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Tri-country physics

The German physicist Stefan Hild lived in the UK for more than 10 years. Last summer, he moved from Glasgow to Maastricht where he is working on the new gravitational wave detector, the Einstein Telescope. ‘The threat of Brexit is a burden that has fallen from my shoulders because the uncertainty seeped through everything.’

In August, you started as Professor of Gravitational Wave Research at Maastricht University. How were the first 100 days in your new job?

‘I still feel like Alice in Wonderland. There are opportunities galore, and that’s fantastic. Research into gravitational waves is booming in the Netherlands, and you notice that in everything. I’ve learned many new things already. We really want to build the new gravitational wave detector, the Einstein Telescope, here. For that, reports need to be written about the subsurface and the necessary construction work. I’ve also been underground, and I now know far more about concrete.’

Nevertheless, you were appointed to do physics?

‘I certainly was, and a lot is happening there as well. My group is working on new equipment for the Einstein Telescope. That is not simply a souped-up version of the existing gravitational wave detectors but a genuinely new technology. That’s why everybody is so excited to work on this. We are busy with the construction of ETpathfinder, a test location for the equipment that will be used in the Einstein Telescope. In addition to that, the standard work for the current generation of detectors continues. For example, we’re trying to refine the measurements from Virgo. That is the existing European detector for gravitational waves in Italy.’

Maastricht was originally a university for law and medicine. Don’t you feel a bit lonely there as a natural scientist?

‘Oh no, not at all. There are many natural scientists here too, such as data analysts. And in the medical corner, there are also a lot of technical people. Furthermore, I work intensely with scientists from other countries, especially Germany and Belgium. Last year, Maastricht University joined the partnership Nikhef. I am often at Nikhef, and the people from Nikhef come here a lot too.
What also makes Maastricht so incredibly interesting is that it exudes Europe. Just around the corner from here, the Maastricht Treaty was signed, which laid the foundation of Europe. And for the ETpathfinder alone we are already working together with 15 partners from three countries.
The Einstein Telescope is the shape of a triangle, with sides of 10 kilometres. The measurement stations are located at the corners. If the telescope comes here, then we want to build one station in Germany, one in Belgium and one in the Netherlands. So it will be genuine international science, and that’s exciting.’

How long will it be before you know whether the Einstein Telescope is actually coming to Maastricht?

‘We are now working on a bid book, and they are doing that on Sardinia too. We will know more in 2023. Of course, I hope it will be here.’

And if Sardinia is chosen?

‘Then we won’t sit around mourning. Because in either case, the ETpathfinder will continue. And we will develop the equipment for the detector on Sardinia here. Furthermore, the ETpathfinder will not stop after the construction of the detector, as some people think. First of all, following the construction, many more years will be needed to calibrate the detector properly. Technology never stands still, and upgrades for the detector will be needed too. That new equipment will once again be tested at ETpathfinder because you do not want to stop the detector itself for too long. The Einstein Telescope will have a lifespan of 50 years. There will be enough work for us to do for the next 30 years irrespective of where the telescope is located.’

How many gravitational wave detectors does the world need?

‘You can better compare detectors to microphones than to telescopes because detectors cannot “see” where the gravitational wave comes from. You can only determine the direction if you have at least three detectors that measure the same wave. As not all of the detectors will be in use at the same time, a minimum of four detectors is needed. There are currently three detectors in use, and shortly there will be another two, one in Japan and one in India. The Einstein Telescope is a new type of detector. It is more sensitive and can therefore perform longer measurements, even up to one day in length. As the earth turns in that time, you will also be able to determine the direction of the signal.’

And if we were to take a time horizon of five years, then when would you be satisfied?

Hild laughs. ‘Oh no, that’s an impossible question for me to answer. So many things can happen in a period of five years. I believe that you should always work in such a way that you can be satisfied with each day’s achievements. That is where I get my energy from. That’s why, in five years time, I still want to be happy about what I do each day. Then things are going well.’

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