What are gravitational waves?

Gravitational waves are ripples in the curvature of space-time that travel outward from the source that created them

13 Feb 2017

Nasa's depiction of gravitational waves emerging from a black hole


Albert Einstein's 100-year-old theory about gravitational waves has been proved correct. And now the breakthrough has been recognised with the 2017 Nobel Prize for Physics.

Scientists working with the Laser Interferometer Gravitational Wave Observatory (Ligo) first confirmed the discovery of gravitational waves in February 2016. A second set of waves was confirmed four months later on June 15.

The first waves detected, spotted in data collected on September 14, 2015 were the result of two black holes 36 and 29 times the mass of our sun merging. The second set of gravitational waves were sent travelling through spacetime when two black holes eight and 14 times the mass of our sun collided.

This collision took place 1.4 billion years ago and created a massive spinning black hole 21 times the mass of the sun. An additional sun's worth of mass was transformed into gravitational energy. The second detection was "very strong" despite the smaller sizes of the black holes.

The scientific collaboration involved around 90 academic and scientific institutions from more than 15 countries, including MIT and Caltech. Professors Kip Thorne, Barry Barish and Rainer Weiss were award the 2017 Nobel Prize for Physics thanks to their "decisive contributions to the Ligo detector and the observation of gravitational waves".

"We’re thrilled to hear that the Nobel Prize in Physics 2017 has gone to gravitational wave detection," said Professor Sheila Rowan, Director of the University of Glasgow’s Institute for Gravitational Research, and one of the UK leads on Ligo. "The discovery of the existence of gravitational waves, just over two years ago, has opened up a whole new way to understand the universe."

Professor Mark Hannam, from Cardiff University’s School of Physics and Astronomy, echoed the sentiment, saying: "We already knew gravitational waves existed. We already knew black holes existed. What Kip Thorne, Barry Barish and Rainer Weiss did was to build the first machine sensitive enough to be able to directly *measure* gravitational waves. It took them over forty years, and the result was the most sensitive measuring device ever made. It is an incredible new tool that has only begun to transform our understanding of the universe."

The gravitational waves research has previously been recognised by a number of institutions. In November 2016, Sheila Rowan and her team at the University of Glasgow took home the WIRED Audi Innovation Award for the best scientific breakthrough of the year for their work on the Ligo project.

And in December 2016, Physics World awarded the discovery with Breakthrough of the Year. “What’s been achieved by Ligo, particularly in a relatively short space of time, is truly incredible,” said Physics World editor Hamish Johnston.

Caltech’s David Reitze, executive director of the Ligo Laboratory, added it is "extremely gratifying" for the efforts and accomplishments to have been recognised, continuing: "Ligo has opened a new window onto the cosmos. We are eager to learn what new secrets the universe reveals to us through gravitational waves!"

What are gravitational waves?

Large optic inspection of the LIGO equipment


One of the LIGO research centres power recycling optics


In 1905, Albert Einstein's groundbreaking work showed that the speed of light in a vacuum is independent of the motion of all observers, and the laws of physics are the same for all non-accelerating observers. This is known as the theory of special relativity.

The theory explains the behaviour of objects in space and time, and it can be used to predict everything from the existence of black holes, to light bending due to gravity and the behaviour of the planet Mercury in its orbit.

According to Einstein, who first predicted them in 1916 after forming his theory of general relativity, gravitational waves are ripples in the curvature of space-time that travel outward from the source that created them. He argued spacetime – any mathematical model that combines space and time – would create ripples that move across the universe at the speed of light.

Ligo claims the waves are "caused by some of the most powerful processes" and carry origins about the universe and the nature of gravity. "With these gravitational waves it is not another part of the electromagnetic spectrum it is a whole new spectrum in itself – it's a completely different way of getting information from things," Anthony Lasenby, from the Cavendish Laboratory at Cambridge University told WIRED.

"They're information carrying and they can tell us about aspects of astrophysical forces and principle things like the early universe, that you just wouldn't be able to get in any other way."

It's possible to create your own version of gravitational waves, to visualise their effects, using lycra, marbles and weights.

Stretch the spandex lycra using the weights and then use wheels attached to a plan of wood to simulate a pair of binary black holes. Rotate them at 18 rotations per second on top of the stretched lycra and the resulting waves are the equivalent to gravitational waves experienced in spacetime. More information and instructions are available from The Kids Should See This website.

Gravitational waves should not be confused with gravity waves. Atmospheric gravity waves form when buoyancy pushes air up, and gravity pulls it back down. As it drops into the low-point of the wave, also known as the trough, the air touches the surface of the ocean. This 'roughens' the water.

This creates long, vertical dark lines that can be seen in satellite images that show where the troughs of gravity waves have roughened the surface. Crests of the atmospheric waves can then be seen as bright areas on the same satellite images. By comparison, water beneath a crest is calm and reflects light towards the sensor. Clouds commonly form at the crests of the waves.

Why do gravitational waves matter?

The discovery of the waves is "profound," Lasenby said. The reason? They create systems that allow us to look at the universe in ways that have not been possible until now. In the same way that infrared and X-ray spectrums have allowed humans to look into the depths of space, gravitational waves open up new possibilities for research.

The existence of the waves is almost universally agreed upon by scientists but until February 2016, they were elusive.

Why are gravitational waves so hard to detect?

As gravitational waves pass through the universe, their interaction with everything around them is "minuscule" when compared to other types of waves. According toCardiff University researchers, "a supernova explosion in our own galaxy would emit pretty strong gravitational radiation, yet a 1 km ring would deform no more than a one thousandth the size of an atomic nucleus."

Lasenby says that "only recently" sensors with the "right sensitivity" detect capabilities to possibly find the waves have been created. The Ligo experiment is using interferometers, which they say can detect tiny amounts of gravitational radiation.

Interferometers are used in many areas of science and engineering and are called interferometers because they merge two or more sources of light to create what's known as an interference pattern.

Interferometers were invented by Albert Michelson and the Michelson Interferometer was first used in 1887 in the 'Michelson-Morley Experiment'. This experiment was set up to study what's known as the 'Luminiferous Ether'. In the late 19th century, this ether was believed to be present across the universe. Lasers then made it possible to study tiny measurements, like those used by Ligo.

What is Ligo?

The LIGO research unit's beam splitter


The research project is one of the largest gravitational wave observatories in the world and is spread across two sites in the US – one in Washington and the other in Louisiana.

It is the world's largest gravitational wave observatory and it studies the properties of light and of space to detect the origins of gravitational waves.

The Advanced Ligo project, which started being used in October 2015 after a seven-year redesign, was designed by California Institute of Technology and Massachusetts Institute of Technology staff but involves researchers from 80 worldwide institutions. "What Ligo is specialising in is looking at these pretty rapid frequencies from 10 hertz up to 1,000 hertz events which correspond to extreme astrophysical things," Lasenby said.

To do this, the interferometer has two 4km long arms which laser beams are shone along, reflecting off mirrors at each end. "Ligo scientists can look for the pattern of arm length changes that we expect from different types of gravitational wave source: if they see the pattern, they'll know a gravitational wave has passed by," says the project.

The system is looking for four different categories of gravitational waves, each of which has their own patterns that could be sensed by the equipment.

In addition to Ligo, the world's largest radio telescope in China was completed in September 2016. Dubbed, Five-hundred-meter Aperture Spherical Telescope (Fast), the telescope features the world's largest aperture, at 500 metres, and has a total area equal to 30 football fields. It not only surpasses the Arecibo Observatory – once the world’s largest single-aperture telescope – in size, but also in sensitivity and overall performance.

Once in full operation, Fast will search space for gravitational waves, galaxies and the origin of life.


Research from last year into the Ligo-Virgo collaboration discovered that the binary black holes it found, and were said to be responsible for the gravitational waves, may be primordial entities that formed just after the Big Bang.

Based on general relativity, the research team from Kyoto University studied how often black holes merge. The binary black holes found by the Ligo-Virgo team would match this theory if they were primordial, and if they made up one thousandth of all dark matter in the universe.

Primordial black hole binaries were heavily discussed in the 1990s but observations suggested they were limited. At present, no-one has found any primordial black holes, possibly making the Ligo-Virgo observations the first of their kind.
