# How were Planck units derived

## "A universal language"

**In 1875, 17 states passed the Meter Convention and thus laid the foundation for the International System of Units. On May 20, 2019 - the day of measurement - the system of units will be reformed: all seven base units will in future be defined using constants. In an interview with Welt der Physik, Jens Simon from the Physikalisch-Technische Bundesanstalt in Braunschweig explains why this redefinition was necessary and what it means for the general public and the sciences.**

**World of Physics: What is the International System of Units?**

Jens Simon: The international system of units basically has a long tradition, starting with the French Revolution. After the revolution, the chaotic system of units of length and weight was to be replaced by a uniform system. With the Meter Convention of 1875 and the first General Assembly in 1889, the original meter and the original kilogram were established as uniform dimensions worldwide. The basic units will now be redefined again on May 20, after all member states of the Meter Convention met in November 2018 to unanimously approve the decision.

General Conference 2018

**Why is a redefinition necessary at all?**

There were some problems with the old system, such as with the electrical units. The ampere - the unit of electrical current strength - is defined, for example, on the basis of some idealized assumptions. The kilogram also causes problems. Because every member of the Meter Convention uses a copy of the original kilogram. Delegates from the states regularly come together with these copies in Paris to compare their copies with the original kilogram. And it was found that the copies tend to get heavier.

**What exactly will change on May 20th?**

The redefinition of the base units also changes all other units, because these are derived from the base units. From May 20th, all base units will be defined using constants, the values of which must first be measured and determined very precisely. Let's take the kilogram as an example: At the moment it is defined using a reference mass - the original kilogram in Sèvres near Paris. After the change, it is defined using Planck's quantum of action - the ratio of energy to the frequency of a light particle. But this approach is not new. In 1983 the meter was officially redefined as the distance that light travels in a specific fraction of a second. For this purpose, researchers have determined the value of the speed of light.

**Which constants will serve as the basis for the new system of units in the future?**

In the future, the basic units will be defined using seven constants. These include those already mentioned, i.e. the speed of light and Planck's quantum of action. But also the elementary charge - the charge of an electron - and the Boltzmann constant, which links temperature and energy. These constants play a crucial role in the fundamental theories of physics and can therefore be called natural constants. Another constant for the new system of units is the Avogadro constant, which indicates the number of particles per amount of substance and thus plays a major role in chemistry. And then there is another parameter that defines the second, namely the frequency for the so-called “clock transition” in the cesium atom. In the official language of the International System of Units, all of these constants are collectively referred to as “defining constants”.

Optical strontium atomic clock from PTB

**In order for the new definitions to work, the values of the constants must be specified. How did you go about it?**

In order to determine the constants as precisely as possible, many experiments were necessary, some of which lasted for decades. For example, to determine Planck's quantum of action, two different experiments were carried out - the watt balance experiment and the Avogadro project with silicon spheres. These are complex experiments that can only be carried out with many scientists from different disciplines. We then determined the value of a constant using the mean of all experiments. We have already published all measurement results in summer 2017.

**How is the new system better than the old system?**

The new system is at least as good as the old one or even better - scientifically this can be expressed in terms of the measurement uncertainties of the values. The uncertainties of the natural constants must therefore not be greater than those of the old units. The participating scientists have now succeeded in implementing this experimentally. This is the first time that metrologists have created a uniform, consistent system - a kind of universal language. Since the natural constants do not change, in contrast to the original kilogram, the new system is constant. With the help of the precisely determined values it will now be possible to measure with an accuracy that was not possible before.

**And what are the effects on everyday life?**

Hopefully none, because that's why we measured the natural constants so precisely that the new system seamlessly connects to the old one. Because a kilogram should still have the same value on the household scales on May 20th as it was on May 19th. So nothing changes for everyday life. But the definition of the units is now very abstract and a little more complicated for the imagination than an original kilogram like the one in the vault near Paris.

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