4 ms·

Optical Atomic Clocks Could Redefine Unit of Time

• ISL 14y ago
  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!
• btilly 14y ago
  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.
  • ISL 14y ago
    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 http://www.sciencemag.org/content/329/5999/1630.short NIST PDF here: tf.boulder.nist.gov/general/pdf/2447.pdf
• deleted 14y ago
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