Albert Einstein’s prediction that the movement of astronomical objects could cause waves of spacetime curvature to be sent rippling through the universe, almost like the waves caused by stones being dropped into a flat pond, was proven in 2015 [1] by the first detection of a gravitational wave (GW) signal arising from the merger of two black holes.

This signal was detected by measuring the induced length change of two four-kilometre long laser arms, as the passing GW stretches and squeezes it.

Since then, a new era of GW astronomy has begun in which many new classes of detectors have been proposed to further the search for GWs arising from various astronomical sources. With the current generation of active detectors featuring strong sensitivity to only low frequency (slowly oscillating) GW signals, the detection of high frequency gravitational waves (HFGWs) remains an unexplored and extremely challenging front in GW astronomy.

Recent theoretical developments [2] describe some potential sources that may produce HFGWs, such as extremely light black holes merging into each other, large events during the universe’s early formation known as “phase transitions” and clouds of particles surrounding black holes which can slow down their rotation through the absorption of energy, emitting HFGWs in the process known as “black hole superradiance”. While sources of HFGWs remain hypothetical and are due to exotic new physics, the lack of active experiments in the field coupled with the increasing amount of theoretical sources motivates us to push GW detection technology into higher frequencies (Mega Hertz and above).

In this experiment the BAW device is connected to a superconducting quantum interference device or SQUID, which acts as an extremely sensitive amplifier for the small voltage signal coming off the quartz BAW. This assembly is placed in multiple radiation shields to protect it from stray electromagnetic fields and then cooled to an extremely low temperature (4 Kelvin or -269.15 degrees Celsius) so that low energy acoustic vibrations of the quartz crystal can be read-out as large voltages given by the SQUID amplifier. With this setup potential incoming HFGWs that stretch and squeeze the device at its high frequency operating point are detectable as large voltage signals across the SQUID.

This device was operational for 153 days in which two strong signal candidate events were detected. We make no claims as to the potential sources of these events but can rule out some possibilities such as violent weather effects, earthquakes, acoustic vibrations relative to the lab and potential high frequency tails of confirmed GW events. We identify the likely cause for such events to be one of the following: Internal solid-state processes attributed to the quartz crystal, impinging charged particles from cosmic showers, meteor showers or similar type events and potentially an unknown source of high frequency gravitational waves. 

With this work we have demonstrated for the first time that these quartz devices can be used as highly sensitive GW detectors. This experiment is one of only two, currently active and searching for HFGWs at these frequencies, with further plans to extend our reach to even higher frequencies where no other experiment has looked before. The development of this technology could potentially provide the first detection of a gravitational wave at these extremely high frequencies, perhaps telling us about some unknown and interesting physics.