Ripples in the Fabric of Spacetime: "Bells That Are Ringing in the Universe"
The LIGO Hanford, Washington Control Room. Photo taken by Tobin Fricke, 2005. Wikimedia Commons
By Bjorn Carey
An international team of scientists excitedly announced that they had directly observed gravitational waves, often described as ripples in the fabric of spacetime. The discovery of gravitational waves confirms a prediction that Albert Einstein made nearly 100 years ago to shore up his general theory of relativity.
The detection was made by the twin Laser Interferometer Gravitational-wave Observatory (LIGO) detectors, an experiment led by researchers at Caltech and MIT that includes more than 1,000 affiliated scientists, including several Stanford physicists and engineers who have played key roles in the program since it was launched. The instrument systems that made the detection possible were built in part on a legacy of interdisciplinary technological advances made by Stanford scientists.
"LIGO is by far the most precise measurement machine that man has ever built," said Robert L. Byer, the William R. Kenan, Jr. Professor of Applied Physics at Stanford, and an original member of the LIGO team. "It's finally sensitive enough to see the bells that are ringing in the universe."
The ringing bell that LIGO heard early in the morning of Sept. 14, 2015, was the result of two massive black holes merging together 1.3 billion light-years away. As the two black holes spiraled around each other, they radiated energy in the form of gravitational waves. While they merged into one even more massive black hole, they released three solar masses of energy.
"This was a huge signal," said Martin Fejer, a professor of applied physics. "It's more energy than our sun will release in its entire lifetime, and it all happened in about a fifth of a second as these two massive black holes coalesced."
Although the peak power output of the event was about 50 times that of the whole visible universe, it required an extremely sensitive device to detect the ensuing gravitational waves. LIGO consists of twin instruments, located 1,865 miles apart in Louisiana and Washington. Each of these instruments involves a single laser, each directed into two 4-kilometer-long arms that run perpendicular to one another.
Two LIGO United States observatories; Hanford, WA and Livingston, LA. Wikipedia
As a gravitational wave passes through a detector, it distorts spacetime such that one arm lengthens, and the other shortens. By comparing the disturbances at the two detectors, the scientists can confirm the direct detection of a gravitational wave.
The detection itself was something of a surprise; the detectors were undergoing final commissioning at the time, and weren't scheduled to enter full-time detection mode for a few days. Brian Lantz, the lead scientist for seismic isolation and alignment systems for Advanced LIGO, was sitting at his desk when the detection appeared in the experiment's online notebook, and it immediately caught his attention.
The scientists had also been on the lookout for a signal that matched gravitational waves from co-orbiting neutron stars, a more anticipated event. But this signal was significantly more energetic and shorter in duration than what would be expected from neutron stars.
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