The age of gravitational wave astronomy has begun

-February 25, 2016

The LIGO (Laser Interferometer Gravitational-Wave Observatory), is the most precise instrument ever built. It's basically a Michelson interferometer with two, 4-km long arms at right angles, shown in Figure 1. A frequency stabilized, 1.06 micron, 220 W laser bounces 400 times back and forth between the mirrors before recombining with the beam in the other arm.

Figure 1. LIGO Hanford, WA, 4 km long Michelson interferometer.

One challenge is to isolate mechanical vibrations of the mirrors from environmental and thermal noise low enough to see motion that is smaller than a millionth the diameter of a proton. A combined passive and active suspension system isolates each of the mirrors.

LIGO consists of two identical interferometers, one in Hanford, WA and the other in Livingston, LA. While a local seismic disturbance would be seen at one location and not the other, a gravitational wave disturbance in space-time would be seen by both detectors, delayed by up to 10 ms, the light travel time between the two observatories.

About 1.3 billion years ago, two black holes, one 29 solar masses, the other 36 solar masses, merged together after a fatal spiral dance. As they orbited closer and closer, they lost energy by gravitational radiation and spiraled in. Only their last 20 ms of separate existence emitted enough gravitational radiation for the LIGO detector to hear it. Figure 2 illustrates the ripple in space time.

Figure 2. Simulation of the space time ripples as the two black holes spiral into their death.

"In that brief, final flash of intense gravitational radiation, as much as three solar masses were converted into pure gravitational energy," said Kip Thorne, Caltech’s Richard P. Feynman Professor of Theoretical Physics, emeritus, and early architect of the LIGO facility. "The peak gravitational radiation had more power than 50 times the total power emitted by all the power output of all stars in the universe. It was a violent storm in space-time and we heard the ripples passing by the Earth."

The entire detected signal lasted only 20 ms. The Livingston detector saw it first and 7 ms later, the Hanford detector saw it. The 7 ms delay is a rough indictor of the direction of the source, in the southern sky in the direction of the Magellantic clouds.

Figure 3. Best guess of the location of two merging black holes in the southern sky.

This strain measurement can only be interpreted in comparison to numerical simulations of possible events. The measured chirp strain signal matches the predicted signature of two black holes of 29 and 36 solar masses resulting in a 62 solar mass black hole after the merger. Figure 4 shows this agreement.

Figure 4. Measured strain data from the two detectors overlaid with the numerical simulation predictions showing the incredible match and the coincidence of measurements separated by 3,000 km.

The range of measurable strain extended from an orbital frequency initially of 15 Hz and ended up at 75 Hz, during the 20 ms final stage. Just before merger, each black hole had an orbital speed of more than half the speed of light.

"The incredible agreement between the simulated response and what was measured is the first ever test of the strong field version of Einstein's general relativity equations," Weiss said. "This discovery is not only a testament to the incredible engineering accomplishment, but also to Einstein’s development of a new way of thinking about gravity."

"Up to now, the field equations of general relativity have only been tested in weak fields, in the solar system, even in the binary pulsar. This is the first test in the extremely intense, strong field of two merging black holes. It is truly amazing that the same theory seems to work over this very large dynamic range."

An interferometer is like a microphone listening to the vibrations of space time. One interferometers is omnidirectional. "Two interferometers mean we can hear in stereo. With a network of gravitational wave detectors, we will get more accuracy to localize future sources," González said. Detectors in Italy, Germany, Japan and India will come on line soon to provide much better directional accuracy.

Listen to the merger of two black holes from their gravitational radiation in Figure 5.

Figure 5. This video lets you hear the sound that occurs when black holes collide.

"This detection is the beginning of a new era: The field of gravitational wave astronomy is now a reality," González concluded.

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