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Gravitational waves are ripples in the fabric of spacetime caused by violent events such as colliding black holes or exploding stars, writes Alison Goddard.

First predicted by Albert Einstein in 1915 in his general theory of relativity, gravitational waves have never been directly detected. They are created by accelerating masses and travel at the speed of light. Any object that has mass will vibrate in response to the waves as they pass but the vibrations are incredibly small - equivalent to changing a person's height by less than the diameter of an atomic nucleus. In principle, experiments to find them should be straightforward, but it is only in the past few years that scientists have developed detectors sensitive enough to pick up the waves.

Researchers are now setting up experiments in several countries, including the United States and Japan, to detect gravitational waves. If successful, they could open a new window for astronomers and cosmologists, just as detecting radio waves and X-rays from space did in the past. In fact, the consequences could be more profound, as gravitational waves are a completely different type of wave.

There is evidence that gravitational waves exist. In 1974, Russell Hulse and Joseph Taylor from Princeton University in New Jersey discovered two stars orbiting one another. They calculated that the period of the orbit should fall at a certain rate if energy was carried away from the system by gravitational waves. Their work, which eventually showed that the orbital period fell as predicted, led to them landing the 1993 Nobel prize for physics.