OptionProbability
No decision made in 2026 conference
Second defined by a (dynamic) weighted sum of atomic transitions
Second defined by a (fixed) weighted sum of atomic transitions
Second defined by 87Sr 698nm transition
Decision made to not redefine the second
Second defined by fixing the Rydberg constant
Second defined by 171Yb 578nm transition
Second defined by 199Hg 265nm transition
Second defined by 27Al+ 267nm transition
Second defined by 199Hg+ 282 nm transition
Second defined by 171Yb+ (E2) 436nm transition
Second defined by 171Yb+ (E3) 467nm transition
Second defined by 88Sr+ 674nm transition
Second defined by 88Sr 698nm transition
Second defined by 40Ca+ 729nm transition
Second defined by 87Rb 6.8GHz microwave transition
Other
38
12
7
6
5
3
2
The cesium 133 F=3 <-> F=4 hyperfine microwave transition (no change)
Any system based on weighted averages of uncertainty or timekeeping contributions
The strontium 87 1S0 <-> 3P0 optical transition
The ytterbium 171 1S0 <-> 3P0 optical transition
Any other optical transition in ions
Any other optical transition in neutral atoms
The thorium 229 metastable nuclear optical transition (any realization)
39
22
8
