Gravitational waves are infinitesimal vibrations of spacetime, extremely weak and therefore very difficult to detect. Produced by extremely energetic events such as the merger of black holes or neutron stars, these ripples propagate at the speed of light and carry energy through space. After decades of attempts and technological advancements, the first signal of a gravitational wave — produced by the merger of two black holes over a billion light-years away from Earth — was observed in September 2015 by the LIGO-Virgo collaboration, thanks to the two twin LIGO interferometers (in the USA). The discovery of gravitational waves, awarded the Nobel Prize in Physics in 2017, is one of the most significant achievements of modern physics, confirming a fundamental prediction of Albert Einstein‘s Theory of general relativity. This result has also opened a new window for observing the universe, giving scientists the opportunity to study cosmic phenomena that do not emit light or other forms of electromagnetic radiation, such as black holes.

To measure the signal of a gravitational wave, highly sophisticated and precise instruments are required: laser interferometers, such as those found in the LIGO and Virgo observatories. These are large, advanced infrastructures consisting of two long arms (4 kilometers at LIGO and 3 kilometers at Virgo) arranged perpendicularly to each other. Inside these arms, laser beams travel through ultra-high vacuum tubes, and thanks to highly advanced suspended mirrors at the ends of the arms, the beams are reflected multiple times to increase their path length before recombining, creating what is known as an “interference pattern”.
The passage of a gravitational wave, by deforming space, alterins the length of the arms and thus the path traveled by the laser beams (one arm becomes longer, the other shorter). As a result, the interference pattern also changes. This variation is extremely small, of much less than the diameter of an atom, but modern interferometers are sensitive enough to detect even such infinitesimal deformations.

The Einstein Telescope (ET) is a European project aimed at constructing a large facility to house a next-generation gravitational wave detector. Its goal is to study the universe through gravitational waves, tracing its history back in time, almost to the moment just after the Big Bang, in order to reconstruct how it formed, evolved, and to understand what its future could be. It is named so because it will be an instrument used to “observe” gravitational waves arriving on Earth from deep space, and it is dedicated to Albert Einstein, who first hypothesized their existence as a consequence of his Theory of general relativity. The Einstein Telescope project has been included in the Roadmap of the European Strategy Forum on Research Infrastructures (ESFRI), the European body that provides advice on which crucial scientific infrastructure investments should be made in Europe.

Currently, there are three instruments in the world capable of detecting gravitational waves: the LIGO interferometers in the United States, Virgo in Italy, and KAGRA in Japan. These will continue to operate for over a decade before passing the baton to next-generation instruments like the Einstein Telescope (ET). ET will be able to detect gravitational waves with unprecedented sensitivity, observing a volume of the universe at least 1,000 times larger than that explored by current detectors, reaching/travel/coming back in time to just a few thousand years after the Big Bang.
Unlike LIGO and Virgo, which operate on the surface, the Einstein Telescope will be underground, significantly reducing seismic noise and other external interferences. Its arms will also be longer, ranging from 10 to 15 kilometers, compared to LIGO and Virgo’s 3-4 kilometers. Furthermore, ET will employ advanced technologies to reduce thermal noise and enhance low-frequency sensitivity. It will operate in a frequency range from 1-3 hertz up to 10 kilohertz, enabling the observation of events that are currently difficult to detect, such as the mergers of supermassive black holes and continuous gravitational waves from unknown sources. On the contrary, current interferometers are more sensitive to higher frequencies (from 10 hertz to 5 kilohertz), typically above 100 hertz, which allow for the detection of events like stellar black hole and neutron star mergers.

ET will study the universe through gravitational waves, for certain types of sources, up to cosmological distances. By observing the gravitational waves produced by the merger of black holes and neutron stars, as well as by other extreme astrophysical events, ET will be able to retrace the evolutionary history of the universe, taking a journey back in time toward the Big Bang. Not only that: thanks to ET, the observation of so-called “multimessenger” events will also become more frequent, where the detection of gravitational waves is associated with that of electromagnetic signals, allowing for the extraction of a vast amount of different types of information about the behavior of matter under extreme conditions. ET’s discoveries could also contribute to the study of some of the great mysteries of the universe, such as the nature of dark matter and dark energy, which together make up over 95% of the entire universe.

The Einstein Telescope will consist of a series of interferometers located in an underground tunnel at a depth of 100 to 300 meters. The scientific community is evaluating two possible configurations for the future instrument: one in the shape of a delta (Δ), with three arms of approximately 10 kilometers extending from the vertices of an equilateral triangle, and to be built on a single site; or an L-shaped configuration, with two perpendicular arms of about 15 kilometers, similar to current detectors. In this second case, two twin interferometers would be constructed and located in two distant distant sites. In both configurations, a series of experimental caverns will house seismic isolation towers, large optical devices, laser systems, cryogenic systems, and vacuum systems, which require advanced electronic and mechanical technologies to ensure optimal performance in detecting gravitational waves.

ET must operate in conditions of absolute silence, far from external interferences. Seismic stability, in particular, is an essential factor to ensure the high performance of the gravitational interferometer, especially in the search for low-frequency oscillations. For this reason, it will be located underground, at a depth of between 100 and 300 meters, further shielding it from seismic and environmental noise, which will be significantly reduced compared to the surface.

The site selection for the Einstein Telescope is still ongoing. Currently, two locations are competing to host the interferometer: the Italian site in the former metalliferous mine area of Sos Enattos in Nuoro, Sardinia, and the Euroregion Meuse-Rhine area at the border between the Netherlands, Belgium, and Germany. The observatory must be built in an area far from both natural sources (seismic activity) and anthropogenic sources (vehicle traffic, industrial operations, transportation) that could mask the weak signal generated by the passage of a gravitational wave. Additionally, the geology of the host site must ensure minimal presence of groundwater to allow for the stable and safe construction of the underground environments that will constitute the ET laboratory. The final choice of the site (or sites) where the experiment will be installed is expected in 2026.

The hinterland region of Sardinia is an ideal location to host the Einstein Telescope. The area around the former Sos Enattos mine, the Sardinian candidate site for the infrastructure, is extremely stable from a seismic standpoint, a crucial factor for ensuring high performance of the interferometer. Additionally, the mine’s rock and surrounding area — mainly composed of granite — along with the minimal presence of groundwater, make the site particularly suitable for the safe construction of an underground laboratory. Lastly, the region of interest, spanning the municipalities of Bitti, Lula, and Onanì, is characterized by vast rural areas with very low population density, further enhancing the “quietness” of the environment, a necessary condition for the operation of the Einstein Telescope.

In the most optimistic scenario, the Einstein Telescope could begin its scientific operations around 2035. The exact timeline will depend on the progress of construction (which will take about 10 years) and funding, which will result from a combination of European funds, contributions from individual participating countries, and global scientific collaborations.

The scientific community will benefit from a new infrastructure for the study of gravitational waves and cosmic phenomena, but not only that. The Einstein Telescope will create new opportunities for scientific research and technological innovation in cutting-edge fields: from precision mechanics and electronics to vacuum technologies, from cryogenics to optics and robotics. The innovative features will not only affect the experimental apparatus of the ET but also all aspects related to its implementation, which will stimulate economic growth and job creation in the host region. All of this will have consequences on the services that the people involved in the project will need, which will certainly strengthen the relationships between research institutions and local universities, also leading to an increase in educational offerings and overall improvements in education.

From the earliest design ideas, the ET’s scientific community has focused on solutions that ensure the efficiency of new infrastructure, the improvement of service quality for local citizens, and the accessibility of the area, with particular attention to environmental and energy sustainability. The goal is to minimize the negative impact on the territory and promote the sustainable use of resources, encouraging landscape-friendly solutions and the construction of low-impact infrastructure. Additional interventions include improving local roads and green energy systems, digital connections, and solar-powered lighting technologies. The Sos Enattos site will be enhanced as a research center and a place for scientific and cultural exploration, with the restoration of spaces and facilities to welcome visitors. Surface buildings and laboratories will also be designed to blend into the landscape, using sustainable construction and engineering solutions.

Italy plays a key role in the Einstein Telescope project, both due to its established experience in the field of gravitational wave detection, and as a possible site for the observatory. The Italian candidacy is supported by the Italian Government, the Ministry of University and Research (MUR), the Autonomous Region of Sardinia, and is scientifically coordinated by the INFN in collaboration with the National Institute for Astrophysics, the National Institute for Geophysics and Volcanology, and research institutions and universities across Italy.

The Italian candidacy is also supported by projects funded by the NRRP:
ETIC (Einstein Telescope Infrastructure Consortium), initiated in 2023, has two main objectives: to conduct a preparatory study for the technical and economic feasibility of the ET observatory, and to create or enhance a national network of laboratories dedicated to developing the technologies necessary for the future interferometer at the INFN locations, universities, and research institutions involved in ET.
FABER/MEET, whose overall objectives are the improvement, technological upgrading and implementation of large scientific networks dedicated to Earth monitoring and observation.  In particular, FABER aims to develop a seismic observatory at the former Sos Enattos mine to record currently unknown seismic signals.
TeRABIT will establish a high-performance network based on fiber optics to ensure rapid data transmission. This network will be crucial to support Sardinia‘s candidacy to host the Einstein Telescope, which will generate large amounts of data that need to be shared with a scientific community spread across the planet.