Gravitational waves: Opening our ears to the Universe

On Thursday 11 February 2016, an international team of scientists announced the first-ever detection of gravitational waves – ripples in the fabric of space-time predicted by Einstein a century ago. But what does this mean? Will the discovery change anything? Are gravitational waves actually useful? Let’s think about it…

The discovery

The Laser Interferometer Gravitational-Wave Observatory (LIGO) detected gravitational waves produced by the collision of two black holes, respectively 29 and 36 times the mass of the Sun. The waves took more than 1.3 billion light years to reach us, so the collision occurred when only multicellular life existed on Earth.

The black holes orbited each other 30 times a second before accelerating to half the speed of light, slamming together and merging in a collision that lasted only 20 milliseconds.

Simulating eXtreme Spacetimes (SXS) Project. A computer simulation of a collision between two black holes.
(Credit: Caltech)

Immense energy was emitted during the merger, equating to around three times the mass of the Sun. Dr Stuart Reid of the University of West Scotland notes that the “emitted energy in gravitational waves is comparable to the total light power being emitted by all the stars in the entire galaxy… this is staggering!”.

The energy caused ripples in the curvature of space-time to propagate outwards at the speed of light.

A computer simulation showing the warping of space and time around two colliding black holes observed by LIGO on September 14, 2015, and the emission of gravitational waves.
(Credit: LIGO/SXS/R. Hurt and T. Pyle)

These gravitational waves, subsequently observed by LIGO, provide vindication of the final major prediction of Einstein’s theory of general relativity.

A new type of astronomy

‘Great!’ I hear you say, ‘is that it then’? Sheila Rowan, Director of the University of Glasgow’s Institute for Gravitational Research, thinks not:

“This detection marks the birth of gravitational astronomy. It expands hugely the way we can observe the cosmos and the kinds of physics and astrophysics we can do – with more discoveries to come!”

Sheila Rowan discusses the UK’s contribution to the LIGO experiment.
(Credit: Caltech)

What is ‘gravitational astronomy’?

Astronomers currently observe the Universe using telescopes that detect light, X-rays and microwaves – all forms of electromagnetic radiation. Gravitational waves however provide an entirely new way of observing the Universe.

Ed Daw of the University of Sheffield usefully explains they are “so completely different from light... a gravitational wave will propagate almost completely unaltered through entire planets, star systems, galaxies, whereas electromagnetic waves are vulnerable to interference.'

As gravitational waves travel unimpeded through matter, they provide information on objects that don’t emit light, such as black holes. This is why gravitational astronomy is described as listening to the Universe.

And indeed Professor Gabby Gonzalez, spokesperson of LIGO, played a (frequency-shifted) audible chirp of the gravitational waves detected by LIGO.

Gravitational waves converted to sound waves: the sound of two black holes colliding.
(Credit: LIGO: Caltech)

As Dr Ik Siong Heng of the University of Glasgow eloquently puts it:

“We can now listen to the symphony of the cosmic orchestra played to us from the darkest, densest regions of the Universe”.

What will we hear?

This is where it gets really exciting.

Kip Thorne, one of LIGO’s founding fathers, said at last Thursday’s conference that, until now, astronomy has looked upon the cosmos as if it is a “calm sea” but gravitational waves will allow astronomers to observe space-time during its most violent storms.

With its first observation, gravitational astronomy has detected the cataclysmic merger of black holes. So what other treats are in store?

Professor B S Sathyaprakash of Cardiff University thinks that LIGO has “opened a new window to observe violent processes, such as merging neutron stars, supernovae, gamma ray bursts and other cosmic phenomena.”

In addition, gravitational astronomy will probe the speed of gravitational waves, the extreme physics of neutron stars, supernovae explosions, defects in space-time known as cosmic strings, and the speed at which the Universe is expanding.

Gravitational waves may also hold the key to understanding the origins of the Universe itself. While the early Universe was opaque to light, it was transparent to gravitational waves. So gravitational waves may contain information about the singularity at the beginning of time. Wow.

Listen up

Gravitational waves are important, useful and may profoundly change our understanding of the cosmos. So keep your ears peeled!

STFC press release

See STFC press release for details of STFCs contribution to this discovery.

Science and Technology Facilities Council Switchboard: 01793 442000