Gravitational waves from merging black holes. 3D illustration. Credit: Peter Jurik/Alamy
Physicists have proposed a new way to detect elusive gravitational waves in the ‘midband’.
These waves are generated by binary systems of white dwarfs and neutron stars in the Milky Way and black hole mergers but are not detectable with current instruments.
The approach uses ‘optical resonator’ technology originally developed for optical atomic clocks, which keep time by measuring the oscillations of light emitted when atoms transition between different states.
Dr Vera Guarrera, from the UK’s University of Birmingham, says: “By using technology matured in the context of optical atomic clocks, we can extend the reach of gravitational wave detection into a completely new frequency range with instruments that fit on a laboratory table.
“This opens the exciting possibility of building a global network of such detectors and searching for signals that would otherwise remain hidden for at least another decade.”
The proposed approach uses 2 ‘optical cavities’ – resonators which bounce laser light between precisely spaced mirrors – at right angles to each other.
“The essential point of our approach is that gravitational waves in the mid-band do not deform the rigid spacer of the cavity … but instead alter the phase of the light as it propagates between the mirrors,” write the authors of the study.
“The cavity length defined by the spacer therefore remains constant, while the optical path through space varies with the passing wave.
“We show that state-of-the-art optical cavities have sufficient sensitivity to be readily employed to search for gravitational waves in the midband.”
Gravitational waves are caused by cosmological events in which massive objects like stars or black holes move around each other or collide, sending out ripples which stretch contract the fabric of spacetime.
“Because astrophysical and cosmological sources of gravitational waves have a rich spectrum covering many decades in frequency – from ∼ 10−18 Hz to ∼ 1 GHz – different techniques and technologies have been used or proposed to target specific frequency regimes,” the authors explain.
But midband gravitational waves, with frequencies of between 0.001 and 1 Hz, are “essentially unexplored” according to the researchers.
Professor Xavier Calmet from the University of Sussex in the UK adds: “This detector allows us to test astrophysical models of binary systems in our galaxy, explore the mergers of massive black holes, and even search for stochastic backgrounds from the early universe.”
There are several new technologies under development to observe midband gravitational waves, such as space laser interferometers LISA (Laser Interferometer Space Antenna), Taiji and TianQin, atom interferometers and optical atomic clocks on satellites.
However, these programs are still more than a decade away from operation. The researchers’ proposed optical cavity detector, while less sensitive, could be used immediately and cost-effectively to explore the midband.
“With this method, we have the tools to start probing these signals from the ground, opening the path for future space missions,” says Calmet.
The research is published in the journal Classical and Quantum Gravity.