Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Canonical Transformations

  1. Apr 30, 2006 #1
    Ive looked through a couple books on this subject and found the basic theory but none actually apply it to a problem. I was wondering if someone would be so kind as to maybe do a practice problem for me? The reason I say this is because I have a homework problem and have solved for the hamiltonian and the canonical equations however, I would like to find a new set of co-ordinates in which the momenta might be constants. In my problem I have a co-ordinate velocity dependant potential and would like to find the transformations in which the hamiltonian is a constant. Is this possible? I have just gotten into the Hamiltonian formalism and am extremely excited, more so when I first learned about lagrangian mechanics. Thank you so much.
  2. jcsd
  3. May 1, 2006 #2

    Physics Monkey

    User Avatar
    Science Advisor
    Homework Helper

    Hi QuantumDefect,

    These methods are really cool, aren't they? I could do a problem for you, something simple like the harmonic oscillator. Such a problem is almost certainly worked in greater detail in your book, but if you're actually interested I could step you through it.

    Unfortunately, there is no good way to just guess or figure out a canonical transformation that leaves the Hamiltonian independent of the coordinates. Is it possible? Sure. Is it easy to find? Almost never. A general approach starts from the Hamilton-Jacobi equation. The general approach says that the solution to the HJ equation (a non-linear differential equation) is the desired generator of the transformation. This method has the advantage of casting the problem in a form (differential equation) familiar to most physicists, but the problem is equally insoluable from an analytic point of view.

    Without some details, I probably couldn't say much more. Feel free to post your problem and we can talk about it.
  4. May 1, 2006 #3
    Thanks Physics Monkey!
    Thats why none of the books took it into greater depth! I'll read up more on the Hamilton-Jacobi equation and if I have any questions, I'll come back. However, you answered my question and I am extremely grateful.

    Many thanks,

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook