Why is physics based on constants

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Today is International Measurement Day or World Metrology Day, which has been a reminder since 2000 that the International Meter Convention was signed on May 20, 1875. In addition, this day recognizes the importance of measurement and metrology for business and society. And this year's 20th World Metrology Day is a very special one for this “science of measurement”. Today the redefinition of the SI units comes into force, all of which are now based on natural constants. For the morning walk on the scales or the punctual appearance at school or at work, of course, it is of no importance, but for science and industry.

On November 16, 2018, the General Conference on Weights and Measures decided to revise the International System of Units so that the units in which we measure everything in the world are placed on the most solid foundation imaginable from a physical point of view: a set of natural constants with precisely defined values ​​now form the basis for defining the seven units. This means that the original kilogram, which is in a safe at the International Bureau of Weights and Measures (BIPM) in Sèvres (Paris) and which differs significantly in its mass from the mass of its copies, has had its day.

  • The national kilogram prototype (left) has had its day when it comes to mass definition. A sphere made from a highly enriched 28Si single crystal (right) was used to precisely measure the Avogadro constant. (Photos: PTB)

The idea of ​​defining a unit of measurement on the basis of natural constants is not new: What began with the definition of the second using atomic clocks 50 years ago and with the definition of the meter using the speed of light over 30 years ago, now applies to everyone other SI units: The Kelvin is now derived from the Boltzmann constant, the ampere from the elementary charge, the candela unit from the photometric radiation equivalent, the mass from Planck's constant and the mole from the Avogadro constant.

In the last few years, extensive experiments have been carried out in the metrological laboratories in order to measure these constants as precisely as possible. These measurements, which were mainly carried out at the large metrology institutes such as PTB in Germany, NIST in the USA, NMIJ in Japan or the NRC in Canada, were successful: The previously set targets, including the measurement uncertainties and the independence of the Experiments from each other were accomplished. Thus, the values ​​of the relevant natural constants could be determined very precisely on the basis of these measurements.

  • The Planck constant h was chosen to define the kilogram. Its value was determined using two independent experiments: the Avogadro experiment (here atoms are counted in a perfect silicon crystal ball, left, PTB) and the watt balance (here a weight is compensated using quantum electrical effects, right, NIST).

If these values ​​can be measured more precisely with new, improved experiments, the units can be implemented more precisely in the new system of units - without changing the underlying definition. In a high-tech world in which the length divisions will not stop at the nanometer or the time divisions at femtoseconds, this technical openness of the new SI to all future advances in accuracy is a great benefit. The new system of units creates the best prerequisites for innovations wherever maximum accuracy is required: in the development of quantum technologies, in the diagnostic possibilities of medicine, the efficiency increases in energy generation or the analysis methods of climate research.

The newly defined system of units eliminates the shortcomings of the previous system, whereby the changes in daily life are not noticeable and the advances in technology will only become apparent in the long term. For science, on the other hand, progress occurs immediately. In addition, the natural constants apply everywhere, so that the new SI system to a certain extent forms a “universal language” that the world community has now agreed on. This is also underlined by DPG President Dieter Meschede: "A universal system of units is many things in one: a diplomatic masterpiece, an aesthetic structure and a technological promise."

Maike Pfalz / PTB

Further information

  • PTB, All Power to the Constants
  • J. Stenger and J. H. Ullrich"For all times ... and cultures", Physik Journal, November 2014, p. 27
  • F. Riehle, Optics make it even more accurate, Physik Journal, March 2018, p. 39
  • K. Saturday, Natural constants as a permanent basis, Physik Journal, January 2019, p. 7
  • DPG fact sheet "Physikkonkret", natural constants as the measure of all things
  • Special issue "Annals of Physics": The Revised SI