Interferometric Systems Detect Gravitational Waves

The observation of gravitational waves — described as ripples in the fabric of spacetime — arriving at the earth confirms a major prediction of Albert Einstein’s 1915 general theory of relativity. The gravitational waves were detected on September 14, 2015 by both of the twin Laser Interferometer Gravitational-wave Observatory (LIGO) detectors, located in Livingston, La., and Hanford, Wash.

Simulation image of two black holes merging into one. Courtesy of LIGO.

The LIGO Scientific Collaboration (LSC) of 15 countries includes the LIGO interferometers, as well as the GEO600 detector, a laser interferometer with 600-m-long arms in Sarstedt, Germany. The discovery was made possible by the enhanced capabilities of Advanced LIGO, a major upgrade that increased the sensitivity of the instruments compared to the first generation detectors, enabling a large increase in the volume of the universe probed. The gravitational waves were discovered during Advanced LIGO’s first observation run.

“This detection is the beginning of a new era: The field of gravitational wave astronomy is now a reality,” said Gabriela González, LSC spokesperson and professor of physics and astronomy at Louisiana State University.

At each of the two LIGO observatories, a 4-km-long, L-shaped interferometer used laser light split into two beams that travel back and forth down the arms, which comprise 4-ft.-diameter tubes kept under a near-perfect vacuum. The beams monitor the distance between mirrors precisely positioned at the ends of the arms.

According to Einstein’s theory, the distance between the mirrors would change by an infinitesimal amount when a gravitational wave passed by the detector. A change in the lengths of the arms smaller than 1/10,000 the diameter of a proton (10⁻¹⁹ m) could be detected.

These plots show the signals of gravitational waves detected by the twin LIGO observatories. The signals came from two merging black holes, each about 30 times the mass of the sun, lying 1.3 billion light-years away. Courtesy of LIGO.

Based on the observed signals, LIGO scientists estimated that the black holes for this event were about 29 and 36 times the mass of the sun, and the event took place 1.3 billion years ago. About three times the mass of the sun was converted into gravitational waves in a fraction of a second, with a peak power output about 50 times that of the whole visible universe.

By examining time of arrival of the signals — the detector in Livingston recorded the event 7 ms before the detector in Hanford — scientists can say that the source was located in the Southern Hemisphere.

The existence of gravitational waves was first demonstrated in the 1970s and ‘80s by Joseph Taylor, Jr. and colleagues. Taylor and Russell Hulse discovered in 1974 a binary system composed of a pulsar in orbit around a neutron star. Taylor and Joel M. Weisberg in 1982 found that the orbit of the pulsar was slowly shrinking over time because of the release of energy in the form of gravitational waves. For discovering the pulsar and showing that it would make possible this particular gravitational wave measurement, Hulse and Taylor were awarded the Nobel Prize in physics in 1993.

According to Einstein’s theory of general relativity, a pair of black holes orbiting around each other lose energy through the emission of gravitational waves, causing them to gradually approach each other over billions of years, and then much more quickly in the final minutes. During the final fraction of a second, the two black holes collide into each other at nearly one-half the speed of light and form a single more massive black hole, converting a portion of the combined black holes’ mass to energy, according to Einstein’s formula E = mc². This energy is emitted as a final strong burst of gravitational waves, which is what LIGO has observed.

The discoveries will be published in Physical Review Letters.

Leoni AG of Nuremberg, Germany, supported the development of laser systems for the LIGO gravitational wave detectors with optical fiber cables that transmit pumped radiation of the laser diodes onto a crystal. They consist of 70- to 100-m-long, assembled bundles of seven LargeCore fibers, each with a core diameter of 400 µm.

The laser head used in the measuring equipment was developed and built by Laser Zentrum Hannover e.V. of Hannover, Germany, and fitted with the assembled cables from Leoni.