Why is the momentum map in the differential geometry of symmetries called the ''momentum'' (or ''moment'') map?



Origin of the name ''momentum map''
According to §1.3 and §11.2 of Marsden and Ratiu [1994] (see detailed citation given below), the momentum map concept can be traced back to [Sophus Lie's](https://en.wikipedia.org/wiki/Sophus_Lie) 1890 book, and is an English translation of the French words *application moment.* To quote directly from Marsden and Ratiu:



> 
> *The notion of the momentum map (the English translation of the French words “application moment”) also has roots going back to the work of Lie.
>  Many authors use the words “moment map” for what we call the “momentum
>  map.” In English, unlike French, one does not use the phrases “linear
>  moment” or “angular moment of a particle,” and correspondingly, we
>  prefer to use “momentum map.” We shall give some comments on the
>  history of momentum maps in §11.2.*
> 
> 
> 


**Add**


For additional clarification, I asked Tudor Ratiu about the origins of the term momentum map. In an email dated Sept 8th 2016, he gave a beautiful response on the evolution of ideas that led to the modern momentum map. At the very end, he made some comments on what Sophus Lie did. Here are the main points from his email, whose contents can mostly be found in the historical notes provided in Marsden and Ratiu [1994]. References are given at the bottom. 




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(1) The momentum map is a generalization of the usual notions of linear and angular momentum from physics, which have a long history.


(2) The first person to introduce the modern version of the momentum
map was [Bertram Kostant](https://en.wikipedia.org/wiki/Bertram_Kostant) at a conference in Japan. He did not give it
a name. He used it to prove what is today called *Kostant's
symplectic covering theorem*.


(3) Just a few weeks later, [Jean-Marie Souriau](https://en.wikipedia.org/wiki/Jean-Marie_Souriau) independently introduced the modern momentum map. He correctly named it in French *application moment.* He also realized its physical significance, and linked it to linear and angular momentum. This was an enormous breakthrough.


(4) Marsden and Weinstein, in their famous 1974 paper on
reduction used the term *moment map*, which was their translation
of the French words *application moment*. This had an immediate reaction from [Hans Duistermaat](https://en.wikipedia.org/wiki/Hans_Duistermaat) who pointed out that this is a misnomer
and physically incorrect. Indeed, even the wiki entry for [moment](https://en.wikipedia.org/wiki/Moment_(physics)), indicates in the first line that moment should not be confused with momentum. It's analogous to confusing force with linear momentum.


(5) For a circle action, the momentum map (without any name)
was introduced by [Theodore Frankel](https://en.wikipedia.org/wiki/Theodore_Frankel) in an Annals paper in the 1950s. This paper is famous because it links the existence of
fixed points of the action to the Hamiltonian character of the
action. This bit of history was not known to Marsden and Ratiu
when they wrote their book, so it won't be found there.


(6) Sophus Lie knew a lot of Poisson geometry and many people now regard him as the founder of Poisson geometry. However, Lie did not introduce symplectic geometry. The link to symplectic geometry, coadjoint orbits, etc. is due to Kostant and Souriau. See the historical notes in Marsden and Ratiu [1994] for details.




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**References**


Frankel, T. [1959]. *Fixed points on Kahler manifolds.* Ann. Math. 70, 1–8.


Marsden, Jerrold E. and Ratiu, Tudor [1994]. [Introduction to Mechanics and
Symmetry](http://www.cds.caltech.edu/~marsden/books/Mechanics_and_Symmetry.html). Second Edition, 1999. Current 2nd Printing, 2003. 
New York, Springer-Verlag.


Kostant, B. [1966] *Orbits, symplectic structures and representation theory.* Proc. US– Japan Seminar on Diff. Geom., Kyoto. Nippon Hyronsha, Tokyo 77.


Ratiu, Tudor. Personal Communication, Sept 8th, 2016.


Souriau, J.-M. [1970]. Structure des systèmes dynamiques, Maîtrises de mathématiques, Dunod, Paris, 1970. ISSN 0750-2435.


