Stress–energy tensor

Contravariant components of the stress-energy tensor.

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The stress–energy tensor (sometimes stress–energy–momentum tensor) is a tensor quantity in physics that describes the density and flux of energy and momentum in spacetime, generalizing the stress tensor of Newtonian physics. It is an attribute of matter, radiation, and non-gravitational force fields. The stress–energy tensor is the source of the gravitational field in the Einstein field equations of general relativity, just as mass density is the source of such a field in Newtonian gravity.

Definition [edit]

The stress–energy tensor involves the use of superscripted variables which are not exponents (see tensor index notation and Einstein summation notation). If Cartesian coordinates in SI units are used, then the components of the position four-vector are given by: x⁰ = t, x¹ = x, x² = y, and x³ = z, where t is time in seconds, and x, y, and z are distances in meters.

The stress–energy tensor is defined as the tensor T^αβ of rank two that gives the flux of the αth component of the momentum vector across a surface with constant x^β coordinate. In the theory of relativity, this momentum vector is taken as the four-momentum. In general relativity, the stress–energy tensor is symmetric,^[1]

$T^{\alpha \beta} = T^{\beta \alpha}.$

In some alternative theories like Einstein–Cartan theory, the stress–energy tensor may not be perfectly symmetric because of a nonzero spin tensor, which geometrically corresponds to a nonzero torsion tensor.

Identifying the components of the tensor [edit]

Because the stress–energy tensor is rank two, its components can be displayed in matrix form:

$T^{\mu\nu} = \begin{pmatrix} T^{00} & T^{01} & T^{02} & T^{03} \\ T^{10} & T^{11} & T^{12} & T^{13} \\ T^{20} & T^{21} & T^{22} & T^{23} \\ T^{30} & T^{31} & T^{32} & T^{33} \end{pmatrix}.$

In the following i and k range from 1 through 3.

The time–time component is the density of relativistic mass, i.e. the energy density divided by the speed of light squared.^[2] It is of special interest because it has a simple physical interpretation. In the case of a perfect fluid this component is

$T^{00} = \rho,$

and for an electromagnetic field in otherwise empty space this component is

$T^{00} = {\epsilon_0 \over 2}\left({E^2 \over c^2} + B^2\right),$

where $E$ and $B$ are the electric and magnetic fields, respectively.^[3]

The flux of relativistic mass across the xⁱ surface is equivalent to the density of the ith component of linear momentum,

$T^{0i} = T^{i0}.$

The components

$T^{ik}$

represent flux of ith component of linear momentum across the x^k surface. In particular,

$T^{ii}$

(not summed) represents normal stress, which is called pressure when it is independent of direction. The remaining components

$T^{ik} \quad i \ne k$

represent shear stress (compare with the stress tensor).

Warning: In solid state physics and fluid mechanics, the stress tensor is defined to be the spatial components of the stress–energy tensor in the comoving frame of reference. In other words, the stress energy tensor in engineering differs from the stress energy tensor here by a momentum convective term.

Covariant and mixed forms [edit]

In most of this article we work with the contravariant form, T^μν of the stress–energy tensor. However, it is often necessary to work with the covariant form,

$T_{\mu \nu} = g_{\mu \alpha} g_{\nu \beta} T^{\alpha \beta},$

or the mixed form,

$T_\mu^\nu = g_{\mu \alpha} T^{\alpha \nu},$

or as a mixed tensor density

$\mathfrak{T}_\mu^\nu = T_\mu^\nu \sqrt{-g} \,.$

Conservation law [edit]

In special relativity [edit]

The stress–energy tensor is the conserved Noether current associated with spacetime translations.

When gravity is negligible and using a Cartesian coordinate system for spacetime, the divergence of the non-gravitational stress–energy will be zero. In other words, non-gravitational energy and momentum are conserved,

$0 = T^{\mu \nu}{}_{,\nu} = \partial_{\nu} T^{\mu \nu}. \!$

The integral form of this is

$0 = \int_{\partial N} T^{\mu \nu} \mathrm{d}^3 s_{\nu} \!$

where N is any compact four-dimensional region of spacetime; $\partial N$ is its boundary, a three dimensional hypersurface; and $\mathrm{d}^3 s_{\nu}$ is an element of the boundary regarded as the outward pointing normal.

If one combines this with the symmetry of the stress–energy tensor, one can show that angular momentum is also conserved,

$0 = (x^{\alpha} T^{\mu \nu} - x^{\mu} T^{\alpha \nu})_{,\nu} . \!$

In general relativity [edit]

However, when gravity is non-negligible or when using arbitrary coordinate systems, the divergence of the non-gravitational stress–energy may fail to be zero. In this case, we have to use a more general continuity equation which incorporates the covariant derivative

$0 = T^{\mu \nu}{}_{;\nu} = \nabla_{\nu} T^{\mu \nu} = T^{\mu \nu}{}_{,\nu} + T^{\sigma \nu} \Gamma^{\mu}{}_{\sigma \nu} + T^{\mu \sigma} \Gamma^{\nu}{}_{\sigma \nu}$

where $\Gamma^{\mu}{}_{\sigma \nu}$ is the Christoffel symbol which is the gravitational force field.

Consequently, if $\xi^{\mu}$ is any Killing vector field, then the conservation law associated with the symmetry generated by the Killing vector field may be expressed as

$0 = \nabla_\nu (\xi^{\mu} T_{\mu}^{\nu}) = \frac{1}{\sqrt{-g}} \partial_\nu( \sqrt{-g} \ \xi^{\mu} T_{\mu}^{\nu})$

The integral form of this is

$0 = \int_{\partial N} \sqrt{-g} \ \xi^{\mu} T_{\mu}^{\nu} \ \mathrm{d}^3 s_{\nu} = \int_{\partial N} \xi^{\mu} \mathfrak{T}_{\mu}^{\nu} \ \mathrm{d}^3 s_{\nu}$

In general relativity [edit]

In general relativity, the symmetric stress–energy tensor acts as the source of spacetime curvature, and is the current density associated with gauge transformations of gravity which are general curvilinear coordinate transformations. (If there is torsion, then the tensor is no longer symmetric. This corresponds to the case with a nonzero spin tensor in Einstein–Cartan gravity theory.)

In general relativity, the partial derivatives used in special relativity are replaced by covariant derivatives. What this means is that the continuity equation no longer implies that the non-gravitational energy and momentum expressed by the tensor are absolutely conserved, i.e. the gravitational field can do work on matter and vice versa. In the classical limit of Newtonian gravity, this has a simple interpretation: energy is being exchanged with gravitational potential energy, which is not included in the tensor, and momentum is being transferred through the field to other bodies. In general relativity the Landau–Lifshitz pseudotensor is a unique way to define the gravitational field energy and momentum densities. Any such stress–energy pseudotensor can be made to vanish locally by a coordinate transformation.

In curved spacetime, the spacelike integral now depends on the spacelike slice, in general. There is in fact no way to define a global energy–momentum vector in a general curved spacetime.

The Einstein field equations [edit]

Main article: Einstein field equations

In general relativity, the stress tensor is studied in the context of the Einstein field equations which are often written as

$R_{\mu \nu} - {1 \over 2}R\,g_{\mu \nu} = {8 \pi G \over c^4} T_{\mu \nu},$

where $R_{\mu \nu}$ is the Ricci tensor, $R$ is the Ricci scalar (the tensor contraction of the Ricci tensor), $g_{\mu \nu}\,$ the metric tensor, and $G$ is the universal gravitational constant.

Stress–energy in special situations [edit]

Isolated particle [edit]

In special relativity, the stress–energy of a non-interacting particle with mass m and trajectory $\mathbf{x}_\text{p}(t)$ is:

$T^{\alpha \beta}(\mathbf{x},t) = \frac{m \, v^{\alpha}(t) v^{\beta}(t)}{\sqrt{1 - (v/c)^2}}\;\, \delta(\mathbf{x} - \mathbf{x}_\text{p}(t)) = E \frac{v^{\alpha}(t) v^{\beta}(t)}{c^2}\;\, \delta(\mathbf{x} - \mathbf{x}_\text{p}(t))$

where $v^{\alpha} \!$ is the velocity vector (which should not be confused with four-velocity)

$v^{\alpha} = \left(1, \frac{d \mathbf{x}_\text{p}}{dt}(t) \right) \,,$

δ is the Dirac delta function and $E = \sqrt{p^2 c^2 + m^2 c^4}$ is the energy of the particle.

Stress–energy of a fluid in equilibrium [edit]

For a fluid in thermodynamic equilibrium, the stress–energy tensor takes on a particularly simple form

$T^{\alpha \beta} \, = \left(\rho + {p \over c^2}\right)u^{\alpha}u^{\beta} + p g^{\alpha \beta}$

where $\rho$ is the mass–energy density (kilograms per cubic meter), $p$ is the hydrostatic pressure (pascals), $u^{\alpha}$ is the fluid's four velocity, and $g^{\alpha \beta}$ is the reciprocal of the metric tensor.

The four velocity satisfies

$u^{\alpha} u^{\beta} g_{\alpha \beta} = - c^2 \,.$

In an inertial frame of reference comoving with the fluid, the four velocity is

$u^{\alpha} = (1, 0, 0, 0) \,,$

the reciprocal of the metric tensor is simply

$g^{\alpha \beta} \, = \left( \begin{matrix} - c^{-2} & 0 & 0 & 0 \\ 0 & 1 & 0 & 0 \\ 0 & 0 & 1 & 0 \\ 0 & 0 & 0 & 1 \end{matrix} \right) \,,$

and the stress–energy tensor is a diagonal matrix

$T^{\alpha \beta} = \left( \begin{matrix} \rho & 0 & 0 & 0 \\ 0 & p & 0 & 0 \\ 0 & 0 & p & 0 \\ 0 & 0 & 0 & p \end{matrix} \right).$

Electromagnetic stress–energy tensor [edit]

Main article: electromagnetic stress–energy tensor

The Hilbert stress–energy tensor of a source-free electromagnetic field is

$T^{\mu \nu} = \frac{1}{\mu_0} \left( F^{\mu \alpha} g_{\alpha \beta} F^{\nu \beta} - \frac{1}{4} g^{\mu \nu} F_{\delta \gamma} F^{\delta \gamma} \right)$

where $F_{\mu \nu}$ is the electromagnetic field tensor.

Scalar field [edit]

Main article: Klein–Gordon equation

The stress–energy tensor for a scalar field $\phi$ which satisfies the Klein–Gordon equation is

$T^{\mu\nu} = \frac{\hbar^2}{m} (g^{\mu \alpha} g^{\nu \beta} + g^{\mu \beta} g^{\nu \alpha} - g^{\mu\nu} g^{\alpha \beta}) \partial_{\alpha}\bar\phi \partial_{\beta}\phi - g^{\mu\nu} m c^2 \bar\phi \phi .$

Variant definitions of stress–energy [edit]

There are a number of inequivalent definitions of non-gravitational stress–energy:

Hilbert stress–energy tensor [edit]

This stress–energy tensor can only be defined in general relativity with a dynamical metric. It is defined as a functional derivative

$T^{\mu\nu} = \frac{2}{\sqrt{-g}}\frac{\delta (\mathcal{L}_{\mathrm{matter}} \sqrt{-g}) }{\delta g_{\mu\nu}} = 2 \frac{\delta \mathcal{L}_\mathrm{matter}}{\delta g_{\mu\nu}} + g^{\mu\nu} \mathcal{L}_\mathrm{matter}.$

where $\mathcal{L}_{\mathrm{matter}}$ is the nongravitational part of the Lagrangian density of the action. This is symmetric and gauge-invariant. See Einstein–Hilbert action for more information.

Canonical stress–energy tensor [edit]

Noether's theorem implies that there is a conserved current associated with translations through space and time. This is called the canonical stress–energy tensor. Generally, this is not symmetric and if we have some gauge theory, it may not be gauge invariant because space-dependent gauge transformations do not commute with spatial translations.

In general relativity, the translations are with respect to the coordinate system and as such, do not transform covariantly. See the section below on the gravitational stress–energy pseudo-tensor.

Belinfante–Rosenfeld stress–energy tensor [edit]

In the presence of spin or other intrinsic angular momentum, the canonical Noether stress energy tensor fails to be symmetric. The Belinfante–Rosenfeld stress energy tensor is constructed from the canonical stress–energy tensor and the spin current in such a way as to be symmetric and still conserved. In general relativity, this modified tensor agrees with the Hilbert stress–energy tensor. See the article Belinfante–Rosenfeld stress–energy tensor for more details.

Gravitational stress–energy [edit]

Main article: Stress–energy–momentum pseudotensor

By the equivalence principle gravitational stress–energy will always vanish locally at any chosen point in some chosen frame, therefore gravitational stress–energy cannot be expressed as a non-zero tensor; instead we have to use a pseudotensor.

In general relativity, there are many possible distinct definitions of the gravitational stress–energy–momentum pseudotensor. These include the Einstein pseudotensor and the Landau–Lifshitz pseudotensor. The Landau–Lifshitz pseudotensor can be reduced to zero at any event in spacetime by choosing an appropriate coordinate system.

Notes and references [edit]

^ On pp. 141–142 of Misner, Thorne, and Wheeler, section 5.7 "Symmetry of the Stress–Energy Tensor" begins with "All the stress–energy tensors explored above were symmetric. That they could not have been otherwise one sees as follows.".
^ Charles W. ,Misner, Thorne, Kip S. , Wheeler, John A., (1973). Gravitation. San Frandisco: W. H. Freeman and Company. ISBN 0-7167-0334-3.
^ d'Invero, R.A, (1992). Introducing Einstein's Relativity. New York: Oxford University Press. ISBN 978-0-19-859686-8.

External links [edit]

Lecture, Stephan Waner
Caltech Tutorial on Relativity — A simple discussion of the relation between the Stress–Energy tensor of General Relativity and the metric

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