[time-nuts] NPR Story I heard this morning

Tom Van Baak tvb at LeapSecond.com
Mon Nov 3 18:19:43 EST 2014

Hi Ken,

That's correct. No two clocks ever agree. If they look like they do, you are not looking close enough or not waiting long enough.

That's also why UTC is based on the combined stability of hundreds of clocks. The weighted average of many cesium clocks is known to be better than any one cesium clock. So a big part of the UTC infrastructure is the inter-comparison of clocks all around the world. Another part is then slowly adjusting local standards to follow the more accurate global mean.

You'll notice too, that many postings to this list are not just about clocks, but also precise time measurement, and about disciplining. Whether UTC at a national lab or a GPSDO at home, there is clock, measurement, and gradual adjustment.

  ----- Original Message ----- 
  From: ken hartman 
  To: Tom Van Baak ; Discussion of precise time and frequency measurement 
  Sent: Monday, November 03, 2014 2:52 PM
  Subject: Re: [time-nuts] NPR Story I heard this morning

  Not to put too fine a point on it, but my practical understanding is that any two or more clocks generally do *not* agree (that is - yield identical phase/frequency information) ever, anyway. So atomic horology - and beyond - means that we continue to ?adjust? ?compensate? clocks of whatever stability and accuracy to the current, agreed upon "ideal" - even as the ideal may move or evolve.

  On Mon, Nov 3, 2014 at 4:27 PM, Tom Van Baak <tvb at leapsecond.com> wrote:

    > I don't see anything in the BIPM definition of the second regarding sea level.

    Hi Mike,

    The usual wording for the definition of the SI second also includes the word "unperturbed". That little word covers a host of physics and engineering effects and can keep graduate students busy for years. You either have to eliminate them from your clock or your lab, or extra carefully measure then and back-out their effects on your clock's operating frequency.

    For a really good example of the sort of corrections that are made inside a cesium clock see: http://tf.nist.gov/general/pdf/1497.pdf

    By the time you read to page 30, you'll see table 3 and 4 which summarize the perturbing corrections.


More information about the time-nuts mailing list