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J g is mass current density or mass flux (J g = ρ g v ρ, where v ρ is the velocity of the mass flow), with SI unit kg⋅m −2 ⋅s −1; J is electric current density; G is the gravitational constant; ε 0 is the vacuum permittivity; c is both the speed of propagation of gravity and the speed of light.
Electromagnetic forces are tremendously stronger than gravity, but tend to cancel out so that for astronomical-scale bodies, gravity dominates. Electrical and magnetic phenomena have been observed since ancient times, but it was only in the 19th century James Clerk Maxwell discovered that electricity and magnetism are two aspects of the same ...
Unification of theories about observable fundamental phenomena of nature is one of the primary goals of physics. [1] [2] [3] The two great unifications to date are Isaac Newton’s unification of gravity and astronomy, and James Clerk Maxwell’s unification of electromagnetism; the latter has been further unified with the concept of electroweak interaction.
The crucial new idea, for Einstein, was the introduction of a tetrad field, i.e., a set {X 1, X 2, X 3, X 4} of four vector fields defined on all of M such that for every p ∈ M the set {X 1 (p), X 2 (p), X 3 (p), X 4 (p)} is a basis of T p M, where T p M denotes the fiber over p of the tangent vector bundle TM.
[2]: 198 Aether theories were alternative proposals to replace apparent action-at-a-distance in gravity and electromagnetism, in terms of continuous action inside an (invisible) medium called "aether". [1]: 338 Direct impact of macroscopic objects seems visually distinguishable from action at a distance.
Continuous charge distribution. The volume charge density ρ is the amount of charge per unit volume (cube), surface charge density σ is amount per unit surface area (circle) with outward unit normal nĚ‚, d is the dipole moment between two point charges, the volume density of these is the polarization density P.
The electromagnetic force is one of the four fundamental forces of nature. It is the dominant force in the interactions of atoms and molecules. Electromagnetism can be thought of as a combination of electrostatics and magnetism, which are distinct but closely intertwined phenomena. Electromagnetic forces occur between any two charged particles.
For the cases of time-independent gravity and electromagnetism, the fields are gradients of corresponding potentials =, = so substituting these into Gauss' law for each case obtains =, = = where ρ g is the mass density , ρ e the charge density , G the gravitational constant and k e = 1/4πε 0 the electric force constant.