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

Why the Lagrangian must involve derivative of field?

  1. Dec 15, 2011 #1
    Please teach me this:
    Why the Lagrangian in QFT must involve derivative of field? Is it correct that because fermions and bosons(meaning all things) obey Dirac and Klein-Gordon equations,then the corresponding Lagrangians include the derivative of field?
    (I know that the derivative has a important role in Gauge Local Symmetries(leading to comparator that reserve the Gauge Symmetry of the derivative))
    Thank you very much for your kind helping.
     
  2. jcsd
  3. Dec 16, 2011 #2

    tom.stoer

    User Avatar
    Science Advisor

    w/o spatial derivatives the field values at neighboring points would be uncoupled, there would be no waves, no propagation in space.
     
  4. Dec 16, 2011 #3

    dextercioby

    User Avatar
    Science Advisor
    Homework Helper

    The Lagrangian contains derivatives, because the Hamiltonian contains momenta.
     
  5. Dec 16, 2011 #4

    haushofer

    User Avatar
    Science Advisor

    Well, try to derive the propagator of the field if there are no derivatives in the Lagrangian, as tom.stoer notes, and you'll see the necessity :)

    It's a same kind of question as "why are there time derivatives in the Lagrangian of Newtonian systems"? What's wrong with an action like, say

    [tex] L[x] = A x_i x^i + B (x_i x^i)^2 + \ldots [/tex]

    for coefficients A,B,...
     
  6. Dec 16, 2011 #5
    In physical terms: derivatives of the field give the notion of "neighborhood" and, therefore, "smoothness".

    Imagine that you have a field in the line, f(x), characterized by a certain energy function. A very simple energy function would be ∫|df/dx|2dx, which is zero for flat fields, and small for "smooth" fields. It's the same for Lagrangians.
     
  7. Dec 16, 2011 #6
    So,''physical'' means ''smoothness''?
     
  8. Dec 18, 2011 #7
    No, physical do not mean smooth. But if field values at nearby points did not have a tendency to "stay together", we would have no notion of space at all. The reason for which we have created the notion of "neighborhood" is because nearby points tend to be more correlated than far-away points. And this is achieved with the derivative terms in the lagrangian.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook