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Lateral thinking vs uncertainty

The chief appeal of working in gravitational-wave research was that it offered just the right amount of uncertainty    …

Lateral thinking vs uncertainty

Lateral thinking vs uncertainty

Just when you think you are getting the hang of it – the search for gravitational waves – the science is turned upside down. The most important factor, you learn, is to measure the distances between pairs of very highly polished mirrors, and if the distances change, you’ve detected a force that may turn out to be gravitational waves. To measure the distances, you also have to be able to fix the positions of the mirrors and eliminate the many other sources of “noise” which may cause the mirrors to move.

Professor Ken Strain has spent over 30 years trying to improve the technology used in the detectors, making sure the mirror suspensions are held in position, and now that the technology has finally delivered – by detecting gravitational waves – the next generation could be designed to “ignore” the position and focus on speed or momentum, thus making it much easier to see what lies beyond.

The fundamental physics may be hard to understand, but Strain and most of the other researchers who have worked on the project over the years could be described as “general-purpose scientists,” because they have to do a bit of everything; not just understanding the theories involved, but also designing and building the mechanical and electronic components which enable the detector to function – including the mirror suspensions.

Strain's first experience was working with the prototypes built in the mid-1980s, with arms only ten metres long, compared to several kilometres in the LIGO (Laser Interferometer Gravitational-wave Observatory) detectors, thousands of miles apart in the US. But despite their modest proportions, the early detectors were “the most sensitive detection instruments in the world at the time,” Strain explains. They were also the progenitors of today's much more powerful systems, with scientists in the UK and Germany leading the way, pioneering the technology which led in a straight line to LIGO, as well as helping out with other similar projects in Italy and Japan.

“In the early days,” says Strain, “there was a huge exchange of ideas, but only about two dozen experimentalists worldwide involved in the project, including PhDs.”

The Glasgow connection

The main contribution of the Glasgow researchers, says Strain, was the mirror suspensions and the development of the “beautiful silica fibres” which hold them and minimise unwanted noise. Scientists working in Scotland have also helped develop the laser technology used in the latest detectors, improving their stability “by more than a million times.” They have also made a huge contribution to the data analysis which makes sense of the signals and also improves the sensitivity of the detectors by helping them focus on their primary target – the enigmatic and elusive gravitational waves.

Strain explains that one of the critical aspects of the LIGO detectors is the use of hydroxide catalysis bonding, which connects the mirror suspensions to their suspension fibres. Each mirror is at the bottom of a chain of four cascaded pendulum stages to isolate it well from ground vibrations. The technique used for bonding was originally developed by a team at Stanford University to make spacecraft more stable, and involves a chemical reaction between the mirror and an “ear” at the end of the silica fibre, so the materials are “perfectly” joined – as if they are one single object.

The mirrors may all have to hang very still, but the technology never stays static for long. Research into the next generation of detectors has been going on for eight years already, says Strain, since before Advanced LIGO was built and began operations. And this will mean further improvements all round, including possibly scaling up the mirrors from 40kg to as much as 160kg.

Pioneering research

Strain first got involved with gravitational waves in the mid 1980s as a student in a summer project, helping to develop electronics and control systems that improved – by a factor of three – the sensitivity of the ten-metre detector in Glasgow. At that time, he says, almost every part of the equipment used had to be custom made and, because they were using non-standard components to build something never attempted before, they faced enormous challenges in every direction. “If we had used off-the-shelf components,” says Strain, “the detector would never have worked.”

Another breakthrough which enabled the detector to work was a revolutionary technique developed by the late Dr Brian Meers in Glasgow called “signal recycling” – “a method of optimising the response of gravitational-wave detectors to the expected astrophysical signals.”

In 1991, Strain and Meers published a paper describing the first experimental demonstration of “an optical system which should improve considerably the performance of proposed laser-interferometric gravitational-wave detectors,” including an enhancement of the signal-to-noise ratio by a factor of seven.

Sadly, Meers was killed in a climbing accident the following year, but the concept of signal recycling lived on in the Hanover-based GEO600 detector and later in Advanced LIGO, helping to fine-tune the system. Strain explains that signal recycling was a way of boosting or recycling the light beams and changing the resonance, to make the system better at detecting target signals in the midst of the “constructive and destructive interference.”

According to Strain, signal recycling works with other optical techniques at the heart of the detector, and with modern ultra-low-loss mirrors, losing only 50 parts per million of the light from the lasers as the light is bounced from mirror to mirror, and together these form the most sensitive probes to measure “modulation of the refractive index of space due to gravitational waves.” Signal recycling enables you to tune the detector to follow waves of a particular frequency, much as an AM radio can be tuned to a particular channel.

Uncertainty rules

“The chief appeal of working in gravitational-wave research was that it offered just the right amount of uncertainty,” Strain explains. Apart from the challenging science itself, funding is always a headache, and there have always been sceptics who said they would never succeed, but Strain and his colleagues have always believed they could do it, and hope the support will continue. “If the funding stops in this country, that would be our loss – other countries will do it,” he says.

According to Strain, the laser interferometer in Advanced LIGO is so sensitive that it operates close to the limits of the Heisenberg Uncertainty Principle, which states you cannot measure the position and the momentum of an object with unlimited accuracy at the same time – for example, if you shine a light on an object to see it, the object will move, thus making it impossible to know its position exactly. Each of the mirrors inside the detector weighs 40kg, but the photons in the detectors can move them. But the battle to get rid of unwanted noise will continue to make further progress, adds Strain, in the quest to build a new kind of detector in which the Heisenberg Principle does not apply – more sensitive and more efficient.

The future generation of detectors may not even measure position, says Strain. They will most likely focus on measuring speed or momentum. “This will help researchers to look deeper into the data they need to identify signals, rather than fighting against noise to extract subtle details of the signals. To do this, we will need a new configuration of optics,” says Strain.

Time is no object

Thirty years ago, when Strain joined the small band of pioneers trying to find gravitational waves, he didn’t know how long the search would take – and didn’t care. “This was the big challenge in physics,” he explains, “and I thought I could make a contribution.”

The project is an “unusual blend of people and an unusual mix of theory and practical work,” Strain concludes. “To succeed, we needed general-purpose scientists.”

When Advanced LIGO was switched on two years ago, most of the researchers expected it would be at least a couple of years before any success, as they gradually boosted the power. When the breakthrough was made in September that year, it was not only unexpected, but a deserved reward for all those “general-purpose scientists” who made a contribution through the years, including Ken Strain and his colleague, the late Brian Meers.


Professor Ken Strain, a Fellow of the Royal Society of Edinburgh and the Institute of Physics, is Deputy Director of the Institute for Gravitational Research (IGR) and was UK Principal Investigator for the Advanced LIGO upgrades, funded by the Science and Technology Facilities Council, that led to the first detections. He is a leading authority in the design, testing and optimisation of signal recycling systems for laser interferometers, which formed a key part of the Advanced LIGO design. Strain is also an External Scientific Member of the Max Planck Institute for Gravitational Physics (Albert-Einstein-Institute) in Hannover.





"Lateral thinking vs uncertainty". Science Scotland (Issue Twenty)
Printed from on 06/07/20 10:52:28 AM

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