe
except by gravitational waves," said Kip Thorne, a Caltech theoretical
physicist and one of the founders of LIGO.
Just as electricity has its electrons and light has its fundamental
particles known as photons, gravity should also exist in particle form as
"gravitons." But because of the nature of gravity, Thorne said, with our
current technical abilities there is no way to actually detect such a
fundamental particle as the ephemeral graviton.
Our best bet is to catch a wave.
"More than offering an additional window on space, gravity waves will
provide a radically new perception," wrote Marcia Bartusiak, author of a
book on the history of the search for gravity waves, "Einstein's Unfinished
Symphony."
Proof is out there
The LIGO station at Hanford, as at the Livingston site, is shaped like an
"L" with two pipeline-style concrete tunnels set at right angles to other --
one tunnel stretching out 2 1/2 miles to the northwest, the other 2 1/2
miles southwest.
Inside the tunnels, precise laser beams bounce back and forth in a
near-perfect vacuum between equally precise mirrors held in place (and
adjustable) by hair-thin steel wires and magnets.
"The instruments have to feel like they're in freefall in space," said Mike
Zucker, an MIT physicist involved in the project.
The way the scientists hope to see the signal of a gravity wave is when the
two lasers of LIGO, set at right angles to each other, interfere with each
other in a particular manner. That's laser interferometry. It was Zucker's
previous boss at MIT, the now-retired Rainier Weiss, who years ago convinced
many in the scientific community that the technique could catch a gravity
wave.
"He came up with the first coherent description of how it could work,"
Zucker said.
But a lot of technological advancements were required in lasers, mirrors,
electronics and the design of isolation systems.
When a gravity wave strikes, the theory goes, it alternately stretches and
compresses matter, space and time at the speed of light. The L-shape of LIGO
means that one of the laser tubes will be stretched while the other one is
being compressed, and then vice versa.
This should knock the two laser beams out of phase to the point where the
passage of a gravity wave can be detected.
The twin LIGO in Louisiana was built to confirm the same signal -- the
1,900-mile separation reducing the chance of seeing identical "fake" waves
produced by local noise.
"We know they exist," said Raab, adding that his confidence in Einstein's
prediction doesn't necessarily translate into confidence the LIGO gang will
get proof anytime soon.
It could be years, maybe many years, of tinkering and improving the system
along with waiting for the right cosmic event before they catch a wave.
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