What are gravitational waves and why is their discovery important?

Gravitational waves were predicted by Albert Einstein in 1916 as part of his general theory of relativity. They are distortions, or 'ripples', in the fabric of space-time – but their discovery has remained elusive for 100 years. Now, scientists from Caltech, the Massachusetts Institute of Technology and the Laser Interferometer Gravitational-wave Observatory (LIGO) Scientific Collaboration are set to make a big announcement about them – possibly their discovery. So what are they and why are they important?

Einstein prediction

Einstein showed that massive accelerating objects, such as black holes and neutron stars, would disrupt space-time so that waves of distorted space would radiate from the source. These ripples were predicted to travel at the speed of light and contain information about where they came from. The physicist said that space-time is not a void but a four-dimensional fabric that can change as objects move through it.

Visualising distortion

A popular way of imagining gravitational waves is if you were to place a heavy object on a trampoline. The trampoline would bend because of the weight of the object. A lighter object placed on the same trampoline would roll towards the heavier one. An even heavier object would further bend the trampoline, pulling both other objects towards it.

This is sort of the same as gravitational waves – the greater the mass of an object, the more it would warp space-time. When any mass moves through space, ripples, or gravitational waves, would be generated. The biggest gravitational waves would be produced by catastrophic events – colliding black holes, for example.

They are yet to be detected

Scientists are fairly sure that gravitational waves exist, but they have never been detected. This is because by the time that gravitational waves reach Earth, the amount of space-time movement they generate is extremely small – thousands of times smaller than an atomic nucleus. This makes recording and measuring them very difficult.

In 1974, astronomers at the Arecibo Radio Observatory in Puerto Rico discovered a binary pulsar – two dense and heavy stars orbiting one another. This was the sort of system that general relativity suggested should generate gravitational waves. So they started measuring how the period of the stars' orbit changed over time. Eight years of observations showed that the stars were getting closer to each other at exactly the same rate predicted by general relativity.

Since then, scientists have continued to monitor this system and astronomers have been looking at the timing of pulsar radio emissions, which again were seen to have similar effects. But confirmation of the existence of gravitational waves has remained confined to mathematical theories with no physical contact.

How to detect gravitational waves?

The LIGO project aims to find physical evidence of gravitational waves. Using LIGO's Interferometer, scientists are looking to detect these tiny measurements. As the gravitational wave passes, it should stretch space in one direction and shrink it in the other.

The Interferometer looks out for these changes by splitting a single laser beam in two and sending these beams from two distant locations. If the beams travelled the same distance when they return, then they have not been distorted in any way. However, if they do not align when they return, something has disrupted them in the process – one potential culprit being gravitational waves.

"Gravitational waves cause space itself to stretch in one direction and get squeezed in a perpendicular direction," the LIGO has explained. "In the wake of a gravitational wave, one arm of an interferometer lengthens while the other shrinks, then vice versa. The arms will change lengths in this way for as long as it takes the wave to pass."

This seems fairly straightforward – but it isn't because of how tiny the changes would be. The Interferometer is able to detect these miniscule changes, down to around 1/10,000th the width of a proton. But other things could also cause disruption, such as earthquakes or road traffic, so these must be discounted before physical evidence can be found.

Why is the discovery of gravitational waves important?

The detection of gravitational waves would allow scientists to observe the universe in an entirely new way. They come from some of the most violent events in the history of the universe – even from the Big Bang (primordial gravitational waves are believed to have been discovered in 2014). Being able to detect and analyse the information carried on them would open up a new area of study of the universe, allowing us to better understand these events. Furthermore, it would help prove Einstein's general theory of relativity and provide an insight into what parts of the theory do not fit in.

"Since gravitational waves don't interact with matter, they travel through the universe completely unimpeded, giving us a crystal-clear view of the gravitational-wave universe," the LIGO has noted. "They will carry information about their origins that is free of the distortion or alteration suffered by electromagnetic radiation as it travels through millions of light years of intergalactic space. With this completely new way of examining astrophysical objects and phenomena, gravitational waves will truly open a new window on the Universe, providing astronomers and other scientists with their first glimpses of previously unseen and unseeable wonders, and greatly adding to our understanding of the nature of space and time itself."