Gravitational waves: a beginner’s guide

Colliding black holes
Credit: NASA

What’s the story?

A collaboration of physicists has announced the first ever direct detection of gravitational waves, thought to have been emitted by merging black holes.

What are they?

In Einstein’s general theory of relativity, gravity is treated as being a result of space and time bending in the presence of mass.

Gravitational waves are ripples in spacetime travelling at the speed of light. They’re produced when bodies with mass accelerate, changing the curvature in spacetime around them, with those changes then spreading outwards from their source as waves.

Why is this a big deal?

Gravitational waves were one of the last unconfirmed predictions of Einstein’s theory.

They hadn’t ever been directly detected before, although there was indirect evidence – for example the orbits of a binary pulsar discovered in 1974 around their common centre of mass were found to be getting smaller at a rate consistent with the loss of energy through gravitational waves.

Why hadn’t they been detected before now?

Any gravitational waves reaching Earth are so weak that they need extremely sensitive equipment to detect them, and that equipment also has to be shielded from other possible sources of interference.

Although predictions of gravitational waves are a century old, the technology to actually detect them has only been around relatively recently.

How are they actually detected?

Gravitational waves passing through an object distort that object’s shape, stretching and squeezing it in the direction the wave is travelling.

Gravitational wave detectors can pick up these distortions by splitting laser light into two perpendicular beams that travel for several kilometres before being reflected back to a detector.

Any differences in the wavelength of the two parts of the beam indicate a distortion suggestive of gravitational waves.

The Ligo detectors on which a group of physicists claims to have detected gravitational waves began operating in 2002 and was upgraded in 2015. They use beams of laser light 4 km long, over which length gravitational waves coming from 10s of millions of lightyears away would stretch and squeeze the detectors by about the width of a proton.

So now we’ve detected them, what’s next?

Ligo uses two detectors at separate sites – one in Louisiana and the other in Virginia. Because gravitational waves arrive and are detected at slightly different times in each, their source can be worked out by triangulation.

Consistent detection opens up the possibility of gravitational wave astronomy – carrying out observations of bodies in space using their gravitational waves rather than the various wavelengths of light. It would open up an entirely new way of looking at, and learning about, galaxies, black holes, and even the beginnings of the universe.

For even greater detector sensitivity and the ability to pick up fainter sources, the European Space Agency is planning to launch a space-based gravitational wave detector in 2034.

Related content from Physics World:

What is a gravitational wave?
Professor Ben Stappers, a University of Manchester astrophysicist, explains how gravitational waves are the astrophysical equivalent of throwing a stone into a pond.

How can we detect gravitational waves?
MIT researcher Professor Nergis Mavalvala explains how LIGO uses interferometry to search for the effect of gravitational waves here on Earth.

What is a black hole?
University of Bristol astrophysicist Dr Andy Young describes a black hole from a classical-physics perspective.

The Advanced LIGO run
Professor Nergis Mavalvala speaks to Louise Mayor in June 2015 about what Advanced LIGO might detect.

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