The new SI defines the second as the hyperfine splitting transition time associated to the frequency of that transition of caesium-133, 9192631770 Hz, that is 12 decimals precision, but...I wonder two things: -Shouldn't it be better to define the second with a pulsar ultraprecise measurement (some of the pulsar timing have 17 decimals of precision I believe)? Could we manage Planck time precision with a good enough "stick"-rule? Surely not with the current SI second definition, does it?

  • $\begingroup$ You ignore that it is an exact definition, with as many zeros after the decimal place as you care to add. $\endgroup$ – Jon Custer Jun 16 '19 at 19:10
  • $\begingroup$ Thus, how could you measure a planck time quantum with caesium-133 atoms? $\endgroup$ – riemannium Jun 16 '19 at 19:15
  • $\begingroup$ Since we do not observe any Planck-time quanta (and physicists are not even in agreement that they exist theoretically) it does not make sense to ask about measuring them accurately. $\endgroup$ – G. Smith Jun 16 '19 at 19:31
  • $\begingroup$ One observation is that the number of digits in the definition of the second is based on the accuracy of measurement available at the time of making the definition. Atomic clocks are better now (see eg the figure in the answer by Emilio Pisanty on this question). $\endgroup$ – jacob1729 Jun 16 '19 at 21:49
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    $\begingroup$ How can we possibly measure nanometers when the SI unit is meters? $\endgroup$ – Jon Custer Jun 17 '19 at 0:05

No, it would not be better. Pulsars make terrible time standards. First, the period of the fastest pulsar is only about a millisecond. Second, the period measurably increases as they lose rotational energy. Third, the rotation is measurably glitchy rather than steady because there can be random starquakes that change the moment of inertia.

Metrologists know what they are doing!

The Planck time is $5.4\times 10^{-44}$ seconds. We are not going be able to measure times to this accuracy in the foreseeable future.

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    $\begingroup$ Also, we want standards that are based on reproducible procedures. Using a natural object like a pulsar is worse than the old metal artifact standards for the metre and kilogram. $\endgroup$ – PM 2Ring Jun 16 '19 at 17:58

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