Stress–energy–momentum pseudotensor: Difference between revisions

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In the theory of [[general relativity]], a '''stress-energy-momentumstress–energy–momentum pseudotensor''', such as the '''Landau–Lifshitz pseudotensor''', is an extension of the non-gravitational [[stress-energystress–energy tensor]] which incorporates the [[energy-momentum]] of gravity. It allows the [[energy-momentum]] of a system of gravitating matter to be defined. In particular it allows the total of matter plus the gravitating energy-momentum to form a [[conserved current]] within the framework of [[general relativity]], so that the ''total'' energy-momentum crossing the [[hypersurface]] (3-dimensional boundary) of ''any'' compact [[space-time]] [[hypervolume]] (4-dimensional submanifold) vanishes.
 
Some people object to this derivation on the grounds that [[pseudotensor]]s are inappropriate objects in general relativity, but the conservation law only requires the use of the 4-[[divergence]] of a pseudotensor which is, in this case, a tensor (which also vanishes). Also, most pseudotensors are sections of [[jet bundle]]s, which are perfectly valid objects in GR.
 
==Landau–Lifshitz pseudotensor==
The use of the Landau–Lifshitz combined matter+gravitational stress-energy-momentumstress–energy–momentum [[pseudotensor]]<ref name="LL">[[Lev Davidovich Landau]] & [[Evgeny Mikhailovich Lifshitz]], ''The Classical Theory of Fields'', (1951), Pergamon Press, ISBN 7-5062-4256-7 chapter 11, section #96</ref> allows the energy-momentum conservation laws to be extended into [[general relativity]]. Subtraction of the matter [[stress-energy-momentumstress–energy–momentum tensor]] from the combined pseudotensor results in the gravitational stress-energy-momentumstress–energy–momentum pseudotensor.
 
===Requirements===
# that it be constructed entirely from the [[Metric tensor (general relativity)|metric tensor]], so as to be purely geometrical or gravitational in origin.
# that it be index symmetric , i.e. <math>t_{LL}^{\mu \nu} = t_{LL}^{\nu \mu} \,</math>, (to conserve [[angular momentum]])
# that, when added to the [[stress-energystress–energy tensor]] of matter, <math>T^{\mu \nu}\,</math>, its total 4-[[divergence]] vanishes (this is required of any [[conserved current]]) so that we have a conserved expression for the total stress-energy-momentumstress–energy–momentum.
# that it vanish locally in an [[inertial frame of reference]] (which requires that it only contains first and not second or higher [[derivative]]s of the metric). This is because the [[equivalence principle]] requires that the gravitational force field, the [[Christoffel symbols]], vanish locally in some frame. If gravitational energy is a function of its force field, as is usual for other forces, then the associated gravitational pseudotensor should also vanish locally.
 
#Since the Einstein tensor, <math>G^{\mu \nu}\,</math>, is itself constructed from the metric, so therefore is <math>t_{LL}^{\mu \nu} </math>
#Since the Einstein tensor, <math>G^{\mu \nu}\,</math>, is symmetric so is <math>t_{LL}^{\mu \nu} </math> since the additional terms are symmetric by inspection.
#The Landau–Lifshitz pseudotensor is constructed so that when added to the [[stress-energystress–energy tensor]] of matter, <math>T^{\mu \nu}\,</math>, its total 4-[[divergence]] vanishes: <math>((-g)(T^{\mu \nu} + t_{LL}^{\mu \nu}))_{,\mu} = 0 </math>. This follows from the cancellation of the Einstein tensor, <math>G^{\mu \nu}\,</math>, with the [[stress-energystress–energy tensor]], <math>T^{\mu \nu}\,</math> by the [[Einstein field equations]]; the remaining term vanishes algebraically due the commutativity of partial derivatives applied across antisymmetric indices.
#The Landau–Lifshitz pseudotensor appears to include second derivative terms in the metric, but in fact the explicit second derivative terms in the pseudotensor cancel with the implicit second derivative terms contained within the [[Einstein tensor]], <math>G^{\mu \nu}\,</math>. This is more evident when the pseudotensor is directly expressed in terms of the metric tensor or the [[Levi-Civita connection]]; only the first derivative terms in the metric survive and these vanish where the frame is locally inertial at any chosen point. As a result the entire pseudotensor vanishes locally (again, at any chosen point) <math>t_{LL}^{\mu \nu} = 0</math>, which demonstrates the delocalisation of gravitational energy-momentum.<ref name="LL"/>