How to Test a Space-Based Gravitational-Wave Detector
The European Space Agency is moving ahead with plans to launch a gravitational-wave detector called “LISA” into space. LISA, which stands for Laser Interferometer Space Antenna, will listen for gravitational waves that are currently undetectable from ground-based facilities such as the Laser Interferometer Gravitational-Wave Observatory. Catching these subtle spacetime ripples will require instrumentation with phenomenally stringent precision. Now, researchers have developed a device to test a core piece of LISA’s laser-based technology and ensure that it meets the requisite performance requirements.
The planned LISA detector consists of three spacecrafts flying in triangle formation. Incoming gravitational waves will change the 2.5 million kilometers between each spacecraft by a few trillionths of a meter. To track those changes, the spacecraft will look for phase shifts in the laser light they receive from their two companions, a feat requiring precise measuring instruments with exceptionally low noise and low distortion.
To test the precision of LISA’s phase-shift-measuring hardware, Thomas Schwarze and colleagues at the Max Planck Institute for Gravitational Physics in Germany built and trialed a calibration device. Their device consists of three lasers whose beams interfere with each other in such a way that their phases—after being extracted by a prototype of LISA’s phase-measuring hardware—should cancel each other out. Any nonzero value reported reflects noise or distortion introduced by the phase-measuring hardware.
Others have suggested using three-laser setups to test LISA’s detectors, but the team’s device introduces an order of magnitude less noise than other proposals. With further refinements to their setup—such as swapping out photodetectors for models with lower noise—the team envisions that they could reduce measurement noise further. Doing that, they say, could enable their setup to serve as a critical performance check for LISA’s hardware.
This research is published in Physical Review Letters.
Christopher Crockett is a freelance writer based in Arlington, Virginia.