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

Signicance of positive and negative frequencies?

  1. Sep 11, 2010 #1
    Reading some QFT in which we are faced with inhomogeneous equation

    [tex](\partial^\mu \partial_\mu + m^2)\phi(x)=J(x)[/tex]

    The solution is given as

    [tex]\phi(x)=\phi^{(+)}_{in}(x)+\phi^{(-)}_{out}(x)+i\int{d^4 x\Delta(x-x')J(x')[/tex]

    where [tex]\Delta[/tex] is the appropriate Green's function. "in" means the solution for early times (when J vanishes) and "out" for late times (when J vanishes). The text states "where the superscripts (+) and (-) indicate the positive- and negative-frequency parts, respectively."

    I don't understand why the in- and out-solutions are restricted to the positive- and negative-frequency parts.
  2. jcsd
  3. Oct 25, 2010 #2
    bump. I'm still interested in this. Maybe someone is around now who can look at it.

    Just one little bump. Any insight is appreciated.
  4. Oct 25, 2010 #3


    User Avatar
    Science Advisor

    The formula is inconsistent with the text. phi plus can't be the solution at early times, because the formula also inlcudes phi minus.

    My recommendation: find a better QFT text.
  5. Oct 25, 2010 #4
    Ok. Thanks.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Threads - Signicance positive negative Date
I Why is position-space favored in QM? Jan 19, 2018
I Negative and Positive energy modes of KG equation Dec 18, 2017
A Transformation of position operator under rotations Nov 27, 2017
B Negative chemical potential Nov 2, 2017