It is a widely accepted view that the canonical energy-momentum and spin tensors are well defined and have relevance only in flat spacetime, and, hence, are usually underestimated and abandoned. However, it is only the analog of these canonical quantities that can be associated with gravity itself. Thus, we first introduce these quantities for the matter fields in a general curved spacetime.
To specify the state of the matter fields operationally, two kinds of devices are needed: the first measures the value of the fields, while the other measures the spatio-temporal location of the first. Correspondingly, the fields on the manifold of events can be grouped into two sharply-distinguished classes. The first contains the matter field variables, e.g., finitely many -type tensor fields , whilst the second contains the fields specifying the spacetime geometry, i.e., the metric in Einstein’s theory. Suppose that the dynamics of the matter fields is governed by Hamilton’s principle specified by a Lagrangian . If is the action functional, i.e., the volume integral of on some open domain with compact closure, then the equations of motion are
Suppose that the Lagrangian is weakly diffeomorphism invariant in the sense that, for any vector field and the corresponding local one-parameter family of diffeomorphisms , one has
for some one-parameter family of vector fields . ( is called diffeomorphism invariant if , e.g., when is a scalar.) Let be any smooth vector field on . Then, calculating the divergence to determine the rate of change of the action functional along the integral curves of , by a tedious but straightforward computation, one can derive the Noether identity: , where denotes the Lie derivative along , and , the Noether current, is given explicitly bycanonical energy-momentum tensor, is defined by [73, 74, 438]. If the torsion is not vanishing, then in the Noether identity there is a further term, , where the dynamic spin tensor is defined by , and the Noether current has a slightly different structure [259, 260].) Obviously, is not uniquely determined by the Noether identity, because that contains only its divergence, and any identically-conserved current may be added to it. In fact, may be chosen to be an arbitrary nonzero (but divergence free) vector field, even for diffeomorphism-invariant Lagrangians. Thus, to be more precise, if , then we call the specific combination (2.3) the canonical Noether current. Other choices for the Noether current may contain higher derivatives of , as well (see, e.g., ), but there is a specific one containing algebraically (see points 3 and 4 below).
However, is sensitive to total divergences added to the Lagrangian, and, if the matter fields have gauge freedom (e.g., if the matter is a Maxwell or Yang–Mills field), then in general it is not gauge invariant, even if the Lagrangian is. On the other hand, is gauge invariant and is independent of total divergences added to because it is the variational derivative of the gauge invariant action with respect to the metric. Provided the field equations are satisfied, the Noether identity implies [73, 74, 438, 259, 260] that
Hence, is also conserved and can equally be considered as a Noether current. (For a formally different, but essentially equivalent, introduction of the Noether current and identity, see [536, 287, 191].)
The interpretation of the conserved currents, and , depends on the nature of the Killing vector, . In Minkowski spacetime the ten-dimensional Lie algebra of the Killing vectors is well known to split into the semidirect sum of a four-dimensional commutative ideal, , and the quotient , where the latter is isomorphic to . The ideal is spanned by the constant Killing vectors, in which a constant orthonormal frame field on , , forms a basis. (Thus, the underlined Roman indices , , … are concrete, name indices.) By the ideal inherits a natural Lorentzian vector space structure. Having chosen an origin , the quotient can be identified as the Lie algebra of the boost-rotation Killing vectors that vanish at . Thus, has a ‘4 + 6’ decomposition into translations and boost rotations, where the translations are canonically defined but the boost-rotations depend on the choice of the origin . In the coordinate system adapted to (i.e., for which the one-form basis dual to has the form ), the general form of the Killing vectors (or rather one-forms) is for some constants and . Then, the corresponding canonical Noether current is , and the coefficients of the translation and the boost-rotation parameters and are interpreted as the density of the energy-momentum and of the sum of the orbital and spin angular momenta, respectively. Since, however, the difference is identically conserved and has more advantageous properties, it is that is used to represent the energy-momentum and angular-momentum density of the matter fields.
Since in de Sitter and anti-de Sitter spacetimes the (ten-dimensional) Lie algebra of the Killing vector fields, and , respectively, are semisimple, there is no such natural notion of translations, and hence no natural ‘4 + 6’ decomposition of the ten conserved currents into energy-momentum and (relativistic) angular momentum density.
Living Rev. Relativity 12, (2009), 4
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