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In special and general relativity, the **four-current** (technically the **four-current density**)^{ [1] } 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 physics, **special relativity** is the generally accepted and experimentally well-confirmed physical theory regarding the relationship between space and time. In Albert Einstein's original pedagogical treatment, it is based on two postulates:

- the laws of physics are invariant in all inertial systems ; and
- the speed of light in a vacuum is the same for all observers, regardless of the motion of the light source.

**General relativity** is the geometric theory of gravitation published by Albert Einstein in 1915 and the current description of gravitation in modern physics. General relativity generalizes special relativity and Newton's law of universal gravitation, providing a unified description of gravity as a geometric property of space and time, or spacetime. In particular, the *curvature of spacetime* is directly related to the energy and momentum of whatever matter and radiation are present. The relation is specified by the Einstein field equations, a system of partial differential equations.

In special relativity, a **four-vector** is an object with four components, which transform in a specific way under Lorentz transformation. Specifically, a four-vector is an element of a four-dimensional vector space considered as a representation space of the standard representation of the Lorentz group, the (½,½) representation. It differs from a Euclidean vector in how its magnitude is determined. The transformations that preserve this magnitude are the Lorentz transformations, which include spatial rotations and boosts.

- Definition
- Motion of charges in spacetime
- Physical interpretation
- Continuity equation
- Maxwell's equations
- General relativity
- Quantum field theory
- See also
- References

Analogously, it is possible to have any form of "current density", meaning the flow of a quantity per unit time per unit area. see current density for more on this quantity.

In electromagnetism, **current density** is the electric current per unit area of cross section. The **current density vector** is defined as a vector whose magnitude is the electric current per cross-sectional area at a given point in space, its direction being that of the motion of the charges at this point. In SI units, the electric current density is measured in amperes per square metre.

This article uses the summation convention for indices. See covariance and contravariance of vectors for background on raised and lowered indices, and raising and lowering indices on how to switch between them.

In mathematics, especially in applications of linear algebra to physics, the **Einstein notation** or **Einstein summation convention** is a notational convention that implies summation over a set of indexed terms in a formula, thus achieving notational brevity. As part of mathematics it is a notational subset of Ricci calculus; however, it is often used in applications in physics that do not distinguish between tangent and cotangent spaces. It was introduced to physics by Albert Einstein in 1916.

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.

In mathematics and mathematical physics, **raising and lowering indices** are operations on tensors which change their type. Raising and lowering indices are a form of index manipulation in tensor expressions.

Using the Minkowski metric of metric signature (+−−−), the four-current components are given by:

The **signature**(*v*, *p*, *r*) of a metric tensor *g* is the number of positive, zero, and negative eigenvalues of the real symmetric matrix *g*_{ab} of the metric tensor with respect to a basis. In physics, the *v* represents for the time or virtual dimension, and the *p* for the space and physical dimension. Alternatively, it can be defined as the dimensions of a maximal positive and null subspace. By Sylvester's law of inertia these numbers do not depend on the choice of basis. The signature thus classifies the metric up to a choice of basis. The signature is often denoted by a pair of integers (*v*, *p*) implying *r* = 0 or as an explicit list of signs of eigenvalues such as (+, −, −, −) or (−, +, +, +) for the signature (1, 3, 0), respectively.

where *c* is the speed of light, *ρ* is the charge density, and **j** the conventional current density. The dummy index *α* labels the spacetime dimensions.

The **speed of light** in vacuum, commonly denoted * c*, is a universal physical constant important in many areas of physics. Its exact value is 299,792,458 metres per second. It is exact because by international agreement a metre is defined as the length of the path travelled by light in vacuum during a time interval of 1/299792458 second. According to special relativity,

In electromagnetism, **charge density** is the amount of electric charge per unit length, surface area, or volume. *Volume charge density* is the quantity of charge per unit volume, measured in the SI system in coulombs per cubic meter (C•m^{−3}), at any point in a volume. *Surface charge density* (σ) is the quantity of charge per unit area, measured in coulombs per square meter (C•m^{−2}), at any point on a surface charge distribution on a two dimensional surface. *Linear charge density* (λ) is the quantity of charge per unit length, measured in coulombs per meter (C•m^{−1}), at any point on a line charge distribution. Charge density can be either positive or negative, since electric charge can be either positive or negative.

In physics, **spacetime** is any mathematical model that fuses the three dimensions of space and the one dimension of time into a single four-dimensional continuum. Spacetime diagrams can be used to visualize relativistic effects such as why different observers perceive *where* and *when* events occur.

This can also be expressed in terms of the four-velocity by the equation:^{ [2] }^{ [3] }

In physics, in particular in special relativity and general relativity, a **four-velocity** is a four-vector in four-dimensional spacetime that represents the relativistic counterpart of velocity, which is a three-dimensional vector in space.

where *ρ* is the charge density measured by an observer at rest observing the electric current, and *ρ*_{0} the charge density for an observer moving at the speed *u* (the magnitude of the 3-velocity) along with the charges.

Qualitatively, the change in charge density (charge per unit volume) is due to the contracted volume of charge due to Lorentz contraction.

Charges (free or as a distribution) at rest will appear to remain at the same spatial position for some interval of time (as long as they're stationary). When they do move, this corresponds to changes in position, therefore the charges have velocity, and the motion of charge constitutes an electric current. This means that charge density is related to time, while current density is related to space.

The four-current unifies charge density (related to electricity) and current density (related to magnetism) in one electromagnetic entity.

In special relativity, the statement of charge conservation is that the Lorentz invariant divergence of *J* is zero:^{ [4] }

where is the 4-gradient. This is the continuity equation.

In general relativity, the continuity equation is written as:

where the semi-colon represents a covariant derivative.

The four-current appears in two equivalent formulations of Maxwell's equations, in terms of the four-potential:^{ [5] }

where is the D'Alembert operator, or the electromagnetic field tensor:

where *μ*_{0} is the permeability of free space.

In general relativity, the four-current is defined as the divergence of the electromagnetic displacement, defined as

then

The four-current density of charge is an essential component of the Lagrangian density used in quantum electrodynamics.^{ [6] } In 1956 Gershtein and Zeldovich considered the conserved vector current (CVC) hypothesis for electroweak interactions.^{ [7] }^{ [8] }^{ [9] }

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.

In the mathematical field of differential geometry, the **Riemann curvature tensor** or **Riemann–Christoffel tensor** is the most common method used to express the curvature of Riemannian manifolds. It assigns a tensor to each point of a Riemannian manifold, that measures the extent to which the metric tensor is not locally isometric to that of Euclidean space. The curvature tensor can also be defined for any pseudo-Riemannian manifold, or indeed any manifold equipped with an affine connection.

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

In differential geometry, a **tensor density** or **relative tensor** is a generalization of the tensor field concept. A tensor density transforms as a tensor field when passing from one coordinate system to another, except that it is additionally multiplied or *weighted* by a power *W* of the Jacobian determinant of the coordinate transition function or its absolute value. A distinction is made among (authentic) tensor densities, pseudotensor densities, even tensor densities and odd tensor densities. Sometimes tensor densities with a negative weight *W* are called **tensor capacity.** A tensor density can also be regarded as a section of the tensor product of a tensor bundle with a density bundle.

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, **geodesic deviation** describes the tendency of objects to approach or recede from one another while moving under the influence of a spatially varying gravitational field. Put another way, if two objects are set in motion along two initially parallel trajectories, the presence of a tidal gravitational force will cause the trajectories to bend towards or away from each other, producing a relative acceleration between the objects.

In differential geometry and mathematical physics, a **spin connection** is a connection on a spinor bundle. It is induced, in a canonical manner, from the affine connection. It can also be regarded as the gauge field generated by local Lorentz transformations. In some canonical formulations of general relativity, a spin connection is defined on spatial slices and can also be regarded as the gauge field generated by local rotations.

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.

The **covariant formulation of classical electromagnetism** refers to ways of writing the laws of classical electromagnetism 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 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.

In electromagnetism and applications, an **inhomogeneous electromagnetic wave equation**, or **nonhomogeneous electromagnetic wave equation**, is one of a set of wave equations describing the propagation of electromagnetic waves generated by nonzero source charges and currents. The source terms in the wave equations makes the partial differential equations *inhomogeneous*, if the source terms are zero the equations reduce to the homogeneous electromagnetic wave equations. The equations follow from Maxwell's equations.

In the theory of general relativity, a **stress–energy–momentum pseudotensor**, such as the **Landau–Lifshitz pseudotensor**, is an extension of the non-gravitational stress–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 of *any* compact space–time hypervolume vanishes.

The **harmonic coordinate condition** is one of several coordinate conditions in general relativity, which make it possible to solve the Einstein field equations. A coordinate system is said to satisfy the harmonic coordinate condition if each of the coordinate functions *x*^{α} satisfies d'Alembert's equation. The parallel notion of a harmonic coordinate system in Riemannian geometry is a coordinate system whose coordinate functions satisfy Laplace's equation. Since d'Alembert's equation is the generalization of Laplace's equation to space-time, its solutions are also called "harmonic".

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.

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.

In mathematics, **Ricci calculus** constitutes the rules of index notation and manipulation for tensors and tensor fields. It is also the modern name for what used to be called the **absolute differential calculus**, developed by Gregorio Ricci-Curbastro in 1887–1896, and subsequently popularized in a paper written with his pupil Tullio Levi-Civita in 1900. Jan Arnoldus Schouten developed the modern notation and formalism for this mathematical framework, and made contributions to the theory, during its applications to general relativity and differential geometry in the early twentieth century.

**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.

- ↑ Rindler, Wolfgang (1991).
*Introduction to Special Relativity*(2nd ed.). Oxford Science Publications. pp. 103–107. ISBN 0-19-853952-5. - ↑ Roald K. Wangsness, Electromagnetic Fields, 2nd edition (1986), p. 518, 519
- ↑ Melvin Schwartz, Principles of Electrodynamics, Dover edition (1987), p. 122, 123
- ↑ J. D. Jackson, Classical Electrodynamics, 3rd Edition (1999), p. 554
- ↑ as [ref. 1, p519]
- ↑ Cottingham, W. Noel; Greenwood, Derek A. (2003).
*An introduction to the standard model of particle physics*. Cambridge University Press. p. 67. - ↑ Marshak, Robert E. (1993).
*Conceptual foundations of modern particle physics*. World Scientific Publishing Company. p. 20. - ↑ Gershtein, S. S.; Zeldovich, Y. B. (1956),
*Soviet Phys. JETP*,**2**576. - ↑ Thomas, Anthony W. (1996). "CVC in particle physics".
*arXiv preprint nucl-th/9609052*.

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