Building New Sensors for Gravitational-Wave Detectors

Did you know that you can build your own Michelson interferometer, the 19^th century device that forms the basis of modern gravitational wave detectors, for under $200? Not that you’ll be able to measure the distortions in space-time that accompany the death throes of stars and the collisions of black holes; but requiring just a low-powered laser pointer, a couple of mirrors, mounts, and some glue, it is a great introduction for curious minds to the properties of light.

And who knows, maybe that will be enough to provide the inspiration for a future astronomer or scientist. Perhaps even one that gets to work in or an observatory of the scale and complexity of the world’s great nuclear physics labs or giant particle accelerators.

Or better still, maybe your future scientist will end up being a part of one of Australia’s premier science organisations, and one that is at the forefront of the emerging branch of observational astronomy known as gravitational-wave astronomy. That organisation is the ARC Centre of Excellence for Gravitational Wave Discovery, also known as OzGrav, and there are scientists there right now working on ways to make future gravitational-wave interferometers even more sensitive to the bangs and pulses going off regularly in the universe around us.

Probing the ripples in the curvature of space-time requires incredible precision, and the first detection of gravitational waves in 2015 involved the most precise distance measurement ever made. But to make such precise measurements, scientists need to account for all manner of earthly vibrations that are also picked up by the detectors.

With scientists already working on the next generation of super-sensitive gravitational-wave detectors, there is a need to develop sensors that will be up to the task of sensing the ripples in space-time originating from cosmic collisions billions of light-years away.

One of the problems that scientists need to deal with is thermal noise. Gravitational-wave signals below a frequency of about 10-Hz are not able to be picked up by current generation detectors, and these long wavelengths are particularly susceptible to thermal noise. Because temperature is a measure of the movement of atoms, even miniscule vibrations at an atomic scale can be a source of noise. Future detectors will need to be cooled to cryogenic temperatures to see an improvement in performance.

Addressing this problem, Dr. Joris van Heijningen from OzGrav recently developed the world’s most sensitive inertial vibration sensor, and now has plans to build another, even more sensitive, device operating at less than 10 degrees above zero in the Kelvin scale – temperatures around -270 degrees Celsius. The sensor would be at least two orders of magnitude more sensitive than current state-of-the-art sensors and operate at frequencies as low as 10 MHz.

But what about at high frequencies? To increase the reach of future gravitational-wave detectors, there are also ideas focussing on improving their high-frequency sensitivity, which is currently limited by the uncertainty inherent in quantum physics.

In quantum mechanics, there is a principle known as Heisenberg’s uncertainty principle that asserts that there is a limit to how precisely we can measure both the position and momentum of a particle. It is a property inherent to all wave-like systems, a fundamental limitation that is written into the laws of nature.

Although we can’t circumvent these laws, OzGrav researcher Parris Trahanas and his team believe that the effects can be reduced by using a new type of optomechanical cavity known as a DEMS (double-end-mirror-sloshing) cavity. Optomechanical cavities are used in gravitational-wave detectors to trap and amplify light, thus enhancing the gravitational-wave signals that are received.

While the mirrors in optomechanical cavities, or resonators, are designed to move, too much movement, for example due to thermal fluctuations, presents itself in the data as noise. The DEMS cavity though uses special techniques to prevent it from becoming easily disturbed by random noise. It is the optical equivalent of connecting two spring mass systems together with a third spring.

These two pieces of innovative research indicate that we are well on the way to being able to build the next generation of gravitational-wave detectors. While the current generation Virgo and LIGO detectors have arm lengths of 3 and 4-km respectively, the next generation Cosmic Explorer will have arms stretching 40-km over the North American countryside.

Australia has its own plans to build a gravitational-wave interferometer, one that is optimised to study the nuclear physics going on in merging neutron stars. That project is called NEMO (Neutron star Extreme Matter Observatory), and its optimum sensitivity will be at high frequencies, in the kHz band. Operating with other gravitational-wave detectors around the world, it will be a necessary part of continuing gravitational-wave discovery while we wait for future-generation detectors like Cosmic Explorer and the orbiting LISA mission to be built.

And while OzGrav scientists continue to play an important role in today’s gravitational-wave research, it will be those curious minds, the ones who built a $200 Michelson interferometer just for fun, that will likely be leading us through that next phase of gravitational-wave discovery.