Gravity-Wave Experiment Modifies Some String Theories
In results announced today, a huge physics experiment built to detect gravitational waves has yet to find any.
Rather than be disappointed by the null findings, physicists say the results were expected, and in fact help them narrow down possibilities for what the universe was like just after it was born.
The Laser Interferometer Gravitational-Wave Observatory Scientific Collaboration (LIGO) is a set of instruments in Louisiana and Washington built to search for evidence of gravitational waves, which are theoretical ripples in space-time thought to be caused by the acceleration of mass. No one has yet directly detected these waves, though they are predicted by Einstein's theory of General Relativity, and are widely thought to permeate our universe.
In theory, every time mass accelerates - even when you rise up out of your chair - the curvature of space-time changes, and ripples are produced. However, the gravitational waves produced by one person are so small as to be negligible. The waves produced by large masses, though, such as the collision of two black holes or a large supernova explosion, could be large enough to be detected.
LIGO has only been running for a few years - the new results are based on measurements taken between 2005 to 2007 - and it is not yet at its highest level of sensitivity. The fact that this first period of observations did not detect gravitational waves allows researchers to rule out the possibility of waves above a certain amplitude threshold. Simply put, if there were any waves big enough for LIGO to have detected them, it would have. Since it didn't, they aren't likely to exist.
"I wouldn't say it's surprising that we're ruling them out," said Vuk Mandic, a physicist at the University of Minnesota who led the new analysis. Most physicists think the models that would have produced gravitational waves above the threshold that could have been seen so far are unlikely, he said.
The next phase of the project, called Advanced LIGO, will improve the experiment's sensitivity, allowing scientists to probe a volume of space about 1,000 times larger than the current project's range.
"If Advanced LIGO doesn't see gravitational waves I think people will be very surprised," Mandic told SPACE.com. "It is likely such a situation would require revision of General Relativity."
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