New era for gravitational waves in Switzerland

Gravitational waves were predicted by Einstein’s general theory of relativity more than a century ago, but they were detected only very recently, in 2015, through the merger of two black holes. When massive objects merge, they create distortions in space and time that propagate across the universe, much like ripples spreading across water after a stone is thrown. These distortions are known as gravitational waves. On Earth, scientists detect them by measuring tiny changes in the path length of a laser beam, using a technique called laser interferometry.

Now that the existence of gravitational waves has been confirmed, scientists are eager to explore them further—and, through them, to learn more about the universe itself. The first detectors, LIGO and Virgo, pioneered this field, but they can capture only a tiny fraction of the signals that we now know are out there: they are only sensitive to the most energetic signals from the nearby universe. This is why researchers are now designing the Einstein Telescope. The goal is to dramatically increase sensitivity while simultaneously reducing external noise. Gravitational-wave signals are extremely faint and can be easily masked by vibrations from human activity or seismic events. To minimize these disturbances, the Einstein Telescope will be built 200-300 meters underground, with its instruments suspended from long chains of pendulums, operated in an ultra-high vacuum, and cooled to extremely low temperatures.

With the sensitivity enabled by these technological advances, the Einstein Telescope will detect gravitational waves before the merger occurs and continue observing them after the event. Instead of capturing a single moment, researchers will be able to follow the entire “dance” of two massive objects: their gradual approach, their slow orbit around each other, and the way their motion accelerates until it culminates in a final merger. This cosmic dance emits a pattern of gravitational waves—at first slow and steady, then increasingly rapid and intense—that fades quickly after the merger.

Detecting these early, low-frequency gravitational waves opens up new research possibilities. For example, by identifying a neutron star merger in advance, researchers can prepare to observe any light emitted during the event, alongside the gravitational signal. Such combined observations offer a powerful new way to study the universe and its history: how cosmic structures have evolved into what we see today, and the role dark energy has played in shaping them.

Observing the full “dance” before and after the merger also provides rich information about the objects involved. Scientists still have many open questions about black holes, the most massive inhabitants of our universe. How do they form? What range of masses can they have? And are all dark merging objects truly black holes—or could any of them be something entirely unknown? Learning more about the properties of these objects will help scientists get closer to answering their questions.

Even more excitingly, the Einstein Telescope will create a link to the earliest—primordial—moments of the universe. Before the birth of the first stars, it was an extremely hot, opaque plasma. In this dense sea of particles, the first black holes may have formed, and their later merging would have sent out gravitational waves. Some of these events may have occurred so far away that their signals have been traveling across the universe ever since and are only reaching us now. After such an immense journey, however, the waves are incredibly weak. Detecting them will require the unprecedented sensitivity and new observational regime of the Einstein Telescope. In doing so, we may finally study objects created in the earliest epochs of the universe.

“The Einstein Telescope will start a new era, completely changing what we can learn about the universe,” says Steven Schramm, professor at the University of Geneva and the Swiss Einstein Telescope community’s representative to the State Secretariat for Education, Research and Innovation of Switzerland.

Although the Einstein Telescope has not yet been built, its design is at an advanced stage of development. Over the past five years, more than 2,000 researchers and engineers from around 100 research institutions worldwide have worked on detailed plans, risk assessments, and alternative design options. Laser interferometers typically have L-shaped forms with two arms, along which a laser beam measures tiny changes in distance caused by gravitational waves. The Einstein Telescope’s arms will be 10-15 km long, a substantial increase from LIGO’s 4 km arms, [1] and will either be built as a triangle of three nested V-shapes or a pair of L-shapes at two different locations. The final infrastructure will require more than 120km of vacuum pipes placed at 200-300 meters underground, showing the immense scale needed to reach such extreme sensitivity to the universe.

Next year, the configuration and the location(s) of the Einstein Telescope will be decided among three candidate sites: the Euregio Meuse-Rhine at the border between the Netherlands, Belgium and Germany, Sardinia in Italy, and Lusatia in Germany. This final decision will complete the proposal, making it ready for approval by European governments and the start of construction soon thereafter.

As with any advanced technological project, some challenges are more complex than others. Current detectors measure gravitational waves at relatively high frequencies, meaning they are sensitive mainly to faster motions and lighter objects. These high-frequency instruments require substantial upgrades to existing designs for greater sensitivity. However, observing the slow “dance” of black holes and the mergers of heavy primordial objects requires detectors operating at much lower frequencies. Such instruments have never been built before—and Swiss researchers are determined to lead their development.

The "SWISS-ET Grand Projet" grant, which represents more than 6 million Swiss francs over five years, places much-needed support in capable hands. Swiss researchers have a strong track record of developing advanced technologies for major experimental facilities such as the Large Hadron Collider and the synchrotron light source at the Paul Scherrer Institute, among others. Through a network of applied research institutions, they combine scientific innovation with industrial efficiency and the ability to scale.

The Einstein Telescope collaboration is confident that its first observations will bring entirely new scientific discoveries. With significant room for future upgrades built into the design, researchers expect the detector to continue opening new windows on the universe for the next 50 years. “With the Einstein Telescope, we might discover completely unexpected things that we cannot even imagine,” says Steven Schramm, sharing his excitement for the future.

Aleksandra Nelson for CHIPP

[1] Laser light that travels inside of these arms gets reflected multiple times, such that it covers more than 1000 km in the LIGO experiment, and is planned to cover over 5000 km in the Einstein Telescope. These vast distances are necessary for the detectors to sense the tiny space distortions caused by gravitational waves.