I’ve recently been writing an assignment on gravitational waves, so I thought I might share some of what I have learned with you!
Gravitational Waves are a big thing in physics. Einstein predicted them in 1916, but nobody was quite sure about their existence till 2016. Einstein – in his Theory of General Relativity – combined space and time and described bodies acting on this space-time plane to attempt to describe Gravity.
I’m going to use a trampoline as an example. If you stand in the middle of the trampoline, it curves under your weight beneath where you stand. This is the same for planets, stars and other astronomical objects, curving space-time. Now imagine you are jumping on the trampoline (not large jumps, but little bounces). If you think about the material of the trampoline as you bounce, you would be right in thinking that your bounces would create little ripples in the trampoline. Similar to when you throw a pebble into water, these waves radiate outwards from you, and depend on how heavy you are and how energetic you are. These ripples are gravitational waves.
To summarise, gravitational waves are created by massive objects and energetic processes in our Universe. They are ripples in space-time and gravitational waves travel at the speed of light.
So, if Einstein predicted them in 1916, why was it such big news in 2016? To put it briefly, nobody believed Einstein. Scientists need evidence to support a theory and despite being a brilliant theory (which turned out to be true in the future), Einstein couldn’t prove his theory was correct.
Many people tried to detect gravitational waves. In 1974, we did find proof that gravitational waves were acting on a binary pulsar system (see http://www.astro.cornell.edu/academics/courses/astro201/psr1913.htm – a brilliant source and an extensive explanation of the Hulse-Taylor pulsar system). It was in 2016 that Physicists directly observed a gravitational wave. LIGO detected a wave of 35 – 250 Hz signal (Read Observation of Gravitational Waves from a Binary Black Hole Merger for more info) that couldn’t be explained by any activity here on Earth, but did match a prediction of gravitational wave emissions from a binary black hole system. A year later, LIGO and other interferometer observatories have detected many signals of gravitational waves. (Future post explaining Interferometers to come!) The evidence is stacking up, therefore scientists are becoming less sceptical of General Relativity and Gravitational Waves.
So, what next? In the immediate future, we are still using interferometers to find new signals and collect more and more data. However, our observations of gravitational waves from Earth’s surface remains limited. There is so much noise and activity on the surface that weaker signals cannot be detected. We can’t exactly ask the human population to sit still and remain quiet whilst we measure gravitational waves! So, Scientists hope to develop space interferometers to detect those weaker signals (such as emissions from single pulsars). Also, we hope to use a collection of pulsars (known as Pulsar Timing Arrays) to observe gravitational waves acting on objects. We can’t see the effect a gravitational wave has on Earth, so we must look out at others.
You might be wondering why gravitational waves are so exciting for Physicists. Well, it might come as a shock to say that we don’t really understand Gravity. It’s a force that governs everything we know and, many people would think, the easiest to understand . You only have to jump up and down to experience Gravity. However, Scientists wouldn’t agree. In this picture of the Universe they are trying to build, Gravity doesn’t fit in. So Scientists hope that by investigating General Relativity and Gravitational Waves, we’ll get closer to understanding and explaining Gravity.