What is LIGO?

Nessie

LIGO is short for “Laser Interferometer Gravitational-Wave Observatory”. At least 3,000 kilometers apart, the two enormous laser interferometers which comprise LIGO are stationed at Livingston, Louisiana and Hanford, Washington. A revolutionary technology for how we observe the universe, LIGO uses the properties of light and space to detect gravitational waves and their origins.

On the same scale of some of today’s giant particle accelerators and nuclear physics laboratories, LIGO is an extraordinary physics experiment which has allowed us to observe the most energetic and violent events in the universe. In doing so, continued observations from LIGO will have dramatic effects to our understanding of cosmology, gravitation, relativity, as well as the physics of nuclear and particle processes.

The original concept of using laser interferometers for gravitational-wave observation was pioneered by American physicist Rainer Weiss. In 1972 at MIT in Cambridge, Weiss completed the invention of an interferometric gravitational-wave detector. By recognizing all potential “noise” sources such an instrument would pick up and remediating them accordingly, Weiss had shown that this principle could lead to detections sensitive enough to detect waves from astrophysical sources.

But what are gravitational waves? For those of you already familiar with Albert Einstein’s general theory of relativity, we understand that the fabric of space-time curves around large bodies of mass and their corresponding gravity. The more massive and dense an object, the greater its distortion and curvature of the fabric of space. From this we can infer that gravitational waves are, in fact, small ripples in the fabric of space-time.

Now, let’s imagine a binary system (two celestial bodies which are gravitationally bound) and then increase the mass of each object to that of a neutron star. Despite such small diameters, averaging around 12.5 miles (20 kilometers), neutron stars have roughly 1.5x the mass of our sun; for reference, a single sugar cube of neutron star would weigh around one hundred million tons while on earth. As these neutron stars orbit closer and closer to one another, their extreme acceleration through space causes ripples in the fabric of space-time itself. Below is an animation depicting such a binary system:

Using the laser interferometric principles developed by Weiss in 1972, LIGO is able to detect mergers such as these via the gravitational waves they create; providing us with a whole new approach to how we observe these celestial bodies. By collecting data from such mergers as well as other observations, scientists can construct sophisticated models to re-create these events to study and understand their implications more deeply.

But theory aside, how does laser interferometry work exactly? By merging two or more sources of light, laser interferometers are able to create an interference pattern; these patterns result from overlapping waves of light. When peaks of two waves of light overlap, they combine to form constructive interference, or a larger peak. However, when the depression of one light wave overlaps with the peak of another light wave, the two waves experience destructive interference and cancel each other out. These interference patterns provide scientists with crucial details about the properties of the sources which emitted the light.

The LIGO observatory lasers, which are directed down its 2.5 mile (4 km) arms, bounce back and are set to cancel each other out completely. As a result, no light reaches the photodetector. If a gravitational wave were to pass through the LIGO facility, however, it would stretch one detector arm and compress the other, throwing off this perfect destructive interference and resulting in some light reaching the photodetector. The pattern of this light provides data on the changes the arms underwent, revealing properties about the incident, its corresponding gravitational waves, and their source. Below is an example of how this works:

A single such observatory, however, would not be able to definitively confirm observations like these on its own. For this reason, a total of two detectors were constructed. In 1992, the Hanford and Livingston sites were selected as the locations for the two LIGO interferometers. Funded by the National Science Foundation (NSF) with cooperative agreement signed between MIT and Caltech, construction of the facilities took place from 1994-1998 with the initial interferometers installed and commissioned between 1999-2002.

Since LIGO’s completion in 2002, however, there had yet to be a single detection of gravitational waves. For this reason the Advanced LIGO project aimed to enhance the detectors initially installed for the two observatories. Still funded of the NSF, Advanced LIGO began installation of enhancements between 2010-2014. Post-installation, Advanced LIGO proved to be more sensitive than Initial LIGO, and observations at the two sites now continued.

On September 14th, 2015 at 5:51 a.m. ET, after just being brought back into operation, the twin LIGO detectors measured ripples in the fabric of space-time, gravitational waves. GW150914 was the very first detection of these waves, a discovery representing decades of research and finally confirming a major prediction of Einstein’s general theory of relativity made in 1915. From the observed signals, scientists calculated that GW150914 was the merger of two black holes, estimated to be about 29 and 36 times the mass of our sun respectively. Originally taking place roughly 1.3 billion years ago, the event converted 3 times the mass of our sun into gravitational waves, a power output of about 50 times that of the whole visible universe. Because the Livingston site detected the event 7 milliseconds before the Hanford site, the event’s location could be isolated to having occurred in the southern hemisphere.

While LIGO may be the first of its kind, it will most certainly not be the last. Since LIGO’s initial completion, other interferometer observatories have gone online. In Germany there is GEO600 , operated by scientists from the Max Planck Institute for Gravitational Physics. Also Virgo, located outside Pisa, Italy, this interferometer is a massive collaboration comprising laboratories from the countries of Italy, France, the Netherlands, Poland, Hungary and Spain. Most importantly, the Laser Interferometer Space Antenna (LISA,) led by the European Space Agency (ESA), is an ambitious space-based project which will consist of three spacecraft separated by millions of miles. By relaying laser beams back and forth between the different spacecraft, the signals are then combined to search for distortions of space-time. Scheduled for launch in the early 2030’s, LISA will usher in a new chapter for the observation of supermassive black holes.

In summary, LIGO and the laser interferometric method, in general, prove to be an exciting new way for humanity to conduct astronomical observations. By the smallest and most subtle of measurements (1/10,000th the width of a proton), we bear witness to the most overwhelming large and extreme events in the cosmos. In doing so, we deepen our knowledge and further our understanding of the cosmos.