# What did Sommerfeld mean by Bohr's magic wand?

## Background

From wikipedia:

"Bohr's correspondence principle demands that classical physics and quantum physics give the same answer when the systems become large.[5] A. Sommerfeld (1924) referred to the principle as "Bohrs Zauberstab" (Bohr's magic wand)."

## Question

Does A. Sommerfeld make a hidden pee pee joke over here? From the context and translation of this German I feel this is likely; since it's used in the context of taking the limit of a system to "large".

I would like to know if this joke would make sense in German? Is there any evidence to further strengthen my case? Or am I reading too much "in between the lines"?

• I'm a native German speaker and I don't see a "pee pee joke" here, or any joke for that matter. Also, I'm not sure if this site is the right place for this question. – Torsten Schoeneberg Sep 11 at 3:54
• Hmmm .. to be fair I don't make as strong a case as I could due to vulgarity ... can you post an answer and I'll accept it? – More Anonymous Sep 11 at 3:56

"Using classical electromagnetism, Rubinowicz computed the angular momentum of a spherical wave and showed that, when renormalized by $$h/2π$$, it had an absolute value equal or less than one. (Rubinowicz, 1918a, pp. 443-444). This led to a selection principle for the azimuthal quantum number: $$\Delta n = 0, ±1$$ (Rubinowicz, 1918a, pp. 444-445)... It was left to Sommerfeld, in the first edition of Atombau und Spektrallinien (1919), to connect Bohr and Rubinowicz's results to his own interpretation of spectral terms as linked to azimuthal quantum numbers. Sommerfeld showed a very clear preference for the theory of his former assistant Rubinowicz, which he discussed at length (Sommerfeld, 1919, pp. 390-411).
Rubinowicz's "selection principle" was for Sommerfeld a means to bridge the gap between classical and quantum physics... He admitted that Bohr's condition $$\Delta n = ±1$$ fitted much better the Rydberg-Ritz formula than Rubinowicz's, but made clear the epistemological gap he perceived between a theoretically significant selection principle like Rubinowicz's, on the one side, and Bohr's empirically successful condition, on the other. The latter, he described as a "magic wand" ("Zauberstab") to make quantum theory useful in practice (Sommerfeld, 1919, pp. 406-411, quote from p. 402)".