A triangle or two L-shapes: What should the Einstein Telescope look like?

Researchers at the AEI in Potsdam contribute to the comparative report on detector geometries

October 09, 2026

To the point

The illustration shows the main science goals of ET. — The illustration shows the main science goals of ET, some of which were used in this study and form the core of the Geometry-Comparison Report (from the Science Added Value Chapter).

The illustration shows the main science goals of ET, some of which were used in this study and form the core of the Geometry-Comparison Report (from the Science Added Value Chapter).

Expertise for governmental decision-making: The Geometry Comparison Report was submitted to the Einstein Telescope Board of Governmental Representatives. The document is intended to support decision-making and to serve as a knowledge resource for the project. Over 200 experts from more than 20 European and U.S. research institutions contributed to the work, which was coordinated by the Einstein Telescope Organization.
Different construction designs: The site-independent report compares two different geometries: a triangle with 10 km arms and two L-shaped detectors with 15 km arms. These are approximately 1,200 km apart and positioned at specified angles relative to one another.
Comprehensive document: The report covers five areas: scientific added value, risk, costing, governance, and socio-economic impact. Alessandra Buonanno, director at the Max Planck Institute for Gravitational Physics in Potsdam, chaired the team that assessed the scientific added value.

The Max Planck Institute for Gravitational Physics (Albert Einstein Institute, AEI), with locations in the Potsdam Science Park and in Hannover, has long been a leading institution in gravitational-wave research and is a co-initiator of the Einstein Telescope (ET). Researchers at the institute hold leading positions within the project and the Einstein Telescope Collaboration.

“The Einstein Telescope will trace black holes across cosmic time, probe matter denser than any lab can create, pair gravitational waves with light, and glimpse the early Universe,” says Alessandra Buonanno, director at the AEI in Potsdam. “As chair of the Scientific Added Value chapter, I had the privilege of assembling and leading a team of thirteen scientists from Europe and the United States. We distilled these goals into a set of 'golden channels', key science cases that we used to test each proposed geometry. On a tight timeline, we worked to give a fair and coherent comparison of the configurations.” Buonanno concludes: ”Every design delivers outstanding science; the choice decides which measurements ET will do best. It was intense, inspiring work, and the decision it informs will shape European science for decades.”

Also part of the team for the Scientific Added Value chapter were Arnab Dhani, postdoctoral scholar at the AEI in Potsdam and Tim Dietrich, Max Planck Fellow at the AEI and professor at the University of Potsdam.

Key insights from the report

The decision on the geometry of the Einstein Telescope will have a significant impact on the project, gravitational-wave science, and the countries involved. It requires thorough information that takes the complexity and multifaceted nature of the case into account. The report is a first compilation and assessment of that information. It already points to the following insights:

Scientific added value: Both the triangular design with 10 km arms and the double-L configuration with 15 km arms successfully meet the Einstein Telescope science requirements. To compare the scientific potential of the designs, the Scientific Added Value team established a portfolio of ‘science golden channels’. The report indicates that the 2L geometry is better suited to address the broadest range of these channels. The analysis also reveals that no single geometry maximizes every aspect of the ET science case. In particular, the angular orientation between the two L-shaped detectors is a further parameter that can be optimized.
Costing: The report provides a cost methodology for the ET project and an estimate for the cost of the scientific instrumentation hardware, referred to as ‘detector costs’. This estimate excludes, for example, the technical and civil infrastructure or the renewal of computing hardware. According to the report, the costs for the scientific instrumentation are estimated to be higher for the triangular geometry.
Risk analysis: The risk analysis does not show one geometry to be preferable over the other. However, the nature of the identified risks differs between the two approaches: while the triangular geometry shows a stronger exposure to risks associated with technical challenges and scientific performance, the 2L geometry is more exposed to risks related to governance and organizational aspects.
Governance: The governance model can and should follow, not drive, the geometry and site decisions. Both geometrical configurations are considered viable and carry distinctive risks and opportunities in terms of governance, but the multi-site option associated with the 2L configuration emerges as offering a more favorable balance of opportunities and risks. The report also highlights that, for both geometries, strong central coordination and management are crucial for successful ET governance.
Socio-economic impact and sustainability: Many details depend on local opportunities and future international partnership agreements. Hence, an in-depth study of environmental sustainability and socio-economic performance is required once the shape and location of the Einstein Telescope observatory have been chosen. This initial analysis, in the form of a differential evaluation, already shows that neither configuration is universally superior; instead, the two present entirely different risk and operational profiles. The analysis therefore calls for the geometry to be decided on scientific and technical grounds first.

More details are available in the publicly available executive summary.
Read the full press release by the Einstein Telescope Organization here.