The detection of gravitational waves has opened the door to a new realm of cosmological discovery.
I have a very clear memory of the moment I knew I wanted to become a scientist – it was when Neil Armstrong stood on the Moon, in 1969. I was in my first year at a not-very-well-to-do high school in Perth, and they had the television on at lunchtime for the first time. And watching that event was just inspirational. You realise what discoveries might be out there. Right then I knew I wanted to try and understand more about the universe.
That was one of those events that stopped the world. And I think we experienced something similar when gravitational waves were detected in 2015 – a discovery that also reverberated around the world. It really captured the imagination of the general public – it was on news shows, on comedy shows, talked about in offices. It was just a fantastic event – and so wonderful to be a part of it.
When I was a student I read a book by a guy called Steven Weinberg – to quote him, he said: “The effort to understand the universe is one of the very few things which lifts human life a little above the level of farce…” His book, The First Three Minutes, was probably my inspiration to want to go on and do a PhD after I completed my honours degree in physics. I remember I went and spoke to my potential supervisor, and said, “I want to understand the universe.” And he said, “Oh that’s nice, David. But do you want a job afterwards?” He suggested I was better off doing experimental physics.
That’s how I became involved with laser physics, which was quite unusual at the time. I ended up getting a job at the Australian National University, where my colleague Hans Bachor was working on quantum optics. And one of its big applications was gravitational wave detectors. So having studied lasers, I found myself working in quantum optics – and realising I can apply this knowledge, the detection of gravitational waves, to an understanding of the universe.
LIGO – our Laser Interferometer Gravitational-Wave Observatory in the US – was an enormous project. There were over 1,100 scientists involved and four major countries, and Australia was one of the partners. My team’s role was firstly to contribute to understanding how to make this interferometer work.
So, what’s an interferometer? What we do is we take a laser beam, split it in two, and send it down perpendicular arms, each four kilometres long. It then hits mirrors, and reflects back to where it was in the first place. And we measure how long it took the laser beam to go down one arm compared to how long it took to go down the other arm. If a gravitational wave passes through our interferometer, that timing is different. Gravitational waves are ripples in spacetime – they cause objects to stretch and squeeze as they move through. As a gravitational wave passes through the laser it’s squeezed in one direction and stretches in the other. So with these two perpendicular arms, one arm will be stretched with the gravitational wave passing through getting a bit longer, the other one will shrink, getting a bit shorter. And we can use interference to make that measurement.
But even from gravitational waves generated in the most energetic events, that change is incredibly small – 10,000 times smaller than size of a proton. It’s an extremely small effect, so you have to build very big devices to measure extremely small effects. And we have to make those very big devices immune to all sorts of things on Earth that could disturb the positions of the mirrors – we only want the mirrors to be affected by the gravitational wave, so we have to put them in big vacuum systems and isolate them from earthly disturbances. We also have to make sure that the light we use is perfectly quiet as well; we don’t want the light to be fluctuating as that would disturb our measurement. A key part of our contribution was to reduce this, even below quantum limits. We then need to be able to control this distance measurement, one arm compared to the other. And that is another area in which the ANU got heavily involved – we worked out a system that controls the mirrors.
There are so many implications from this research across different fields of science. Proving Einstein’s general relativity right was just the very beginning – we call that a classical theory. There’s a mismatch between the classical theory of gravity and other types of forces which are in the world of quantum mechanics. We have to resolve these questions at some stage, and measuring gravitational waves might be able to help us do that. Can we use LIGO detectors to find out if there is a size scale above which quantum effects do not persist? Gravitational wave detector technology itself has found application across many areas – from perimeter and security sensing to Earth observation from space – and led to a number of spinoff companies, such as Canberra’s homegrown Liquid Instruments Pty Ltd.
When we first turned the detectors on we not only learned that Einstein was right – we observed something we hadn’t predicted. And that was two black holes of about 30 solar masses spiralling around each other and crashing together. That was what caused the gravitational wave that we measured. We didn’t expect there to be black holes of that type of mass. We hadn’t actually thought about that being the very first signal we would see, so it was quite serendipitous that this first signal was from an event that we hadn’t actually expected. We’ve since discovered that there are a whole range of black holes of different masses that spiral around each other – black holes of masses that we hadn’t predicted from any previous observations of astronomy. We’re now trying to understand where they came from and how they formed. We know they come from massive stars – but the process, which produced 30 to 100 solar-mass black holes, we don’t yet understand.
So we proved relativity, and we found this brand new signal – a signal, by the way, which only emits gravitational waves. It doesn’t emit light. Everything we understand from the universe up until then had been from the light given off – the optical light, or radio, X rays, gamma rays. But black hole collisions don’t emit light.
So my big excitement for the future is learning what else is out there in the dark side of the universe, just waiting to be discovered by these massive detectors.
It could be something like wormholes. Wormholes are truly fascinating – they’re predicted by relativity as well. A black hole is something which just ends in a singularity – space and time cease. But a wormhole potentially connects to another part of the universe, and it could give off a different gravitational wave signal. They’re still pure science fiction, but they exist in theory in general relativity.
There could also be cosmic strings – these are very long strings of gravitational energy which might be the seeds around which galaxies form. These cosmic strings might have a tension in them, and they could give off gravitational waves when they release that tension. We haven’t seen any of those.
We’re hoping that one day we will see gravitational waves from the very beginning of the universe. We know that there’s a stochastic background out there, called the microwave background, that is pervasive throughout the universe. Again, it’s not optical radiation. And we know that this “microwave background” was formed when the universe was about 300,000 years old. So that’s as far back in time as we have currently been able to look. But from the gravitational waves background, we can learn about the universe less than a second after the Big Bang! So we’d be learning about the nuclear physics and the particle physics that happens in this enormously energetic event inaccessible to particle accelerators on Earth – an event that happened at the beginning of time. However that’s a really hard one. I don’t think we will see the stochastic background of gravitational waves for many years. All sorts of other sources will obscure it, but sometime in the future we’ll learn about the universe when it was really, really young.
Many such discoveries will require a global array of detectors, featuring a full-scale future generation interferometer in Australia, to accurately locate the source on the sky. We call the pathfinder for this detector NEMO – the Neutron star and Extreme Matter Observatory. After this, the next big thing might be the LISA project. It’s very exciting – that’s the Laser Interferometer Space Antenna. LISA is a bit like LIGO but it’s in space. Instead of the mirrors being four kilometres apart, LISA will use three spacecraft in formation flying five million kilometres apart. Each spacecraft fires laser beams that are received by the other spacecraft. LISA will discover a new raft of sources that emit gravitational waves at lower frequencies than those observed by LIGO. That’s something in which Australia is becoming more involved now.
There’s a whole lot of interest in our gravitational wave detectors because we’re beginning to understand how the most massive objects behave, discover objects on the dark side of the universe and understand the fundamental forces in nature. But it takes a village – now over 2000 scientists, engineers and students from across every continent. It’s truly amazing what can be achieved by a global community working together. I have been fortunate to work with many outstanding colleagues in my centre at the Australian National University, including Bram Slagmolen, Robert Ward, Kirk McKenzie, Daniel Shaddock, Jong Chow and Susan Scott over many years, through OzGrav (the ARC Centre of Excellence for Gravitational-wave Discovery), and the LIGO Scientific Collaboration. I’m just thrilled to be involved.
Gravitational waves were predicted by Einstein’s general theory of relativity more than 100 years ago. After 40 years of experimentation, in 2015, the Laser Interferometer Gravitational-wave Observatory (LIGO) detected the death spiral of two stellar-mass black holes as the gravitational waves they emitted almost a billion years ago passed through its two detectors in the US, verifying those predictions. This is the edited transcript of a conversation David McClelland had with Cosmos contributor Graem Sims.
Professor David McClelland
As the lead Australian investigator in LIGO, Professor David McClelland has done work that significantly expanded our view of the Universe.