Shaking the Universe
In search of gravity waves
Every now and again something calamitous happens out there in space. Stars undergo massive supernova explosions or two black holes collide. Physicists believe that such massive acts of violence literally send ripples across the universe in the form of gravitational waves - perturbations of space-time itself.
Einstein predicted gravitational waves over 90 years ago but to date, no one has directly observed one. Their detection would open a new window for astronomy giving us a completely new way to probe the universe – akin to a deaf person being able to hear sounds for the very first time.
The reason Gravitational waves have proved so difficult to detect is that in spite of their violent origins, their impact on Earth based detectors is incredibly small. The change in the shape of space as a wave passes by is smaller than a million, million, millionth of a metre. And movements on that scale are incredibly difficult to measure.
Instruments currently operational, such as the Laser Interferometer Gravitational Observatory (LIGO), are within a factor of 10 of the required sensitivity. However with conventional detectors, most signals would be masked by the inherent quantum noise in the interferometer.
Facilities, like LIGO are already both expensive and physically huge. Increasing the size by a factor of ten or a hundred is out of the question. So scientists need to look at ways to do better with what they have. And one of those ways is to try to get round that problematic quantum noise on the laser.
That’s exactly what scientists like Professor David McClelland of the ANU Centre for Gravitational Physics are doing. “You can’t simply eliminate quantum noise in the way you can eliminate interference in electronics because it’s a fundamental property of the light. That’s just the way the universe is put together. But what you can do is use the Heisenberg uncertainty principle to decrease the quantum noise in one parameter of the light at the expense of increasing it in another. Then make your measurements using the less noisy parameter.”
This technique is known as squeezing the light and is a speciality of the ANU group. It’s rather like trying to squash a balloon: it gets smaller in the direction you’re squeezing but expands elsewhere.
Over the past decade, Professor McClelland’s group have been building a gravity wave laboratory in which to develop the squeezed light interferometer parts. This has required the construction of huge seismically isolated tanks in which prototype instruments are isolated from vibration in the environment by 200dB. To put that into perspective, it’s like wearing earplugs that make a jet engine exhaust sound a thousand times quieter than a snowflake falling.
Having eliminated all the physical sources of vibration, the scientists are able to detect the minute buffeting of the mirrors caused quantum vacuum fluctuations. “This quantum buffeting may set a limit on the sensitivity of gravity wave interferometers at low frequencies,” Professor McClelland says, “Though we’re using squeezing technology to reduce this too.”
In optical astronomy replacing human eyes with sensitive photographic or CCD detectors enabled the same telescopes to achieve millions of times better sensitivity. Squeezed light detectors may do the same thing for our existing gravitational wave observatories.