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

A ambiguous variation of Einstein-Hilbert action

  1. May 19, 2010 #1
    A ambiguous variation of Einstein--Hilbert action

    Variation of EH action is:
    \delta S_{EH}=\int_{\Omega}{\delta(R\sqrt{-g})dx^4}=
    is symmetric einstein's tensor.

    The action have to be extremal for each volume [tex]\Omega[/tex]. This implicate
    Becouse variation of metric is arbitrary and [tex]G_{\mu\nu}[/tex] is principal independed on [tex]\delta{g^{\mu\nu}}[/tex], latest equation is equivalent with
    This are ordinary einstein's vacuum equations.

    But variation of metric is symmetric tensor, therefore more general form of vacuum
    field equations are
    where [tex]F_{\mu\nu}[/tex] is whatever antisymmetric tensor build it from metric and its derivations.

    Why we can ignore this tensor (it is proven that there is not exist)?
  2. jcsd
  3. May 19, 2010 #2


    User Avatar
    Science Advisor

    Re: A ambiguous variation of Einstein--Hilbert action

    Because the righthand side is the energy momentum tensor, which per definition is symmetric (well, for special-relativistic field theories this is not entirely true, which is the cause for the need of something called the Belinfante tensor).

    Also, the metric is symmetric, so theoretically you could add ANY antisymmetric tensor to your expression, because contracting it with [itex]\delta g_{\mu\nu}[/itex] gives zero anyway. However, this term has to be divergenceless due to covariant energy-momentum conservation. Which tensor should that be?
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook