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General four-tensors are usually written in tensor index notation as
In special and general relativity, many four-tensors of interest are first order (four-vectors) or second order, but higher order tensors occur. Examples are listed next.
In special relativity, the vector basis can be restricted to being orthonormal, in which case all four-tensors transform under Lorentz transformations. In general relativity, more general coordinate transformations are necessary since such a restriction is not in general possible.
First order tensors
In special relativity, one of the simplest non-trivial examples of a four-tensor is the four-displacement
a four-tensor with contravariant rank 1 and covariant rank 0. Four-tensors of this kind are usually known as four-vectors. Here the component x0 = ct gives the displacement of a body in time (coordinate time t is multiplied by the speed of light c so that x0 has dimensions of length). The remaining components of the four-displacement form the spatial displacement vector x = (x1, x2, x3).
For a particle with relativistic mass m, four momentum is defined by
with τ the proper time of the particle.
Second order tensors
The Minkowski metric tensor with an orthonormal basis for the (−+++) convention is
used for calculating the line element and raising and lowering indices. The above applies to Cartesian coordinates. In general relativity, the metric tensor is given by much more general expressions for curvilinear coordinates.
The angular momentum L = x ∧ p of a particle with relativistic mass m and relativistic momentum p (as measured by an observer in a lab frame) combines with another vector quantity N = mx − pt (without a standard name) in the relativistic angular momentum tensor
The stress–energy tensor of a continuum or field generally takes the form of a second order tensor, and usually denoted by T. The timelike component corresponds to energy density (energy per unit volume), the mixed spacetime components to momentum density (momentum per unit volume), and the purely spacelike parts to 3d stress tensors.
The three field tensors are related by
which is equivalent to the definitions of the D and H fields.
The Ricci curvature tensor is another second order tensor.
Higher order tensors
- Lambourne, Robert J A. Relativity, Gravitation and Cosmology. Cambridge University Press. 2010.
- R. Penrose (2005). The Road to Reality. vintage books. pp. 437–438, 566–569. ISBN 978-0-09-944068-0. Note: Some authors, including Penrose, use Latin letters in this definition, even though it is conventional to use Greek indices for vectors and tensors in spacetime.
- M. Fayngold (2008). Special Relativity and How it Works. John Wiley & Sons. pp. 137–139. ISBN 3-527-40607-7.
- Vanderlinde, Jack (2004), classical electromagnetic theory, Springer, pp. 313–328, ISBN 9781402026997
- Barut, A.O. Electrodynamics and the Classical theory of particles and fields. Dover. p. 96. ISBN 978-0-486-64038-9.
- Barut, A.O. Electrodynamics and the Classical theory of particles and fields. Dover. p. 73. ISBN 978-0-486-64038-9. No factor of c appears in the tensor in this book because different conventions for the EM field tensor.