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The noise hunter

We had to learn that things don't always go the way you think they'll go ...…

The noise hunter

The noise hunter

From painting walls and cleaning a laboratory in Germany to being part of the team which detected the first gravitational waves and helping to design the future generation of laser interferometers, Professor Stefan Hild has come a long way on his journey to Glasgow, along the way tackling the effects of “virtual photons” and battling a “mystery noise.”

To describe what he’s doing today at the Institute for Gravitational Research, Hild asks the following questions: “How can you make photons stiffer than diamond? How can we measure space-time without encountering the Heisenberg Uncertainty Principle? What does the Universe sound like?” As an undergraduate student, however, Hild’s experience of gravitational research was not quite as exotic – he was employed for eight euros an hour to fit out the laboratory in Germany, building cleanrooms with simple sheets of acrylic glass and silicone sealant, never dreaming that one day he would be a member of the international team, designing and developing the laser interferometers (detectors) used to discover gravitational waves.

“What first attracted me,” says Hild, “was the spirit of what they were doing. When building a gravitational-waves detector and getting it to work, you need to know a bit of everything.”

Hild was the last student to work on the two Garching Prototype Interferometers, test facilities with arms only 10 and 30 metres long, compared to 4km in the detectors which made the first discovery. At the end of his undergraduate studies, he also had “the sad task” of helping to dismantle the Garching machine, which had provided “a playground” for ground-breaking gravitational waves work for several decades. During his PhD studies, Hild also worked at the core of the British–German Gravitational-wave detector GEO600: “I was lucky to have the opportunity to become one of only a handful people at that time who could operate GEO600, enabling me to watch it grow, subsystem by subsystem, and become more and more sensitive.”

During this period, Professor Ken Strain (now Deputy Director of the Institute of Gravitational Research – IGR) was a regular visitor, spending one week every month at the German facility as part of the team running GEO600 in Hanover. “For three years, there were six of us (I was the most junior and least experienced) sharing a desk in a very small room, with one eye on the monitors, asking questions and sharing ideas with each other, and trying to keep the interferometer ‘happy.’ I don’t think I will ever experience such an environment again,” Hild explains.

There is a lot of “healthy competition” between the different international projects, while at the same time scientific collaboration flourishes, supporting the sharing of new ideas, exchange of hardware, and pooling of the recorded data across LIGO, GEO and Virgo. “Everybody knew that they could not discover gravitational waves on their own – we needed a network of detectors,” says Hild.

As a “noise hunter,” Hild tries to track down the hundreds of possible sources of noise in the system – including everything from dust in the laserbeams to earthquakes and tractors in neighbouring fields, misbehaving electronics, lightning strikes or even modulations in the power grid caused by smart electricity meters. “We are trying to put together a super-sensitive system with several hundred control loops, many of which interfere with each other,” says Hild. “You might fix a problem on one end of the system, but if you’re not extremely careful, the same fix might cause some noise at the other end of the interferometer. We had to learn that things don’t always go the way you think they’ll go!”

Hild's career has taken him from Germany to the University of Birmingham (where he was amused to realise that he was a German researcher being funded by the French and Italians in a British laboratory), to help to develop Advanced Virgo. “I had a lot of freedom then because at that time there were really only two people working full time on the design of Advanced Virgo,” says Hild. Two years later, he was offered a position as a Lecturer at the University of Glasgow, where he found himself supervising five post-doctoral researchers who were all of a similar age to himself, and giving lectures in astronomy, despite the fact that he had never done any undergraduate courses in the subject himself.

The fact that Hild now focuses on building the next generation of laser interferometers, including Europe’s billion-euro Einstein Telescope, should reassure his students that he knows what he’s talking about.

According to Hild, the Einstein Telescope will establish a completely new and “fabulously sensitive” class of gravitational-wave detectors, designed to work as a “self-sustained” observatory for at least 30 years. It will also be constructed underground to reduce seismic noise and its influence on interferometer mirrors.

Another new feature of future detectors will be what Hild calls a “xylophone” design – a kind of dual-band microphone to help detect a range of different frequencies. The problem, he says, is that powerful lasers and cooled optics, optimised for certain frequencies at opposite ends of the scale, cannot work well together, so “building two instruments, each optimised for a certain frequency range or tone, will greatly improve our ability to listen to the exotic phenomena in the Universe.” This will allow the team, he adds, “to find gravitational waves from sources we know of, but hopefully – and I am even more excited about this – to find the unexpected.”

As he looks forward to future designs, Hild is reminded of the “mystery noise” once detected by GEO600. No matter how much the researchers looked into the possible source of the noise, they never identified what it was or where it came from. “It's a curious puzzle,” he says. “The most likely explanation is that it is something completely boring, such as some noise in an electronic circuit, or – less likely – it could be a strange phenomenon from a holographic Universe, as suggested by researchers in Chicago.” But as Hild and his colleagues continue to study the Cosmos with smarter and smarter detectors, the hunt for evidence will never be boring – especially if they discover the source of the mystery noise
and other strange phenomena never detected before.

How to see the whole Universe

Hild is leading a team in Glasgow aiming to establish a new way of building more sensitive gravitational-wave detectors – so-called speedmeters. The problem, says Hild, is the Heisenberg Uncertainty Principle. When you try to establish with high precision the position of objects, such as the mirrors in LIGO, the act of observation is likely to change things – i.e., shining a light on a target will move it. Similarly, when you are trying to observe gravitational waves, continuously measuring the distance between the mirrors hanging inside the detector, the light from the laser amplifies so-called vacuum fluctuations so that they cause tiny movements – the result of a phenomenon called “radiation pressure.”

The mirrors used in the detector weigh 40kg each, but even though most people struggle to lift them, individual virtual photons can cause a disturbance, i.e., push the mirrors around. So the challenge, says Hild, is to configure the mirrors so the Heisenberg Principle does not apply, and the solution is to measure the position of the mirrors twice in short succession – effectively measuring speed, not position, to cancel out the possible effects of the unwanted noise, thereby making the detectors more sensitive to gravitational waves.

The result, says Hild, is an “amazingly better performance,” and a new approach to laser interferometry which is now accepted as a “fruitful avenue to pursue.” When designing the next generation of future detectors, “speedmeters” were not part of the original plans, but it is likely that they will be used when the detectors are finally built, contributing to “cubed improvements” in performance.

“The new detectors, such as the Einstein Telescope, will be 1,000 times more sensitive,” says Hild, “and this means we will see more stuff, more often, and detect more events. For some sources, such as the binary black hole detected in September 2015, we will be able to see the whole Universe!”


Professor Stefan Hild, a Fellow of the Institute of Physics and a member of the RSE Young Academy of Scotland, is a leading authority in laser interferometry and carried out research that led to crucial advances in the sensitivity of both the Advanced LIGO and Virgo detectors. He is also involved in the design of future ground-based interferometers, particularly with regard to developing innovative techniques to exploit some of the weird rules and relationships in quantum physics to further enhance the sensitivity of future detectors. For his outstanding workin physics and astronomy, Hild was awarded the RSE Makdougall Brisbane Medal in 2016.







"The noise hunter". Science Scotland (Issue Twenty)
Printed from on 05/04/20 05:20:20 AM

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