4 mins read 16 Mar 2021

New Technology To Improve Gravitational Wave Detection

New technology from a collaboration involving OzGrav combines photons and phonons to improve the sensitivity of gravitational wave detectors.

An artist’s illustration of two black holes merging, an event which is a source of gravitational waves. Credit: LIGO/Caltech/MIT/Sonoma State (Aurore Simonnet).

Gravitational waves - a strange phenomena, first described as the outcome of Einstein’s General Relativity theory, where “ripples” are formed in the fabric of space-time by the movement of very large masses, such as the merging of in-spiral black holes or neutron stars. 

Measuring these waves, however, is a very delicate science which uses extremely accurate interferometers that serve as detectors, capable of measuring the vibrating effects of a passing gravitational wave that shifts the detector by a value as small as 1/1000th the width of a proton.

In a new paper published in Communications Physics, researchers from the University of Western Australia (UWA) as part of an international team have figured out how to make these detectors even more sensitive to high frequencies. 

The team is led by the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) at UWA, and is joined by the ARC Centre of Excellence for Engineered Quantum Systems, the Niels Bohr Institute in Copenhagen, Denmark, and the California Institute of Technology in Pasadena, California. 

Measuring Gravitational Waves

A schematic of the Laser Interferometer Gravitational-Wave Observatory (LIGO) interferometer. The world’s first observed gravitational waves were created in a violent collision between two black holes, 1.3 billion lightyears away. When these waves passed the Earth, 1.3 billion years later, they had weakened considerably: the disturbance in spacetime that LIGO measured was thousands of times smaller than an atomic nucleus. Credit: LIGO.

Gravitational waves are measured using a device called an interferometer. It uses a formation of mirrors kilometres apart to direct a laser beam to form an interference pattern of light, which moves if any of the mirrors are disturbed. The movement that these mirrors, caused by passing ripples in space-time around Earth, can detect are even as small as gravitational waves, which are much too subtle for humans to notice. 

The existing interferometers, with several now operating around the northern hemisphere, are tuned into detecting gravitational waves from a certain frequency - generated by objects like merging black holes or colliding neutron stars. However, gravitational waves can be generated by any accelerating masses (including humans running around each other!) but these are too feeble to detect. The larger the mass, the more it will shake the fabric of space-time more. 

The existing interferometers have been detecting and confirming binary black hole mergers and neutron star mergers since 2015, with advancements in the technology since then, improving detection capabilities. However, this system only has limited sensitivity. 

Phonons, Photons, and Physics, Oh My!

An artist’s impression of laser photons interacting with the OzGrav white light cavity. Credit: Carl Knox / OzGrav / Swinburne University.

Astrophysicists from OzGrav were motivated to improve the sensitivity of gravitational wave detectors when they noticed they were unable to view the formation of a black hole (an event that would have caused gravitational waves) in the 1 - 5 kHz range. This range is limited by an effect called shot noise, which makes it difficult to detect the waves. The solution that astrophysicists came up with involves cycling the signal billions of times through an interferometer by combining phonons and photons in a crystal-like structure that can be added to existing interferometers. This device is called a “white light cavity”. 

“More than a hundred years ago Einstein proved that light comes as little energy packets, which we now call photons,“ Emeritus Professor David Blair from UWA’s Department of Physics said. 

“Two years after Einstein's prediction of photons, he proposed that heat and sound also come in energy packets, which we now call phonons.”

An artist’s impression of the white light cavity device from OzGrav. Credit: Carl Knox / OzGrav / Swinburne University.

Lead author Dr Michael Page explained: “The trick is to combine phonons and photons together in such a way that we can amplify a broad range of gravitational wave frequencies simultaneously.” 

“The  new  breakthrough will  let  physicists  observe the  most  extreme  and  concentrated  matter  in  the known  universe as  it  collapses  into  a  black  hole.  This  happens  when  two  neutron  stars  collide.  The waveforms  sound  like  a  brief  scream  that  is  pitched  too  high  for  current  detectors  to  hear  them.  Our technology will make it audible,” Emeritus Professor Blair said.

“These sounds will also reveal whether the neutrons in neutron stars get split up into their constituents called quarks when they are in this extreme state.” 

This is not the first innovation in gravitational wave detection to come from OzGrav. Scientists at OzGrav have also developed devices such as the world’s most sensitive inertial vibration sensor to combat thermal noise in interferometers, and a device to increase interferometer sensitivity at high frequencies. OzGrav scientists have also contributed to research involving ‘squeezed light’, which has allowed them to observe the effects of quantum mechanics on large objects.