Mirror, mirror underground

The telescope actually consists of 6 so-called interferometers, which can ‘sense’ the smallest fluctuations in the gravitational waves travelling through the universe. On each corner point of the triangle, 2 interferometers will be built on top of each other. One interferometer will be tuned to detect very low-frequency signals and the other to high-frequency signals. They work with light and only produce a signal as soon as a minute difference in length occurs in the tubes of the interferometer telescope.

“Normally there is no signal and nothing can be seen on the measuring equipment,” says Professor Michaël Vervaeke of the B-PHOT research group at VUB. “At the corner points where the tubes come together and where the light in the different tubes can interfere with each other, a signal only arises in the form of a ring of light at the moment when minuscule changes occur in the length of the tube system. The measuring equipment will be able to detect differences of 10⁻¹⁸ metres and even better, dimensions on an atomic scale.”

These small differences in length are caused by gravitational waves, which we know cause length variations here on Earth. “They are the result of cosmic events taking place somewhere far out in space, such as colliding black holes or exploding stars,” Vervaeke explains. “It was not that long ago that the first gravitational wave was detected. Nowadays we find at least one every week. We suspect there are many more and that they may also be the result of much lighter and smaller events in space, but that we simply do not register them. We hope that the Einstein Telescope will make that possible.”

For that, it is important that the components of the Einstein Telescope are manufactured with extreme precision. They must meet almost impossible standards. “In Maastricht they are working on the production of a laser light source with a very stable wavelength of 1550 nanometres,” says Vervaeke. “At the same time, work is being carried out in Aachen in Germany on a stable light source for light with a wavelength of 2090 nanometres. At the VUB research group B-PHOT, on our campus in Gooik, we are working on the input and output mode cleaners, the instruments used to stabilise and filter those laser sources to an extreme degree, so that exactly 1 mode is emitted and received at a very well-defined frequency. We are also working on the mirrors for the prototype and will contribute to the mirrors for the final telescope. Those for the prototype will have a diameter of 15 centimetres and a thickness of 8 centimetres. They are made from extremely pure monocrystalline silicon. That purity must be preserved throughout all processing steps, from raw silicon ingot to the ultra-precise mirror, which will have to reflect very large powers of several megawatts of light.”

The final version of these mirrors will have a diameter of 45 centimetres, a thickness of 57 centimetres and a weight of around 200 kilograms. The mirrors must be and remain so pure because they need to absorb as little light as possible. When light is absorbed, it is converted into heat, and the Einstein Telescope must be able to maintain an operating temperature of 15 degrees Kelvin, that is fifteen degrees above the lowest possible temperature. “Any disturbance, any minute heating, and even the motion of the atoms in the silicon, can lead to a deterioration of the signal-to-noise ratio,” Vervaeke explains.

There is still no certainty about the location of the new telescope. At present, there are three possible sites: one near the tri-border area of the Euregio Meuse-Rhine at the Belgian-Dutch-German border, one on the island of Sardinia, and possibly one in Saxony, Germany. “A stable subsoil with little seismic activity is essential,” says Vervaeke. “The vacuum tubes, which are intended to form a kind of triangular CERN, must be buried deep underground to avoid picking up seismic noise from ocean waves and to prevent interference from all kinds of human activity. At the points where the tubes of the immense triangle meet, underground spaces will be constructed with cathedral-like proportions to house the measuring equipment. These are therefore major underground construction works.”

The scientists at VUB have not waited for that final decision. At the end of 2025, the FWO-IRI project E-TECH was approved, an important step in the development of technological infrastructure to support the Einstein Telescope. Within that project, several Flemish partners are working together to build research and testing capacity that positions Belgium in the preparation of the future detector. For VUB, the contribution mainly lies in its expertise in photonics and precision optics, including through the B-PHOT research group, which works on optical components with extremely low surface roughness and high accuracy.

In September, an Ion Beam Figuring machine was delivered in Pajottegem for the final shape correction of mirrors for the Einstein Telescope. The installation makes it possible to adjust optical surfaces at nanometre level after polishing, which is crucial for the extremely high accuracy required to detect gravitational waves. The technology forms the final step in the production process of ultra-precision mirrors and allows surface deviations to be corrected at atomic level. The installation is part of the activities of the VUB photonics infrastructure in the Pajottenland and ties in with work on optical components for the Einstein Telescope Pathfinder. With this machine, local capacity is strengthened to develop and validate high-quality mirror optics, which is essential for the future Einstein Telescope.

“This summer, the completion of a cleanroom in Halle is also expected, which forms part of the ETpathfinder Smart Skills Lab ecosystem,” says Vervaeke. “This facility will be used for training, and for assembling and testing optical components under controlled conditions. The cleanroom makes it possible to organise training for engineers, technicians and companies wishing to be involved in the development of the Einstein Telescope. At the same time, the infrastructure also supports demonstration and development activities in precision optics and photonics.”