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Need flat spacetime to define mass and spin?

  1. Jun 28, 2013 #1
    In his book on GR, Wald writes "the notion of the mass and spin of a field requires the presence of a flat background metric...which one has in the linear approximation but not in the full theory..."

    Why do we need a flat background metric to define a field's mass and spin?
  2. jcsd
  3. Jun 29, 2013 #2


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    Mass and spin are essentially the 2 Casimirs of the (restricted) Poincare' group, which is the isometry group of a flat space-time.
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