Gravitational waves

One of the most exciting new discoveries in physics has been gravitational waves. Generated by the most extreme events in the cosmos (like the crashing together of neutron stars and black holes), gravitational waves are ripples in the very fabric of the Universe. They are disruptions in space-time* that can only be detected by the most sensitive instruments around the world.

*Space-time is the four-dimensional universe that includes time as well as the three spatial dimensions we are used to.

LIGO: The first observation of gravitational waves (Credit: Caltech)

What are gravitational waves?

LIGO: Journey of a gravitational wave (Credit: Caltech)

Isaac Newton recorded one of the first theories about gravity in his famous Law of Universal Gravitation. This law states that every object with mass – from the famous apple falling from a tree to the Moon orbiting the Earth – has its own gravitational field, which interacts with the gravitational fields of every other object in the universe.

There is nothing, however, that describes how the effects of gravity are transferred from one place to another. In fact, according to this law, if we move one of the objects to a different point in space, then the other mass “knows” and reacts instantly over any distance (meaning that this “information” is travelling faster than the speed of light). These problems couldn’t be explained until Einstein’s Theory of General Relativity, hundreds of years later.

Gravity and space-time

Gravity is one of the four fundamental forces of nature (along with electromagnetic, weak and strong nuclear forces). In general relativity, it is described as the way in which matter perceives distortions in space-time. In the words of the eminent theoretical physicist, John Wheeler: “mass/objects tell(s) space-time how to curve, and space-time tells mass how to move”.

As matter moves, it changes the curvature of the space-time in the form of waves. These disruptions spread out across the universe like ripples across a pond, travelling at the speed of light. However, because gravity is the weakest of the fundamental forces and space-time is very stiff, gravitational waves are incredibly small. Whilst any object with mass that accelerates produces gravitational waves (including humans and apples, and the Moon), the gravitational waves made by us here on Earth are much too small to detect.

Our current level of sensitivity only allows us to observe the gravitational waves generated by massive astronomical objects, accelerating extremely quickly. Even the most energetic events known to astronomy – such as two orbiting black holes about to collide into one another – create gravitational waves that move our detectors on Earth by an amount 1000 times smaller than the size of a proton.

Events that are sufficiently powerful to create measurable waves are rare, and are scattered across the universe, potentially billions of lightyears away. So, how can we detect waves which are so small, from such a great distance, that have so much to tell us about the universe?

How can we detect gravitational waves?

When gravitational waves travel past us, they stretch space-time slightly in one direction and compress it at right angles. Therefore, if we can detect an object being stretched and compressed in this way, we can detect gravitational waves. Due to the tiny size of gravitational waves, the amount of stretch and compression of space is correspondingly tiny.

These minute changes are detected using a piece of equipment called an interferometer. This instrument works by splitting and recombining a beam of light and creating a pattern (called an interference pattern) that can be studied and analysed. In this case, the patterns can reveal information about gravitational waves.

Basic schematic of LIGO's interferometers depicted with an incoming gravitational wave (Credit: Caltech/MIT/LIGO Lab)

One of LIGO's two detector sites, located near Livingston in Louisana, USA (Credit: Caltech/MIT/LIGO Lab)

The science behind wave detectors

Gravitational wave detectors consist of two right-angled ‘arms’ of several kms length in an “L” shape, with a mirror at each end. Here’s how they work:

A laser beam is split into two and sent down the two arms
The beams then bounce back from the mirrors and they recombine at a light detector, where they interfere with each other and create a pattern. The interferometer is carefully configured so that, normally, the recombined beams of light cancel each other out.
When a gravitational wave interacts with the interferometer, it stretches one arm and compresses the other, changing the distance each laser beam travels and the interference pattern of light that is detected. This process provides us with valuable information about the source that originated the waves millions of years ago.

Ultimate precision

The interferometers that we use to detect gravitational waves need to be extremely precise, to detect this sub-atomic stretch and compression. To do this, the detectors are extremely large: the LIGO detectors are the largest gravitational wave detectors ever made; each of its interferometer arms is 4km long and the entire system is designed to minimise any source of noise.

Noise, such as physical vibrations from the environment (from cars driving on nearby roads to waves crashing on distant ocean shores), fluctuations within the laser itself, even molecules crossing the path of the laser could hamper LIGO's efforts to make its sensitive detections. Keeping this noise at bay creates many engineering challenges:

Firstly, the 4km beam tubes are kept under ultra-high vacuum, which prevents sound waves and temperature changes from altering the path of the laser light.
Secondly, the mirrors at each end are held in place by a very complex seismic isolation system to counteract any vibrations using both electromagnetic and hydraulic systems.
Finally, LIGO detectors can't work alone. One interferometer can only tell you that a potential gravitational wave has been detected, and at least two detectors are required to be certain that it is not random noise. Because gravitational waves move at the speed of light, when a potential gravitational wave has been detected, we can check that the same signal arrived at the next detector with the right time difference (a few milliseconds). The detectors are sensitive enough to be affected by events like a tree falling, or someone dropping a hammer; but the tiny likelihood of this same event happening in different places, with the correct time delay, means that these detections can be safely discarded.
There are also thousands of sensors detecting these environmental noises and to compare with any potential gravitational wave signal.

Global scientific collaboration

With a third detector we can use the difference in signal arrival time across the three sites to triangulate where the source comes from. In fact, located across the world is an even bigger network of detectors, working together, in the effort to pinpoint where a gravitational wave has come from in the cosmos.

This network comprises two LIGO detectors in Washington and Louisiana in the USA, Virgo in Italy, GEO 600 in Germany and KAGRA in Japan.

Unlocking the universe's mysteries with gravitational waves

Gravitational waves might enable us to see the universe like never before. This is because:

Telescopes that observe the universe usually rely on some kind of electromagnetic waves like visible light, radio waves or infrared radiation to function; but these waves are easily blocked, bent or reflected by matter in their path.
The waves are virtually unaffected by matter, which means that they travel through the universe, effectively unchanged, providing incredibly accurate information of the sources billions of lights years away.
Scientists can use gravitational wave detectors to understand the information that is coded into these waves – they hold many secrets about the conditions that created them, potentially from events that we’ve never been able to observe before.

By tapping into this new source of information about the universe, gravitational wave astronomy might be able to solve some of the biggest puzzles in physics, including: how black holes form, how matter acts in extreme conditions, illuminating the nature of dark matter, and looking at the beginning of the universe long before light existed.