The dream of the Einstein telescope
A huge detector, hidden in the European underground, is to detect the space-time quakes of the future. But it is still unclear whether and where the prestige project will become a reality.
It seems to be a kind of law of nature: The longer physicists work on a topic, the larger measuring apparatus they need. The particle physicists at CERN, for example, are thinking about a 100-kilometer-long ring accelerator, since the current 27-kilometer machine cannot answer the open questions of the discipline. Astrophysicists, on the other hand, are eagerly awaiting the James Webb Space Telescope, the most expensive and largest instrument of its kind to date.
And even in the hunt for gravitational waves, the urge for something bigger and better is visible. Inspired by the first evidence of such a space-time ripple exactly five years ago, the researchers are currently planning an ambitious space project called LISA, which is to be launched into space in 2034. On the other hand, they are preparing for a new generation of measuring instruments on the ground, which will even make the two four-kilometer-long laser interferometers of the LIGO observatory look contemplative.
Gravitational waves are tiny tremors of space-time, which Albert Einstein predicted in 1915 in his general theory of relativity and which humans first proved in 2015. The space-time quakes occur when huge masses are accelerated somewhere in space, for example when two black holes collide. The oscillations that are released in this process spread out at the speed of light in all directions in space — and can penetrate the entire universe almost unhindered. On Earth, gravitational waves can be detected with laser interferometers: The systems consist of two tunnels, each three to four kilometers long, arranged like an “L”, in which laser beams travel back and forth. If a gravitational wave hits one of these arms, the distance is minimally compressed, causing the light to travel the distance to the end of the tunnel a fraction of a second faster.
The European version of such a dream detector is called “Einstein Telescope”, researchers usually speak briefly and concisely of the “ET”. It is said to have three ten-kilometer arms, which form a huge triangle underground. Possible locations include the Mediterranean island of Sardinia — and the Dutch-Belgian-German border triangle between Eindhoven, Leuven, and Aachen.
Beyond one billion light-years
The considerable dimensions and a position 200 to 300 meters below the earth’s surface should make the facility even more sensitive to gravitational waves than current detectors. The Einstein telescope should be able to detect space-time quakes at least ten times weaker than LIGO in the USA and VIRGO in Italy, the ET team advertises on the project’s website.
This would allow us to look much further into space than before, explains Harald Lück, co-chair of the ET Steering Group at the Leibniz University of Hannover. Today, terrestrial gravitational wave observatories primarily detect signals from a good billion light-years away. “In a sense, however, this is just our cosmic neighborhood, the observable universe is much larger,” says Lück.
In the vast majority of cases, the gravitational waves detected so far originate from black holes. The invisible mass lumps sometimes drift through space in pairs, orbiting each other and getting closer and closer over time. At some point, they merge into an even heavier black hole. Since masses are accelerated extremely strongly for a short time, space-time becomes hotter and hotter — gravitational waves spread out in all directions at the speed of light.
The amplitude of the waves decreases with increasing distance. A more sensitive detector like the Einstein telescope could therefore detect signals that have traveled a much longer distance through space. According to calculations, the researchers would even have to be able to look back into the “Dark Age”, when no stars had formed yet.
The ET could also set new accents in terms of gravitational-wave frequencies. Unlike the previous facilities, it is to consist of two separate laser interferometers. The first, strongly cooled specimen is supposed to catch low frequencies, while the second one operates at room temperature and could detect space-time quakes of higher frequency. The larger bandwidth promises better measurement results.
Black holes in the ingot gap
In particular, the ET would be active at very low frequencies of two to three Hertz — a range in which current detectors can detect practically no gravitational waves. In this part of the spectrum, the researchers expect to find signals mainly from the fusion of “moderately heavy” black holes with a few hundred solar masses. That such objects exist at all was one of the surprises of recent years. They are regarded as “seeds” for the much more massive black holes that are at the center of many galaxies — and for which it is not yet really clear how they were able to form in the young universe.
So far, LIGO and VIRGO have measured dozens of black hole mergers, each of which had between a handful and several dozen solar masses. “The Einstein telescope is expected to record some 100,000, maybe even a million of these events, from a wide variety of times,” says Lück. “This will enable us to check whether the history of the origin of the universe is happening as we imagine it to.”
The ET may even detect gravitational waves from the time before the universe became transparent. So far, the earliest insights into the very young universe have come from the cosmic background radiation. It escaped 380,000 years after the Big Bang when the radiation-matter mixture had cooled down to such an extent that protons and electrons “recombined” to form neutral hydrogen. Unlike charged atomic nuclei and particles, this hydrogen simply allowed light to pass through, allowing electromagnetic waves to propagate undisturbed for the first time since the creation of the universe. Gravitational waves must have been able to travel freely earlier — and in the ideal case could provide a view of the processes during the first 380,000 years.
Neutron stars are another field of research for the Einstein telescope. These are ultra-compact corpses of stars that are only two dozen kilometers in size. They can also emit detectable gravitational waves — and not only when pairs of them merge. “Neutron stars rotate 100 times faster than a washing machine drum in the spin cycle,” says Lück. If their shape is not perfectly spherical, gravitational waves are also generated.
“The ET could determine deviations from the spherical shape down to a hair’s breadth and help us to better understand these extreme objects.”
Start of measurements in the 2030s
The researchers estimate that the construction of the Einstein telescope will cost around 1.9 billion euros — it would thus be considerably cheaper than a 100-kilometer particle accelerator, which is said to cost more than ten times as much. Work on the new gravitational wave detector could begin as early as the end of the decade, Lück and his colleagues hope. The facility would then be ready for operation in the 2030s.
“It’s an infrastructure that is constantly being expanded with new technologies and should be in operation for 50 years,” says Stefan Hild from the University of Maastricht. Under Hild’s leadership, a team is currently sounding out what initial equipment the huge facility should have. In a warehouse on the site of an old Maastricht print shop, a much smaller prototype called “ETpathfinder” is currently being built with an arm length of 20 meters. “That is too short to measure gravitational waves,” says Hild. “Instead, we want to test various technologies that will give the ET the desired sensitivity.”
The physicist uses an example to demonstrate the extreme quantities that need to be mastered: “To measure a gravitational wave, we have to detect a relative change in length of 10–23,” he says. “For comparison: If a raindrop falls into the Ijsselmeer (the largest lake in the Netherlands, editor’s note): the water level rises by 10–19 meters. Lake Constance, which is half that size, is 2 by 10–19 meters.
These figures illustrate how any vibration on earth can destroy the hunt for gravitational waves. After all, such vibrations shift the mirrors of the interferometer and thus change the travel time of the light a little bit. Just like a gravitational wave that minimally compresses the space between the mirrors.
For the researchers, even the Brownian motion of atoms is a problem. To slow down the heat-induced fidgeting on the mirror surface, the scientists want to cool some of the reflectors down to minus 263 degrees Celsius. “At these temperatures, however, the mechanical quality of the synthetic glass used so far deteriorates, and the noise increases,” says Hild. He and his team want to counter this with a more rigid silicon single crystal. However, this is not transparent for the wavelength of common near-infrared lasers, which is why the researchers are focusing on fiber lasers with a slightly longer wavelength.
Another source of interference is the so-called quantum noise. Roughly simplified, this is the momentum transfer of light particles to the mirrors, which sets the latter in motion minimally. To reduce the deflection, mirrors weighing 200 instead of 40 kilograms are to be used in the ET. The researchers also want to improve the suspensions.
Even if Maastricht is “only” about technology tests, the ETpathfinder team does not need any disturbances. “When the wind pushes against the hall wall, the floor also moves minimally,” says Hild. That’s why the researchers replaced it without further ado. The new floor rests on 169 pillars and is thus decoupled from the rest of the building. The group is currently having an extremely sterile cleanroom built, and tests are scheduled to begin in spring 2021.
Search for the best location
At which location the fully-grown Einstein telescope will eventually hunt gravitational waves is still unclear. What is certain is that it will go underground. On the one hand, because it is unlikely to find an easily accessible area in Europe with room for a triangular detector with sides ten kilometers long on each side. On the other hand, because the seismic disturbances of settlements, industry, and traffic are less severe at depth. According to geological and socio-economic studies, planners are currently tending towards Sardinia or the aforementioned Euregio Meuse-Rhine around the border region of the Netherlands, Belgium, and Germany.
The latter would be a belated satisfaction for many German physicists. For if things had gone differently, the first gravitational wave might have been discovered not in the USA but Germany. After all, it was German researchers, among others, who in the 1970s laid the foundation for the later proof of such a space-time quake.
First in Munich, later in Garching, they sounded out the capabilities of possible detectors for gravitational waves, explains Walter Winkler, who was a research associate at the Max Planck Institute for Quantum Optics in Garching at the time. In 1987, the group even wrote a letter to Heinz Riesenhuber, then Federal Minister of Research, proposing the construction of a laser interferometer. The device was to have three arms, each three kilometers long. According to Winkler, the physicists renewed their proposal in 1989, this time together with British colleagues.
“But we didn’t even get a response from the ministry,” the researcher says. Certainly, the 300 million D-Mark that was at stake represented a considerable investment. The project was also a risk at the time, “more hope than knowledge,” as Winkler admits. Nevertheless, he and other German physicists still feel that a large German gravitational wave detector did not work at that time.
In the USA, on the other hand, the National Science Foundation took the risk in the 1990s — and was rewarded three decades later with the detection of the first gravitational wave, for which a Nobel Prize was promptly awarded.
A question of the underground
“Big science always has a political dimension. And so it is by no means certain that the Einstein telescope will be built in the Aachen-Limburg-Maastricht region, just as it is not in Sardinia. “We have just applied to put the project on the ESFRI roadmap for important European research infrastructures,” says Lück. This would require the support of individual states, including Germany. However, the ET has not yet been included in the German government’s research plans.
The researchers expect a decision on the location within the next five years. In the end, many criteria are likely to play a role. For example, the local seismic and the geological composition of the subsoil, which determines the costs for underground construction. But socioeconomic factors such as the economic influence on the region and the countries involved also play a role — as well as, of course, the political interests and financial possibilities of the countries that come into question.
Also, ET advocates must explain why their plant is needed as much as other projects. For example, scientists are planning not only the LISA satellite observatory but also an earth-bound detector called “Cosmic Explorer”. In a way, it is the US-American vision for the future of gravitational-wave search. With two 40-kilometer-long arms above ground, it would be just as ambitious as the Einstein telescope.
“Our detector will be technologically more advanced,” says Alessandra Buonanno of the Max Planck Institute for Gravitational Physics in Potsdam, with a view to the ET. “Especially in the range of low-frequency gravitational waves, it will be able to observe much more precisely and thus look deeper into the “Dark Age” of the universe”. Ideally, both devices would be built, and a third one somewhere else on earth. “This is necessary to locate a source in the sky very precisely,” says Buonanno. In this case, radio telescopes and optical observatories could be quickly aligned with it, which would sometimes make it easier to document and understand what is happening.
The LISA space project, on the other hand, observes in a different frequency range from a little less than one to about 100 millihertz and is therefore receptive to other sources, says Harald Lück. “For example, it will be able to detect mergers of supermassive black holes, which are usually found in the centers of galaxies. Because the processes observed by next-generation Earth-based detectors and LISA are so different, the researchers believe they complement each other.
“A small group of sources, however, will first be observed with LISA and later with earthbound detectors.”
One example is the first gravitational wave event GW150914, which LIGO knocked out exactly five years ago. “This could have been seen with LISA about ten years earlier if the mission had been in space at that time. Instead of a big bang, the gravitational waves, in this case, would already have shown the death spiral on which the two objects came ever closer together — for gravitational wave researchers, it is almost as exciting as the big bang at the end. For them, it is clear that they need both types of detectors, in the sky and the earth.