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Symmetry properties under arbitrary field redefinitions of the metric energy–momentum tensor in classical field theories and gravity

Guido Magnano1 and Leszek M Sokolowski2

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We derive a generic identity which holds for the metric (i.e. variational) energy–momentum tensor under any field transformation in any generally covariant classical Lagrangian field theory. The identity determines the conditions under which a symmetry of the Lagrangian is also a symmetry of the energy–momentum tensor. It turns out that the stress tensor acquires the symmetry if the Lagrangian has the symmetry in a generic curved spacetime. In this sense, a field theory in flat spacetime is not self-contained. When the identity is applied to the gauge invariant spin-2 field in Minkowski space, we obtain an alternative and direct derivation of a known no-go theorem: a linear gauge invariant spin-2 field, which is dynamically equivalent to linearized general relativity, cannot have a gauge invariant metric energy–momentum tensor. This implies that attempts to define the notion of gravitational energy density in terms of the metric energy–momentum tensor in a field-theoretical formulation of gravity must fail.


PACS

03.50.-z Classical field theories

04.40.-b Self-gravitating systems; continuous media and classical fields in curved spacetime

04.20.Fy Canonical formalism, Lagrangians, and variational principles

11.30.Qc Spontaneous and radiative symmetry breaking

MSC

70S05 Lagrangian formalism and Hamiltonian formalism

83C05 Einstein's equations (general structure, canonical formalism, Cauchy problems)

Subjects

Gravitation and cosmology

Particle physics and field theory

Dates

Issue 2 (21 January 2002)

Received 12 July 2001, in final form 11 October 2001

Published 2 January 2002



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