In recent years different solitary waves, like solitons, vortex rings, solitonic vortices, and more complicated, were predicted. They can move in superfluid ultracold gases along elongated traps. A theoretical description of the moving demands knowledge of two functions, the inertial mass of the soliton and the effective number of particles. They can be calculated on the basis of a microscopic theory. However, one can express them in the the terms of observable quantities - the phase increment and the depletion of number of particles in soliton. In this note a transparent derivation of the corresponding equations is suggested. The results are applied to the "magnetic soliton" in mixtures of Bose-gases in different spin states,which was predicted recently.

PACS: 03.75.Lm, 3.75.Kk, 67.85.De (all) DOI: 10.3367/UFNe.2016.08.037891 URL: https://ufn.ru/en/articles/2016/10/f/ 000391228300005 2-s2.0-85007415265 2016PhyU...59.1028P Citation: Pitaevskii L P "Dynamics of solitary waves in ultracold gases in terms of observable quantities" Phys. Usp. 59 1028–1033 (2016) BibTexBibNote ® (generic)BibNote ® (RIS)MedlineRefWorks Received: 16th, August 2016, accepted: 21st, August 2016 Оригинал: Питаевский Л П «Динамика уединённых волн в ультрахолодных газах в терминах наблюдаемых величин» УФН 186 1127–1132 (2016); DOI: 10.3367/UFNr.2016.08.037891