Laser Interferometer Gravitational-Wave Observatory (LIGO)

The Laser Interferometer Gravitational-Wave Observatory (LIGO) is a system of two large laser telescopes designed to detect gravitational waves, ripples in the fabric of space-time caused by massive objects in outer space. Gravitational waves were first predicted by physicist Albert Einstein in 1915, though their existence remained purely theoretical for decades. In the 1960s and 1970s, scientists began to think about using laser beams to detect the minuscule changes in physical space caused by such waves. The idea for LIGO developed from these discussions, and in the 1990s, the LIGO observatories were built at sites in Louisiana and Washington State. After years of searching and a series of upgrades, LIGO accomplished its goal in 2015 when it detected gravitational waves caused by the collision of two black holes billions of light years away.

Background

German physicist Albert Einstein developed the concept of space-time in 1905 in his theory on special relativity. His theory sought to understand the laws of physics and energy at speeds approaching that of light—186,000 miles per second. Einstein found that the speed of light was absolute but that time and space were not constants and would change relative to the motion of an observer. For example, time appears to slow down to observers traveling at speeds close to light. To observers in a stationary position, time moves at its normal pace. This led Einstein to theorize that physical space is made up of not just three dimensions—width, length, and depth—but a fourth dimension, time. His ideas, however, did not account for acceleration or the presence of gravity. He solved the problem a decade later in his general theory of relativity.

Einstein theorized that massive objects created a gravitational distortion in the fabric of space-time, like a dip in a trampoline when a heavy object is placed upon it. If large objects such as black holes—the remains of supermassive stars that are so dense that their gravity will not allow light to escape—were to accelerate or decelerate, they would produce ripples in the fabric of space-time. These ripples, called gravitational waves, would spread away from their source at the speed of light, passing through matter and causing small vibrations in atoms.

Overview

While Einstein proposed the existence of gravitational waves in 1915, the technology was not available to prove his theory correct. Scientists struggled for decades to develop a method to confirm that the waves were real. Even Einstein himself briefly doubted their existence. In the 1960s, Joseph Weber, a physicist from the University of Maryland, built a pair of xylophone-like sensors designed to detect resonating waves and convert their energy into electrical signals. The devices were separated by hundreds of miles to eliminate the possibility of localized noise. In 1969, Weber announced that he had found the echo of a gravitational wave, but scientists were never able to confirm the findings, and his discovery was disputed.

Inspired partly by Weber's work, physicist Rainer Weiss from the Massachusetts Institute of Technology (MIT) developed an idea to use laser beams to measure the faint distortions caused by gravitational waves. His plan consisted of bouncing laser beams among three masses laid out in an "L" shape. Since passing gravitational waves would compress space in one direction and expand it in another, the device would measure the slight shifts in space caused by the wave. In 1975, Weiss met Kip Thorne, a theoretical astrophysicist from the California Institute of Technology (Caltech), and the two scientists began discussing the construction of an observatory that could utilize Weiss's idea of a laser-based detector. They recruited a team that included Scottish scientist Ronald Drever, an expert in experimental physics, and received funding for the LIGO project from the National Science Foundation.

After years of setbacks and squabbling among the scientists, the project was finally inaugurated in 1999 and began searching for gravitational waves two years later. LIGO consisted of two facilities located 1,865 miles apart in Livingston, Louisiana, and Hanford, Washington. Each of LIGO's detectors consisted of a pair of two-and-a-half-mile vacuum tubes arranged in an "L" shape. Laser beams were sent through the tubes and bounced off mirrors to return to their precise starting point. If gravitational waves were to pass through the detectors, the wave should stretch out the beam in one tube and compress it in the other. Because the waves would be incredibly faint, the amount of change detected by the lasers would be minuscule.

The observatories' first attempt to detect gravitational waves lasted nine years until 2010. In that time, they found nothing, but that was somewhat by design as the project was viewed as a way to prove the equipment could work and to set the stage for future improvements. The upgrade to the advanced LIGO system was completed in 2014 and underwent testing for a proposed start date in September of 2015.

On September 14, 2015, a few days before the project was to officially go online, an engineering check detected a signal at the installation in Louisiana. Seven milliseconds later, the same signal was picked up in Washington. The signal, recorded as a "chirp" resembling the sound of a heartbeat, was believed to have originated from the sky in the Southern Hemisphere. After reviewing the signal to confirm its authenticity, scientists determined it was indeed a gravitational wave created by a collision between two black holes about 1.4 billion light-years away. Since a light-year is the distance light travels in the course of a year, the collision LIGO detected occurred about 1.4 billion years ago. Light from the event took that long just to reach Earth.

In February of 2016, the LIGO team published its findings and announced the discovery to the public with much fanfare. The scientists had not only recorded the first direct evidence of gravitational waves and confirmed Einstein's century-old theory but also proved to critics that LIGO's combined price tag of more than $1 billion was justified. In June of 2016, scientists announced that LIGO had detected another set of gravitational waves, this time a smaller signal resulting from a collision between two less massive black holes. Further analysis of LIGO data has revealed other significant findings including gravitational waves (GW) from two collisions of black holes and neutron stars, identified as GW200105 and GW200115; in both cases the black holes swallowed the neutron stars. LIGO was also collaborating with researchers at other observatories including the Kamioka Gravitational Wave Detector (KAGRA) in Japan, which went onlin in February 2020, and the European Gravitational Observatory's (EGO) gravitational wave interferometer in Italy, Virgo, which received an upgrade that allowed it to begin searching for gravitational waves in 2016.

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