### Gravitational Waves

The universe is home to some amazing things! Over the past century scientists have found light from the most distant reaches of the universe using telescopes on earth and in space - they have seen the mergers of galaxies and the birth and death of stars. However orthodox techniques only *shine light* on a fraction of the universe. Many of the most interesting things - like black holes - are invisible to optical telescopes. We need a new window: opened not by using light waves, but gravitational waves. Not only will we be able to observe some of the most exotic objects in the universe with gravitational waves, but we'll also be able to probe the very nature of space and time! It works like this:

Gravity can be described as a curvature of space-time.

Imagine that space-time is like a giant rubber sheet. When massive object (like a star) sits on the sheet, the sheet (and therefore space-time) bends and becomes curved, and anything placed near the large object will accelerate and fall in towards the massive object.

From Einstein we know that gravity works a lot like this. When we experience a gravitational force accelerating us towards the earth, we're actually travelling along curves in space and time!

If two black holes were placed close enough to each other, they would accelerate towards each other; they would start circling around each other in a spiral and eventually merge in a giant collision. During this process the space they occupy becomes disturbed and this disturbance propagates at the speed of light to the rest of the universe. This phenomenon is exactly what a wave is, except these are waves of gravity.

So what do these gravitational waves do? The simplest way to describe their effect is that they "stretch" and "squash" the space they're travelling through - or, in other words, they change distances between objects.

A gravitational wave stretching and squashing an apple. Image credits: Paul Fulda.

Amongst the most promising sources of gravitational waves are colliding black holes, but even though they are extremely strong sources of gravitational waves, they cause things to stretch and squash by only a tiny amount. For example, a 1m long ruler might change in length by only around 10-22m (or 0.0000000000000000000001 m). So we need really sensitive instruments to detect this effect.