Add to collection
  • + Create new collection
    • Rights: Measurement Standards Laboratory of New Zealand
    Published 15 August 2019 Referencing Hub media
    Download

    The metre1 (m) defines length (roa).

    The metre is defined using a fundamental constant of nature – the speed of light (c), which is given a fixed numerical value of 299,792,458 m s-1, so a metre can be defined as the distance covered by light in a vacuum2 in exactly 1/299,792,458 of a second.

    The definition also relies on another fundamental constant – the frequency3 of a 133Cs atom4 (represented as ΔνCs), which has a fixed value of 9 192 631 770 Hz (or s-1).

    Discussion point: The metre has gone from being defined by a physical artefact5, to waves of light from a krypton6 atom, to a fraction of the speed of light. Do you think that this definition will last – or will we redefine it again?

    Transcript

    PETER SAUNDERS

    The metre used to be defined in terms of the length of a metal7 bar – it’s in a vault in Paris – that was based on the measurement of the circumference of the Earth. But the problem with having an artefact standard like that is it becomes infinitely valuable. And if for example, it drifts in length or if somebody damages it, then by definition, it is still a metre so everything else in the universe8 must change size. And it’s also having an artefact standard that everyone has to go back and compare with limits technological advances.

    And so this was recognised quite early on with the metre, and so in 1960, they redefined the definition of the metre to be a certain number of wavelengths of light that’s emitted by a krypton atom. And defining it that way improved the accuracy9 of length measurements by about a factor of 25. But even that wasn’t enough, and so soon after, they redefined it again.

    And so we know from Einstein’s theory10 of relativity that the speed of light is a constant, no matter how you observe it or where you observe it, and so it’s a really good foundation for a definition of a unit because it is unchanging in the universe. And so in 1983, the definition of the metre was defined by setting the speed of light to be this exact number here [c = 299,792,458 m s-1]. So previously, people used to be able to measure the speed of light, now this is it exactly, and it’s never going to change. And so the metre now is defined as the distance that light travels in a very short, small fraction of a second, where that fraction of a second is one divided by this number [299,792,458]. And so having the metre defined that way actually improved the accuracy by another factor of about 40. But it means now that anyone can use any method they like to determine what a metre is as long as they can relate, have the apparatus and can relate their measurements back to the speed of light.

    And essentially we’ve now taken the metre out of the vault – both literally and figuratively – so you know, and the metre now defines, well the speed of light defines not just the metre but the kilometre or the nanometre11. Any scale that you want to measure, you can measure by relating your measurements back to the speed of light.

    Acknowledgements

    This video clip is from a recording of a presentation by the Measurement Standards Laboratory of New Zealand (MSL12) in celebration of the redefinition of the International System of Units (SI13), 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 metrology14 development.

    1. metre: The base unit of length in the International System of Units (SI).
    2. vacuum: An absence of matter. In practice, a space that contains a very low density of matter (very low pressure) is often referred to as a vacuum.
    3. 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.
    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. artefact: 1. A product of human art and workmanship. 2. An object remaining from a particular period. 3. In the context of the international prototype kilogram (IPK), a 1 kilogram cylinder of platinum-iridium alloy was manufactured in France in the 1880s to serve as the mass standard. 4. Something observed during a scientific investigation that is not naturally present but has been introduced as a result of the experimental procedure. Artefacts are commonly introduced during preparation of specimens for microscopy.
    6. krypton: Krypton is a noble gas with the symbol Kr and atomic number 36. It occurs in trace amounts in the Earth’s atmosphere.
    7. metal: Any of a category of elements that usually have a shiny surface, are generally good conductors of heat and electricity and can be melted or fused, hammered into thin sheets or drawn into wires (for example, copper).
    8. universe: All matter and energy, including the Earth, the galaxies and the contents of intergalactic space, regarded as a whole.
    9. 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.
    10. theory: To scientists, a theory provides a coherent explanation that holds true for a large number of facts and observations about the natural world. It has to be internally consistent, based upon evidence, tested against a wide range of phenomena and demonstrate problem solving.
    11. nanometre: Nanometre (nm) is a billionth of a metre.
    12. MSL: The acronym for the Measurement Standards Laboratory.
    13. SI: A measurement system used worldwide in the scientific community. SI stands for Système International d’Unités.
    14. metrology: The science of weights and measures.

    Appears in

    INTERACTIVE

    The International System of Units (the SI)

    Information about the International System of Units – the SI – including their definitions using fundamental constants.

    READ MORE
    MORE
    Add to favourites
    Add to collection
  • + Create new collection
      • Go to full glossary
      Download all

      metre

    1. + Create new collection

      2. The base unit of length in the International System of Units (SI).

      atom

    2. + Create new collection

      3. 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.

      metal

    3. + Create new collection

      4. Any of a category of elements that usually have a shiny surface, are generally good conductors of heat and electricity and can be melted or fused, hammered into thin sheets or drawn into wires (for example, copper).

      theory

    4. + Create new collection

      5. To scientists, a theory provides a coherent explanation that holds true for a large number of facts and observations about the natural world. It has to be internally consistent, based upon evidence, tested against a wide range of phenomena and demonstrate problem solving.

      SI

    5. + Create new collection

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

      vacuum

    6. + Create new collection

      7. An absence of matter. In practice, a space that contains a very low density of matter (very low pressure) is often referred to as a vacuum.

      artefact

    7. + Create new collection

      8. 1. A product of human art and workmanship.

      2. An object remaining from a particular period.

      3. In the context of the international prototype kilogram (IPK), a 1 kilogram cylinder of platinum-iridium alloy was manufactured in France in the 1880s to serve as the mass standard.

      4. Something observed during a scientific investigation that is not naturally present but has been introduced as a result of the experimental procedure. Artefacts are commonly introduced during preparation of specimens for microscopy.

      universe

    8. + Create new collection

      9. All matter and energy, including the Earth, the galaxies and the contents of intergalactic space, regarded as a whole.

      nanometre

    9. + Create new collection

      10. Nanometre (nm) is a billionth of a metre.

      metrology

    10. + Create new collection

      11. The science of weights and measures.

      frequency

    11. + Create new collection

      12. 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.

      krypton

    12. + Create new collection

      13. Krypton is a noble gas with the symbol Kr and atomic number 36. It occurs in trace amounts in the Earth’s atmosphere.

      accuracy

    13. + Create new collection

      14. 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.

      MSL

    14. + Create new collection

      15. The acronym for the Measurement Standards Laboratory.

      Email Us

      See our newsletters here.