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Einstein predicted gravitational waves in his theory of relativity, but the first gravitational wave was not detected until 2015. Gravitational waves are ripples in spacetime caused by cataclysmic events in the universe. The wave detected in 2015 was caused by two black holes colliding with each other.

“It’s a very, very new field,” says Associate Professor Kirk McKenzie from the Australian National University (ANU). And it’s a field that took him to NASA’s Jet Propulsion Laboratory (JPL) for 11 years.

Because gravitational waves cannot be seen, detecting them requires a new type of telescope. The instrument used in the 2015 detection is called LIGO (Laser Interferometer Gravitational-Wave Observatory). It is an L-shaped concrete structure, each arm measuring four kilometres. The concrete arms house vacuum tubes with mirrors at each end, as well as at the join. Lasers are bounced off the mirrors to attempt to detect gravitational waves.

“When a gravitational wave comes through, it moves the mirrors a miniscule amount,” Kirk says. “If you can detect that, you can detect things that can’t be seen any other way.”

Since 2008, Kirk has been working on a project to put a version of LIGO into space. This version is called LISA (Laser Interferometer Space Antenna). LISA will consist of three spacecraft in a triangle formation. They will be positioned millions of kilometres apart, spinning cartwheels behind the Earth as it orbits the Sun. The spacecraft will search for gravitational waves by relaying laser beams between one another.

LISA is not due to launch until the 2030s, but the technology has other applications. While at JPL, Kirk applied the same ideas to another mission called GRACE Follow-On. (GRACE stands for Gravity Recovery and Climate Experiment.) GRACE Follow-On consists of two satellites that follow each other in orbit around Earth. The satellites can detect changes in the amount of water and ice on the Earth’s surface by measuring the variations in gravity’s strength.

“If you go over the Himalayas in winter, for example, the mountains are a bit heavier than in summer because there’s more ice,” Kirk says. “They measure things like sea level rise, ice cap melts, and the effects of drought. This measurement of water moving around the Earth over time gives us a record of climate change.”

The most exciting part of working on the project was helping build the satellites. “I put the hardware on the spacecraft, helped bolt it down,” Kirk says. “It’s up there in space operating as planned.”

Kirk left JPL for ANU in 2019, but he is still involved with both LISA and GRACE Follow-On. “I’m lucky enough to work with a great bunch of people at ANU, but also colleagues in the US and Europe,” he says. “The fact that I can work on an earth science mission, even though I’m not an earth scientist, is really inspiring.”

Kirk’s journey

  • Kirk completed a Bachelor of Science with honours at the Australian National University in 2002.
  • In 2008, he completed his PhD in experimental physics, looking at gravitational wave detection.
  • After completing his PhD, Kirk moved to California to be a postdoctoral fellow at NASA’s Jet Propulsion Laboratory.
  • He stayed at JPL for 11 years, holding positions as an Optical Engineer and NASA Instrument Manager. He worked on the LISA and GRACE Follow-On missions during that time.
  • Kirk returned to Australia in 2019. He is now an Associate Professor and Head of the Space Interferometry Group at ANU.
  • Kirk is a Chief Investigator at two Australian Research Council Centres of Excellence: OzGrav and EQUS.
  • In 2019, Kirk received a NASA Exceptional Public Achievement Medal for his work on the GRACE Follow-On mission.