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10 mins read 06 Nov 2019

Australian Scientists Put The Pinch on Black Hole Collisions

Australian researchers have developed a technique to reduce quantum noise, allowing gravitational-wave interferometers more precision at detecting stellar-mass black hole mergers.

Two merging Blackholes warping space time surrounding them creating arcs of light in their local region
Artist impression of two merging black holes, causing localised distortions of space-time. Credit: LIGO.

In September 2015, emanating from a region in the direction of both Magellanic Cloud galaxies in our southern skies – an event occurred that no one in history had previously witnessed. Described by Einstein in his General Theory of Relativity 100 years earlier (1915), the fabric of space-time distorted for 0.2 seconds, enough time to be picked up by some of the most sensitive equipment on Earth.

The event that caused the nature of space-time, the plane all existence lies within, to be squeezed and stretched took place approx. 1.4 billion years ago – long before our species came into existence.

The world had just witnessed its first detection of a gravitational wave.

This paradigm shift heralded in a new field in astronomy, as gravitational waves moved from theory to reality – rewarding Rainer Weiss, Kip Thorne and Barry Barish with the Nobel prize for their work in the field.

The Most Sensitive Devices In the World

Aerial view of LIGO interferometer building showing L-Shape design
The LIGO Livingston observatory, with 4km tunnels in L-Shape configuration. Credit: LIGO

The detection of this enormous event was made possible by advancements in laser interferometer technology that allowed the extremely sensitive and subtle ripples through space-time from the ancient event, to be observed.

To do so, two facilities in the United States - separated by 3,002 km (Hanford, Washington, and Livingston, Louisiana) were built by the Laser Interferometer Gravitational-Wave Observatory (LIGO) Scientific Collaboration. Unlike the regular image of observatory domes housing optical telescopes, or even large paraboloid radio telescope dishes – the LIGO observatories are two 4km vacuum tubes lying perpendicular to each other and connected where they meet through a building.

Within the observatory vacuum tubes, lasers fire towards each tube-end and onto massive, almost perfectly smooth, hanging mirrors known as the “Test Mass” where it is reflected back into a photodetector creating an inference pattern. If any passing gravitational waves sweep by Earth, then effectively they will cause the perpendicular arms to stretch and squeeze into ever so slightly changing distances, which manifests itself as a change in the laser’s inference patterns.

Diagram showing how the lasers are fired into the tunnels and hit mirrors and bounce back to create an inference image.
The internal workings of the LIGO lasers – fired into a splitter then sent down the 4km tunnels, reflected of test masses and returned to be combined to form an inference image. Credit: LIGO.

These shifts, however, are unfathomably small – 0.0000000000000000018 meters of change-making the detectors the most sensitive devices ever built by humans. At the time of the discovery, LIGO’s sensitivity was as accurate as 1/1,000th the width of a proton. At its most sensitive, the accuracy will be 1/10,000th the width of a proton – which is analogous to measuring the distance between Proxima Centauri (closest star system to us at 4.2 light-years) to within a width of a human hair.

Squeezing Lasers

So minuscule is the signal from the interferometers that they can be drowned out by local disturbances, such as a passing truck off in the distance or tiny temperature fluctuations within the tubes. As such, the engineers have had to devise clever ways to protect the sensitive measurements carried out by the detectors during observing runs – to ensure that findings are not the result of false data.

However, photons in the laser can’t be counted and the transfer of momentum that these photons apply onto the test masses as they hit them causes another type of disturbance known as ‘quantum noise’ – an outcome of the random nature of quantum physics – and requires a different approach to managing uncertainty.  

Researchers at the Australian National University have revolutionised a method called ‘squeezing’ – which dampens quantum noise and makes measurements from the interferometer more precise – a critical breakthrough for the next generation of detectors (or when current detectors are upgraded).

New experiments were now underway to confirm the findings in a new proof of concept that would be tested as a prototype before upgrading detectors.

“The detectors use particles of light called photons from a laser beam to sense the change in position of widely separate mirrors,” said Dr. Robert Ward, from the ANU Research School of Physics and the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav).

“However, the detectors are so sensitive that just the random quantum variability in the number of photons can disturb the mirrors enough to mask the wave-induced motion.”

The work that the ANU has produced has shown that the detectors can be made many fold more sensitive by applying the upgraded squeezing method to reduce the variability.

Black Hole Death Spiral Signature

Illustration of orbiting black holes showing gravitational waves radiating away
Two orbiting black holes radiating gravitational waves. Credit: LIGO.

Gravitational waves are caused when objects are accelerated in the universe. These waves are invisible and travel away from their source at the speed of light, roughly 300,000km/s, squeezing and stretching anything in their path.

However, these waves are so tiny that they are extremely unnoticeable for familiar objects like humans, cars, airplanes or even the Earth itself. This changes as the mass and acceleration of said objects increase to extremes – such as those associated with Neutron stars or black holes. For these much more massive objects, we can start to detect their gravitational wave disturbances on the universe.

Scientists have devised a set of four categories to classify gravitational waves, each based on its own unique ‘signature’ that would be detected by the LIGO interferometers:

  • Continuous Gravitational Waves – these are created when a single spinning massive objects rotate with an imperfection on its surface – such as a neutron star with a small bump. The spin rate remains the same, so the gravitational waves are emitted at the same, continuous frequency and amplitude
     
  • Compact Binary Inspiral Gravitational Waves – these are created by orbiting massive objects like white dwarfs, neutron stars or black holes with a decaying orbit. There are three sub-classes in this category:
    • Binary Neutron Stars (BNS)
    • Binary Black Holes (BBH)
    • Neutron Star-Black Hole Binary (NSBH)
       
  • Stochastic Gravitational Waves – these are the sum of all tiny passing gravitational waves from all over the universe, producing a random gravitational-wave background. It is thought that these could be relics from the original big bang or from supermassive black holes colliding in galaxy mergers
     
  • Burst Gravitational Waves – these are for everything else that we don’t know about (the “unknown-unknowns”). These types of events burst into detectors and scientists do not know what could cause them, nor expect them. To date, none have been detected

GW150914

Image of the wave signature produced by the detectors showing a rising peak to the right of the graph
The ‘Chirp’ that LIGO heard – the gravitational wave signature of GW150914. Credit: LIGO.

The 2015 BBH event (named GW150914) contained two rather large black holes – one 35 times the mass of the Sun, the other 30 times the mass of the Sun – which fell into an accelerating orbit and on a path for collision with each other.

During their in-spiral plummet, the black hole's velocity reached 60% that of the speed of light (180,000km/s), and upon merging – they produced an even larger black hole with the mass of 62 Suns. The remaining 3 solar masses were converted into energy that radiated away from the event – much like ripples expanding away from a heavy stone being dropped into a flat, calm pond – except these waves were packed with as much as 50 times the energy of all the combined power of radiated starlight in the observable universe.

The squeezing of light for the purpose of reducing its quantum noise effects has been a part of ongoing discussions within the science community for decades. One proposal which was put forward in that time was to consider continually measuring a mirror’s position with light – a method that could have given rise to achieving squeezed light.

Prior to the discovery of the first gravitational wave in 2015, a study published in Nature by Safavi-Naeini et al. (2013) describes how silicon sculptures, built on microchips could improve the reduction in quantum noise in light, thus improving the sensitivity of the LIGO detectors.

Another study from the same year (2013), by the LIGO Collaboration, described how the injection of squeezed light into one of the detectors improved the capability beyond the quantum noise limit for the frequency region down to 150 Hz.

After the discovery of GW150914, physicists at LIGO published further findings about using a device known as a ‘filter cavity’ that would allow simultaneous reduction of quantum noise for both the position and momentum of photons – which would improve the overall sensitivity of the LIGO detectors across entire observational bands.

The key differences that the ANU team have now achieved is that their paper describes how the team presents an engineered state of light (i.e. the squeezing) to directly manipulate the continuous measurement of the position of photons within the 10 – 50 kHz range and a result of 1.2 dB reduction of quantum noise. In turn, this should improve the detection sensitivity of the gravitational-wave interferometers.

LIGO Results

Image black holes masses and 11 detections showing gravitational wave signatures for 10 black hole mergers and 1 neutron star merger
LIGO results from O1: 10 binary black hole (BBH) mergers and 1 binary neutron star (BNS). Credit: LIGO/Frank Elavsky/Northwestern

During the first two observation runs (O1 and O2) which ended on 25 August 2017, the LIGO collaboration detected a total of 11 confirmed gravitational wave events (10 BBH + 1 BNS).

LIGO is currently in its third observation run (O3), which commenced on 1 April 2019 and running through to 30 April 2020, and this time including collaboration with European Interferometer observatory called E-VIRGO – which aids with localising gravitational wave events in the sky. In October 2019, it was announced that Japan’s Kamioka Gravitational-Wave Detector (KAGRA) would also be joining the collaboration soon.

Currently, in O3, there have been a total of 22 candidates who are now undergoing further review, but O3 is the first run where LIGO has utilised the ‘squeezed light’ method to counteract noise at higher frequencies.

Future Applications of ANU’s Results

The ANU team has played a vital role within the LIGO observations and overall project, as one of a few Australian partners to the global collaboration. In addition to Dr. Ward, other members of the team from ANU include Professor David McClelland, Ph.D. scholar Min Jet Yap and Dr. Bram Slagmolen.

“The ‘quantum squeezers’ we designed at ANU along with other upgrades for the current LIGO detectors have greatly improved their sensing capabilities,” Dr. Slagmolen said.

“The new-generation LIGO detectors will have the capability to detect every black-hole smash in the Universe at any given moment,” added Mr. Yap.

With these new findings, the upgraded quantum squeezers will be designed and built by the LIGO collaboration over the next few years, then fitted into any new detectors, in addition to being retrofitted into existing detectors. The upgrade should allow all gravitational wave observatories to be able to detect a larger number of these cataclysmic events, much deeper into the universe.

In the space of just over 100 years since Einstein predicted gravitational waves as an outcome of his General Theory of Relativity, scientists from around the world have been able to build highly sensitive observatories, and learn about violent events occurring in our universe billions of years before humans even evolved on our planet.

Everything about gravitational wave astronomy is extreme – from the massive objects and events that cause these space-time distortions to ripple across space, down to the extremely delicate and small measurements required to capture these ever-so-subtle shifts in our reality, unknown to our everyday presence.

Australian research continues to provide a leading role in these frontiers of science and continue to add value in improving global astronomical infrastructure that allows humanity to learn more about itself and its place in the universe.

There are plans to bring orbiting interferometers online – separated by millions of kilometres and able to detect with even more sensitivity. Resolving issues like quantum noise with squeezing will surely play a pioneering role in these future observatories and studies.

In the meantime, sit back and relax – and know that at this very second your body, your environment, and your reality is being stretched and squeezed by far distant monsters, battling in the sky.

The paper, titled 'Broadband reduction of quantum radiation pressure noise via squeezed light injection' is currently available on Nature