On a table next to the lectern in the Paulskirche is the microscope through which Otto Stern saw the future of quantum physics in February 1922.

Neither he nor his colleague Walther Gerlach could have been fully aware of the implications of their discovery.

Through the microscope, the two researchers were able to see a pattern left by silver atoms on a glass plate.

The conclusions that could be drawn from this pattern have enabled new insights into the nature of elementary particles and pioneering developments in technology.

This is commemorated by the ceremony with which the city of Frankfurt, the Goethe University and several specialist societies celebrated the 100-year anniversary of the Stern-Gerlach experiment on Tuesday evening.

Sasha Zoske

Sheet maker in the Rhein-Main-Zeitung.

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With their experimental setup in the institute's rooms at today's Senckenberg facility, the two scientists actually wanted to refute a theory put forward by the Danish physicist Niels Bohr on the behavior of electrons in the atomic shell.

For this purpose, Stern and Gerlach, together with the master mechanic Adolf Schmidt, had constructed an apparatus that was as sophisticated as it was expensive for the time: Silver was vaporized in a small furnace at 1000 degrees Celsius.

The free atoms flew through a slit into a vacuum tube, which was exposed to a strong magnetic field, and then hit a glass plate.

There they left traces that could be made visible by reacting with a chemical containing sulfur.

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Instead of the expected even distribution within an oval, the researchers saw two left-and-right curves touching each other at the ends, with nothing in between.

This meant that the silver atoms had assumed only two distinct magnetic states, and not many different ones, as suggested by Stern.

The physicists had thus experimentally proven the principle of so-called directional quantization - and had to admit: Niels Bohr was right.

Stern's and Gerlach's discovery made it possible to better understand the magnetic properties of electrons and whole atoms.

This opened the door for a series of further experiments, which in turn resulted in technical applications that are now indispensable in everyday life.

Dorothée Weber-Bruls, President of the Physical Society, mentioned some of them in the panel discussion at the ceremony.

One of the best-known of these achievements is magnetic resonance imaging, or nuclear spin tomography, which is now one of the most important imaging processes in medicine.

Magnetic resonance spectroscopy plays a major role in basic research;

at the Goethe University, for example, it is used to elucidate the structures of large biomolecules.

Electron spin resonance spectroscopy, which Weber-Bruls says can be used, among other things, to measure the oxygen partial pressure in living tissue – a way of detecting tumors that are poorly supplied with oxygen is important for medicine.

34 Nobel Prize winners

The benefit that Klaus Blaum and his colleagues at the Max Planck Institute for Nuclear Physics in Heidelberg derive from the Stern-Gerlach experiment is literally fundamental: they use refined variants of the experimental setup to more precisely determine fundamental physical constants, such as the mass of an electron determine.

According to the Max Planck Director, the precision of these measurements is now so high that, extrapolated to large dimensions, they could record the increase in weight of the Eiffel Tower that occurs when a bee lands on it.

If you add up the Nobel Prize winners whose work was somehow related to the Stern-Gerlach experiment, you get 34 in physics and 11 in chemistry.

This was calculated by professor emeritus Horst Schmidt-Böcking, one of the best experts on the history of physics in Frankfurt.

In his keynote address, he recalls that the development of the laser would have been inconceivable without the historical experiment.

Otto Stern himself received the Nobel Prize in 1943, but Walther Gerlach had no prospect of such an honor at the time.

Unlike the Jew Stern, who was forced to flee the "Third Reich" in 1933, Gerlach served the Nazi academic system despite opposing Nazi ideology.

He was involved in the uranium program of the brown rulers, but spoke out after the war with other researchers against nuclear armament of the Bundeswehr.

Otto Stern stayed in the United States.

Thanks to Stern's niece, his microscope later found its way to Frankfurt from his estate.

He himself wanted nothing more to do with Germany and his former colleague Gerlach.

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