Redefinition of SI Units

Overview

• Meeting on November 16th in Versailles, France
• Representatives from nearly 60 nations to vote on changes in four SI base units:
  • Kilogram
  • Kelvin
  • Mole
  • Ampere
• Transition to using fundamental constants of nature instead of physical artifacts

Current Definition of the Kilogram

• Defined since 1799 as the mass of a metal cylinder stored in Paris (International Prototype Kilogram or Big K)
• Issues:
  • Mass divergence among identical cylinders
  • Limited access to Big K

Proposed Solution

• Set Planck's constant to a fixed, exact value
• Planck's constant relates photon frequency to energy and connects energy to mass (E=mc²)
• Current uncertainty in Planck's constant

Measurement Techniques for Planck's Constant

• Multiple techniques used by scientists to measure Planck's constant accurately:
  • Watt balance: balances weight of a kilogram with electromagnetic force
  • Creation of a round silicon object for precise measurements
• Results will lead to a consistent value for Planck's constant

Impact of the Redefinition

• Kilogram will be defined in relation to Planck's constant, not by the physical object
• Other units will also be redefined:
  • Mole: based on Avogadro's constant, which will also have a fixed value
  • Ampere: will depend on the charge of an electron
  • Kelvin: defined by the fixed Boltzmann constant

Practical Implications

• Most people will not notice changes in daily life
• Consistency and reliability of measurements will improve
• Electrical metrologists will see minor changes due to previous fixed values of Planck's constant

Significance of Measurement in Science

• Measurements are foundational to scientific understanding
• Historical examples:
  • Kepler’s work on planetary orbits
  • Discovery of the Higgs boson and gravitational waves
• Science builds on itself; redefining units based on nature enhances precision

Conclusion

• Movement away from reliance on physical objects to abstract definitions based on natural constants
• A step towards better, more precise scientific measurements

Acknowledgments

• Support from Audible and viewers
• Recommendation for the book "An Absolutely Remarkable Thing" by Hank Green.