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A recent comment called my attention to the Google Ngram for 'angular momentum', which shows a very strong and rather sharp peak in the usage of the phrase shortly after 1960, followed by a steady decline with a loss of ~60% of the original frequency in today's usage.

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I find this extremely surprising, and I'm coming up short in terms of possible explanations for why this should be the case. Is there some specific aspect of physics that saw particularly strong research or dissemination then and which has then fallen into disuse? Did the focus shift from the concept into slightly adjacent ones? Was there a boom in the writing of textbooks? (None of those feel particularly satisfying or plausible to me.) Or is this just an artifact of the measuring tool?

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    $\begingroup$ My guess: Sputnik ... 4 October 1957 ... followed by immense response in the US ... the "missile gap" became a dominant issue in the 1960 Presidential election. see books.google.com/ngrams/… $\endgroup$
    – Gerald Edgar
    Jan 21, 2018 at 11:37
  • $\begingroup$ @GeraldEdgar nails it... Still mysterious why “angular momentum” correlates so much better than others...? $\endgroup$
    – Francois Ziegler
    Jan 21, 2018 at 16:27
  • $\begingroup$ @GeraldEdgar Yeah, that feels like it has a good deal to it. The ngrams for 'satellite' and 'orbit' show similar bumps. $\endgroup$
    – Emilio Pisanty
    Jan 21, 2018 at 16:43
  • $\begingroup$ Might this be explained with a relative increase of "non scientific" literature around 1960? Otherwise I would ask the same question about "integral" or "differential" $\endgroup$
    – Michael Bächtold
    Jan 22, 2018 at 9:29

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First thing, we have to realize that the n-grams show the percentage of some sequence of characters in all the available literature.

If you take out some constant background signal, you will notice that the increase in the frequency of "angular momentum" takes place around 1900, when quantum theory is born.

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You can notice that "angular momentum" correlates extremely well with "energy levels" and "hydrogen atom".

So my guess is simply that the literature about atomic theory started to increase immediately after quantum mechanics was born in the first years of 1900 and eventually reached a plateau around 1960, with a consequent decrease in the relative frequency in the literature. In those years, quantum electrodynamics was developed, and not much was left to be said about atomic theory.

Given the nature of physics, there is no real point in writing many new books about old theories, so it only makes sense that once atomic theory had reached its full development the production of new books about it decreased.

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  • $\begingroup$ I'm skeptical of these justifications. There are similar spikes around 1960 in the ngrams of "equation" and "integral"... $\endgroup$
    – Michael Bächtold
    Jan 22, 2018 at 15:06
  • $\begingroup$ @MichaelBächtold Of course this is only an hypothesis. But you are right, many topics related to math and physics show the same kind of bizarre behavior...Maybe someone will come up with an explanation more convincing than mine. $\endgroup$
    – valerio
    Jan 22, 2018 at 15:13
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I looked at the contexts of the "angular momentum" cluster around the spike: angular momentum of electrons; angular momentum about the nucleus; anti-neutron differs from the neutron in that its intrinsic magnetic moment is directed parallel to the spin angular momentum; the quantum number of angular momentum, the spin Hamiltonian, and the angular momentum Hamiltonian; quantum theory of scattering, etc. Orbital dynamics contributed some, but much less, it seems. I would not be surprised, however, by many spurious correlations. The turn of 1960-s was a "cultural moment" that lifted all boats, the romance with nuclear physics and space exploration were only two aspects of it. So were high hopes for thinking computers, loosening of the iron curtain, and the rise of youth counterculture and rock-n-roll.

The bulk seems to come from expositions of quantum theory, presumably prompted by its technologization that created a new audience of engineers (lasers, semiconductors, transistors, nuclear energy, etc.). The so-called magic-angle spinning in nuclear magnetic resonance was also discovered in 1958-59, see Timeline of quantum mechanics.

The second contributing factor seems to be the great search for the new subatomic particles in 1950-s after the large accelerators were built, followed by the efforts to make sense of the "particle zoo" in 1960-s. "Had I foreseen this, I would have gone into botany", Pauli quipped. Soviet movie Nine Days of One Year, that romanticized the endeavor, came out in 1962 and became a hit. The spin of course is the distinction between bosons and fermions so it came up a lot.

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