Representation of conjugate momentum

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Discussion Overview

The discussion revolves around the representation of conjugate momentum in various coordinate systems, particularly focusing on the transition from Cartesian coordinates to curvilinear coordinates such as cylindrical or spherical coordinates. Participants explore the implications of quantization in these different frameworks and the mathematical representation of momentum operators.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant proposes that the conjugate momentum \( p_\theta \) can be represented by \(-i\hbar \frac{d}{d\theta}\) in the \(\theta\) basis for a particle constrained to move on a circle.
  • Another participant notes that canonical quantization is more straightforward in rectilinear coordinates and suggests that quantizing generalized coordinates can be complex, although it may work for periodic coordinates.
  • A different participant states that the momentum operator in position representation is \(-i\hbar \vec{\nabla}\) and asserts that this operator can be expressed in any coordinate system, maintaining its meaning.
  • One participant argues that quantization is applicable in any coordinate system as long as a coordinate transformation exists, emphasizing the use of generalized coordinates in various fields of physics.
  • Another participant suggests that on a circle, the coordinate \(\theta\) can be treated similarly to a linear coordinate by considering a large circle, and discusses the transformation between quantized systems.

Areas of Agreement / Disagreement

Participants express differing views on the ease and applicability of quantization in various coordinate systems. While some agree that quantization can be performed in generalized coordinates, others highlight the complexities involved, indicating that the discussion remains unresolved.

Contextual Notes

Participants mention the dependence on coordinate transformations and the potential complications that arise when moving from Cartesian to curvilinear coordinates. There is also an acknowledgment of the limitations of applying canonical quantization in non-rectilinear systems.

AlonsoMcLaren
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We know that in Cartesian position basis the representation of momentum is -ihbar (d/dx)

Consider a cylindrical/spherical/whatever curvilinear coordinates. To make life simple, consider a particle constrained to move on a circle so that its position can described by θ only. Suppose we express the wavefunction as a function of θ, not x. The system has an Lagrangian from which we can find the conjugate momentum pθ

Can we thus declare that pθ can be represented by -ihbar (d/dθ) in the θ basis?
 
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Canonical quantization sort of only works well in rectilinear coordinates iirc. Quantizing generalized coordinates is a messy subject. However, in the case of cylindrical coordinates in one dimension, All you have to do is turn x into a periodic coordinate and it should work fine.
 
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The momentum operator in position representation is always
$$\hat{\vec{p}}=-\mathrm{i} \hbar \vec{\nabla}.$$
Since ##\vec{\nabla}## is a vector operator, it can be expressed in any coordinates you like. It always has the same meaning, but perhaps I don't understand the question right.
 
Quantization will work in any coordinate system as long as there exists coordinate transformation between different system, that is the whole pt. and the physics does not depend on a specific coordinate system. Generalized coordinates are used all the time in QFT, QM and Stat.Physics, you name it.

On a circle, theta can be replaced by x, just consider the circle is very large and is almost a straight line btw 2 pts. A & B. Why not? There is only one basis vector in theta-basis, since 1-dim. If the radius of the circle is changing then use r and theta as basis.

You can also transform the coordinates from x-quantized system to theta system with simple coordinate transformation and that's it, you have a theta-quantized system.

Bottom line is that you quantize in one system of coordinates and just transform into any coordinates, the system remain quantized.
 

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