When should we change the definition of the second?

@article{Gill2011WhenSW,
  title={When should we change the definition of the second?},
  author={Patrick Gill},
  journal={Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences},
  year={2011},
  volume={369},
  pages={4109 - 4130}
}
  • P. Gill
  • Published 28 October 2011
  • Physics
  • Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences
The microwave caesium (Cs) atomic clock has formed an enduring basis for the second in the International System of Units (SI) over the last few decades. The advent of laser cooling has underpinned the development of cold Cs fountain clocks, which now achieve frequency uncertainties of approximately 5×10−16. Since 2000, optical atomic clock research has quickened considerably, and now challenges Cs fountain clock performance. This has been suitably shown by recent results for the aluminium Al… 
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