Wednesday, February 8, 1922, was a glorious winter day in Frankfurt am Main.

After a bitterly cold night, the sun was shining on the fresh snow.

The physics institute at the university had already been cleared when a doctoral student jumped over the swept-up bank of snow to get quickly to the post office on Viktoria-Allee, today's Senckenberg-Anlage.

His professor, the only 33-year-old Extraordinarius Walther Gerlach, had commissioned him to post a telegram there to his colleague Otto Stern in Rostock.

The text read: "Bohr is right!"

Ulf von Rauchhaupt

Editor in the “Science” section of the Frankfurter Allgemeine Sunday newspaper.

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Gerlach had worked through the whole winter night.

He'd stayed in his lab for eight hours between whirring pumps by a small glass-and-metal contraption, then released the vacuum to remove a cooled slide of glass on the right, treated it with a sulphurous chemical, and then placed it under a microscope.

It turned out the picture that is shown on the postcard shown below.

What looks like kissing lips is the first evidence of a genuine quantum physical effect.

The Frankfurt experiment of Otto Stern (1888-1969) and Walther Gerlach (1889-1979) showed an experimental consequence of quantum theory, which was originally developed to explain something other than this observation.

With the Stern-Gerlach experiment, the surprising statement of a theory that philosophers of science are still talking about to this day became a hard empirical fact.

But what - and to what extent - was Bohr right?

The Danish physicist Niels Bohr had advocated a daring hypothesis since 1913: In order to explain why the negatively charged electrons in the shells of the atoms do not fall onto the positively charged atomic nucleus - which they should actually do according to the laws of classical electrodynamics - Bohr had this Laws here simply declared partially invalid: If electrons orbit the nucleus at certain distances, according to Bohr, then they remain stable.

It is precisely those orbits on which the orbital angular momentum of such an electron, i.e. the momentum with which it whizzes around the atomic nucleus, assumes certain discrete or, as one also says, quantized values.

That would also explain why atoms always only emit light in certain energy packages, the quanta,

emit or absorb and thus produce the phenomenon of spectral lines.

This idea of ​​a quantum nature of light was first formulated by Max Planck in 1900 and recognized five years later by Albert Einstein as absolutely correct.

Otto Stern was Einstein's first postdoc in Prague and Zurich, as we would say today, but what Bohr suggested to simply suspend tried and tested classical physics when it came to atoms went too far for him and others.

As early as 1913, Stern discussed the idea in Zurich with his colleague Max von Laue, and they both swore: "If this nonsense by Bohr turns out to be correct, we'll give up physics."