When we can understand the densest material in the universe, at the heart of a neutron star, we unlock a fundamental understanding of all matter.
I remember watching the first lunar landing on TV as a child, in 1969 – we’d been sent home from school for the day, and it went on for hours. Eventually my friends got bored and took off outside to play, but I sat there mesmerised watching those guys leaping about on the surface, as if in slow motion. I was fascinated in that aspect of the gravity of the Moon compared to Earth. I probably didn’t tell myself then that I was going to become a gravitational scientist, but I think that event really sparked my interest.
At the end of primary school, I won a scholarship to Methodist Ladies College in Melbourne. It was there I was able to follow my interests in mathematics and physics. Towards the end of secondary school I knew I wanted to go down the path of trying to be a scientist, but it seemed pretty crazy – nobody I knew was a scientist, and certainly no-one in my family. It seemed a bit ridiculous – but I thought I would give it a go and see where it ended up.
I never enjoyed the practical experiments we had to do in physics at university. I’m definitely a hardcore theorist – I think in very abstract ways scientifically. I studied special relativity in first year at Monash, then finished up doing my Honours year in general relativity.
Nobody I knew was a scientist, and certainly no-one in my family. It seemed a bit ridiculous – but I thought I would give it a go.
At that stage, general relativity had only been tested in what we call the weak gravity regime. And there were pretty good tests of Einstein’s theory in the weak field – we’re coming up to the centenary of the famous solar eclipse at Wallal in Western Australia, which I’m involved with, where we tested the bending of light from stars by the Sun. There was also the test of the precession of the perihelion of Mercury. But at the time I was a student, nothing had been done in testing in the strong gravity regime – near a black hole, for instance. And so the jury was still out, and nobody was even sure if it could ever be tested.
There was David Blair in Western Australia, and others, with gravitational wave bar detectors. But there weren’t the big-scale instruments we have now – they were trying to get lucky with something going off in the galaxy. So for me it was a fun time to be starting out in relativity, because there had been a certain amount of good stuff done, but some of the really big questions were yet to be answered.
When I began to be involved with gravitational wave detection in the 1990s, there was an experimental basis to our activities – a few groups were working on interferometer aspects and so on. But there was nobody involved in doing the science that was going to come out of these big gravitational wave detectors that were planned. I started that side of the field in Australia in the late 1990s – obviously, there was a long time between then and our first detection in 2015.
We thought that first detection would be two neutron stars smashing together, but it turned out to be two black holes.
The gravitational wave detectors sit there on the surface of the Earth and collect an incredible amount of data. First question: how do you process that data? In the early days, we were involved in producing data analysis systems for LIGO, contributing components. Then we became interested in possible noise sources that would contaminate the data – that’s the main problem with these detectors. There’s noise everywhere, which affects our data stream.
Then you go on to the real sciencey side, which is, okay, well, we want to have a first detection, so we need to have a signal to look for in the data. So you have to involve general relativity and calculate the exact signals that might come out of a system to actually make a first detection.
We thought that first detection would be two neutron stars smashing together, but it turned out to be two black holes. But all that side of it had to be developed. It was no easy thing. That was a big deal.
In 2017, we got gravitational waves from two neutron stars colliding – and that was spectacular, and gave us so much new physics.
The “next big thing” will be detecting gravitational waves from neutron stars, which I’ve been very closely associated with for some years now. In 2017, we got gravitational waves from two neutron stars colliding – and that was spectacular, and gave us so much new physics. But now we want to detect the continuous wave stream from a single neutron star as it spins. Obviously, the strength of those continuous waves is less than you get from the cataclysmic collision of two big black holes, which is why we haven’t detected them yet. But we desperately want to detect them because there’s so much about neutron stars we don’t know.
Neutron stars are about the size of Canberra, but with about 1.4 times the mass of the Sun packed into them.
I mean, they’re the densest type of star in the universe. They’re about the size of Canberra, but with about 1.4 times the mass of the Sun packed into them. So we don’t understand what the properties are of the material that they’re made of and their composition. We know they’re partly made of neutrons, at least. But we don’t know the exact size of them, and we don’t know how many types there are. We will unlock a lot of that information once we can detect this continuous stream of gravitational waves from these single spinning neutron stars.
It’s a completely new form of material. The nuclear physicists are crazy for us to achieve this because they cannot produce these conditions in an Earth-based laboratory – it can only be found out from these neutron stars out in space.
Obviously, it has to be a mission of us as humans to find out what is this densest material in the universe. It’s going to be vital for our fundamental understanding of the universe at large.
Distinguished Professor Susan Scott is Professor of Theoretical Physics in the Centre for Gravitational Astrophysics at the Australian National University. Her research expertise is in gravitational physics, general relativity theory, cosmology and gravitational waves. In 2022 she became the first Australian to be elected Fellow of the International Society on General Relativity and Gravitation (ISGRG), joining an exceptional club of scientists including Stephen Hawking and Nobel Laureates Roger Penrose and Kip Thorne.