Abstract
After discussing several issues in a future redefinition of the kilogram, this paper considers the lessons that one might have learned from the analogous redefinitions of the metre and the second. The progress of length metrology was slow and steady, from seven digits reproducibility with the 1889 X-shaped metre prototype, to nine digits with Kr lamps, to 11 digits with the 1983 redefinition of the metre using the speed of light. With laser cooling, the Cs clock improved to 15, now 16, digits (and so also astronomical distance measurements could improve). Laser-cooled ions, and now atoms captured and cooled in an optical lattice, enable accuracy capability of three different optical frequency references to exceed 17 digits, i.e. better than time itself. The optical comb and related techniques vastly simplify frequency comparisons. Such progress stimulates a new satellite experiment, the STAR Mission (Space-Time Asymmetry Research). The goal is to test at the 1E-18 level frequency shifts owing to spatial anisotropy, position, gravitational potential and boost. The onboard optical clock will use stabilization to a molecular transition in I(2) or HCCH or CO(2). The length etalons will be multiply redundant, with stability at the thermo-mechanical mirror motion limit. For a ULE glass etalon spacer (1987), I measure length creep approximately -1.5E-12/d, i.e. below 1E-14 over the 500 s satellite spin period.
Highlights
With the great progress in the international ‘Avogadro project’, working to make the connection between atomic mass and the International System of Units (SI) mass scales, it will be appropriate to consider a redefinition of the SI kilogram unit [1]
Let me make explicit the parallel with the present SI redefinition issue: what we offered was the higher precision which was already available to all high-skill workers in the field, who would naturally share our conviction that the frequency of the HeNe/CH4 standard was not going to change
There is a long-term creep of the cavity length, getting always shorter, which may be a dozen hertz per second of the optical frequency drift soon after the mirrors have been optically contacted to the spacer and the system mounted in vacuum
Summary
With the great progress in the international ‘Avogadro project’, working to make the connection between atomic mass and the International System of Units (SI) mass scales, it will be appropriate to consider a redefinition of the SI kilogram unit [1]. Let me make explicit the parallel with the present SI redefinition issue: what we offered was the higher precision (in the IR) which was already available to all high-skill workers in the field, who would naturally share our conviction that the frequency of the HeNe/CH4 standard was not going to change It was known in SI units with an uncertainty of ‘only’ 5 × 10−10, but could be relied upon securely to ‘keep’ any measurements almost 100-fold better, even though they could not be expressed in SI Hz to the full precision limit. New dreams and good meetings stimulate people and open the new highways
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