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In differential geometry, the four-gradient (or 4-gradient) is the four-vector analogue of the gradient from vector calculus. In special relativity and in quantum mechanics , the four-gradient is used to define the properties and relations between the various physical four-vectors and tensors .
A four-vector A is a vector with a "timelike" component and three "spacelike" components, and can be written in various equivalent notations: [3] = (,,,) = + + + = + = where A α is the magnitude component and E α is the basis vector component; note that both are necessary to make a vector, and that when A α is seen alone, it refers strictly to the components of the vector.
If the object has mass, so that its speed is necessarily less than the speed of light, the world line may be parametrized by the proper time of the object. The four-velocity is the rate of change of four-position with respect to the proper time along the curve. The velocity, in contrast, is the rate of change of the position in (three ...
The proper time interval between two events on a world line is the change in proper time, which is independent of coordinates, and is a Lorentz scalar. [1] The interval is the quantity of interest, since proper time itself is fixed only up to an arbitrary additive constant, namely the setting of the clock at some event along the world line.
In non-inertial coordinates, which include accelerated coordinates in special relativity and all coordinates in general relativity, the acceleration four-vector is related to the four-velocity through an absolute derivative with respect to proper time.
Given that in general for a closed system with generalized coordinates q i and canonical momenta p i, [3] = =, = =, it is immediate (recalling x 0 = ct, x 1 = x, x 2 = y, x 3 = z and x 0 = −x 0, x 1 = x 1, x 2 = x 2, x 3 = x 3 in the present metric convention) that = = (,) is a covariant four-vector with the three-vector part being the ...
Therefore, in thermo-mechanical situations the time component of the four-force is not proportional to the power but has a more generic expression, to be given case by case, which represents the supply of internal energy from the combination of work and heat, [2] [1] [4] [3] and which in the Newtonian limit becomes +.
Rindler chart, for = in equation (), plotted on a Minkowski diagram.The dashed lines are the Rindler horizons. The worldline of a body in hyperbolic motion having constant proper acceleration in the -direction as a function of proper time and rapidity can be given by [16]