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  • The second (s) defines time (wā).

    Caesium atomic clocks operate by exposing caesium atoms to microwaves1 until they start to respond at one of their transition frequencies. By determining this frequency2 (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 radiation3 corresponding to the transition between the two hyperfine levels of the unperturbed ground state of the 133Cs atom4.

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

    Discussion point: GPS5 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 accuracy6 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 – MSL7 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 (SI8), 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 metrology9 development.

    1. microwaves: Invisible electromagnetic waves with wavelengths ranging from 1 mm to 1 m. Microwaves occur between radio and infrared waves on the electromagnetic spectrum.
    2. frequency: 1. How often something occurs within a specified time. 2. The number of waves per second that pass a given point or the number of waves produced per second by a source.
    3. radiation: Energy that is transmitted (radiates) from a source in the form of rays or waves or particles.
    4. atom: The smallest possible unit of matter that still maintains an element’s identity during chemical reactions. Atoms contain one or more protons and neutrons (except hydrogen (H), which normally contains no neutrons) in a nucleus around which one or more electrons move.
    5. GPS: Global positioning system. Uses satellites, computers and receivers to determine the exact position of a receiver on Earth by calculating the time difference for signals from different satellites to reach the receiver.
    6. accuracy: In science, accuracy indicates the closeness of the measurements to the true or accepted value. In general usage, accuracy can mean precise, exact or correct.
    7. MSL: The acronym for the Measurement Standards Laboratory.
    8. SI: A measurement system used worldwide in the scientific community. SI stands for Système International d’Unités.
    9. metrology: The science of weights and measures.
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      microwaves

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    2. Invisible electromagnetic waves with wavelengths ranging from 1 mm to 1 m. Microwaves occur between radio and infrared waves on the electromagnetic spectrum.

      atom

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    4. The smallest possible unit of matter that still maintains an element’s identity during chemical reactions. Atoms contain one or more protons and neutrons (except hydrogen (H), which normally contains no neutrons) in a nucleus around which one or more electrons move.

      MSL

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    6. The acronym for the Measurement Standards Laboratory.

      frequency

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    8. 1. How often something occurs within a specified time.

      2. The number of waves per second that pass a given point or the number of waves produced per second by a source.

      GPS

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    10. Global positioning system. Uses satellites, computers and receivers to determine the exact position of a receiver on Earth by calculating the time difference for signals from different satellites to reach the receiver.

      SI

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    12. A measurement system used worldwide in the scientific community. SI stands for Système International d’Unités.

      radiation

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    14. Energy that is transmitted (radiates) from a source in the form of rays or waves or particles.

      accuracy

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    16. In science, accuracy indicates the closeness of the measurements to the true or accepted value. In general usage, accuracy can mean precise, exact or correct.

      metrology

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    18. The science of weights and measures.