Einstein’s telescope could usher in a new era in astronomy

It’s just a plan for now, but a new telescope could soon measure gravitational waves. Gravitational waves are something like the sound waves of space. They are created, for example, when black holes or neutron stars collide.

The future gravitational wave detector, the Einstein Telescope, will use the latest laser technology to better understand these waves, and thus our universe. One possible location for the construction of this telescope is the border triangle of Germany, Belgium and the Netherlands.

How the universe makes gold

The summer of 2017 was an extremely exciting day for astronomers: on August 17, three gravitational wave detectors registered a new signal. Hundreds of telescopes around the world were immediately pointed at the supposed point of origin, and indeed the luminous celestial body was seen there. For the first time, the collision of two neutron stars has been detected both optically and as a gravitational wave.

Neutron stars are something very special in the universe: They are burned-out stars that no longer emit any visible radiation. They weigh slightly more than our Sun, but compress their mass into a sphere less than 20 km in diameter. The force of their collision is so great that atomic nuclei are torn apart, ejecting gigantic amounts of matter and heavy atoms such as gold can be formed.

“Compared to the mass of neutron stars, not much gold is formed – only a few lunar masses,” explains Professor Achim Stahl, astrophysicist at RWTH Aachen University, with a smile.

“But researchers are pretty sure that most of the gold in the universe was created in such gigantic explosions.” Therefore, the gold ring we wear on our finger has already experienced galactic history.

Gravitational wave detectors open a new chapter in astronomy

Thanks to gravitational wave detectors, we already know more about neutron star collisions. By galactic standards, these are very fast processes. In the past, if we were very lucky, we could register gamma-ray bursts that lasted less than a second. When black holes collide, the signal that can be measured by current gravitational wave detectors is very short.

The first gravitational wave signal measured in 2015 was just over 0.2 seconds long. Such waves are created when ultra-heavy objects orbit each other in space and then collide.

The signal detected in the summer of 2017 was 100 seconds long, so it was immediately clear that it must be something new. Shortly after the gravitational signal stopped, a gamma-ray burst was detected; later, the afterglow of the explosion was observed at different wavelength ranges, and traces of heavy elements such as gold and platinum were detected.

The event was identified as the collision of two neutron stars. The simultaneous observation of gravitational waves and electromagnetic signals opened a new chapter in observational astronomy. “In fact, the optical signal was crucial in finding the star in the sky,” explains astrophysicist Stahl.

Our ‘ears’ to space

For centuries, astronomy was limited to the observation of visible radiation. With a better understanding of the electromagnetic spectrum, astronomers have added many new observational methods, detected radio waves, and greatly expanded human knowledge through calculations and simulations.

When Albert Einstein postulated his general theory of relativity a good hundred years ago, he also came up with the idea that there could be waves that have nothing to do with the electromagnetic spectrum. Similar to a sound wave, they were meant to cause the test specimen to “wobble” a bit over a long distance.

Large accelerated masses should send such waves through space. On Earth, however, the fluctuation caused by gravitational waves is so weak that the motion is much smaller than the diameter of an atom. However, it is now possible to measure gravitational waves. This is a new era for astronomers.

This is made possible by so-called laser interferometers. They consist of two arms with mirrors at the ends. The laser beam enters the interferometer and is split into a beam splitter in the middle.

It travels to the end mirrors in the two arms and back to the beam splitter. If the position of the mirror at the end of the arm changes, the transit time of the respective laser beam changes slightly. This quantity can be measured by comparing the laser beam from the affected mirror with the laser beam from the other arm of the interferometer where the mirror has not moved.

The precision of this measurement in current gravitational wave detectors is always astonishing, even for physicists: “We measure with an accuracy of less than one two-thousandth of the diameter of a proton,” explains Professor Stahl.

“Ironically, we need precision on the scale of the smallest particles we know to detect the biggest events in the universe, black hole mergers,” he adds.

The first attempts to measure gravitational waves were already made in the 1960s. However, this extreme precision can only be achieved by the current second generation of laser measuring devices, which have now detected around 100 collisions of black holes or neutron stars.

Einstein’s telescope

Professor Stahl is a member of the German Einstein Telescope community and is currently working on a new generation of gravitational wave detectors. Measuring devices of this third generation should be ten times more sensitive than those currently in use. The planned gravitational wave observatory was named “Einstein Telescope” after the founder of general relativity.

“With its help, we want to explore an area that is a thousand times larger than what is currently possible in the universe for gravitational waves. And then we should find significantly more sources for which the current instruments are not sensitive enough,” explains the astrophysicist. . This also applies to heavier objects that emit gravitational waves at lower frequencies.

The Einstein telescope will consist of three nested detectors. Each of these detectors will have two laser interferometers with 10 km long arms. In order to shield interference as much as possible, the observatory will be built 250 m underground.

However, scientists are already thinking much further ahead. “The Einstein telescope will work with a new, innovative generation of observatories in the electromagnetic spectrum from radio to gamma rays. We call this multi-messenger astronomy,” Professor Stahl describes the vision.

“In addition to ‘ears’ for gravitational waves, we will also have ‘eyes’ that detect very different signals. Together, this will provide a live broadcast of cosmic events that no one has ever seen before.”

Until now, you could casually watch the sky and hope for a brief flash. In the future, gravitational wave detectors will run continuously and “listen” when a signal appears. If several such detectors pick up a signal, its region of origin can be calculated and other optical telescopes aligned with it. As with the neutron star collision in the summer of 2017, several systematic measurements will then be possible.

Scientists hope to gain many new insights from it, for example about the early universe or about the collisions that produced all the elements heavier than iron.

Detectors in Europe and the world

Such complex measurements require global cooperation. Accordingly, a conceptual design for a third-generation detector is also being developed in the US

“Cosmic Explorer” will create a global network of detectors with the Einstein telescope. In 2021, the Europeans included the Einstein Telescope in the plan of the European Strategic Forum for Research Infrastructures (ESFRI). ESFRI was established in 2002 to enable national governments, the scientific community and the European Commission to jointly develop and support the concept of research infrastructures in Europe.

With its inclusion in the ESFRI Roadmap, the Einstein Telescope entered the preparatory phase. The budget is estimated at 1.8 billion euros. The operation should cost around 40 million euros per year. Construction is scheduled to begin in 2026, with observations scheduled to begin in 2035.

Site selection studies are currently underway. A decision is expected in 2024. Two possible sites are currently being investigated: one in Sardinia and one in the Euregio Meuse-Rhine in the border triangle between Germany, Belgium and the Netherlands. When evaluating sites, research partners must consider not only the feasibility of construction, but also anticipate how the local environment will affect detector sensitivity and operation.

The project promises a number of benefits for the region: A large part of the 1.8 billion costs will go to construction measures. Three by ten kilometers of tunnels and twelve by ten kilometers of vacuum tubes are needed, to name just two examples. A significant number of companies are already involved in the project.

A large team is already working on the measuring device itself in various locations. In addition to RWTH Aachen University, this also includes the Fraunhofer Institute for Laser Technology ILT in Aachen. New lasers are currently being developed there, without which new measurements would not be possible.

“What we are developing here for potential use in the Einstein Telescope is unique in its design and is intended exclusively for measuring gravitational waves,” confirms project manager Patrick Baer of Fraunhofer ILT, who represents the research groups as Research Unit Leader in the Einstein Telescope community. from the Fraunhofer Institutes for Laser Technology ILT and for Production Technology IPT as well as the Departments for Laser Technology LLT and for Optical Systems Technology at RWTH Aachen University.

“However, in a simplified version, the laser technology developed for this area of ​​use may also be interesting for other applications, e.g. in quantum technology. However, the knowledge gained may also be useful for the development of lasers in medical technology: A wavelength of 2 µm is suitable, for example, for crushing kidney stones and urinary stones.”

After all, this is what Fraunhofer ILT has been doing since its foundation: producing cutting-edge lasers from research suitable for industrial applications.

Funding is not yet fully secured. Professor Stahl expects a final decision in the next two years. First the planners get to work, then the tunnel builders, and finally the laser physicists. “I estimate that we will be able to make the first measurements in 2035.”

What fascinates a researcher like Achim Stahl? “With gravitational waves, we can look much further into space than with normal telescopes,” explains the astrophysicist.

“In astrophysics, looking deeper into space means – above all – looking back in time. With the Einstein telescope, we will receive signals from the time when galaxies and the first stars formed. This goes further than is possible with optical. And we will hear cosmic explosions live with gravitational waves before we see them.”

The Einstein Telescope’s more sensitive detectors will “hear” the signals sooner, giving the other telescopes more time to align. In the past, it was more of a happy accident to see such action. Systematic measurements are now possible for the first time. Exciting times ahead – and not just for astrophysicists.