Thursday, 23 April • 17:00 – 18:15 • Room Rotonde

The Consultative Committee for Time and Frequency (CCTF) is actively advancing its roadmap towards the redefinition of the SI second, provided that a preferred option for the new definition is consensually selected and that all mandatory criteria are met. The goal is to ensure both a clear improvement for users and a sustainable, long‑term definition.

The CCTF roadmap was endorsed by the 27th General Conference on Weights and Measures (CGPM) in 2022. An updated version of this roadmap underpins the resolution to be submitted for approval at the 28th CGPM in October 2026, with the target date for the redefinition maintained in 2030.

This presentation will cover the key elements of the updated CCTF roadmap, focusing on recent progress, the current status and main challenges, as well as the workplan to move forward and reach the objective of taking an important decision on the new definition in 2030.

Full abstract

This paper presents ongoing historical research into the technological developments and the process that led to an agreement, in 1967, to redefine the SI second for the first time on the basis of an atomic reference.

Drawing on a close analysis of official reports and an inventory of all frequency standards and atomic clocks constructed prior to 1967, the research explores how cesium was ultimately chosen as the basis for the new definition.

Special attention is given to the emerging methods, infrastructures, and nomenclatures developed during this period, which enabled effective comparison of these devices. While such technical considerations were vital in demonstrating the maturity of atomic clocks for redefining the second, the final decision also depended on institutional negotiations and broader strategic concerns.

This examination of the debates and consensus‑building strategies of that period offers insights relevant to current discussions about a new redefinition.

Full abstract

Davide Calonico – INRIM

Toward a Redefinition of the Second: Criteria

The Time and Frequency Consultative Committee of the CIPM has set up a Task Force dedicated to the roadmap towards a possible redefinition of the second. The Task Force is organised in three groups, respectively focusing on:

• the actual definition
• dissemination and communication
• the criteria considered mandatory or recommended before a redefinition takes place

At the conference, the criteria will be presented, along with their main challenges, the engagement of research groups and institutions, and the advances needed to achieve the relevant milestones before the possible redefinition.