What is waving in gravitational waves?

2 min read

We’re all familiar with gravity. We’re also familiar with waves (image source 1|2): 

But gravitational waves?
To begin, a gravitational wave observatory is very different from a normal observatory.  Here is how it looks like! Those detector arms are 4km long! 

Image source

So, what are these arms detecting?
What is actually waving in gravitational waves?
Spacetime! Not space by itself. Not time by itself, but space + time = spacetime!

Scientists imagine spacetime as 4-dimensional: 3 dimensions of space and 1 dimension of time. Einstein’s theories of relativity in the early 20th century changed our view of space and time. 

What is gravity? 
With relativity, we imagine gravity as motion within spacetime. 
Mass and energy distort or curve spacetime, and objects “falling in gravity” are simply moving along the shortest path in spacetime. 

So, what can these arms detect? 
When distant massive objects such as black holes collide and merge, they send out gravitational waves: ripples in spacetime! When they reach Earth, the waves are tiny. The arms detect these small vibrations in spacetime using super-precise lasers and mirrors. 

Hear black holes colliding
The sound of two black holes colliding can be heard here: 
Video | The Sound of Two Black Holes Colliding | LIGO Lab | Caltech (menlosecurity.com)

Why are gravitational waves exciting? 

Gravitational waves were first observed in 2015, after decades of hard work by scientists. 
As gravitational waves are different from light – which traditional observatories use to observe the universe – they open a new window to the universe and help scientists answer some of the biggest questions!

Illustrated by Eric Lua
Written by Seow Koon Sng


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