Long arms capture gravitational waves

Gravitational wave detectors – interferometers – have two long arms. Laser pulses travel to mirrors at the ends of the arms, then reflect back. When they meet, their patterns reveal whether a gravitational wave has passed by.

On 7 January 1610, when Galileo Galilei aimed his new telescope at Jupiter, he was the first to see three of the gas giant’s biggest moons. At the time his invention revolutionised astronomy, allowing us to see objects too faint or small to the naked eye. Telescopes have grown over the four centuries since Galilieo; scientists have mapped the entire visible universe, and have peered back through developments in the galaxy all the way to the formation of the first stars.

Today astronomers stand once again on the threshold of a new era, in which they could for the first time gain ‘sight’ into the dark side of the universe. Ground-breaking technology will allow the new telescopes to see what no ordinary telescope can see.

The messengers heralding this transformation are gravitational waves. Space itself oscillates when large compact masses such as black holes and neutron stars accelerate tremendously – or collide. It was as recently as 2015 that scientists first identified gravitational waves, using two detectors in the US. Those waves came from two black holes that merged in a nearby galaxy.

The discovery was a triumph for the scientists, and for the two detectors. But within a few decades, that equipment will seem as primitive as Galilei’s telescope does today. Astrophysicists have already scaled up and have much more sensitive detectors in the pipeline. That means they will reveal gravitational waves from events that took place much further into time and space.

So far, in fact, that the new detectors will tell us about the universe’s dark childhood, before any stars had yet begun to burn. And they may also teach us more about two phenomena that were central to the development of the universe.

The first is as-yet unknown dark matter, which we cannot see, yet which holds together the ordinary matter of the galaxies with its gravity. The other is the mysterious dark energy that must exist for the universe to be expanding at its known acceleration.

Black holes distort Earth

Scientists hope to understand both these phenomena by studying gravitational waves from black holes that merged in the young universe. When gravitational waves from a collision between two black holes travel through Earth, they make our planet rhythmically expand and contract. The effect is very slight. The detectors must be able to measure the difference with an accuracy to one hundred-thousandth of a nanometer.

Existing detectors – interferometers – are L-shaped, with arms 3-4km long, through which scientists send laser pulses. At the end of each arm a mirror reflects the pulses back to the centre of the detector, where the beams meet again. This normally results in negative interference, where the light waves cancel out. But when a gravitational wave travels through the device, the length of the arms changes slightly, so the mirrors move. This displaces the laser pulses in relation to each other, causing positive interference in which they amplify each other, producing a pattern that the detector records.

Future detectors will be more sensitive by a factor of 10

The mirrors are built on shock-absorbing systems, but even so, other things could make the mirrors move – anything from a local earthquake to trucks passing nearby. So the trick is to build two interferometers, and have them far apart. Then they will not feel the same local vibrations, but will both feel gravitational wave. By comparing data from the two sites, scientists can ignore the vibrations that differ and look for signals that appear at both detectors the same time.

The two American detectors – LIGO – that picked up the first gravitational waves in 2015 were around 3000km apart. Then in June 2017 the European Virgo detector entered service in Italy, and in 2020 the Japanese KAGRA detector followed suit. Scientists now have access to a total of four detectors, positioned across the world.

Detectors cooperate

When gravitational waves make all of Earth oscillate, the signal is recorded near-simultaneously by all the detectors, with the spread of detectors allowing a more accurate estimation of where the waves come from, and so where in the sky astronomers should search for their source.

In the first six years of gravitational wave detection, the international team of scientists, which includes researchers from the Australian National University, captured 90 detections over three observing runs from 2015 and 2020. In November 2021 the team announced 35 new detections of gravitational waves caused by pairs of black holes and neutron stars smashing together. The detectors have just been upgraded, and

The mirrors at the ends of the detector’s arms are built on shock-absorbing suspensions, to neutralise slight earth vibrations. Using a second distant detector allows scientists to further cancel out local effects.

Detector captures the wave 3 If the gravitational wave is strong enough, the laser beams are displaced so much that their light waves are in phase. The result is positive interference, in which the gravitational wave is revealed.

A gravitational wave passes by 2 When a gravitational wave passes through the detector, one arm is slightly prolonged, whereas the other one becomes slightly shorter. Their light waves are then displaced when they return.

Laser beams cancel each other out 1 In the interferometer’s basic position, the light waves of the two laser beams are in phase opposition. The result is negative interference, in which the two laser beams cancel each other out.