It happened about one and a half billion years ago, long before the birth of man. Let alone that astronomy already existed, the science with which mankind hopes to fathom the cosmos. Yet we have to go that far back in time to unravel the moment when two colliding black holes — super-massive objects that swallow everything, including light — launched an entirely new branch of astronomy.
The insane amount of energy released in the collision set the surrounding space and time in motion. That space-time shiver set off at the speed of light in all directions, like waves in a rock pool, arriving at Earth just before 11 a.m. on an otherwise unremarkable Monday in September 2015.
The effect on our planet was very small. It kneaded her a tiny bit, comparable to the change in the water level of the IJsselmeer when a single raindrop falls in it. And yet physicists were able to catch this gravitational wave, as the vibration of space and time is called. It hit two newly launched measuring instruments in the United States, the only ones that could fish the signature of that old blow from the background noise.
These so-called Ligo detectors did this by means of tunnels that are kilometers long through which laser beams pass, which are reflected off mirrors at the ends. They are tuned to distract from shifting the laser path when a cosmic ripple passes. Two years after the measurement, the pioneers behind Ligo promptly won the Nobel Prize in Physics.
That first measurement opened a whole new window on the universe. Until then, astronomers viewed space mainly using light, heat radiation, radio waves, microwave radiation, and so on. Each and every appearance of the same electromagnetic waves, in which only the wavelength differs. The swaying of space and time suddenly gave astronomers a new cosmic sense. It was as if until recently they could only see the universe, but now they could suddenly hear it.
Million times more sensitive
Fast forward to now, a handful of years and a few dozen confirmed measurements later, and physicists and astronomers are already dreaming of more. From a measuring instrument for the next generation, a European gravitational wave detector that is – for some vibration frequencies – up to a million times more sensitive than its predecessors, so that you can pick up thousands of signals per day. Such an instrument, it is expected, can pluck even the faintest tremors from the cosmic depths and study the universe during its first moments, a split second after the Big Bang.
That period is completely inaccessible to even the most powerful new “ordinary” telescopes, because the universe at the time was so densely packed and hot that it was completely impenetrable to everything from visible light to microwave radiation. Gravitational waves, however, pierce through them effortlessly.
The Limburg hilly landscape – or rather: a place more than 250 meters below that landscape – is one of two possible locations for the scientific mega project that can absorb these primordial waves. The Dutch government recently released over 900 million euros through the National Growth Fund. Of this, 42 million will be paid out shortly, intended for preliminary research and rigging the organization. If successful, another 870 million will follow later, when the instrument is actually built in the Netherlands.
Because Italy is also competing, with the mines of Sos Enattos in Sardinia as a possible location. Preliminary studies show that both locations are suitable for housing the instrument. The telescope has to be deep underground, so that it is protected against the vibrations of, for example, passing trucks. In addition, the region itself must be stable, so that it is ‘quiet’ enough to measure gravitational waves. The final decision will be made around 2025 about where the Einstein Telescope, because that is what it will be called, will be placed.
Control room full of boxes
At Maastricht University’s Faculty of Science and Engineering – where English is the official language – physicists are building a test setup that will answer the biggest technological issues surrounding the new mega telescope. A few hundred meters from the banks of the Meuse, in the inky black building on the outside that used to be the editorial and printing press of the daily newspaper The Limburger houses, the ETpathfinder (in full: Einstein Telescope pathfinder) is now being built, a mini variant of the Einstein Telescope with tunnels of 20 meters, where the real one will soon have ‘arms’ of 10 kilometers.
‘This setup is too small to measure gravitational waves’, says physicist Stefan Hild, project leader of ETpathfinder, while watching from a room full of glass in the brand new faculty – until about five years ago, Maastricht University did not conduct any research in the fundamental physics – looking down on the first contours of the future mini detector. “This will be the only place in the world where you can test all the necessary technology, vacuum tubes, lasers, mirrors and so on together,” he says. ‘In that regard, it doesn’t matter to us whether the Einstein Telescope comes around the corner, or in Italy. We are even going to do research for the Americans, who are working on their own successor to Ligo.’
In the room itself you see half and full towers, in which the mirrors will soon be hung. The plastic-wrapped connectors lie on the floor. A control center right next to the room for the new instrument, which will soon resemble the well-known control rooms at rocket launch sites, is still full of unpacked boxes. ‘Everything has to be finished here by the end of the year,’ says Hild.
Or well: ‘finished’… he actually prefers not to speak of that. ‘This experiment is never finished. It can be used for decades to come to test increasingly modern technology for all kinds of gravitational wave experiments,” he says.
Starting with the Einstein Telescope, which is not only growing bigger than the Ligo detectors or its European counterpart Virgo, but also undergoing some major changes under the hood. ‘There’s no other way,’ says Hild. ‘We want to measure ten thousand to a million times more accurately. The Einstein Telescope will be a factor of three larger – and therefore more sensitive – but you are still a long way from there. You therefore have to adapt the technology.’
For example, the physicists try to keep out even the minute brown motion, the heat-fed motion of individual particles, by cooling everything, including the mirrors, to well over 260 degrees below zero. ‘But the glass mirrors that are now used in Ligo and Virgo do not function well at those extremely low temperatures,’ says Hild. That is why they are switching to silicon and replacing the lasers with super stable variants with a different wavelength. Only: nobody has much experience with that yet. ‘So you need something like ETpathfinder for that,’ says Hild.
The true nature of gravity
Hild is also looking forward to what the big brother of ‘his’ pathfinder, the Einstein Telescope itself, will find in the future. “If you ask me what gives me energy, what gets me out of bed in the morning, it’s all the unexpected discoveries you can make with such a large detector,” he says. ‘With Ligo and Virgo, for example, we discovered black holes with a totally unexpected mass. There are currently two thousand specialist articles that try to explain this,’ he says.
But, Hild emphasizes, unexpected discoveries alone are not enough. “Let’s face it: with just such a vague promise, no one will give you money,” he says with a laugh. ‘That is why there are also dozens of important subjects in physics and astronomy where we know for sure that the Einstein Telescope can make a decisive contribution.’
For example, the device can help explain what dark matter consists of, an invisible ‘something’ that you can only discover indirectly, for example because its gravity prevents galaxies from whipping apart. Dark matter may consist of small black holes that formed shortly after the Big Bang. And it could find the Einstein Telescope.
‘We may soon be able to measure what is inside black holes,’ says Hild. Until now, this was considered impossible because nothing can escape from a black hole, not even information about its interior. ‘But we may be able to deduce something about the interior from the echoes of a collision between two black holes, which the Einstein Telescope will soon be able to hear.’ All it takes for that? A gigantic detector, 200 meters below the Limburg hills. The answer to the deepest astronomical riddles has never been so far and so close at the same time.
Earthly Challenges
Before the new Einstein Telescope can uncover the earliest secrets of the cosmos, its initiators must first shift their gaze from the heavens to the terrestrial. “Everyone is excited about the science and the opportunities the telescope brings to the region in terms of jobs and economic activity,” said physicist Jo van den Brand, Einstein Telescope project director.
But there are also challenges. For example, the region where the telescope is to be located is a quiet area. ‘We have to ensure that everything fits well in this environment, where tourism is also important.’ Furthermore, just like any other construction project in the Netherlands, the mega project will have to deal with strict rules regarding, among other things, nitrogen and CO .2†
And then there is the risk that someone else will want to do something in the area of the future detector. For example, mining company Walzinc wanted to open a zinc mine in Plombières, near the Drielandenpunt in 2019. The activity in such a mine would cause so much background vibration that it could drown out the sensitive detector. ‘And windmills can also be problematic if they come too close to the observatory,’ says Van den Brand.