How do canonical transformations relate to Hamiltonians?

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Discussion Overview

The discussion revolves around the relationship between canonical transformations and Hamiltonians, specifically in the context of a harmonic oscillator. Participants explore the implications of these transformations on the forms of the Hamiltonians involved and the conditions under which they can be considered equivalent.

Discussion Character

  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions how K(P, Q) can equal H(p, q) when they appear in different forms, suggesting a need for clarification on their equivalence.
  • Another participant notes that K(P, Q) and H(p, q) have different forms due to the use of different parameters, indicating that one set uses Cartesian coordinates while the other uses polar coordinates.
  • A participant seeks to clarify that K(P, Q) = H(p, q) means evaluating H at specific values of p and q should yield the same numerical result as evaluating K at the transformed values of P and Q, without implying that K and H share the same functional form.
  • It is mentioned that if K and H had the same functional form, it would imply a symmetry in the canonical transformation.
  • Another participant points out that Q does not appear in Hamiltonian K, labeling it as a cyclic coordinate, and provides the Hamilton equation of motion for the conjugate momentum.

Areas of Agreement / Disagreement

Participants express uncertainty regarding the equivalence of K and H, with some agreeing on the numerical equivalence under transformation but disagreeing on the implications for functional forms. The discussion remains unresolved regarding the broader implications of these transformations.

Contextual Notes

Participants highlight that the differences in functional forms may be due to the choice of coordinates and the nature of the canonical transformation, but the implications of these differences are not fully resolved.

dyn
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Hi
The Hamiltonian for a harmonic oscillator is H = 1/(2m) ( p2+m2ω2q2). A canonical transformation is then made to a new Hamiltonian K( P , Q )

It is said that K ( P , Q ) = H ( p , q ) but K ( P , Q ) = ωP ( cos2Q +sin2Q ) = ωP

I don't understand how K ( P , Q ) = H ( p , q ) when they have different forms ? I thought if K = H then they must have the same form but H is a sum of 2 squares but K just equals ωP

Thanks
 
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dyn said:
I don't understand how K ( P , Q ) = H ( p , q ) when they have different forms ?
They have different forms for different parameters, i.e. (P,Q) and (p,q). One uses Cartesian coordinates. Another uses polar coordinates.
 
Last edited:
Thank you. I think i might be getting confused with symmetries , so let me see if i have got this right.

K ( P , Q ) = H ( p , q ) means that if i evaluate H at a certain value of p and q and then evaluate K at the transformed values of P and Q i get the same numerical answer ? There is no implication that K and H have the same functional form ?

If K and H had the exact same functional form then i could write H ( P , Q ) = H ( p , q ) and this occurs when the canonical transformation is a symmetry ?

Is that right ? Thanks
 
K and H does not have the same function form. Q does not appear in Hamiltonian K, which is called cyclic coordinate. Hamilton equation of motion for conjugate momentum is
\dot{P}=0
The generating function of transformation is
\displaystyle W_{1}(q,Q)={\frac {1}{2}}m\omega q^{2}\operatorname {cot} {Q}
 
Last edited:
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dyn said:
Thank you. I think i might be getting confused with symmetries , so let me see if i have got this right.

K ( P , Q ) = H ( p , q ) means that if i evaluate H at a certain value of p and q and then evaluate K at the transformed values of P and Q i get the same numerical answer ? There is no implication that K and H have the same functional form ?

If K and H had the exact same functional form then i could write H ( P , Q ) = H ( p , q ) and this occurs when the canonical transformation is a symmetry ?

Is that right ? Thanks
This post is a general question. It is not specific to the harmonic oscillator. I am just trying to find out if i understand the concept in general terms ?
 

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