The Einstein Telescope (ET) GW observatory project has entered a decisive phase. The detection of gravitational waves in 2015 (GW150914) and the beginning of the multi-messenger era (GW170817) by LIGO Scientific Collaboration and the Virgo Collaboration demonstrated the potential of gravitational wave observatories and provide a solid basis for the future scenario of GW observation. ET is the pioneer of a new generation of facilities, the so-called 3rd generation GW observatories network, that will observe the entire Universe. In order to realise it in the decade 2030+, we are carrying out a series of actions and the letter of intent (LOI) has been the first step. The aim of the LOI has been to constitute the basis of the ET collaboration. The process signature process is concluded and the results are available here:





Letter of Intent for the Einstein Gravitational Wave Telescope (ET) collaboration.


This letter of intent concerns the study and the realization of the Einstein Telescope (ET), a 3rd Generation (3G) Gravitational Wave Observatory (GWO). The scientists signing this LOI agree with the framework and in the targets described below.



1.      Prologue

The key elements of the Einstein Telescope project are described in the ET conceptual design study report (ET-0106C-10,

That document sets out the basic concepts of the scientific case, the technology and the infrastructures, although they are currently being updated taking into account the changed scientific and international framework.



2.      The ET Collaboration

  • The ET Design Study created an ET community in Europe and a global interest in the 3rd generation of GW observatories. In this LOI, we agree to transform the ET community into a collaboration with the goals outlined in the following sections. The ET collaboration for the proposal and pre-construction phase will indicatively be structured according to the following scheme:

    • An Executive Board (EB), chaired by a “Project Leader” and co-chaired by a “Deputy Project Leader”. The EB is responsible to structure the work, to liaising with international bodies to approve the ET project in an international context and to have it developed in the pre-construction phase. The project leader and the Deputy-Project leader are appointed according to the rules defined in the ET consortium agreement document (see point 3).
    • A General Assembly (GA), consisting of one or more representatives per funding agency/Institute. The duty of the GA is primarily to interface with the funding agencies and with the ET teams.
    • The details of the organisation of the ET collaboration for the initial phase are defined in the ET consortium agreement document, to be realised by a dedicated committee led by H. Lueck and M. Punturo. There will be a consultation phase on the draft consortium agreement document.
    When the consortium agreement document will be released the above mentioned organisation shall enter into force and scientists can sign up for membership.


3.      The ET project

We promote the approval and funding of the ET project by all the potentially interested institutes, funding agencies and national governments. The common target is to have the facility under construction for the middle of the next decade (ca. 2025) and to complete the research infrastructure before the end of the next decade (<2030). We define a stepwise procedure to support the approval of the project and the funding of the R&D needed to implement the new technologies in the first phase supporting and profiting from Advanced Virgo + and Advancd LIGO +. Scientists signing this LOI are engaged to submitting the ET proposal to their research institution within the times and the modalities agreed in the collaboration. At the national level, scientists are engaged to submit the ET project in their national roadmap for future research infrastructures. The main target in the next year is the submission of the ET proposal to the 2020 update of the ESFRI roadmap; scientists signing this LOI agree to collaborate to support the submission of a joint ET proposal and to stimulate the support of their institutes and national governments.


4.      Toward the ET technical design report

The ET design study has been delivered in 2011. Although it contains all the key elements of the ET project, the global scenario has changed due to the GW detection and renewed global interest in 3G observatories.

We will collaborate on updating  the design study by developing a Technical Design Report (TDR) for the ET project over the next few years, providing a renewed science case in a global network scenario, developing enabling technologies, investigating and selecting the appropriate site for the ET research infrastructure, including socio-economic impacts. This must be done within a European framework, considering the European Commission’s support for ESFRI projects, and within an international framework, in particular in synergy with the GWIC 3G efforts, depending on the development of the global scenario.


5.      ET site selection

Site selection will be one of the high priority activities of the ET collaboration, with the aim of taking the final decision in the period 2021-2022. The ET proposal will be submitted to the ESFRI roadmap with a restricted list of candidates and some criteria that will be used to make the selection. Scientists signing this LOI agree to support the characterisation, assessment, and endorsement of all promising candidate sites until the final site will have been agreed on.



GWIC: Gravitational Wave International Committee
ESFRI: European Strategy Forum on Research Infrastructures


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