System and phase space trajectory

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Discussion Overview

The discussion revolves around the role of phase space trajectories in describing physical systems, particularly in classical mechanics. Participants explore the relationship between phase space, parametrization, and the equations of motion.

Discussion Character

  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions the extent to which phase space trajectories fully describe a system, noting that different initial conditions can lead to different parameterizations of the same trajectory.
  • Another participant asserts that a phase trajectory requires a specific parametrization, arguing that a curve like ##x^2 + y^2 = 1## is not a trajectory without a defined path through configuration space.
  • A third participant suggests that the concept of phase space may not be necessary for describing physical systems, as solutions to equations of motion can be derived directly without it.
  • In contrast, a later reply states that trajectories in phase space serve as geometric representations of the solutions to equations of motion, implying that both approaches are valid and interchangeable.

Areas of Agreement / Disagreement

Participants express differing views on the necessity and role of phase space in describing physical systems, indicating that multiple competing perspectives remain unresolved.

Contextual Notes

There are unresolved assumptions regarding the definitions of trajectories and curves, as well as the implications of parametrization on the description of physical systems.

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To what extent do phase space trajectories describe a system? I often see classical systems being identified with (trajectories in) phase space, from which I get the impression these trajectories are supposed to completely specify a system. However, if you take for example the trajectory x^2+p^2=1 for a one-dimensional harmonic oscillator, it is still left open if x(t=0)=0 or x(t=0)=1 which corresponds to two different parameterizations of the circle. This leads me to ask: what is the role of phase space trajectories in the description of physical systems?
 
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A phase trajectory, by definition, includes a particular parametrization in its specification. It's a path through configuration space, with a path being defined as a continuous map from an interval in the real numbers to the path's range. So, ##x^2 + y^2 = 1## isn't a trajectory, it's just a curve. A corresponding trajectory would be ##t\in[0,1) \rightarrow (\cos t, \sin t)##, etc. You can always reparametrize, but then you have a different trajectory.
 
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I see. So there really isn't any need for the concept of phase space for describing physical systems, since the trajectory can be found by just solving the equation of motion directly. My question was motivated by the classical variant of the Dirac-Von Neumann axioms where a classical system is associated with phase space, but maybe I'm reading too much into it.
 
Trajectories in phase space are just geometric representations of the solutions to the equations of motion. It's not one or the other, they're interchangeable.
 

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