The stress–energy tensor, sometimes called the stress–energy–momentum tensor or the 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. This density and flux of energy and momentum are the sources 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.
The stress–energy tensor involves the use of superscripted variables (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 Tαβ of order two that gives the flux of the αth component of the momentum vector across a surface with constant xβ coordinate. In the theory of relativity, this momentum vector is taken as the four-momentum. In general relativity, the stress–energy tensor is symmetric,
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 tensorEdit
Because the stress–energy tensor is of order two, its components can be displayed in 4 × 4 matrix form:
In the following, k and ℓ 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. Its components have a direct physical interpretation. In the case of a perfect fluid this component is
where is the relativistic mass per unit volume, and for an electromagnetic field in otherwise empty space this component is
where E and B are the electric and magnetic fields, respectively.
The flux of relativistic mass across the xk surface is equivalent to the density of the kth component of linear momentum,
represent flux of kth component of linear momentum across the xℓ surface. In particular,
In solid state physics and fluid mechanics, the stress tensor is defined to be the spatial components of the stress–energy tensor in the proper frame of reference. In other words, the stress energy tensor in engineering differs from the relativistic stress–energy tensor by a momentum-convective term.
Covariant and mixed formsEdit
Most of this article works with the contravariant form, Tμν of the stress–energy tensor. However, it is often necessary to work with the covariant form,
or the mixed form,
or as a mixed tensor density
This article uses the spacelike sign convention (−+++) for the metric signature.
In special relativityEdit
The divergence of the non-gravitational stress–energy is zero. In other words, non-gravitational energy and momentum are conserved,
When gravity is negligible and using a Cartesian coordinate system for spacetime, this may be expressed in terms of partial derivatives as
The integral form of this is
where N is any compact four-dimensional region of spacetime; is its boundary, a three-dimensional hypersurface; and is an element of the boundary regarded as the outward pointing normal.
In flat spacetime and using Cartesian coordinates, if one combines this with the symmetry of the stress–energy tensor, one can show that angular momentum is also conserved:
In general relativityEdit
When gravity is non-negligible or when using arbitrary coordinate systems, the divergence of the stress–energy still vanishes. But in this case, a coordinate-free definition of the divergence is used which incorporates the covariant derivative
Consequently, if is any Killing vector field, then the conservation law associated with the symmetry generated by the Killing vector field may be expressed as
The integral form of this is
In special relativityEdit
Given a Lagrangian Density that is a function of a set of fields and their derivatives, but explicitly not of any of the spacetime coordinates, we can construct the tensor by looking at the total derivative with respect to one of the generalized coordinates of the system. So, with our condition
By using the chain rule, we then have
Written in useful shorthand,
Then, we can use the Euler–Lagrange Equation:
And then use the fact that partial derivatives commute so that we now have
We can recognize the right hand side as a product rule. Writing it as the derivative of a product of functions tells us that
Now, in flat space, one can write . Doing this and moving it to the other side of the equation tells us that
And upon regrouping terms,
This is to say that the divergence of the tensor in the brackets is 0. Indeed, with this, we define the stress–energy tensor:
By construction it has the property that
Note that this divergenceless property of this tensor is equivalent to four continuity equations. That is, fields have at least four sets of quantities that obey the continuity equation. As an example, it can be seen that is the energy density of the system and that it is thus possible to obtain the Hamiltonian density from the stress–energy tensor.
Indeed, since this is the case, observing that , we then have
We can then conclude that the terms of represent the energy flux density of the system.
Note that the trace is defined to be . Note that
When we use our formula for the stress–energy tensor found above,
Using the raising and lowering properties of the metric, and the fact that ,
Since , so that we can conclude that
In general relativityEdit
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: kinetic 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 equationsEdit
In general relativity, the stress tensor is studied in the context of the Einstein field equations which are often written as
where is the Ricci tensor, is the Ricci scalar (the tensor contraction of the Ricci tensor), is the metric tensor, Λ is the cosmological constant (negligible at the scale of a galaxy or smaller), and is the universal gravitational constant.
Stress–energy in special situationsEdit
In special relativity, the stress–energy of a non-interacting particle with rest mass m and trajectory is:
where is the velocity vector (which should not be confused with four-velocity, since it is missing a )
Stress–energy of a fluid in equilibriumEdit
where is the mass–energy density (kilograms per cubic meter), is the hydrostatic pressure (pascals), is the fluid's four velocity, and is the reciprocal of the metric tensor. Therefore, the trace is given by
The four-velocity satisfies
the reciprocal of the metric tensor is simply
and the stress–energy tensor is a diagonal matrix
Electromagnetic stress–energy tensorEdit
The Hilbert stress–energy tensor of a source-free electromagnetic field is
where is the electromagnetic field tensor.
The stress–energy tensor for a complex scalar field which satisfies the Klein–Gordon equation is
and when the metric is flat (Minkowski in Cartesian Coordinates) its components work out to be:
Variant definitions of stress–energyEdit
There are a number of inequivalent definitions of non-gravitational stress–energy:
Hilbert stress–energy tensorEdit
The Hilbert stress–energy tensor is defined as the functional derivative
where is the nongravitational part of the action, is the nongravitational part of the Lagrangian density, and the Euler-Lagrange equation has been used. This is symmetric and gauge-invariant. See Einstein–Hilbert action for more information.
Canonical stress–energy tensorEdit
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 tensorEdit
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.
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 referencesEdit
- 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."
- Misner, Charles W.; Thorne, Kip S.; Wheeler, John A. (1973). Gravitation. San Francisco, CA: W.H. Freeman and Company. ISBN 0-7167-0334-3.
- d'Inverno, R.A. (1992). Introducing Einstein's Relativity. New York, NY: Oxford University Press. ISBN 978-0-19-859686-8.
- Landau, L.D.; Lifshitz, E.M. (2010). The Classical Theory of Fields (4th ed.). Butterworth-Heinemann. pp. 84–85. ISBN 978-0-7506-2768-9.
- W. Wyss (2005). "The Energy–Momentum Tensor in Classical Field Theory" (PDF). Colorado, USA.