Why is the mass constant and not varied

History of the international system of units

How long is a meter, how heavy is a kilogram and how long is a second? The answers to these questions have often varied greatly over the course of history. Most units can now be precisely defined, but scientists are still pondering more precise definitions.

Until around 125 years ago, there were no binding length measures in Germany - and even if two measures had the same name, they did not have to match. The Bremen cubit, for example, was 55.372 centimeters and could be measured directly on the market square: It was determined by the distance between the knees of the Roland statue. The Frankfurt cubit, on the other hand, was 54.37 centimeters, the short Hamburg cubit 57.31 centimeters and the long Hamburg cubit 68.71 centimeters.

The Roland statue in Bremen

With the French Revolution, the desire for a uniform system of measurement prevailed in France. First they looked at a new measure of length - the meter - which should apply to the entire world and also be unchangeable. The earth itself was chosen as a reference: one meter corresponds to one forty-millionth of the circumference of the earth. In 1799, the first folding rule was cast in platinum and declared the current measure of the country. Later it turned out that the measurements of the circumference of the earth on which the "original meter" was based were incorrect, but this did not affect the success of the metric system.

On May 20, 1875, 17 states signed an international treaty - the Meter Convention - with the aim of standardizing all weights and measures internationally. In 1889 the “General Conference for Weights and Measures” met for the first time to approve the prototypes for the original meter and the original kilo and distribute them to all member states. In 1948 the general conference met for the ninth time and designed a system of units with six basic units, which was then confirmed in the tenth conference in 1954: the meter for length, the kilogram for mass, the second for time, the ampere for the current strength, the kelvin for the temperature and the candela for the light intensity. In 1960 the eleventh conference determined the name of the new system:Système International d'Unités with the abbreviation SI, which should be used in all languages. The mole as a measure of the amount of substance was added as the seventh SI unit only in 1973.

Even today, basic research is still concerned with the SI units. Scientists try to find a reference - a so-called normal - for each unit that does not depend on either the location or the time of the user and with which the unit can be reproduced again and again without errors. The cubit is therefore a bad normal because the length of the forearm varies from person to person. But the earth itself is not an ideal normal either, because in the course of the earth's history continents shift, mountains arise or disappear, so that their extent changes minimally.

Therefore one tries to fix the units to natural constants. This undertaking is not only technically complicated, it also touches on a fundamental question in physics: How constant are natural constants anyway?

Despite the actually binding SI system, there are still breakdowns in the use of units: In September 1999 the probe crashed Mars Climate Orbiter on Mars. Troubleshooting soon revealed that the orbit had been miscalculated. Two working groups were involved in the calculation, one of which had calculated in meters and kilograms, the other in feet and pounds.