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

Conserved charge

  1. Mar 18, 2007 #1
    Is there an inconsistency between the definition of conserved charge and conserved current in Hamiltonian and Lagrangian formulation?

    For example, [tex]H = \int T^{00} d^3x[/tex] is a conserved charge,
    [tex]\frac{dH}{dt} = \{H, H\} = 0[/tex]

    But we have [tex]\partial_\mu T^{\mu\nu} = 0[/tex] implies
    [tex]\int (\partial_\mu T^{\mu 0}) d^3x = \int (\partial_0 T^{00} + \partial_i T^{i0}) d^3x = 0[/tex] so it seems
    [tex]\frac{d}{dt}\int T^{00}d^3x = - \int \partial_i T^{i0} d^3x \neq 0[/tex]

    I'm very puzzled.
    Last edited: Mar 18, 2007
  2. jcsd
  3. Mar 18, 2007 #2
    Why is the last integral non-zero? If the charge is constant, shouldn't the current be zero?
  4. Mar 18, 2007 #3
    So there's a contradiction. In general if there is a boundary, the last integral is not zero.
  5. Mar 19, 2007 #4


    User Avatar
    Science Advisor
    Homework Helper

    Yes, of course. It's the case on curved manifolds which occur in GR, for example. The boundary terms are very important. However, as it's usually presented in field theory in Minkowski space, the hypersurface integrals are always chosen to be 0.
  6. Mar 19, 2007 #5
    So is there a condition
    [tex]T^{i0} = 0[/tex] at boundary?
  7. Mar 19, 2007 #6
    If you take a volume in which the charge is conserved that will mean there is no net charge flowing in or out of the boundary, wich is the last condition you mention (integrated over the surface).
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook