LIGO - Laser Interferometer Gravitational-Wave Observatory

Aerial image of a research facility.
The LIGO Laboratory operates two detector sites, one near Hanford in eastern Washington, and another near Livingston, Louisiana. This photo shows the Livingston detector site. LIGO

The Laser Interferometer Gravitational-Wave Observatory, called LIGO, is an American national scientific collaboration to study astrophysical gravitational waves.  The LIGO observatory consists of two different interferometers, one of them in Hanford, Washington, and the other in Livingston, Louisiana. On February 11, 2016, LIGO scientists announced that they had successfully detected these gravitational waves for the first time, from the collision of a pair of black holes over a billion lightyears away.

The Science of LIGO

The LIGO project that actually detected the gravitational waves in 2016 is actually known as "Advanced LIGO," due to an upgrade that was implemented from 2010 to 2014 (see the timeline below), which increased the original sensitivity of the detectors by an amazing 10 times. The effect of this is that the Advanced LIGO equipment is the most precise measuring device in the universe. To use just one of the many amazing facts available on the LIGO website, the level of sensitivity in their detectors is equivalent to measuring the distance to the nearest star to within the width of a human hair!

An interferometer is a device for measuring the interference in waves traveling along different paths. Each of the LIGO sites contains L-shaped vacuum tunnels that are 2.5 miles long (the largest in the world, except for the vacuum maintained at CERN's Large Hadron Collider). A laser beam is split so that it travels along each section of the L-shaped vacuum tubes, then bounce back and are reunited together. If a gravitational wave propagates through the Earth, rippling spacetime itself as Einstein's theory predicts it should, then one part of the L-shaped path would be squeezed or stretched in comparison to the other path. This would mean that the laser beams, when they meet back up at the end of the interferometer, would be out of phase with each other, and therefore would create a wave interference pattern of light and dark bands... which is precisely what the interferometer is designed to detect. If you're having trouble visualizing this explanation, I suggest this great video from LIGO, with an animation that makes the process more clear.

The reason for the two different sites, separated by nearly 2,000 miles, is to guarantee that if both detected the same effect, then the only reasonable explanation would be an astronomical cause, rather than some environmental factor in the region of the interferometer, such a truck driving nearby.

The physicists also wanted to be sure that they didn't accidentally jump the gun, so they implemented protocols to try to prevent that, such as double-blind secrecy internally so that the physicists analyzing the data didn't know if they were analyzing real data or fake sets of data that were tailored to look like gravitational waves. This meant that when a real set of data showed up from both of the detectors representing the same wave pattern, there was an added degree of confidence that it was real.

Based on the analysis of the gravitational waves detected, LIGO physicists have been able to identify that they were created when two black holes collided together nearly 1.3 billion years ago. They had a mass about 30 times that of the sun and each were about 93 miles (or 150 kilometers) in diameter.

Key Moments in LIGO History

1979 - Based on initial feasibility research in the 1970s, the National Science Foundation funded a joint project from CalTech and MIT for extensive research and development on building a laser interferometer gravitational-wave detector. 

1983 - A detailed engineering study is submitted to the National Science Foundation by CalTech and MIT, to build a kilometer-scale LIGO apparatus.

1990 - The National Science Board approved the construction proposal for LIGO

1992 - The National Science Foundation selects the two LIGO sites: Hanford, Washington, and Livingston, Louisiana.

1992 - The National Science Foundation and CalTech sign the LIGO Cooperative Agreement.

1994 - Construction begins at both of the LIGO sites.

1997 - The LIGO Scientific Collaboration is officially established.

2001 - LIGO interferometers are fully online.

2002-2003 - LIGO conducts research run, in collaboration with interferometer projects GEO600 and TAMA300.

2004 - National Science Board approves the Advanced LIGO proposal, with design ten times more sensitive than initial LIGO interferometer.

2005-2007 - LIGO research run at maximum design sensitivity.

2006 - Science Education Center at the Livingston, Louisiana, LIGO facility is created.

2007 - LIGO enters into an agreement with the Virgo Collaboration to perform joint data analysis of interferometer data.

2008 - Begin of construction on Advanced LIGO components.

2010 - Initial LIGO detection comes to an end. During the 2002 to 2010 data collection on the LIGO interferometers, no gravitational waves were detected.

2010-2014 - Installation and testing of Advanced LIGO components.

September, 2015 - The first observation run of LIGO's advanced detectors begins.

January, 2016 - The first observation run of LIGO's advanced detectors comes to an end.

February 11, 2016 - LIGO leadership officially announces the detection of gravitational waves from a binary black hole system.