Based on his General Theory of Relativity, Einstein predicted ripples in the fabric of spacetime, called gravitational waves. It took a hundred years, the input of 1,000 scientists from around the world, loads of money and an enormous risk to observe them directly - and Australia played a major role in this remarkable global collaboration.
The discovery was made on 14 September 2015 in the US, at the Laser Interferometer Gravitational-wave Observatory (LIGO). But the gravitational waves, which were detected by both of LIGO's twin detectors located in Louisiana and Washington state, came from an event far away and a long time ago - in fact from the most violent event ever recorded in the universe: the collision of two black holes around 1.3 billion years ago.
The beams are used to monitor the distance between mirrors precisely positioned at the ends of the arms.
According to Einstein's theory, the distance between the mirrors will change by an infinitesimal amount when a gravitational wave passes by the detector. A change in the lengths of the arms smaller than one-ten-thousandth the diameter of a proton (10-19 meter) can be detected.
Independent and widely separated observatories are necessary to determine the direction of the event causing the gravitational waves, and also to verify that the signals come from space and are not from some other local phenomenon.
At present, a third Advanced LIGO detector awaits approval to be established on the Indian subcontinent.
According to the physicists involved in the discovery, the gravitational waves were produced during the final fraction of a second as the two black holes merged to single, more massive spinning black hole.
LIGO scientists estimate that the black holes were between 29 and 36 times the mass of the sun, and as they merged around 3 times the mass of the sun was converted into gravitational waves.
This confirms what Einstein had predicted in his theory on general relativity - that space-time is a four-dimensional fabric that can be pushed or pulled as objects move through it. Accordingly, a pair of black holes orbiting around each other will lose energy through the emission of gravitational waves, causing them to gradually approach each other over billions of years, and then much more quickly in the final minutes.
During the final fraction of a second, the two black holes collide into each other at nearly one-half the speed of light and form a single more massive black hole.
According to Einstein's formula E=mc2, a portion of the combined black holes’ mass is converted to energy, which is emitted as a final strong burst of gravitational waves. It is these gravitational waves that LIGO has observed.
There was already strong evidence for gravitational waves. In the 1970s and 80s, Joseph Taylor and colleauges made the discovery of a binary system composed of a pulsar in orbit around a neutron star, a discovery rewarded with the Nobel prize in 1993. The scientists observed that the orbit of the pulsar was slowly shrinking, and they concluded that energy was released in form of gravitational waves.
However, it is now the first time that the effects of gravitational waves have been directly observed on Earth. This was made possible through a major upgrade of LIGO costing around $200 million, which increased the sensitivity of LIGO's detectors, and now allows the scientists to probe a much larger volume of the universe.
The gravitational waves were detected in the first observational run of the upgraded detectors.
The Australian contribution to this breakthrough through the Australian Partnership in Advanced LIGO, which was led by the Australian National University and was funded by the ARC, is considerable, both to the development of the detector technologies and the sophisticated data analysis.
And the excited reactions from the scientific community suggest it was worth the effort. There is a general believe that the discovery will open up new fields in physics and astrophysics, and will give scientists a new way to study the universe, black holes, dark matter and gravity.
Here a few comments:
Sensing for the first time these rumbles in space-time will go down as one the major events in the history of physics, made possible by a close-knit, world-wide collaboration using instruments whose sensitivities are approaching limits imposed by quantum mechanics. And this is just the beginning.
With this detection we have shifted from the realms of theory to the beginning of a new astronomy. Hopefully this first observation will accelerate the construction of a global network of detectors to enable accurate source location for multi-messenger astronomy.
This verification of Einstein's general relativity in the nonlinear strong gravity regime was done with massive instruments whose amazing sensitivities approach limits imposed by quantum mechanics - a fact Einstein would no doubt have found amusing* (* In regard to quantum mechanics, Einstein is quoted as saying that “God does not play dice with the universe” Of course his ‘God’ was the laws of physics.)
We built the most massive scientific instruments in the world and and made them so sensitive that they approach limits set by quantum mechanics. On September 14 last year, they directly detected for the first time the weakest signals in the universe – gravitational waves – generated in the most violent event yet recorded – the collision of two solar mass black holes.
The energy released in this binary black hole collision was equivalent to 10 billion billion billion times the world's nuclear arsenal.
What's even more fascinating is that this event (BBH collision) did not (and does not) emit electromagnetic waves or neutrinos – the only way to observe it was with spacetime change sensors - our giant laser interferometers.
The ANU designed, constructed, installed and commissioned the lock acquisition subsystem. This crucial subsystem initially fixes the interferometer mirrors with respect to each other. Once this has been established, mirror separations induced by a passing gravitational wave can then be read out from the change in the laser light leaving the interferometer.
With skilled craftsmen in our workshop, we also built, installed and commissioned 30 small optics steering mirrors for routing the signal beam around the interferometer and into the photo-detectors where the optical signal is turned into a voltage.
When I first heard of GW150914 from one of my post docs, I thought it was probably a blind injection. When I was informed that this was not the case, the excitement was palpable and my group monitored "the chatter" 24/7. Once a signal was confirmed, I was overwhelmed by the enormity of what our international collaboration of over a thousand scientists and engineers had achieved. Waves in spacetime really do exist. They do propagate over astronomical distances. And they can be detected – they do detectably modulate the optical path of our interferometers.
Twenty five years ago, the idea of building giant optical sensors limited by quantum mechanics to detect the weakest signals in the universe to help us understand it in a new way, drove me to initiate an Australia-wide collaboration. We are now at the dawn of that new era and I am proud to have Australian technology in the Advanced LIGO detectors.
Whilst a new field of astronomy is the most enduring outcome of all our work, the brilliant young scientists and engineers we have produced and the contributions they will make to science and technology will also be long lasting.
Detection may not change my work but it will change the Australian physics and astronomy community's view of my work. Thankfully, we will never again have to address the damning remark - 'but what if gravitational waves cannot be detected?'.
Our current model of the universe is derived largely from information carried by electromagnetic waves emitted by only a small component of the universe. The gravitational wave LIGO detected was emitted by objects we can't see. Now we will be able to eavesdrop on the violent dark side of the universe. Who knows what else we will find now that we can both look and listen to the universe?
The Advanced LIGO detectors are a technological triumph and the discovery has provided undeniable proof that Einstein's gravitational waves and black holes exist. I have spent 40 years working towards this detection and the success is very sweet. We are on the threshold of a potential revolution in which gravitational astronomy could dramatically change our understanding of the universe and its evolution.
The discovery confirms Einstein's prediction that gravitational waves exist, validating one of the pillars of modern physics. It confirms that black holes exist and orbit each other in binary systems, teaching us important lessons about how stars are born and live their lives.
It is incredibly exciting to be participating as scientific history is being made. Every aspect of this research is elegant and beautiful. The LIGO detectors are genuine marvels of precision engineering. Einstein's theory of relativity, which predicts the existence of gravitational waves, brings together the concepts of geometry and gravity in a wonderfully inspiring way. The sources that LIGO detects, like black holes, are the home of some of the most fascinating physics in the Universe. It is very exciting to think that we now have a new and powerful tool at our disposal to unlock the secrets of all this beautiful physics.
Humanity is at the start of something profound and perpetual. We now have a new way of looking at the Universe and we will never stop looking. Gravitational waves are neither scattered nor absorbed by the material they pass through, so they let us peer right into the heart of some of the most extreme environments in the Universe, like black holes and neutron star, to do fundamental physics experiments under conditions that can never be copied in a lab on Earth.
The possibilities are endless.
Gravitational waves are akin to sounds that travel through space at the speed of light. Up to now humanity has been deaf to the universe. Suddenly we know how to listen. The universe has spoken and we have understood!
We have just passed through the threshold from being deaf to the universe, to being able to hear and understand. This is the tip of an iceberg. A whole new spectrum is open to us. This is like Heinrich Hertz's first detection of radio waves. He never guessed that it would revolutionise life in the next century.
We have opened a whole new frontier by creating exquisite and almost unimaginable technologies that have allowed us to measure vibrations as small compared with atoms as atoms are compared to people.
By measuring the smallest amount of energy ever measured, we have detected the most powerful explosion ever observed in the universe, in which three times the total mass energy of the sun was emitted in pure explosion of gravitational energy in a time of less than one tenth of a second.
The discovery of this gravitational wave suggests that merging black holes are heavier and more numerous than many researchers previously believed. This bodes well for detection of large populations of distant black holes – research carried out by our team at Monash University. It will be intriguing to see what other sources of gravitational waves are out there, waiting to be discovered."
Einstein's General Relativity has been a highly successful theory passing all tests conducted in our Solar System in the weak gravity regime. With the detection of gravitational waves from this binary black hole merger, it has passed with flying colours its first test in the strong gravity regime which is a major triumph.
We now have at our disposal a tool to probe much further back into the Universe than is possible with light, to its earliest epoch.
This discovery is the first direct detection of gravitational waves, predicted in 1916. It is a further confirmation of the validity of general relativity as the correct theory of gravity.
The most exciting thing is that it opens the door to a new window on the Universe. In the same way that radio astronomy led to the discovery of the cosmic microwave background, the ability to 'see' in the gravitational wave spectrum will likely to lead to unexpected discoveries.
This detection marks the beginning of the age of gravitational wave astronomy.
If you run your hand through a still pool of water waves will follow in its path, spreading throughout the pool. Now that we've caught these waves, we can use them to see the Universe in entirely different ways to what was previously possible.
The interferometer system includes a series of mirrors which are coated with multiple precisely controlled layers of optical materials to give the required reflective properties and lastly a top layer of gold, designed for thermal shielding.
The coatings, which were developed and applied at CSIRO, are among the most uniform and highly precise ever made. This precision ensures that LIGO's laser remains clean and stable as it travels through the detectors.
We really are world-leaders in this area, and are thrilled to play a part in this discovery.