Gravitational waves allow astrophysicists to probe phenomena that are difficult to observe through conventional astronomy techniques. However, observing gravitational waves at frequencies below 10 Hz is practically challenging. Ground-based detectors are unsuitable for this task because of Earth’s seismic noise, making space-borne laser interferometers, such as LISA, our safest bet.

While several space-borne laser interferometer designs have been proposed, none is able to operate at low frequencies—particularly at around 0.1 Hz—with high sensitivity, because they would require spacecrafts flying in formations with unrealistic precision.

In a recent study, a pair of scientists from Japan Aerospace Exploration Agency and the National Astronomical Observatory of Japan have proposed a novel space-borne laser interferometer design that addresses this problem. Their topology was named the ‘back-linked Fabry–Pérot interferometer.’

It consists of three identical spacecrafts, each carrying two independent laser sources and two test masses for a total of three Fabry–Pérot cavities. These cavities are used to capture the effect of incident gravitational waves on the test masses and laser photons. Inside each spacecraft, the two independent laser sources are connected by a ‘back link’.

Whereas conventional Fabry–Pérot interferometers have to precisely maintain the distances between spacecrafts to ensure the cavities are at resonance, the proposed topology can do so by simply adjusting each laser’s frequency through the back link. This design, however, introduces the problem of laser frequency noise, which significantly degrades performance if left unchecked. The scientists solved this problem by developing a novel scheme to subtract these noises.

If successfully implemented, the proposed interferometer could help scientists probe ancient gravitational waves, which could revolutionize how we understand the history of our Universe. It will also let us observe astronomical phenomena involving heavier binary systems and provide useful information on the dark and relativistic sides of the Universe.