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Twenty

A billion years, three minutes later

It was a narrow escape. But now that we have done it, it has opened up an entirely new field of science…

A billion years, three minutes later

A billion years, three minutes later

Some media reports suggest that after gravitational waves were detected for the first time (at 9:51 am Greenwich Mean Time on September 14, 2015), it took just three minutes to “see” what had happened, using powerful data analysis tools. But those three minutes stretched to months before the international team of scientists working for LIGO (the Laser Interferometer Gravitational-Wave Observatory) were confident enough to make the public announcement on February 11 the following year.

Three minutes after it happened, in fact, the only thing they knew was that a “candidate event” had been recorded by the instruments. According to the data, gravitational waves had travelled at the speed of light through Space, after two black holes collided and merged, about a billion years ago. The scientists had been waiting for decades for evidence of gravitational waves, but this was just the first step in a rigorous process that would take a thousand people in the LIGO Scientific Collaboration (LSC) several months to confirm, sometimes even doubting they were dealing with genuine data – suspecting that the whole thing was another training exercise designed to test their technical and human resources.

When he first found out about the probable detection, Dr Siong Heng was in China. The news was something Heng had been waiting a long time to hear, but he soon wished he had never even opened the email which gave him the first hint that something important had happened – Heng and his family were on holiday that week. It was hard not to feel the excitement, however, even though it could be yet another false alarm – perhaps another “bump” in the detector. Heng, who is now a Professor in the Institute for Gravitational Research (IGR) based in the University of Glasgow and co-Chair of the 'burst' working group, crunching the numbers for LIGO, likes his free time like everyone else, but the email was hard to ignore. When you have a leading role in one of the world’s most advanced scientific experiments, on the brink of discovering something of historic importance, the holiday just has to wait.

Heng's colleague Professor Martin Hendry – Head of the School of Physics and Astronomy at the University of Glasgow and the Chair of the Education and Public Outreach Group in LIGO – was also in Beijing for a conference that week, and when they met, the two men exchanged knowing looks, even though they had to contain their excitement in front of other scientists who might spill the beans before the team was ready to make an official announcement. Every individual directly involved in the LSC project had been sworn to secrecy until the results could be verified, and everyone knew that the process could take several months.

The software did its job by identifying what had probably happened, comparing the data with “an extensive bank of theoretically predicted waveforms,” using a process known as “matched filtering" to find the waveform that best matched the data, but no-one could be sure yet if the data did show gravitational waves, or even if the data was what it appeared to be. More checks still had to be made, and no-one could be sure how long the next step would take, or if it would lead to a positive outcome.

Later that day, Heng and Hendry escaped for a beer, and were able to share their excitement in private, but both knew that the waiting game had only begun...

Blind injections?

Meanwhile, on the opposite side of the planet, Professor Jim Hough – a veteran “cosmic detective” and the Associate Director of the IGR in Glasgow – and Professor Sheila Rowan, Director of the IGR, were chatting on the phone to a colleague at the University of Cardiff when they heard the first rumours that something momentous had happened. A few days earlier, a burst of gamma rays had been observed by several astronomers and Rowan and Hough were curious to know if that had been detected by LIGO – such phenomena are relatively rare, and among the most energetic and mysterious events in the Universe. Little did they know at first, but something much bigger had happened, and the news was beginning to gather momentum in LSC circles. When the project started, the scientists had thought the first detection would be neutron stars colliding, but this was something even more dramatic, as well as less likely – gravitational waves emitted by the coalescence of two black holes.

“A scientist in Germany emailed to say that he had seen the data,” says Hough, “and even though we didn’t know it straight away, it turned out he had reached the right conclusion.” Rowan and Hough later learned how lucky they had been to detect anything that day. The detectors were supposed to be in “engineering mode,” but fortunately the equipment had been left operating.

Hough, Rowan, Heng and Hendry had been through this nail-biting process before – many times. To tighten procedures and prepare the team for any future public announcement, the project managers had regularly organised what they call “blind injections” – a training exercise like an emergency drill. To stage these dress rehearsals, false data is “injected” into the system to make it seem a candidate event has just occurred, triggering the process of verification. Only a few people know it is only a test run, but the rest of the team must believe it is real so that they behave exactly the same as they would if it had really happened. And the process goes down to the wire – until the academic paper is written to describe the false “discovery” and present the “evidence,” ready to make the official (fictitious) announcement, with everyone convinced the announcement is real.

“A thousand people are involved in the process,” says Heng, “so you need to be sure and communicate clearly before going public.” Heng also says the blind injections make people calmer and “sociologically less likely” to spread any rumours. “It makes us more grounded,” he adds, “and more scientifically rigorous in our approach.” Sometimes, the exercises also lead to technical improvements and statistical refinements, to fine-tune the whole operation, learning from experience.

Hough recalls one blind injection, when he and the rest of the team had been “fooled” till the very last moment. “Data colleagues had almost finished writing the paper,” he says, “when we were invited for a celebration drink. But I knew straight away that it wasn’t the real thing when they used plastic glasses and offered us all sparkling wine – not champagne.”

The real thing?

In September 2015, however, the team soon began to suspect that this was the real thing. The initial data may have seemed “too good to be true,” but they also knew the new detector (Advanced LIGO) had only been running for a very short time and the set-up wasn't ready to do blind injections. Perhaps this was no accident – perhaps no dress rehearsal, after all. It was tempting to push for a public announcement, but Heng and Hough had been here in the past, and were more aware than most of the need to be cautious. “When something significant happens, it’s easy to jump to conclusions,” says Heng.

Almost two decades earlier, Heng had learned his lesson the hard way, as a PhD student in Australia, writing his Thesis on data analysis for the detection of gravitational waves, using bar detectors (an earlier version, using solid metal cylinders isolated from outside vibrations). “My supervisor and I detected what we thought were gravitational waves,” Heng explains. “It was my first taste of the kind of excitement you get when you make such a discovery – eight tantalising events which together suggested we’d done it.” But the news was released before the final checks were made which showed that it had been a false alarm.

Hough and his research supervisor, Ron Drever, and other younger colleagues, had a similar experience in 1972, soon after building their first prototype detectors in his lab at the Department of Physics and Astronomy in the University of Glasgow. He and his colleagues had already seen the pioneer in the development of gravitational-wave detectors, the American physicist Joseph Weber, discredited for prematurely claiming success, and this meant they were careful before they even started to conduct their experiments. Using what they thought was an improvement on Weber’s design, another type of bar detector, Hough, Drever and colleagues detected a “beautiful signal,” on two separate systems. But even though it looked as if they had detected gravitational waves, this could not be confirmed – all the other detectors in various countries were not operating that day, so no-one else was in a position to confirm or deny the event. Even today, a detection would not be confirmed unless it was confirmed by another detector hundreds or thousands of miles away, to eliminate local effects.

Hough is still convinced it’s highly likely that they did detect gravitational waves, but in their academic paper they avoided such claims, opting instead to describe what had happened in clinical detail – a good example of the scientific method in action. “Weber missed out on the Nobel Prize in Physics,” says Hough, “for his work on the development of the hydrogen maser, so perhaps he was too anxious to announce his results – several other scientists pointed out discrepancies in Weber’s data analysis which later proved fatal, and some of them even accused him of being economical with the truth.”

Fast forward 45 years...

Forty-five years later, Hough and the rest of the team who were working on LIGO were getting ready for the public announcement. The group had lots of work to do to verify the data and write the paper to back up their claim. And everyone was interviewed to find out exactly what happened that day – for example, to determine if there had been any accidents or human errors.

Heng says he was “75% sure” after three or four weeks, then 95% sure after two or three months. The final 5% was writing up the evidence and discussing the details before publication. Hough says he made up his mind by Christmas and when a second, similar, event occurred on December 26, 2015, most of the team were convinced – the signs were unmistakable.

It had taken five decades to do it, but Hough only felt “great relief” when the great moment finally came. At that time, many members of the project team were getting more and more concerned their funding would not carry on if there wasn’t a significant breakthrough soon. “It was a narrow escape,” says Hough. “But now that we have done it, it has opened up an entirely new field of science.”

“It took just three minutes to flag the event,” Heng explains, “and a few weeks to be sure that the data were right.” And when the news finally broke five months later, in February 2016, it was not just another scientific announcement, but a step towards a new kind of science which will change people’s lives – including all the scientists themselves.

“Now we can find out how galaxies formed, and perhaps also find out more about dark matter,” says Rowan. “It is a whole new way of looking at the Universe, and I hope it will also be a great inspiration to children and parents, enthusing the next generation of scientists in Scotland and beyond.”


Jumping the gun

“Joseph Weber spent time in the Second World War in a submarine-chaser. If they detected a sub, they dropped a depth charge, and if there was no sub, they still dropped a depth charge – just in case. Weber also missed out on the Nobel Prize for Physics for his work in the development of hydrogen masers. Maybe that explains why he was so keen to announce that he had detected gravitational waves – just in case it was true so he wouldn't miss
getting the credit.”

Professor Jim Hough


Einstein in Glasgow

Even though he couldn’t have known at the time that the University of Glasgow would play such a key role in proving his General Theory of Relativity just over 80 years later, the great theoretical physicist, Albert Einstein, was awarded an Honorary Degree by Glasgow in 1933 – visiting the very same campus where scientists pioneered some of the systems that detected the first gravitational waves, a hundred years after the ground-breaking Theory was published.

The Scotsman newspaper reported Einstein saying: “I was very glad to accept the invitation to say something about the history of my own scientific work. Not that I have an unduly high opinion of the importance of my own endeavours… it would be a mistake, from a sense of false modesty, to pass by an opportunity to put the story on record.”

According to The Scotsman, “The cheering which greeted his appearance lasted for several seconds, and was acknowledged by a shy smile from the famous savant.”

Image of Einstein in the Glasgow University quadrangle after his Honorary graduation. Image credit: University of Glasgow Archives & Special Collections, John S Allison collection, GB248 DC399

 

 

 

 

 

 

 

 

"A billion years, three minutes later". Science Scotland (Issue Twenty)
Printed from http://www.sciencescotland.org/feature.php?id=310 on 24/09/17 11:20:58 AM

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