The ripple that caused a universal wave
A mere month after the 2017 Nobel Prize in Physics was awarded for the first ever detection of gravitational waves, another extraordinary announcement was made. Strange ripples in space-time were detected coming from a whole new – and far more illuminating – source, and Australian researchers were among the first in the world to watch it play out.
In 2015, almost exactly 100 years after Albert Einstein predicted the existence of gravitational waves, an international team of physicists detected strange ripples in space-time emanating from a violent collision between two monster black holes.
It was hailed as the “discovery of the century”. It won the 2017 Nobel Prize in Physics, and offered up a whole new way of understanding the Universe. But since then an even more extraordinary gravitational wave has been captured, and Australian scientists were at the heart of it.
"It’s entirely possible that this recent discovery will produce its own Nobel Prize,” says theoretical physicist Professor Susan Scott, leader of the General Relativity Theory and Data Analysis Group at the Australian National University (ANU), and one of the first groups in the world to respond to the signal.
“There’s been so much new physics that we’ve been able to get from this event. This is just the beginning.”
More than a century ago, Einstein used his theory of general relativity to predict the existence of gravitational waves: ripples in the fabric space-time that originate from objects moving about the Universe.
The only problem was that Einstein described them as being so faint that no human would ever be able to prove his predictions correct.
Or, as Scott puts it, “He didn’t think we’d have a hope in hell of detecting them."
But where Einstein saw the impossible, modern physicists saw a challenge, and with critical assistance from Australian researchers, we’ve been able to build detectors sensitive enough to catch gravitational waves radiating from the biggest, most catastrophic events in the Universe.
“Australians developed key components of the detectors, and enabled them to become sensitive enough to detect gravitational waves,” says Scott. For example, the ANU team has introduced quantum optical techniques to enhance interferometer performance and the University of Adelaide team has developed a sensor that could measure the change in shape of mirrors, buried deep within the detector, due to low-level absorption of laser power, with an unprecedented precision and accuracy.
The US-based Laser Interferometer Gravitational-wave Observatory (LIGO) was switched on in 2002, and by 14 September 2015, it had detected its first ever gravitational wave. The find was announced to the world five months later.
Since then, four more events have been recorded by LIGO, all resulting from cataclysmic black hole collisions.
As well as revealing a completely new form of radiation in the Universe, the five events also proved that general relativity – the most famous of Einstein’s theories and a cornerstone of modern physics – could pass every test thrown at it.
A new kind of gravitational wave
Fast-forward to 17 August 2017, and a sixth gravitational wave appeared when no one was expecting it.
“We were just eight days from the end of the nine-month second observing period for LIGO, and we thought, “We’re not going to get it this time,’” Scott recalls. “But sure enough, one came in.”
Three detectors – two LIGO detectors in the US and the Virgo interferometer in Italy – caught signals from this gravitational wave, and it sent the physics and astronomy worlds into overdrive.
“It was amazing,” says Scott. “This avalanche of telescopes and satellites all immediately trying to get onto this patch of sky. Everyone knew it was going to be the real deal and we couldn’t afford to miss it."
Scientists from around the world pulled together to analyse the signal, including dozens of Australian researchers from Monash University, ANU, the University of Melbourne, Swinburne University, the University of Sydney, the University of Western Australia, the CSIRO, and the LIGO-Virgo collaboration, and found that this one was different from all the other signals that came before it.
When Scott received an email from her US partners at LIGO, she had to re-read it four or five times before the news sunk in. This wasn’t just any old gravitational wave – it was the first one to be detected from a neutron star collision!
Better than black holes
A neutron star is all that remains of a star after it detonates into a supernova – one of the largest explosions in the Universe. This explosion leaves behind a small, heavy core about the size of Adelaide, but with up to twice the mass of our Sun, making it the densest form of matter known to science.
While black holes famously don’t emit any light, neutron star mergers emit a whole lot of it, which means this was the first time that physicists have been able to simultaneously detect gravitational waves and light, and actually ‘see’ the event.
Australian scientists were among the first to detect the explosion electromagnetically, with light and radio waves. ANU scientists captured images of the explosion’s afterglow via two telescopes at the Siding Spring Observatory in northwestern NSW. The ANU team reported the fireball's colour and relayed details of the heat generated. A couple of weeks later Sydney University researchers using an advanced CSIRO telescope were one of the first groups to confirm radio emissions from the explosion.
Australian researchers will continue to play a significant role through technology development for current and future gravitational wave detectors and data analysis conducted at numerous universities as part of the Australian Research Council Centre of Excellence for Gravitational Wave Discovery (OzGrav).
"The impact of this discovery is even greater than the collision of black holes,” says OzGrav Chief Investigator Scott. “And we were an absolutely key player.”
The fact that neutron stars are made of matter – and black holes technically aren’t – means that the information gleaned from this gravitational wave could tell the researchers so much more about the secrets of the Universe.
The discovery helped them solve a 50-year-old mystery by confirming that neutron star mergers are the source of rapid jets of high-energy gamma ray bursts, and also the likely origin of heavy metals in the Universe, such as gold, platinum, and uranium.
Having located the actual source of the gravitational wave – a galaxy 130 million light-years away – they were also able to make a direct measurement of the expansion rate of the Universe, and once again confirmed Einstein’s calculations showing that the speed of gravitational waves equals the speed of light.
"Einstein’s theory is standing up like a mountain,” says Scott. “It’s phenomenally accurate."
More on Professor Scott: With the detectors currently offline for an upgrade, Scott is taking time to focus on the 2018 Homeward Bound international leadership program for women in STEM, an initiative that she is passionate about. “Homeward Bound trains cohorts of about 80 women per year from around the world, to give them a voice in making science policy, and the leadership skills they need to take a major role in decision-making in to the next decade, particularly to do with the health of the planet”.
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