On a formal viewpoint on the D’Alembert–Lagrange Principle

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Discussion Overview

The discussion centers around the D’Alembert–Lagrange Principle, exploring its mathematical formulation and implications within classical mechanics. Participants examine terminology related to phase space in both Lagrangian and Hamiltonian contexts, as well as the relevance of classical mechanics to modern physics, particularly quantum theory.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant shares an article aimed at building a modern mathematical theory of the D’Alembert–Lagrange Principle.
  • Another participant questions the terminology used in Russian literature regarding phase space, specifically whether the tangent bundle is referred to as phase space.
  • Some participants argue that in the Lagrangian context, phase space is typically considered the tangent bundle, while in the Hamiltonian context, it is the cotangent bundle.
  • A participant mentions their experience in geometrical foundations of field theory and notes they have not encountered the tangent bundle referred to as phase space.
  • Another participant agrees that the tangent bundle is not commonly called phase space, emphasizing that it is usually the cotangent bundle.
  • There is speculation about the possibility of avoiding the Hamiltonian formalism in classical analytical mechanics, suggesting that the focus for physicists may be on quantum theory.
  • One participant highlights the existence of significant fields such as dynamical systems and control theory, which are also relevant to the study of classical mechanics.

Areas of Agreement / Disagreement

Participants express differing views on the terminology of phase space, with no consensus reached on whether the tangent bundle can be considered phase space. The discussion also reflects a divergence in perspectives regarding the relevance of classical mechanics to modern physics.

Contextual Notes

The discussion includes various assumptions about terminology and the applicability of classical mechanics to quantum theory, which may not be universally accepted. The exploration of these concepts is ongoing and lacks definitive conclusions.

wrobel
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I would like to discuss the following article: https://web.ma.utexas.edu/mp_arc/c/20/20-87.pdf
This article is pure educational and it does not contain any research level results. My aim was to build the theory of the D’Alembert–Lagrange Principle in mathematically closed and modern form.
Thanks for any comment.
 
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Is it traditional in Russian literature to call the tangent bundle of configuration space the "phase space"? I am asking, because in "Western" literature it is the cotangent bundle getting that name (V.I. Arnol'd calls this on page 68 of this book in English on mathematical mechanics, and he is Russian, too).
 
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mmmmmmm I get used to think that in Lagrangian context phase space is the tangent bundle because ##L=L(x,\dot x)## and in Hamiltonian context phase space is the cotangent bundle since ##H=H(x,p)##
 
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Hm, I did my masters in geometrical foundations of field theory (Tulczyjew triples) and I read a lot of papers about foundations of mechanics as well and I've never seen anyone calling tangent bundle a "phase space".
 
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True, but that's semantics ;-)). I've also never heard the tangent bundle called phase space. It's always the co-tangent bundle.

I often wonder whether one can avoid the Hamiltonian formalism entirely. The main purpose to learn classical analytical mechanics for physicists in the 21st century of course is quantum theory, and I often wonder, whether you can do the heuristics with the operator formalism avoiding phase space and Poisson bracketes and only using configuration space and Lagrange brackets.

Usually the Lagrange formulation in quantum theory is only used in the context of the path integral, where it is derived by integrating out the canonical momenta in the Hamiltonian version of the path integral, which is always necessarily the same starting point, because the naive Lagrange version only works for special cases, but that's another story, more suitable for another thread in the QM forum.
 
weirdoguy said:
I read a lot of papers about foundations of mechanics as well and I've never seen anyone calling tangent bundle a "phase space".
Nonholonomic Mechanics and Control (Interdisciplinary Applied Mathematics) (Anthony Bloch, et al)
 
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vanhees71 said:
The main purpose to learn classical analytical mechanics for physicists in the 21st century of course is quantum theory
for physicists -- yes, but there is a big field called dynamical systems and there is a big field called control theory
 
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