In June 2024, the European Southern Observatory Council approved the process of identifying its next flagship project, which would follow the Extremely Large Telescope (ELT). This process is named "Expanding Horizons: Transforming astronomy" in the 2040s". Considering a decision to be made at the end of the current decade and another decade of construction, the new facility will enter into operation at the end of the 2030s and throughout the 2040s.

The astronomical infrastructure landscape in the early 2040s will differ significantly from today's landscape. At the ground, the Vera Rubin observatory, the ESO ELT, and the SKA Phase 1 will be in operation for well over a decade, and the US ELT programme will also be up and running. The same applies to the Cerenkov Telescope Array. In space, the JWST has operated for almost two decades, and Euclid has completed its sky survey. PLATO and ARIEL planet surveys will be delivered. The LISA mission has started operations mapping gravitational waves across the universe.

In this framework, optical long baseline interferometry presents a unique parameter space. Currently, the Very Large Telescope Interferometer delivers angular resolution of up to 1 milliarcsecond and astrometric precision of up to 10 micro-arcseconds. It can reach objects as faint as 22 mag in the K-band. In the 2040s, a factor of 10 in this performance is within reach.

What science can be achieved with 0.1 milliarcsecond angular resolution, 1 microarcsecond astrometry and very high sensitivity from the blue to the thermal infrared? Which technologies need to be matured? Which infrastructure concepts can deliver these performances? This workshop, jointly organised by the European Interferometry Initiative, aims to start the discussion towards answering these questions.

This workshop is also a commemoration of the 1985 ESO VLTI Interim Report and the visionaries who contributed to it. Forty years later, the vision, for example, of laser guide star adaptive optics optical long baseline interferometry will become a reality with GRAVITY+.

This meeting has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 101004719. support from Centre National de la Recherche Scientifique and Max Planck Gesellschaft.