Possible redefinition of the second

A few days ago, a news story appeared about the possible redefinition of the second. The BBC reported that scientists have invented a clock that loses just one second in 300 million years.

This new atomic clock is called the optical lattice clock. Its accuracy compares to the loss of one second every 100 million years by the caesium atom clock which is used for the current definition of a second. Given that the metre is defined as the distance that light travels in 1/299 792 458 seconds, a redefinition of the second will also change the definition of the metre.

Given the accuracy of the caesium atomic clock, we may ask why we need a more accurate definition of a second and what relevance has this to our daily lives. The BBC news report answers this as follows: “Many technologies such as telecommunications, satellite navigation and the stock markets rely on ever-better time measurements.”

This demonstrates that SI, the modern metric system, continues to evolve to meet the world’s ever-increasing demands for more accurate measurements. It is a measurement system that is fundamental to all measurements, both old and new.

The BBC report can be found at:

This news story has also appeared in UK newspapers, which can be found at the following links:



7 thoughts on “Possible redefinition of the second”

  1. I am just sorry that this is in Paris rather than Greenwich. That was another world opportunity this country missed.
    We ruled the world with time and gave it away through lack of investment and forward thinking.


  2. A standard atomic clock takes advantage of the fact that an atom can absorb electromagnetic radiation such as light at certain frequencies as its internal structure jumps from one “quantum state” to another. The clock essentially exposes atoms to radiation tuned to such a frequency, which then serves as the ticking of the clock. The atomic clocks that keep official world time are accurate to 3 parts in 10^16, so they would gain or lose less than a second in 100 million years.


  3. By coincidence, yesterday’s London Metro free newspaper had an article to commemorate the 220th birthday of the metre, see pages 24 and 25:

    Unfortunately, the author, Ben Gilliland, failed to explain very well the essence of a triangulation survey, namely that distances are difficult to measure accurately (done only twice over short distances for the meridian survey of 1792-98) whereas angles can be measured comparatively easily. He also failed to make clear that star shots aka astronomical observations are difficult to carry out to the required accuracy and were restricted to a few key locations.

    His final statement does not appear to have been too well researched either:
    “America has yet to acknowledge the existence of the metric system.” Did not America made metric legal for all purposes in 1866, thirty years before the UK?


  4. Indeed, the Metric Act of 1866 did so.

    The Mendenhall Order of 1893 made our metric standards the reference for all Customary measure. Customary units are merely declared fractions of metric units (tweaked slightly in 1958 to reconcile with UK and other English-speaking nations).

    Finally, the FPLA (and UPLR) require prepackaged goods to be dual-labeled in SI and USC (since 1994). Oh, and several US industries are entirely or primarily metric.


  5. Yes, the USA did acknowledge the metric system in 1866. More, United States Customary is in fact a second hand metric system, as the Mendenhall Order of 1893 defined the yard and the pound in terms of the international metric standards. Then came the agreement of 1959 that also linked British Imperial to the metric standards. Thus, for instance, the inch was defined as exactly 25.4 mm. Some opponents of metric brag that the USA went to the moon without using metric. If the inch had still been defined as the length of three barleycorns, using US Customary for high technology would have been impossible. So, the USA went to the moon, using second hand metric.


  6. It depends on what you mean by “went to the moon.” The navigation system was internally metric and then was converted to Ye Olde English/Second Hand metric for readout.


  7. Re: the 1960s American space programme.

    If it was indeed the case that the systems were calibrated and programmed in SI with USC front end, then I would guess that was for the benefit of the astronauts who were used to working in those units due to their background in aviation.


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