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

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.