In 1609, Galileo applied the recently invented telescope to looking at the stars, and since then, scientists have been looking to the sky to discover more about the universe that we live in.
Telescopes have become increasingly advanced, with the super sensitive James Webb telescope planning to be launched in 2021. Using them, we have plotted out more and more of the universe, discovering new solar systems and the weird and wonderful things that inhabit them. But how? Scientists today still use the same medium as Galileo did 400 years ago - the electromagnetic spectrum. This uses the waves as information carriers, each one containing information about where it’s come from. While this is by far the most common way of studying the stars, scientists are looking for other ways of exploring the universe, like using neutrinos and gravitational waves.
What are gravitational waves?
To understand what a gravitational wave is, first, we must understand what makes up our universe. While we are all familiar with the traditional model of space with its neat three dimensions, the reality is that there are four dimensions. The three dimensions of space and the one dimension of time are actually two parts of the same thing - imaginatively named ‘spacetime’. In his general theory of relativity, Einstein predicted its existence, as well as the fact that massive (‘having a lot of mass’) or rapidly moving objects can distort it. Figure 1 shows a simplified diagram of the warping of spacetime by a massive object. He also predicted that massive accelerating objects, like neutron stars or black holes, that orbit each other would send out ‘ripples’ in spacetime, as demonstrated in Figure 2. These would travel at the speed of light, containing information about their origins, just like EM waves. The biggest ripples in spacetime would be caused by violent events like the collision of two neutron stars. These ripples came to be known as gravitational waves.
Gravitational waves were originally detected indirectly in 1993, when three astronomers, Joseph Taylor, Joel Weisberg and Lee Fowler, were studying two neutron stars in orbit around each other. They noticed that they took less and less time to orbit each other at the exact rate predicted by Einstein’s theory of general relativity. Other such examples of this phenomenon were investigated, and after further analysis, gravitational waves were deemed as proven.
However, it took 40 years to actually detect one. A team of scientists from Caltech built a very specialised observatory known as LIGO, or Laser Interferometer Gravitational-wave Observatory. This consists of two identical interferometers placed 3000km apart across America. Each station uses two huge lasers located at right angles to each other along long shielded tunnels (4km each). There is a complex system of mirrors that amplify the laser, which itself is specially tailored using a very large amount of kit (see Figure 3). When a gravitational wave passes through, one axis will be compressed while the other will be extended (only by a length smaller than a proton), which LIGO can detect. Due to the sensitivity of the equipment, it picks up a variety of noise, for example earthquakes, local traffic and even internal laser fluctuations.
One of the ways one can differentiate between these gravitational waves and noise is by comparing the results from each detector. A gravitational wave would affect both detectors almost simultaneously, whereas noise is local, so its results would be different between detectors. For a wave to have been detected, both detectors must have experienced identical contraction/elongation at the same time. A second observatory, Virgo, in Italy works similarly. Figure 4 shows the simplified lay out of the detectors. In September 2015, LIGO finally detected its first gravitational wave, confirmed by the Vigo detector. These waves were later attributed to the collision of two neutron stars.
But why put so much effort into detecting them? What’s wrong with using EM waves? Well, a few things. Firstly, they can only show us one side of the universe. EM waves are great for detecting lots of things, like stars and planets, but they can’t tell us anything about distortions in spacetime, so by using these waves, we have a whole new window into the universe. This is like only seeing the world, and then suddenly being able to hear it too - it reveals a different side of the universe. EM waves also can’t detect things like black holes, as EM waves can’t escape the event horizon, but they will emit gravitational waves. This shows that we could learn so much about the universe, both phenomena we know exist, and ones we have only predicted.
Furthermore, gravitational waves barely interact with matter. This means that the information that they carry about where they come from is left undisturbed, giving us a clearer picture of the universe. This is unlike EM waves, which interact with many things. For example, they can be bent in the presence of large masses, and they can be absorbed or reflected off surfaces. All of these factors contribute to their relative unreliability compared to gravitational waves, which travel straight to us from the furthest reaches of the observable universe.
Overall, whilst they are hard to detect, gravitational waves could revolutionise the way that astronomers study the sky, allowing us to look at our universe in a completely new way.