What are gravitational waves?

• What is a gravitational wave?

They are ripples in the fabric of spacetime that move at the speed of light. Imagine them like ripples on the surface of a pond after you throw a stone into the water.

Gravitational waves carry energy away from a massive objects through space. They are caused by the force of gravity stretching and squeezing space itself. Their intensity depends on how much energy the waves are carrying - if two enormous black holes collide, for example, or a star explodes into a supernova, the gravitational waves from the event would spread far and wide across the galaxy.

Albert Einstein predicted that gravitational waves must exist a century ago. It was part of his general theory of relativity, which describes how gravity works in the universe. General relativity has been proved correct time and time again in the previous 100 years and gravitational waves are the last missing piece of evidence.

• Why has it taken so long to find them?

Gravitational waves are so weak and no instrument has yet been sensitive enough to detect them. Even a massive event such as a supernova explosion within our galaxy would produce gravitational waves that only change the distance between the sun and the Earth by about the diameter of an atom for a few hundredths of a second.

• How have we been looking for them?

Ever since Einstein predicted them a century ago. But the technology really got going in the 1990s when German and British scientists began work on GEO600, an experiment in Hanover. There are also several other detectors run by Italian and Japanese teams.

The US-led Laser Interferometer Gravitational-Wave Observatory (LIGO) project has been in operation since 2002 and consists of two detectors spaced very far apart: one in Hanford, Washington and the other in Livingston, Louisiana. They operate together to hunt for the tiny fluctuations caused by gravitational waves.

• What can we do with gravitational waves?

Don't hold your horses for any practical applications of gravitational waves. But they will be important for astronomers who might now be able to build telescopes that can use gravitational waves to make images of parts of the universe that have been completely hidden to us until now. They will give us insights into dark matter and dark energy, for example, mysterious substances that makes up 96% of the universe but which we cannot see with any modern instruments.

Gravitational waves will also let scientists look further back in time than ever before - possibly even to take pictures of the moment of creation itself, the Big Bang.

Whoever first detects gravitational waves will a dead-cert for a Nobel prize. It will be one of the most significant scientific discoveries of the past century.