Gravitational wave detection ‘comparable to Galileo’

2 August 2016

Observing gravitational waves is as revolutionary as when Galileo first looked through a telescope, says the University of Glasgow’s Professor Martin Hendry.

Professor Martin Hendry

Asked after his IOP Summer Sessions public lecture on 1 August about which other scientific discoveries the detection of gravitational waves measures up to, Hendry said it was at least comparable to the discovery of the Higgs boson, adding: “If I were to be provocative I’d say it’s greater than that. It gives us a whole new way of looking at the universe. It’s like 1609 when Galileo first looked through a telescope.”

Hendry’s lecture, Making Waves, held at the IOP’s London premises rather than Granary Square due to wet weather, recounted how gravitational waves had been detected in September 2015, with that detection publicly announced in February 2016 – a century after they were first predicted to exist in Albert Einstein’s general theory of relativity.

Hendry, professor of gravitational astrophysics at the University of Glasgow and a member of the LIGO gravitational wave collaboration, recapped Newtonian gravity and problems with that theory, how Einstein reformulated gravity as the curvature of space and time, how that theory predicts waves in spacetime, and how efforts to detect them developed – and were ultimately successful.

Newton treated gravity as being due to mutually attractive forces between bodies with mass. But he couldn’t explain why those forces existed, and his theory was problematic as it implied that gravitation propagates instantaneously, when it should be limited by the speed of light.

Hendry explained how Einstein showed that mass causes spacetime to curve, and that bodies then follow a curved path instead of travelling in a straight line. He used a picnic blanket and a basketball in a variation of the rubber sheet analogy, and said that in Newtonian physics objects would follow the lines of the blanket’s tartan pattern, but the basketball creates a gravity well and their paths curve instead.

Picnic blanket gravity well

Continuing the analogy by dropping the ball on the stretched-out blanket and watching it vibrate, Hendry explained that gravitational waves are ripples in spacetime caused by massive bodies accelerating, which spread out across the blanket. This gets around the problem of instantaneity as they do take a finite time to travel

As the effect is too weak to generate in the lab, we have to wait to detect gravitational waves coming from space. “What we need to do is to look at the cosmos itself,” Hendry said. “That’s what we detected on 14 September last year.”

Detectors are looking for astrophysical events that are very energetic and very far away. They are expected to be violent occurences such as black-hole collisions or inspiralling binary neutron stars.

The gravitational waves detected last year and announced in February this year were from just that kind of event – thought to be the merging of two black holes of 36 and 29 times the mass of the Sun, located a billion lightyears from the Earth. The change in local spacetime that the detector had to pick up was about a trillionth of the width of a human hair. This is “a huge technological challenge”, Hendry said.

The problem is not only spotting an effect that is so small, but also isolating it from background noise. To illustrate the difficulty, Hendry had the lecture’s audience create background noise while vocally imitating the chirp in the gravitational wave signal – which was virtually impossible to distinguish above the din of the crowd.

To detect gravitational waves, LIGO’s two detectors use laser interferometry, splitting beams of laser light and sending them down 4 km long perpendicular arms before reflecting them back to their source and recombining the beams. If one path is longer than the other, this is evident in the recombined beams’ interference pattern. And one arm lengthening relative to the other is “just what would happen if a gravitational wave went by,” Hendry said.

For the detectors to be sensitive enough to detect such a tiny change they have to be large, and the mirrors have to be isolated from local disturbances. LIGO’s recent upgrade – it’s now known as Advanced LIGO – involved greater isolation of the detector’s mirrors using silica-fibre wires. The UK played a major role in developing the mirror suspension system, led by Hendry’s group at Glasgow. “Getting to that point was the culmination of decades of painstaking work,” Hendry said.

Hendry went on to explain that the five months between the detection of gravitational waves and announcing them publicly were spent checking the data, confirming that the signal was real, and figuring out where it originated. The time taken for the wave to travel between the detector in Louisiana and the one in Washington gives an idea of the direction the wave came from, and physicists can analyse the shape of the interference pattern and work out the nature of the source.

A second detection was made in December 2015 and confirmed in June 2016. Hendry said that we could be seeing hundreds of events by 2019 as the detector gets more sensitive, and, as more gravitational wave detectors come online around the world, we’ll be able to triangulate their positions more and more accurately.

He added that it’s likely we’ll see gravitational wave sources that we don’t yet know the nature of: “One of the exciting things about doing this is building a detector and seeing what we find. The really exciting stuff is still to come. There’s bound to be more surprises up the universe’s sleeve.”

Responding to an audience member’s question about why the public might care about gravitational waves, Hendry added that beyond having opened a new window on the universe and being worth detecting for their own sake, the spinoff technology of the lasers and mirrors has applications in industry, medical physics and material science. He speculated that there is also an even more exotic possibility of one day being able to directly harness gravitational waves themselves for “hoverboards, antigravity and interstellar travel”.

Speaking after the event, Hendry said that having been involved in the first direct detection of gravitational waves was “a tremendous experience to be part of”. “I was blown away by all the interest”, he said. “Professionally it’s been the most interesting six months of my life.”

The IOP’s public engagement manager, Manisha Lalloo, said: “The IOP Summer Sessions have given us a chance to explore how physics affects our lives with top physicists working in the field. Already this year we’ve seen how physics is helping us to create new electronics, understand our changing climate and uncover our archaeological past.

“Martin’s talk was a great end to the series, giving us an insight into gravitational waves – and explaining why such a big discovery is important to us all.”