Wednesday, November 21, 2007

LIGO scanning the skies

Searching for one of Einstein's greatest predictions:

Gravitational waves are produced when massive objects in space move violently. The waves carry the imprint of the events that cause them. Scientists already have indirect evidence that gravitational waves exist, but have not directly detected them.

The Laser Interferometer Gravitational-wave Observatory, or LIGO, consists of detectors at two U.S. sites managed by the California Institute of Technology (Caltech) and Massachusetts Institute of Technology (MIT).

The LIGO observatories use lasers to accurately monitor the distance between a central station and mirrors suspended three miles away along perpendicular arms. When a gravitational wave, a traveling ripple in space-time, passes by, the mirror in one arm will move closer to the central station, while the other mirror will move away.

The change in distance caused by stretching and squeezing is what LIGO is designed to measure, says Alan Wiseman - associate professor @ UWM's Center for Cosmology and Gravitation.

Those changes will be inconceivably tiny. LIGO can record distortions at a scale so small, it is comparable in distance to a thousandth of the size of an atomic nucleus.

LIGO records a series of numbers - lots of them - and feeds them to several supercomputer clusters around the country, including UWM's Nemo cluster.

The computer's job is to sort out the numerical patterns representing gravitational waves buried in ambient noise produced by lots of other vibrations - from internal vibrations of the equipment itself, to magnetic fluctuations from lightning storms, to seismic vibrations from trains rolling along the tracks a few miles from the observatory, or from earthquakes on the other side of the world.

There are thousands or even millions of different signals that could be emitted from space. So you have to take each segment of data individually. Nemo performs many billions of calculations per second in its search for these signals.

The strings of numbers from LIGO are like tracks on a compact disk. Once detected, gravitational-wave signals can be converted into sound. Scientists have already simulated, based on mathematical predictions, what certain events in space will sound like.

When two black holes are merging, for example, you might expect to hear a "chirp" that represents the spiraling together of the black holes just before they collide. "The spiral can go on for thousands of years," says Brady, a UWM professor of physics. "The sound is the identifying signal of the last few seconds of the process!"

Gravitational waves may hold secrets to the nature of black holes, the unknown properties of nuclear material, and maybe even how the universe began.

"We've only been able to find out about the universe since it became cool. But with gravitational waves, we'll see the universe when it was much younger -- and hotter." "I think we're in for a surprise," says Siemens from UWM's Center for Cosmology and Gravitation. "We have all these ideas about what we think we will find, but it could be something completely different."
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