Gravitational waves from nearby pulsars could be detected using just a few kilograms of superfluid helium-4, according to physicists in the US. Their detector, which is yet to be built, would measure sound waves in the superfluid caused by gravitational waves in the 0.1–1.5 kHz range.

Gravitational waves are ripples in space–time that are created when massive objects are accelerated under certain conditions. The first gravitational-wave detection was made in 2015, when the LIGO observatory spotted a signal from a coalescing binary black hole. Two more gravitational waves have since been detected by LIGO, both associated with binary black holes.

LIGO is a wideband detector that can detect signals in the 10 Hz–5 kHz range. It is particularly good at detecting transient signals (that change in frequency) associated with coalescing black holes.

Low-noise measurement

Swati Singh of Williams College, Laura DeLorenzo and Keith Schwab of Caltech and Igor Pikovski of Harvard University want to build a detector that can focus on a relatively narrow frequency band to detect gravitational waves from pulsars. A pulsar is a rapidly rotating neutron star that is expected to continually broadcast gravitational waves at a specific frequency in the 1 Hz–1 kHz range – with the frequency depending on the physical characteristics of the pulsar. By making a narrow-band measurement over a long period of time, a very low noise signal from a pulsar could in principle be detected.

Singh and colleagues' detector comprises several kilograms of superfluid helium held in a cylindrical container that is coupled to microwaves in a superconductor resonator. Confinement in the container means that the superfluid will resonate with sound waves at certain frequencies – just like a musical instrument.

This acoustic resonance also means that the superfluid should act like an antenna that is tuned to detect gravitational waves at specific frequencies. When such a gravitational wave travels through the detector it would create a strain field that would create sound waves in the helium. The microwave resonator would then convert these waves into a measureable signal.

Adjustable frequency

Although others have tried to make such antennas using metal bars, the team says superfluid helium offers several benefits – including the fact that the frequency of the detector can be changed by adjusting the pressure of the helium.

Writing in New Journal of Physics, the team reckons that, using state-of-the-art microwave transducer technology, the detector could measure signals from certain types of pulsars after running several months.