On the 11th February 2016, scientists at the Laser Interferometer Gravitational-Wave Observatory (LIGO) made an amazing discovery. After years of theorising and developing their gravitational wave interferometers in locations across the USA, January 2016 was the moment the team had been waiting for: the final moments of the merger of two black holes.

Two Black Holes Merge


Einstein’s Gravitational Waves

Gravitational waves were first predicted by Einstein in his theory of general relativity. In his theory, he compares the universe to a single fabric, hewn from space and time. He theorised that the force of gravity is caused by the curvature in space-time. Gravitational waves are the ripples in space-time and are produced as a result of massive objects like black holes colliding.

While Einstein’s theory has been around since 1915, finding a way to observe the effect of two black holes merging was a difficult problem to solve. An event on such a massive scale doesn’t happen every day so timing would be key to observing the effects.

Luckily for the LIGO team, such a merger was predicted to happen.

Refining LIGO’s Detectors

The original gravitational wave detectors were completed in 1999 and the first search for gravitational waves took place between 2002 and 2010. Though no waves were detected, the search revealed that the instruments were not sensitive enough. Over the next four years, the team completely redesigned their gravitational wave detectors.

The sensors in the improved LIGO were 10 times more sensitive than their predecessors. This meant that the detector would be able to listen for gravitational waves 10 times further away. While it may not sound like much, it is actually a vast improvement as this gives LIGO access to 1000 times more volume of space.

Where are the Interferometers Located?


Though LIGO is considered as one observatory, it is actually made up of 4 facilities: two gravitational wave detectors (the interferometers) are in Hanford, Washington and Livingston, Louisiana and two university research centres at Caltech, Pasadena and MIT in Cambridge.

The interferometers are remotely located with 3002 km between them. This is important because the detectors are so sensitive that they can pick up vibrations from 1000s of kilometres away: anything from a truck passing nearby to an earthquake happening can mask or mimic a passing gravitational wave.

The interferometers observe and collect data simultaneously so that should one interferometer pick up a signal that the other doesn’t, it won’t be mistaken for a gravitational wave. A gravitational wave should stand out as an anomaly because both detectors will pick up identical signals at the same time.

LIGO is also in collaboration with VIRGO, an interferometer set up in Italy near Pisa which adds a third set of results to increase the confidence that a detected signal is a gravitational wave.

How Does an Interferometer Work?

An interferometer is essentially a device that merges two or more sources of light to create an interference pattern. This is where the name comes from: interfere-ometer. An interferometer is usually used to make tiny measurements and LIGO’s interferometers can measure a distance of 1/10,000th the width of a proton!

This is a schematic for an interferometer:

Interferometer diagram

Image: LIGO

The laser is pointed at the beam splitter. The light then travels down the perpendicular arms of the interferometer. At the end of each is a mirror that the light bounces off and returns to merge again into a single beam. If the arms are exactly the same length and the mirrors are perfectly aligned then the two beams will cancel each other out and the photodetector will see nothing.

However, if the distance changes even slightly, then the two beams won’t cancel each other out but will be made visible to the photodetector. This is what happens when a gravitational wave passes through.

gravitational wave


This video from LIGO explains things in more detail.

When Two Black Holes Merge

All objects emit gravitational waves when they orbit each other but as the two black holes drew closer, the energy they were losing to gravitational waves was enough to pull them closer together leading them to distort space-time further and even more gravitational waves being emitted.

The closer the black holes get the faster they spin and the shorter their orbit of each other until their orbits last only a few milliseconds before they merge. When they do merge, the resulting enormous black hole has to settle into an sphere, releasing even more gravitational waves.

When these gravitational waves reach Earth, the interferometers all pick up the same signal which, when translated into a sound frequency sounds ‘something like a thump’.

Can Gravitational Waves Help Us to Explain Our Universe?

The detection of a gravitational wave has helped to confirm the last part of Einstein’s theory of general relativity and could pave the way to extending the theory to further explain the cosmos. The study of gravitational waves could even begin to reconcile two of the major theories of the universe: Einstein’s general relativity and quantum mechanics.

LIGO is now into its second ‘advanced’ run and scientists are trying to predict what the signals of other things will look like so that when the signals arrive, we will be able to identify them. Throughout this run, they have been building a large data bank and have two event candidates worthy of further study.

Looking into the Future

Since detecting the black hole collision last year, LIGO have set their sights on finding ways to search for and categorise neutron stars. The theory is that these stars are so big that if they aren’t perfectly spherical, then any small bump could cause a gravitational wave. Another candidate for the interferometer is a supernova which should also produce detectable gravitational waves.

The progress the LIGO team are making and the data bank they are creating is incredibly exciting. With so many unanswered questions about the fabric of our universe on the brink of being answered, we will be following the progress of the interferometer with rapt attention.