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

Unit Operator

  1. Nov 26, 2006 #1
    Time dependent perturbation theory...

    second order term...

    For some reason they replace




    I know why they are allowed to do this, what I don't understand is how it makes my life better?
  2. jcsd
  3. Nov 27, 2006 #2

    Meir Achuz

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    The fact that \sum|E_i><E_i|=1 is called the completeness property of the states E_i>. It means that you have enough states to expand a function in.
    Look at the fourier sin series. There sin(2pi x/L) form a complete set.
    It becomes useful when the |E_i> are eigenstates of H_0, while the |E_m> are not.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook