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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.