Imagine you are floating on a raft at sea on a calm summers day. There is no wind
and (for the moment), the sea is flat and motionless, no waves, no turbulence, just
flatness as far as the eye can see. This is what spacetime, the fabric of the universe
that links our three physical dimensions as well as time itself, looks like. Well I’m
simplifying a little here. This is what space time looks like for the vacuum. You
may have heard that planets and stars ”bend” or ”warp” spacetime. What is
happening here is akin to how on you in your raft are causing a (all be it slight)
displacement of the water beneath you. Your raft in order to float has sunk
down a little into the sea, bending and warping the water’s surface!
Now just as you are enjoying your day along comes someone to mess it all up.
You hear an obnoxiously loud whirring of an engine, as speed boat enters the bay.
This is speed boat is captained by its new and untrained owner, who has taken
to riding the boat around in circles just about a kilometre away from you. After a
short period of time you begin to notice that your raft is beginning to rock back
and forth. Much to your annoyance small waves from the speed boat’s antics
have rippled across the water and are shattering the tranquillity of what was a
wonderful day. These waves that occur due to the motion of large objects are also
seen in the fabric of spacetime! These are, as you may have guessed from the
title, Gravitational waves!
Gravitational waves were predicted as far back as 1916, by the well known
Albert Einstein, using his newly developed theory of general relativity. But
it wasn’t until almost 100 years later in 2015, that gravitational waves were
detected in the Laser Interferometer Gravitational-Wave Observatory (LIGO).
LIGO detected waves, which are theorised to be produced from the collision
of two black holes. This detection is done by sending light beams down two
perpendicular long tunnels and measuring the length change of the tunnels,
by detecting any changes in the time it takes for the light to travel down the
tunnel and back.
Gravitational waves are slightly different to the waves on water that we were
describing in our analogy. First of all the surface of water is a 2D surface where
as we live in 3D! The water waves also only have one way to oscillate, that is
they can move up and down. However gravitational waves have two possible
directions of oscillation. If a gravitational wave was to pass through your body,
it could warp space so that you get taller, or it could warp space so that you get
wider!
As researchers continue to refine their techniques for detecting and
interpreting gravitational waves, we can expect to unlock even more secrets
about the nature of spacetime and the universe at large. It’s a thrilling journey
that promises to revolutionise our understanding of the cosmos and inspire awe
and wonder for generations to come.
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