The most accurate clock ever made

Cosmos Magazine ^ | May 4, 2015 | Cathal O'Connell

[C19fan]

Scientists have succeeded in making a clock so precise it could tick for 15 billion years – longer than the age of the Universe – without gaining or losing a second. The new research, described in Nature Communications in April, sets a world record for timekeeping and is a three-fold improvement over the previous record, set by the same clock in Boulder, Colorado, last year.

On a practical level, the optical lattice atomic clock Jun Ye and his colleagues at the US National Institute of Standards and Technology are developing could replace the caesium atomic clocks used in GPS systems, internet communications and other technologies that rely on accurate time keeping.

[I want the USA back]

What is a second? What is the external reference by which this is calibrated or has its accuracy checked?
So how would they know it doesn’t lose accuracy?

[egfowler3]

According to the Dictionary of Units of Measurement, maintained by Prof Russ Rowlett and the University of North Carolina at Chapel Hill, a second is defined as follows:

“In 1967, scientists agreed to define the second as that period of time which makes the frequency of a certain radiation emitted by atoms of cesium-133 equal to 9,192,631,770 hertz (cycles per second). In other words, if we really want to measure a second, we count 9,192,631,770 cycles of this radiation. This definition allows scientists to reconstruct the second anywhere in the world with equal precision.”

The International System of Units (SI) is a short list of base units defined in an absolute way without referring to any other units. The seven base units are: the meter for distance, the kilogram for mass, the second for time, the ampere for electric current, the kelvin for temperature, the mole for amount of substance, and the candela for intensity of light.

On the web, the dictionary can be found at:

http://www.unc.edu/~rowlett/units/index.html