

Optical Atomic Clocks Could Redefine Unit of Time - ISL
http://physics.aps.org/articles/v5/126

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btilly
_The availability of a frequency standard with 10^−18 fractional
uncertainty—still beyond the accuracy of existing clocks—would allow one to
measure Earth’s geoid (the shape of its gravitational potential) at a level of
1 cm, with the proper application and calculation of general relativistic
corrections for the clocks and their comparisons._

If I understand this correctly, they are discussing using the fact that
general relativity says that time goes slower inside of a gravitational
potential well to measure their height in the Earth's gravitational well to an
uncertainty of 1 cm.

Wow. They have got to line things up fairly precisely vertically to avoid
having time dilation from gravity invalidating their experiments. That's all
kinds of impressive to me.

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ISL
It is precisely the time dilation effect that they measure. As long as two
clocks are comoving, the only difference in their rates is due to the
difference in their gravitational potential. Without a clock very far from any
gravitating body, it's impossible to measure the absolute depth of the
gravitational potential.

This paper was downright incredible when it came out:
<http://www.sciencemag.org/content/329/5999/1630.short>

NIST PDF here: tf.boulder.nist.gov/general/pdf/2447.pdf

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ISL
At 5 parts in 10^20, the frequencies measured in this work will no longer be
representable with an unsigned 64-bit int.

They're at 2 parts in 10^17 now. Less than a factor of 1000 to go!

