How do you make a kilogram? Gravity can provide new answers

Claus Lämmerzahl, Professor of Gravitational Physics at the University of Bremen, and Dr. Sebastian Ulbricht, scientist at the Natural Metrology Institute, have proposed in a new article that gravity could be the basis for the quantum-physical realization of quantities.

The article was published in Physical Review Letters and focuses on metrology, the science of measurement. In 2019, this field experienced a revolution: Units such as the kilo, the meter, and the second were redefined on the basis of quantum physics.

Previously, they had often been determined by comparison with natural phenomena or specific objects. For example, the reference for the kilogram was the “original kilogram,” a metal cylinder kept in a safe in Paris.

Today, units are defined on the basis of fundamental constants. To realize a unit such as the kilogram, that is, to construct it so that it can be used in everyday life, it must be traced back to these fundamental constants.

This can be done, for example, with a special scale called a Kibble scale. This connects mass with electrical quantities. It measures the mechanical power and thus also the weight of an object, and balances this with an electrical power. The resulting electrical voltage and the electrical current can then be determined using quantum effects, the quantum Hall and the Josephson effect, and thus traced back to the basic electrical quantities of quantum physics.

“Units such as the kilogram are thus traceable to electrical units,” explains Lämmerzahl. “But it would also be possible to realize them in terms of gravity. After all, a gravitational field is to masses what an electromagnetic field is to charges.”

Electromagnetism and gravity have interesting parallels: Both are described by fields that mediate certain fundamental interactions in nature. In their new publication, Lämmerzahl and Ulbricht therefore introduce a gravitational Josephson and a gravitational quantum Hall effect.

“We were able to transfer the auxiliary effects of metrology, which previously only applied to electromagnetic fields, to gravitational fields,” explains Lämmerzahl. The work of Lämmerzahl and Ulbricht could thus serve as a basis for realizing mechanical units such as the kilo on the basis of gravitational force.