European physicists are edging closer to realizing their ambitious vision for the Einstein Telescope, a groundbreaking underground gravitational wave observatory.
Today, three leading gravitational wave detectors operate worldwide: the two LIGO sites in the United States (Louisiana and Washington State) and Italy's VIRGO. These instruments rely on laser interferometry to capture elusive signals from the cosmos.
Gravitational waves arise when massive objects, like colliding black holes or neutron stars, distort the fabric of spacetime, sending ripples hurtling through the universe at light speed. Upon reaching Earth, these waves cause tiny, alternating compressions and stretches in space itself.
Detectors work by splitting a laser beam with a semi-reflective mirror, directing the two beams along perpendicular arms to bounce off distant mirrors before recombining at a sensor. Gravitational waves shift the beams out of phase, revealing their passage (see the explanatory CNRS video below for details).
In the last five years, scientists have detected dozens of black hole and neutron star mergers, including two confirmed black hole-neutron star events, using LIGO and VIRGO. These observatories spot collisions billions of light-years away, but researchers now seek to probe even deeper into cosmic history.
To push these boundaries, European experts are developing the Einstein Telescope: an equilateral underground triangle equipped with six V-shaped interferometers (two per corner), each boasting 10-kilometer-long arms.
By contrast, LIGO's arms measure 4 kilometers, and VIRGO's are 3 kilometers—making the Einstein Telescope far more sensitive.
Though still conceptual, the project has gained traction. The European Strategy Forum on Research Infrastructures (ESFRI) has now added this €1.9 billion initiative to its roadmap of priority scientific projects, signaling strong political momentum.
"This isn't funding yet, but it demonstrates clear commitment," notes Harald Lück, gravitational wave physicist at Germany's Gottfried Wilhelm Leibniz University Hannover and co-chair of the Organizing Committee.
Over the next 3-4 years, the team will refine technical designs and expand international partnerships beyond current collaborators—Belgium, Italy, the Netherlands, Poland, and Spain.
If timelines hold, construction could start in the mid-2030s, unlocking unprecedented views of the universe's most violent events.
