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Geodetic Survey of NIST and JILA Clock Laboratories
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2019
[PDF-2.42 MB]
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Description:Einstein’s theory of general relativity indicates that when any clock is operated at a location “higher” than another (“up” is measured away from the mass that generates the local gravity field) it will be observed to run faster. That is, it will appear to “tick” at a higher frequency to those observers “below.” In our everyday experience this effect is unnoticeable, but groups at the National Institute for Standards and Technology and JILA (University of Colorado) in Boulder, Colorado are developing atomic clocks (so-called “optical” clocks and “optical lattice” clocks) with accuracies approaching a few parts in 1020. [1, 2, 3] At this level, changes in a clock’s height of even a few millimeters will cause a noticeable difference in its output frequency and must be accounted for. Technically, it is changes in the potential of the Earth’s gravity field – known as “geopotential” – to which the clocks are sensitive. To facilitate comparisons amongst clocks within a laboratory and/or between different laboratories, NOAA’s National Geodetic Survey (NGS) agreed to measure geopotential differences at various locations between the NIST and JILA facilities. This includes both the laboratory for the existing time standard as well as the new, experimental clock laboratories. With geodetically-determined geopotential differences in hand, a prediction of the expected frequency difference between any two NIST-JILA laboratories will be possible immediately. This is an update to a technical memorandum (NOS-NGS-73) describing the NIST-only survey of 2015[4]. In 2018, NGS extended the NIST geopotential network to new sites on the University of Colorado campus, including the lattice clock laboratories in JILA. Those results, along with the existing NIST results, will all be presented here for ease of use. The techniques and instruments used in 2018 were more or less identical to those of 2015, and any differences will be noted below. Looking forward, the hope is that one day these clocks can then be linked across continental or even global scales. Once the difference in local geopotential values is taken into account, it will allow for the direct comparisons of clocks for metrological, time distribution purposes. Further, the process can also be reversed: observed differences in the frequencies of clocks operating at far-flung locations can be used to infer geopotential differences directly; so-called “Chronometric Leveling.” [5, 6] This real-time “geo potentiometer” would revolutionize the field of geodesy.
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Rights Information:CC0 Public Domain
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