Breakthrough in hunt for gravitational waves

A new physical device makes it possible to observe – and hear – quantum effects at room temperature, a breakthrough that could help the search for gravitational waves.

Currently, gravitational waves from colliding black holes are detected by bouncing laser beams off mirrors at each end of the four-kilometre-long arms of the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the US.

However, the sensitivity of these detectors in limited by a trade-off. Increasing the power of the laser beams theoretically results in more precise measurements, but in doing so the mirrors themselves are moved by the radiation pressure of the light, known as back action. This motion in turn presents a quantum radiation pressure noise that ultimately limits the sensitivity of the detector.

Investigating such an effect, and potentially any solutions to address it, has been made easier in a breakthrough by physicists from Louisiana State University (LSU) in the US.

Typically, quantum mechanical effects are only apparent at ultra-low temperatures, but in a paper published in the journal Nature, the researchers demonstrate a table-top experiment that explores the noise at room temperatures.

Researcher Thomas Corbitt and his team constructed a miniature version of LIGO, with a tiny mirror pad the size of a pinprick suspended from a cantilever at the end of each tiny arm.

A laser beam was directed at one of the mirrors and as the beam was reflected, they report, the fluctuating radiation pressure was enough to bend the cantilever structure, causing the mirror pad to vibrate, thus creating noise.

“It really is amazing to step back and think about the fact that quantum mechanics – something that seems otherworldly and removed from the daily human experience – is the main driver of the motion of a mirror that is visible to the human eye,” lead author Jonathan Cripe explains.

Corbitt adds that the work offers clues for LIGO physicists about how to avoid the back action interference during gravitational wave detection.

“Given the imperative for more sensitive gravitational wave detectors, it is important to study the effects of quantum radiation pressure noise in a system similar to Advanced LIGO, which will be limited by quantum radiation pressure noise across a wide range of frequencies far from the mechanical resonance frequency of the test mass suspension,” he explains.

Advanced LIGO is the name given to the latest upgrade of the massive facility. Further work will have to overcome the back action barrier.

Pedro Marronetti, a physicist and director of the US National Science Foundation, the agency responsible for LIGO, notes that it can be tricky to test new ideas for improving gravitational wave detectors, especially when reducing noise that can only be measured in a full-scale interferometer.

“This breakthrough opens new opportunities for testing noise reduction,” he says.

“The relative simplicity of the approach makes it accessible by a wide range of research groups, potentially increasing participation from the broader scientific community in gravitational wave astrophysics.”