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The covariant formulation of classical electromagnetism refers to ways of writing the laws of classical electromagnetism (in particular, Maxwell's equations and the Lorentz force) in a form that is manifestly invariant under Lorentz transformations, in the formalism of special relativity using rectilinear inertial coordinate systems. These expressions both make it simple to prove that the laws of classical electromagnetism take the same form in any inertial coordinate system, and also provide a way to translate the fields and forces from one frame to another. However, this is not as general as Maxwell's equations in curved spacetime or non-rectilinear coordinate systems.

In multilinear algebra and tensor analysis, **covariance** and **contravariance** describe how the quantitative description of certain geometric or physical entities changes with a change of basis.

**Classical electromagnetism** or **classical electrodynamics** is a branch of theoretical physics that studies the interactions between electric charges and currents using an extension of the classical Newtonian model. The theory provides a description of electromagnetic phenomena whenever the relevant length scales and field strengths are large enough that quantum mechanical effects are negligible. For small distances and low field strengths, such interactions are better described by quantum electrodynamics.

**Maxwell's equations** are a set of coupled partial differential equations that, together with the Lorentz force law, form the foundation of classical electromagnetism, classical optics, and electric circuits. The equations provide a mathematical model for electric, optical, and radio technologies, such as power generation, electric motors, wireless communication, lenses, radar etc. Maxwell's equations describe how electric and magnetic fields are generated by charges, currents, and changes of the fields. An important consequence of the equations is that they demonstrate how fluctuating electric and magnetic fields propagate at a constant speed (*c*) in a vacuum. Known as electromagnetic radiation, these waves may occur at various wavelengths to produce a spectrum of light from radio waves to γ-rays. The equations are named after the physicist and mathematician James Clerk Maxwell, who between 1861 and 1862 published an early form of the equations that included the Lorentz force law. Maxwell first used the equations to propose that light is an electromagnetic phenomenon.

- Covariant objects
- Preliminary 4-vectors
- Electromagnetic tensor
- Four-current
- Four-potential
- Electromagnetic stress–energy tensor
- Maxwell's equations in vacuum
- Maxwell's equations in the Lorenz gauge
- Lorentz force
- Charged particle
- Charge continuum
- Conservation laws
- Electric charge
- Electromagnetic energy–momentum
- Covariant objects in matter
- Free and bound 4-currents
- Magnetization-polarization tensor
- Electric displacement tensor
- Maxwell's equations in matter
- Constitutive equations
- Lagrangian for classical electrodynamics
- Vacuum 2
- Matter
- See also
- Notes and references
- Further reading

This article uses the classical treatment of tensors and Einstein summation convention throughout and the Minkowski metric has the form diag (+1, −1, −1, −1). Where the equations are specified as holding in a vacuum, one could instead regard them as the formulation of Maxwell's equations in terms of *total* charge and current.

For a more general overview of the relationships between classical electromagnetism and special relativity, including various conceptual implications of this picture, see Classical electromagnetism and special relativity.

The theory of special relativity plays an important role in the modern theory of classical electromagnetism. First of all, it gives formulas for how electromagnetic objects, in particular the electric and magnetic fields, are altered under a Lorentz transformation from one inertial frame of reference to another. Secondly, it sheds light on the relationship between electricity and magnetism, showing that frame of reference determines if an observation follows electrostatic or magnetic laws. Third, it motivates a compact and convenient notation for the laws of electromagnetism, namely the "manifestly covariant" tensor form.

Lorentz tensors of the following kinds may be used in this article to describe bodies or particles:

- where
*γ*(**u**) is the Lorentz factor at the 3-velocity**u**.

- where is 3-momentum, is the total energy, and is rest mass.

- The d'Alembertian operator is denoted , .

The signs in the following tensor analysis depend on the convention used for the metric tensor. The convention used here is `+−−−`, corresponding to the Minkowski metric tensor:

In the mathematical field of differential geometry, a **metric tensor** is a type of function which takes as input a pair of tangent vectors v and w at a point of a surface and produces a real number scalar *g*(*v*, *w*) in a way that generalizes many of the familiar properties of the dot product of vectors in Euclidean space. In the same way as a dot product, metric tensors are used to define the length of and angle between tangent vectors. Through integration, the metric tensor allows one to define and compute the length of curves on the manifold.

The electromagnetic tensor is the combination of the electric and magnetic fields into a covariant antisymmetric tensor whose entries are B-field quantities. ^{ [1] }

In mathematics and theoretical physics, a tensor is **antisymmetric on****an index subset** if it alternates sign (+/−) when any two indices of the subset are interchanged. The index subset must generally either be all *covariant* or all *contravariant*.

and the result of raising its indices is

where **E** is the electric field, **B** the magnetic field, and *c* the speed of light.

The four-current is the contravariant four-vector which combines electric charge density *ρ* and electric current density **j**:

The electromagnetic four-potential is a covariant four-vector containing the electric potential (also called the scalar potential) *ϕ* and magnetic vector potential (or vector potential) **A**, as follows:

The differential of the electromagnetic potential is

The electromagnetic stress–energy tensor can be interpreted as the flux density of the momentum 4-vector, and is a contravariant symmetric tensor that is the contribution of the electromagnetic fields to the overall stress–energy tensor:

where *ε*_{0} is the electric permittivity of vacuum, *μ*_{0} is the magnetic permeability of vacuum, the Poynting vector is

and the Maxwell stress tensor is given by

The electromagnetic field tensor *F* constructs the electromagnetic stress–energy tensor *T* by the equation:

where *η* is the Minkowski metric tensor (with signature +−−−). Notice that we use the fact that

which is predicted by Maxwell's equations.

In vacuum (or for the microscopic equations, not including macroscopic material descriptions), Maxwell's equations can be written as two tensor equations.

The two inhomogeneous Maxwell's equations, Gauss's Law and Ampère's law (with Maxwell's correction) combine into (with +−−− metric):^{ [2] }

while the homogeneous equations – Faraday's law of induction and Gauss's law for magnetism combine to form:

where *F*^{αβ} is the electromagnetic tensor, *J*^{α} is the 4-current, *ε*^{αβγδ} is the Levi-Civita symbol, and the indices behave according to the Einstein summation convention.

Each of these tensor equations corresponds to four scalar equations, one for each value of *β*.

Using the antisymmetric tensor notation and comma notation for the partial derivative (see Ricci calculus), the second equation can also be written more compactly as:

In the absence of sources, Maxwell's equations reduce to:

which is an electromagnetic wave equation in the field strength tensor.

The Lorenz gauge condition is a Lorentz-invariant gauge condition. (This can be contrasted with other gauge conditions such as the Coulomb gauge, which if it holds in one inertial frame will generally not hold in any other.) It is expressed in terms of the four-potential as follows:

In the Lorenz gauge, the microscopic Maxwell's equations can be written as:

Electromagnetic (EM) fields affect the motion of electrically charged matter: due to the Lorentz force. In this way, EM fields can be detected (with applications in particle physics, and natural occurrences such as in aurorae). In relativistic form, the Lorentz force uses the field strength tensor as follows.^{ [3] }

Expressed in terms of coordinate time *t*, it is:

where *p*_{α} is the four-momentum, *q* is the charge, and *x*^{β} is the position.

In the co-moving reference frame, this yields the 4-force

where *u*^{β} is the four-velocity, and τ is the particle's proper time, which is related to coordinate time by *dt* = γ*d*τ.

The density of force due to electromagnetism, whose spatial part is the Lorentz force, is given by

and is related to the electromagnetic stress–energy tensor by

The continuity equation:

expresses charge conservation.

Using the Maxwell equations, one can see that the electromagnetic stress–energy tensor (defined above) satisfies the following differential equation, relating it to the electromagnetic tensor and the current four-vector

or

which expresses the conservation of linear momentum and energy by electromagnetic interactions.

In order to solve the equations of electromagnetism given here, it is necessary to add information about how to calculate the electric current, *J*^{ν} Frequently, it is convenient to separate the current into two parts, the free current and the bound current, which are modeled by different equations;

where

Maxwell's macroscopic equations have been used, in addition the definitions of the electric displacement **D** and the magnetic intensity **H**:

where **M** is the magnetization and **P** the electric polarization.

The bound current is derived from the **P** and **M** fields which form an antisymmetric contravariant magnetization-polarization tensor ^{ [1] }

which determines the bound current

If this is combined with *F*^{μν} we get the antisymmetric contravariant electromagnetic displacement tensor which combines the **D** and **H** fields as follows:

The three field tensors are related by:

which is equivalent to the definitions of the **D** and **H** fields given above.

The result is that Ampère's law,

- ,

and Gauss's law,

- ,

combine into one equation:

The bound current and free current as defined above are automatically and separately conserved

In vacuum, the constitutive relations between the field tensor and displacement tensor are:

Antisymmetry reduces these 16 equations to just six independent equations. Because it is usual to define *F*^{μν} by

the constitutive equations may, in *vacuum*, be combined with the Gauss–Ampère law to get:

The electromagnetic stress–energy tensor in terms of the displacement is:

where *δ _{α}^{π}* is the Kronecker delta. When the upper index is lowered with

Thus we have reduced the problem of modeling the current, *J*^{ν} to two (hopefully) easier problems — modeling the free current, *J*^{ν}_{free} and modeling the magnetization and polarization, . For example, in the simplest materials at low frequencies, one has

where one is in the instantaneously comoving inertial frame of the material, σ is its electrical conductivity, χ_{e} is its electric susceptibility, and χ_{m} is its magnetic susceptibility.

The constitutive relations between the and *F* tensors, proposed by Minkowski for a linear materials (that is, **E** is proportional to **D** and **B** proportional to **H**), are:^{ [4] }

where *u* is the 4-velocity of material, ε and μ are respectively the proper permittivity and permeability of the material (i.e. in rest frame of material), and denotes the Hodge dual.

The Lagrangian density for classical electrodynamics is

In the interaction term, the four-current should be understood as an abbreviation of many terms expressing the electric currents of other charged fields in terms of their variables; the four-current is not itself a fundamental field.

The Euler–Lagrange equation for the electromagnetic Lagrangian density can be stated as follows:

Noting

- ,

the expression inside the square bracket is

The second term is

Therefore, the electromagnetic field's equations of motion are

which is one of the Maxwell equations above.

Separating the free currents from the bound currents, another way to write the Lagrangian density is as follows:

Using Euler–Lagrange equation, the equations of motion for can be derived.

The equivalent expression in non-relativistic vector notation is

- Covariant classical field theory
- Electromagnetic tensor
- Electromagnetic wave equation
- Liénard–Wiechert potential for a charge in arbitrary motion
- Moving magnet and conductor problem
- Inhomogeneous electromagnetic wave equation
- Proca action
- Quantum electrodynamics
- Relativistic electromagnetism
- Stueckelberg action
- Wheeler–Feynman absorber theory

- 1 2 Vanderlinde, Jack (2004),
*classical electromagnetic theory*, Springer, pp. 313–328, ISBN 9781402026997 - ↑ Classical Electrodynamics by Jackson, 3rd Edition, Chapter 11 Special Theory of Relativity
- ↑ The assumption is made that no forces other than those originating in
**E**and**B**are present, that is, no gravitational, weak or strong forces. - ↑ D.J. Griffiths (2007).
*Introduction to Electrodynamics*(3rd ed.). Dorling Kindersley. p. 563. ISBN 81-7758-293-3.

- Einstein, A. (1961).
*Relativity: The Special and General Theory*. New York: Crown. ISBN 0-517-02961-8. - Misner, Charles; Thorne, Kip S.; Wheeler, John Archibald (1973).
*Gravitation*. San Francisco: W. H. Freeman. ISBN 0-7167-0344-0. - Landau, L. D.; Lifshitz, E. M. (1975).
*Classical Theory of Fields (Fourth Revised English Edition)*. Oxford: Pergamon. ISBN 0-08-018176-7. - R. P. Feynman; F. B. Moringo; W. G. Wagner (1995).
*Feynman Lectures on Gravitation*. Addison-Wesley. ISBN 0-201-62734-5.

The **stress–energy tensor**, sometimes **stress–energy–momentum tensor** or **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.

An **electromagnetic four-potential** is a relativistic vector function from which the electromagnetic field can be derived. It combines both an electric scalar potential and a magnetic vector potential into a single four-vector.

In mathematics, the **Hamilton–Jacobi equation** (**HJE**) is a necessary condition describing extremal geometry in generalizations of problems from the calculus of variations, and is a special case of the Hamilton–Jacobi–Bellman equation. It is named for William Rowan Hamilton and Carl Gustav Jacob Jacobi.

In differential geometry, the **Einstein tensor** is used to express the curvature of a pseudo-Riemannian manifold. In general relativity, it occurs in the Einstein field equations for gravitation that describe spacetime curvature in a manner consistent with energy and momentum conservation.

In special and general relativity, the **four-current** is the four-dimensional analogue of the electric current density. Also known as **vector current**, it is used in the geometric context of *four-dimensional spacetime*, rather than three-dimensional space and time separately. Mathematically it is a four-vector, and is Lorentz covariant.

In differential geometry, the **four-gradient** is the four-vector analogue of the gradient from vector calculus.

The **mathematics of general relativity** refers to various mathematical structures and techniques that are used in studying and formulating Albert Einstein's theory of general relativity. The main tools used in this geometrical theory of gravitation are tensor fields defined on a Lorentzian manifold representing spacetime. This article is a general description of the mathematics of general relativity.

In electromagnetism, the **electromagnetic tensor** or **electromagnetic field tensor** is a mathematical object that describes the electromagnetic field in spacetime. The field tensor was first used after the four-dimensional tensor formulation of special relativity was introduced by Hermann Minkowski. The tensor allows related physical laws to be written very concisely.

In general relativity, a **geodesic** generalizes the notion of a "straight line" to curved spacetime. Importantly, the world line of a particle free from all external, non-gravitational force is a particular type of geodesic. In other words, a freely moving or falling particle always moves along a geodesic.

In general relativity, the **Gibbons–Hawking–York boundary term** is a term that needs to be added to the Einstein–Hilbert action when the underlying spacetime manifold has a boundary.

In mathematics, mathematical physics, and theoretical physics, the **spin tensor** is a quantity used to describe the rotational motion of particles in spacetime. The tensor has application in general relativity and special relativity, as well as quantum mechanics, relativistic quantum mechanics, and quantum field theory.

A **theoretical motivation for general relativity**, including the motivation for the geodesic equation and the Einstein field equation, can be obtained from special relativity by examining the dynamics of particles in circular orbits about the earth. A key advantage in examining circular orbits is that it is possible to know the solution of the Einstein Field Equation *a priori*. This provides a means to inform and verify the formalism.

In relativistic physics, the **electromagnetic stress–energy tensor** is the contribution to the stress–energy tensor due to the electromagnetic field. The stress–energy tensor describes the flow of energy and momentum in spacetime. The electromagnetic stress–energy tensor contains the negative of the classical Maxwell stress tensor that governs the electromagnetic interactions.

In physics, **Maxwell's equations in curved spacetime** govern the dynamics of the electromagnetic field in curved spacetime or where one uses an arbitrary coordinate system. These equations can be viewed as a generalization of the vacuum Maxwell's equations which are normally formulated in the local coordinates of flat spacetime. But because general relativity dictates that the presence of electromagnetic fields induce curvature in spacetime, Maxwell's equations in flat spacetime should be viewed as a convenient approximation.

There are various **mathematical descriptions of the electromagnetic field** that are used in the study of electromagnetism, one of the four fundamental interactions of nature. In this article, several approaches are discussed, although the equations are in terms of electric and magnetic fields, potentials, and charges with currents, generally speaking.

In continuum mechanics, a **compatible** deformation **tensor field** in a body is that *unique* tensor field that is obtained when the body is subjected to a continuous, single-valued, displacement field. **Compatibility** is the study of the conditions under which such a displacement field can be guaranteed. Compatibility conditions are particular cases of integrability conditions and were first derived for linear elasticity by Barré de Saint-Venant in 1864 and proved rigorously by Beltrami in 1886.

In theoretical physics, **relativistic Lagrangian mechanics** is Lagrangian mechanics applied in the context of special relativity and general relativity.

In theoretical particle physics, the **gluon field strength tensor** is a second order tensor field characterizing the gluon interaction between quarks.

**Lagrangian field theory** is a formalism in classical field theory. It is the field-theoretic analogue of Lagrangian mechanics. Lagrangian mechanics is used for discrete particles each with a finite number of degrees of freedom. Lagrangian field theory applies to continua and fields, which have an infinite number of degrees of freedom.

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