Need flat spacetime to define mass and spin?

copernicus1
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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?
 
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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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