From transmission spheres to silicon spheres to Planck's constant to a constant kilogram
Arnold Nicolaus
Physikalisch-Technische Bundesanstalt Braunschweig
How
long is a kilogram? You'll say nonsense - how heavy is a kilogram!
And no. Since 2019, one of the best ways to determine a kilogram is by using an optical interferometer.
For 150 years, you could only measure a kilogram "correctly" if you brought a reference piece to the BIPM in Paris, where the experts compared it with the one and only kilogram – the international prototype of the kilogram, the IPK.
Since the redefinition of 4 of the 7 SI units (Système International d’unités) – in particular kg, A, mol, K – the kilogram is now based on the natural constant h, Planck's constant, which is fixed in its numerical value.
You will hear and perhaps also 'experience' why and then how a – well, "almost" – classical Fizeau interferometer can be operated to realize a kilogram – with the today smallest measurement uncertainty and: more stable and inviolable than ever before.
If you need something in black and white:
Bartl, G.; Becker, P.; Beckhoff, B.; Bettin, H.; Beyer, E.; Borys, M.; Busch, I.; Cibik, L.; D'Agostino, G.; Darlatt, E.; Di Luzio, M.; Fujii, K.; Fujimoto, H.; Fujita, K.; Kolbe, M.; Krumrey, M.; Kuramoto, N.; Massa, E.; Mecke, M.; Mizushima, S.; Müller, M.; Narukawa, T.; Nicolaus, A.; Pramann, A.; Rauch, D.; Rienitz, O.; Sasso, C. P.; Stopic, A.; Stosch, R.; Waseda, A.; Wundrack, S.; Zhang, L.; Zhang, X. W.; "A new
28Si single crystal: counting the atoms for the new kilogram definition", Metrologia 54, pp. 693–715, (2017), https://doi.org/10.1088/1681-7575/aa7820
