Kavli Affiliate: Anthony Lasenby
| Summary:
For the Fierz-Pauli action the Bessel-Hagen construction does not produce a preferred local gauge-invariant energy-momentum tensor; it identifies only a gauge-invariant equivalence class of Noether currents, because the action is built from first derivatives of $h_μν$ whereas the first local gauge-invariant spin-2 field strength, the curvature, contains two. It is natural to ask whether a theory built directly from the linearised curvature recovers the electromagnetic-like feature of a strictly gauge-invariant local representative. We examine linearised Weyl-squared (conformal) gravity, whose action is built from the linearised Weyl tensor $C^(1)_μνρσ$, the irreducible spin-2 part of the curvature and hence the curvature counterpart of the spin-1 field strength of electromagnetism. The Bessel-Hagen construction extends naturally from the Poincaré to the full conformal group, realised actively on the fixed Minkowski background, and the resulting Noether current is gauge invariant as a class for every conformal generator. Nevertheless there exists no nonzero strict local, polynomial, symmetric, dimension-four rank-two tensor quadratic in $h_μν$ that is invariant under both gauge symmetries and conserved on the Bach shell: the two gauge symmetries force any candidate to be built from $C^(1)$ and its derivatives, while the dimension-four and quadratic-order hypotheses leave only expressions quadratic in $C^(1)$ with no additional derivatives, and the four-dimensional Weyl identities collapse these to a pure trace, which cannot be conserved. Linearised Weyl-squared gravity therefore behaves like Fierz-Pauli, not electromagnetism. This suggests that a first-order gauge-covariant field strength, rather than a curvature-built action, is the natural route to an electromagnetic-like local representative.
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