Resolution 4 : The definition of the second
Since the 13th General Conference on Weights and Measures (CGPM) in 1967, the second has been defined as follows : ‘The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the undisturbed caesium-133 atom. ”
The CGPM had then decided that time would no longer be defined by the Earth’s motion but on the basis of the quantised energy levels of atoms and the frequency corresponding to the energy differences.
It was then that the oscillation of radiation, whose frequency is tuned to the atomic transition of caesium – chosen for its stability – made it possible to establish a standard : 9,192,631,770 oscillations = 1 second !
But research did not stop there, and although the current level of precision is in itself a remarkable achievement and meets a wide range of needs, it must be borne in mind that an error of one microsecond results in a 100-metre error on Earth when determining a satellite’s position !
To take this further, therefore, other transitions that would correspond to higher – and thus more precise – frequencies are being studied in various atoms : strontium, mercury, ytterbium, calcium, aluminium and many others.
One of the greatest obstacles faced by research teams is the extreme mobility of atoms. Several techniques – cooling, the ‘magic frequency’ devised in the 2000s, and the frequency comb in 2005 – have led to major advances.
The CGPM has encouraged the CIPM to continue the necessary work in order to propose a new definition of the second, along with its implementation date, at the 29th meeting of the CGPM (2030).
https://www.doi.org/10.59161/CGPM2026RES4F
Resolution 5 : Abolishing the leap second ? It’s decided !
The time used by our clocks – from wristwatches to satellites – is Coordinated Universal Time (UTC), which has been the standard underpinning civil time in most countries since 1972, the year it was established.
Its role is to maintain consistency between the two measures of time we still use today, namely : the length of the astronomical day and atomic time. There is a discrepancy between these two measures of time, which until now has been resolved by adding a so-called ‘leap second’.
Until now, the length of the day has always been slightly longer than atomic time. However, since 2025, the Earth’s rotation has been speeding up ! It would therefore no longer be necessary to add a second, but rather to remove one between 2030 and 2035 if this phenomenon were to continue.
Whilst adding a second already posed serious difficulties for computer systems, removing one would be even more complicated and could have a considerable impact on several critical sectors (satellites, communication networks, banking services, GPS).
At present, a leap second is added when the difference between the two times exceeds 0.9 seconds. The proposal is to switch to a tolerance interval of 3,600 seconds, or 1 hour, which would enable UTC to be maintained continuously for several centuries.
The CGPM has adopted this proposal to move to 1 hour, and decided that ‘continuous UTC will come into effect on 20 May 2027’ !
https://www.doi.org/10.59161/CGPM2026RES5F
But why, in fact, has the Earth’s rotation accelerated ?
To date, there are only hypotheses.
Until now, the gravitational pull exerted by the Moon caused what is known as the ‘tidal effect’, which acted as a brake and slowed the Earth’s rotation by 2 milliseconds per century.
No one had anticipated that the opposite would happen – that instead of slowing down, it would speed up !
Yet this is precisely what has been observed since 2020. Researchers cannot say for certain that the phenomenon will continue, as they do not know its causes.
The most likely hypothesis – though one that will be difficult to verify – is that of internal movement within the Earth’s core, the dynamics of which are still very poorly understood.
Resolution 6 : Lunar time scale
The third resolution in which the LTE is involved concerns the definition of an international lunar reference time scale and its traceability to UTC.
It emphasises that the desire of space agencies to develop lunar infrastructure, satellite positioning, navigation and synchronisation systems in the lunar environment requires a common time reference compatible with the precision requirements of general relativity. The resolution notes that the direct use of the UTC scale would be unsuitable for the most demanding applications, due to relativistic effects. It further emphasises that the proliferation of coordinated time scales must be avoided so as not to lead to a proliferation of numerical values for mass parameters.
It notes that Lunar Coordinated Time (LCT), defined by the International Astronomical Union (IAU), fully meets the identified scientific requirements. Finally, it recommends that the CIPM monitor pioneering lunar projects and that the BIPM collaborate with space agencies and national metrology institutes to coordinate the realisation of the lunar time scale and ensure its traceability to UTC.
For further information :
https://www.doi.org/10.59161/CGPM2026RES6F
https://lte.observatoiredeparis.psl.eu/Quelle-heure-est-il-sur-la-Lune
As we have just seen with the three resolutions discussed at the 28th CGPM, the levels of precision have reached such heights that they must be accompanied by new infrastructure to enable these clocks to be compared without any degradation caused by the method used.
Refimeve
This is one of the roles of the REFIMEVE network, in which the LTE plays an active part, enabling European laboratories to overcome the limitations of satellite communications and further improve the accuracy of comparisons.
The REFIMEVE+ team at the Time and Space Laboratory (LTE) is building on a world-first scientific achievement : the long-distance transmission of an ultra-stable optical frequency over an internet network without disrupting traffic.
This infrastructure enables the distribution of ultra-stable frequency and time signals generated at the Time and Space Laboratory (LTE) (LNE-OP), via over 9,000 km of RENATER fibre-optic cable, to more than 30 laboratories and research facilities located throughout France. The aim is to be able to provide a global time reference.
As the relative precision of the distributed signals can exceed the 10⁻¹⁸ range, this enables a wide variety of scientific applications in fundamental physics, quantum technologies, photonics, geosciences and climatology, as well as for the synchronisation of large-scale instruments (such as accelerators or telescope networks) used in high-energy physics and astrophysics.
Similarly, the sensitivity of the optical fibre is such that the teams have realised that the network could be used to detect movements such as earthquakes, and to track the movement of bodies of water, magma and other solids and liquids. It is in this context that they are participating in the European Sensei project :
https://www.projetsensei.com/fr/
The future societal benefits also look set to be significant.
As part of this drive, an initiative led by France is now aiming to expand this infrastructure to a European scale. The FOREST project aims to establish a European research infrastructure centred on the transfer of optical time and frequency references on a continental scale.
Find out more :
https://www.refimeve.fr/
https://lte.observatoiredeparis.psl.eu/La-seconde-du-futur-se-precise-avec-un-reseau-mondial-d-horloges
Projects that push the boundaries of our measurements
And because this is research, the questions are open-ended and experiments are welcome, whether they confirm theories or reveal their flaws.
Thus, some teams at the LTE are working on building portable atomic clocks to measure how the value of a second varies with altitude, following in the footsteps of the theory of general relativity, which states that the passage of time depends on the gravitational field.
Similarly, the Pharao clock, installed on the ISS, compares its measurements from space with those taken on Earth : https://lte.observatoiredeparis.psl.eu/Lancement-reussi-d-ACES-PHARAO
Fundamental physics is being scrutinised, and it is in this way that it will be able to answer the major questions that remain about matter, time and space.
