On 9 January 2025, we welcomed Prof. Dr Härtig, Vice President of the Physikalisch-Technische Bundesanstalt (PTB), to our MSE research seminar at ZeHS for an exciting lecture entitled "The new International System of Units - now also for extraterrestrials". Prof Härtig gave us fascinating insights into the redefinition of SI units and explained their fundamental importance for our daily lives.

SI units play a crucial role in our everyday lives. Whether weighing food in the supermarket or filling up with petrol, we rely on measurements being standardised and reliable worldwide. This requires a common, precise reference system so that, for example, a kilogram is actually the same kilogram anywhere in the world.

Historically, units of measurement were defined by artefacts, such as the prototype metre or the prototype kilogram, but these varied. However, with the comprehensive redefinition of the International System of Units in 2019, all SI units are now based on unchanging natural constants, such as the speed of light or Planck's constant. This means that the units can be realised uniformly not only on Earth, but theoretically everywhere in the universe - even by extraterrestrial civilisations.

One particularly interesting example of the new realisation of the kilogram is the Avogadro experiment. In this experiment, an almost perfect sphere is produced from high-purity silicon and the number of atoms in this sphere is precisely determined. Such sophisticated experiments impressively demonstrate the combination of basic research, precision and technology behind modern measurement technology.

One of the highlights of the lecture was that Prof Härtig had brought a specimen of the silicon sphere to the seminar. The participants had the unique opportunity to see this extraordinary object up close and even hold it in their hands. The sphere is very probably the roundest object on earth - if not in the entire universe.

We would like to thank Prof Härtig for his visit and the insightful presentation on the redefinition of the SI units. His lecture impressively demonstrated the central role that measurement technology and metrology - the science of measurement - play in our lives.

Further information at:
https://www.ptb.de/cms/forschung-entwicklung/forschung-zum-neuen-si/ptb-experimente/kilogramm-und-mol-atome-zaehlen.html