Searching for the fundamental building blocks of the universe with an underground telescope

Which physical forces and subnuclear reactions gave rise to the universe? What happens when two neutron stars collide? Have the laws of physics always been the same? These are just a few of the questions that scientists across a range of disciplines are eager to answer, yet with current technologies they are, unfortunately, still left wanting. Since the first detection of gravitational waves—barely 11 years ago—those answers have suddenly begun to come within reach.

Studying gravitational waves does, however, require a very particular kind of telescope. Not the large dish structures you might imagine, but tunnels stretching for kilometres, through which laser beams are fired. A few already exist in different parts of the world, but as their capabilities are limited, plans have recently been approved for a new one: the Einstein Telescope. This new type of observatory will be located... deep underground. It is—without exaggeration—one of the most exciting scientific projects of this century.

Gravitational waves are the key to unlocking many of the universe’s secrets. They are generated when enormous masses move extremely rapidly (and accelerate)—for instance by spinning on their axis or colliding with another mass—and then travel through the cosmos much like ripples spreading across a pond after a stone has been thrown in.

Gravitational waves make it possible to study phenomena that do not produce light and therefore cannot be observed with conventional telescopes. Although Einstein first theorised their existence around a hundred years ago, it was not until 14 September 2015 that they were detected for the very first time.

“It was an unexpected breakthrough,” says Alexander Sevrin, Professor and Chair of the Department of Physics and Astronomy at the VUB. “Gravitational waves are incredibly weak compared with, for example, electromagnetic radiation. I still remember warning my students that gravitational waves would never be detected in my lifetime—nor, most likely, in theirs. I am very pleased to have been wrong, but this truly is science at the very limits of what is possible—perhaps even beyond.

“What we are trying to measure are deviations on the scale of one attometre—that is 10⁻¹⁸ metres, or less than a thousandth of a proton—using interferometers that are several kilometres long. To put that into perspective, it is comparable to measuring the thickness of a human hair across the distance from Earth to the nearest star outside our solar system—around five light-years away.”

The design of the Einstein Telescope centres on 3 tunnels, each 10 kilometres long, forming a triangle at a depth of 250 to 300 metres underground. Laser beams are sent through these tunnels and reflected back by extremely precise mirrors. The presence of gravitational waves can then be inferred from minute deviations in the reflected beams.

Since 2018, Sevrin—together with professor Hugo Thienpont of B-PHOT—has been working to bring the construction of the Einstein Telescope onto the right track. The VUB is involved in 3 aspects of the project: conducting geological surveys at one of the potential sites, building the mirrors, and developing the measurement instruments.

“These mirrors are an extraordinary feat of engineering that only a handful of laboratories worldwide are capable of producing,” Sevrin explains. “They are made from silicon—something no one has done before—and have a precision of less than one ångström, effectively the scale of a single atom. This is achieved by using ion beams to remove atoms that could cause imperfections.”

Sevrin’s own department is, of course, also directly involved in the project. “Our task is to define the kinds of research we will be able to carry out with the Einstein Telescope. It’s quite an extensive list. For instance, it will allow us to study the universe’s ‘dark ages’. That term should be taken quite literally: it refers to the period between the Big Bang and the formation of the first stars, roughly 150 million years later. There was therefore a substantial span of time during which the universe already existed, but no light had yet emerged. We can only investigate that period with the Einstein Telescope, because gravitational waves were already present. The Big Bang itself, however, is unlikely to be observable in the coming decades—that will, unfortunately, be a challenge for the generation after mine.”

It is already clear that the Einstein Telescope will bring about major breakthroughs in cosmology and particle physics. It will make it possible to study the behaviour of matter under extremely high density and pressure—conditions that cannot be reproduced in a laboratory. In addition, scientists will gain deeper insights into astrophysics, as the Einstein Telescope will allow for highly detailed observations of supernova explosions.

Both Europe and the United States have previously invested in detectors to observe gravitational waves. Advanced Virgo (in Europe) and Advanced LIGO (in the US) succeeded in August 2017 in detecting gravitational waves produced by the merger of two neutron stars. These are extremely dense remnants of massive stars that have collapsed following a supernova explosion, during which protons and electrons combine to form neutrons.

The EMR consortium, which aims to build the Einstein Telescope at the tri-border area of Belgium, Germany and the Netherlands, estimates the project cost at around €3 billion. “That may sound like a huge amount, but for milestones of this kind in the history of science, it is actually quite reasonable. The construction of the Large Hadron Collider (LHC), the best-known particle accelerator at CERN, cost roughly €4 billion in total, with the experiments carried out using it costing almost as much again. The construction and development of the James Webb Space Telescope amounted to approximately €8.5 billion.”

In the meantime, the necessary technologies for the future operation of the Einstein Telescope are already being developed. According to Sevrin, this is by no means a gamble: even if the contract to build the Einstein Telescope is not awarded to the consortium involving Belgium, the investments will still pay off.

“For the metal required to build the 120-kilometre vacuum tubes through which those extremely precise laser beams will be sent, a strong collaboration has already emerged between metallurgical specialists at Ghent University and Belgian steel companies. Even if the project ultimately goes to another consortium, there is a good chance they will still be selected to supply the metal for those tubes. And that expertise will undoubtedly prove valuable in other applications as well. The same applies to the development of silicon-based optics being carried out at the VUB.”

The economic return on this project could be substantial. “Every euro spent on it will generate around three euros for our economy,” Sevrin calculates. “For me personally, the intrinsic scientific value of this project is enough, but in the current economic climate, such public investment can, of course, only be justified if it also directly benefits society.”