“I feel like Alice in Wonderland”

Professor Stefan Hild on the Einstein Telescope, Sardinia and the cosmos


The right man in the right place. This would be an accurate description of Professor Stefan Hild, who is currently working to get ETpathfinder – a testing facility for the Einstein Telescope – off the ground. Hild’s entire career has been devoted to gravitational waves, detecting signals in particular. “There is always the risk of detecting a passing commuter train rather than a black hole.”

During his years as a professor of experimental physics in Glasgow, Hild could already see dark clouds on the horizon. When his fears came true and the UK voted to leave the EU, the Hild family decided to leave as well. The United Kingdom – the place where Hild and his wife had moved twelve years earlier, where they had hoped to raise their two children and spend the rest of their lives – had changed forever.

No sooner had they made their decision than Hild found out that Maastricht University was looking for a professor of gravitational research. As a physics student in Hannover, he had worked on the experimental gravitational wave detector GEO600, painting the cleanrooms where tests were conducted. After graduating, he did research there.

Several universities offered Hild (born in Celle, Germany, in 1978) a position when he left Glasgow. Why did he decide to come to Maastricht? Because UM offered him a “unique opportunity” to set up the testing facility ETpathfinder, where new ideas and applications will be tested for the next generation of gravitational wave detectors, such as the Einstein Telescope. 

All this will take place in the Black Box, as people call the building at Duboisdomein 30. This is also where the nine members of Hild’s research team have their offices.

On the way to the coffee machine - just before the corona crisis - Hild half-jokingly remarks that giving interviews has been one of his main tasks since he came to Maastricht. He doesn’t seem to mind, though. Back in his office, Hild enthusiastically talks about his work.

You’ve been here for over eight months now. What has exceeded your expectations and what has disappointed you? 

I feel like Alice in Wonderland. I could mention a million different things that exceeded my expectations, like all the positive energy, the buzz and the support I get from my colleagues, including those at other universities participating in the project. At the same time, building a testing facility like this from scratch is quite a challenge. Everything we do and make here is special, as we’re perfecting the technology as much as possible. For example, the setup includes these huge laser cleaners in-vacuum that consist of hundreds of specialised parts. These parts have to be tailor-made. We can’t keep outsourcing that, so I would like us to have our own workshop here. I’m currently trying to convince the directors we need one.”

Do you miss the scientific community in Glasgow? It was probably much larger than the one in Maastricht.

“No, not at all. After all, it’s not like ETpathfinder is a UM-only project. We’re working together with fifteen different organisations, including universities in Belgium, Germany and the Netherlands, the Dutch National Institute for Subatomic Physics (Nikhef) and companies. Pretty much every single physics institute interested in gravitational waves in Europe is participating in the Einstein Telescope.”


The news flashed around the world: on 14 September 2015, scientists detected gravitational waves for the first time. These ripples in space-time resulted from two black holes merging 1.3 billion light years from Earth.

Some media boiled the observation down to “Einstein was right”, says Hild, who was one of the people involved. “Which is true – Einstein did predict the existence of gravitational waves a hundred years ago. But at the same time, we made astonishing discoveries. We thought some black holes were twenty times more massive than the sun, but they turned out to be forty times more massive. Astronomers had to go back to the drawing board and reconsider their theories of the evolution of stars. Our view of the cosmos was definitively changed.”

Soon after, scientists detected signals of a spectacular collision of two neutron stars. “Thinking back to that moment still gives me goose bumps. Telescopes like Hubble, legendary instruments from my childhood, pointed to the exact location where we had detected something.”

The signals were detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the United States. In the 2030s LIGO and other current detectors will be succeeded by the Einstein Telescope, which will be ten times more sensitive and survey a volume of space more than a thousand times greater. But this doesn’t mean that other detectors will become redundant, says Hild. “Gravitational wave observations are most reliable when multiple observatories can confirm them. Otherwise, there is always the risk of detecting a passing commuter train rather than a black hole. Detectors currently pick up gravitational waves once a week, even if they only last a few seconds.”

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The testing facility ETpathfinder will house a prototype of the Einstein Telescope (see photo) with twenty-metre-long arms. Just like in the real detector, whose underground arms will be ten kilometres long, a laser beam is split in two and will shine on a semi-transparent mirrors, and bounce back and forth. The two beams then fade out – unless a gravitational wave passes. If this happens, the length of the tunnels changes by a miniscule amount and the laser beam remains intact.

“In ET Pathfinder, we’ll experiment with things like vibration dampers, the laser light, and the material and temperature of the mirrors. We’ll cool them to 250 degrees Celsius below zero. Heat produces vibrations, which we want to reduce. But this also means that we’ll need a cooling system, which means we’ll have to figure out how to connect one.”

A cleanroom will be built at the back of the Black Box in a 800-square-metre hall. The newspapers of Dagblad de Limburger, a local newspaper, used to be distributed from here. A new, vibration-free floor will be installed in the coming weeks, supported by 169 5.5-metre deep columns. “The floor won’t touch the walls. It will essentially be an island. This way, the wind blowing against the building won’t affect our observations. Construction and equipment combined will cost 14.5 million euros. The experiments will begin in 2022.”

FSE dean Thomas Cleij once said, “ETpathfinder is much more interesting to UM than the Einstein Telescope in South Limburg”. Do you agree?

“It depends on how you look at it. All the Einstein Telescope will do is survey the universe. There’s not much for instrumentalist like me to do there once it is running and observing – so yes, a testing facility is more interesting to us. It’s like an arena for experimentation. The testing facility, where new technology will be developed and students will be trained, is also a very valuable addition to UM. But the Einstein Telescope is interesting on a whole different level. It will allow us to reconstruct the history of the universe and will radically change our view of space. It’s also a project thousands of people are working on, attracting worldwide attention.”

ETpathfinder will attract a lot of talent. Scientists from all over the world will come to Maastricht to conduct their experiments.

“Yes, it has already attracted a lot of interest. Germany has already made research grants available to build hardware for ETpathfinder. But don’t forget about our own staff members, either. Let me give an example. The LIGO Scientific Collaboration, the largest in the field of gravitational waves, has six research groups, each with its own specialisation in building equipment. Three of these groups were led by a UM scientist. Not bad, right?”

Contrary to what many people think, the testing facility will not be closed when the Einstein Telescope becomes operational. “We’ll take stock after five years, in 2035. Which technology has been improved by that time? How can the equipment be upgraded? Pathfinder will contribute to all that, too. And if it’s up to me, it will continue to do so until 2082, the expiration date of the Einstein Telescope.” 

In 2022, an international panel will decide whether the Einstein Telescope will be built in South Limburg or in Sardinia. What if they decide on Sardinia?

Hild smiles. “If they do, everyone will just be glad we’ve found a location. To be honest, this is a gigantic, 1.9-billion-euro project. We’ve been trying to get governments to make funds available since the design was completed in 2011, but no country has yet committed to it. I’ll be glad if the project gets off the ground at all.” 

Do you think there’s a chance it won’t get off the ground?

“Um, not really, no. Considering the reaction to those observations in 2015, I can’t imagine the project will end here. Science magazine gave our field the Breakthrough of the Year award two years in a row, in 2016 and 2017 – first for confirming that gravitational waves exist and then for observing those neutron stars. And if you also consider the fact that the observatories LIGO (in the US) and Virgo (in Europe) are quite old and will only be operational for fifteen more years at most, it’s inconceivable that we would consciously decide not to start using another gravitational wave detector by 2035.”

But if we’re going to build one, wouldn’t you prefer it to be here?

“Of course. There are two advantages to South Limburg as a location. If I put myself in the shoes of the hundreds of scientists who will relocate their families to live near the telescope, I would rather live in a bustling, high-tech region than a remote island like Sardinia. It’s a nice place to go on holiday – good food, beautiful beaches – but it’s not a great place to work, in my opinion. Here, you’re close to Aachen, and Eindhoven, and so many different institutes and companies…”

What’s the second advantage?

“It has to do with soil composition. The soil composition of South Limburg is perfect: a foundation of rock, in which the tunnels would be dug, with soft clay soil on top. The loam would muffle the sounds of the world above. Sardinia is all rock, stretching hundreds of metres underground. It’s a quiet island, so we’d definitely be able to detect gravitational waves there. In fact, there are slightly fewer vibrations than here, although that may change in the future if the activity around the telescope increases. The Italians have proposed an industrial exclusion zone of fifty kilometres around their location,  but to me that’s not realistic. The government will also invest in the observatory because of its socio-economic impact on the region. Governments want activity. All in all, I think choosing South Limburg would be safer in the long term. But then again, I’m quite biased.”

“I feel like Alice in Wonderland”
Stefan Hild