According to Isaac Newton, gravity is a force which acts everywhere in the Universe, attracting massive bodies – including planets, stars, galaxies – towards each other. Gravity: Newton’s Picture Gravity makes apples fall from trees…. ….and the Moon ‘fall’ towards the Earth.

Gravity: Newton’s Picture But the Moon’s orbit means that (fortunately for us) it keeps on missing the Earth!

Gravity: Einstein’s Picture But how does the Moon know it’s supposed to orbit the Earth in the first place? How is the force of gravity between them transmitted across space and time? These questions troubled Albert Einstein, until his General Theory of Relativity provided an elegant answer.

According to Einstein gravity isn’t a force acting between bodies but a curving of spacetime in their vicinity. So the Moon is simply following the natural, contours of the curved spacetime around the Earth – like a marble rolling across a stretched sheet of rubber.

If the motions of matter are changing violently in time, this causes ‘ripples’ in spacetime – like a wobbling trampoline. We call these ripples gravitational waves. We can sum up Einstein’s picture with the phrase: “Matter tells spacetime how to curve and spacetime tells matter how to move”.

Gravitational Waves Although we haven’t detected gravitational waves directly yet, we can already clearly see their effect. Using the giant Arecibo radio telescope, astronomers Russell Hulse and Joe Taylor have mapped precisely the orbits of a close pair of neutron stars. Russell Hulse and Joe Taylor

Relativity predicts that the orbit of these neutron stars should be shrinking: energy is leaking away from the stars as they radiate gravitational waves. Their orbits are shrinking exactly as relativity predicts, although the stars are still too far apart for us to detect the gravitational waves directly.

But the stronger gravitational waves from other binary systems – where the neutron stars are already much closer together – should be detected by Advanced LIGO. As the gravitational waves spread out through the Universe, they become weaker – like ripples spreading across a pond.

By the time the gravitational waves reach the Earth, they cause only tiny movements in the LIGO detectors…. ….Which we look for as tiny changes in the length of path travelled by laser light, along the two 4-km arms of LIGO.