The second

The second (s) defines time (wā).

Caesium atomic clocks operate by exposing caesium atoms to microwaves until they start to respond at one of their transition frequencies. By determining this frequency (ΔνCs), an unchanging measurement of time can be established. ΔνCs has a fixed numerical value of 9,192,631,770 Hz, which leads to the following official definition: One second is equal to the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the unperturbed ground state of the 133Cs atom.

Caesium clocks are very stable and are accurate to 1 second every 30 billion years.

Discussion point: GPS satellites have onboard caesium and rubidium atomic clocks. What role does time play in how GPS systems work?

Transcript

FARZANA MASOULEH

The second is now defined based on a physical constant, which is the transition frequency of a caesium atom. It might not sound as intuitive as, for example, an older definition of the second, which was one specific fraction of a day. But clocks that work with this technology are called atomic clocks and have the accuracy of 1 second per 30 billion years, and this high accuracy gives us the ability to use technologies like GPS and internet these days, which our lives really rely on.

And New Zealand – MSL in New Zealand keeps the time for New Zealand with three atomic clocks.

Acknowledgements

This video clip is from a recording of a presentation by the Measurement Standards Laboratory of New Zealand (MSL) in celebration of the redefinition of the International System of Units (SI), which happened on 20 May 2019. The presentation by Peter Saunders and Farzana Masouleh of MSL was filmed at Unleash Space, Faculty of Engineering, Auckland University.

Filming and editing by Jonathon Potton of Chillbox Creative. MSL produced these videos to share the story of metrology development.