
Tidal tensor

In Newton's theory of gravitation and in various relativistic classical theories of gravitation, such as general relativity, the tidal tensor represents 
The tidal tensor represents the relative acceleration due to gravity of two test masses separated by an infinitesimal distance. The component formula_1 represents the relative acceleration in the formula_2 direction produced a displacement in the formula_3 direction.
Tidal tensor for a spherical body.
The most common example of tides is the tidal force around a spherical body ("e.g.", a planet or a moon).
Here we compute the tidal tensor for the gravitational field outside an isolated spherically symmetric massive object. According to Newton's gravitational law, the acceleration "a" at a distance "r" from a central mass "m" is 
Let us adopt the frame in polar coordinates for our three-dimensional Euclidean space, and consider infinitesimal displacements in the radial and azimuthal directions, formula_5 and formula_6, which are given the subscripts 1, 2, and 3 respectively.
We will directly compute each component of the tidal tensor, expressed in this frame.
First, compare the gravitational forces on two nearby objects lying on the same radial line at distances from the central body differing by a distance "h":
Because in discussing tensors we are dealing with multilinear algebra, we retain only first order terms, so formula_9. Since there is no acceleration in the formula_10 or formula_11 direction due to a displacement in the radial direction, the other radial terms are zero: formula_12.
Similarly, we can compare the gravitational force on two nearby observers lying at the same radius formula_13 but displaced by an (infinitesimal) distance "h" in the formula_10 or formula_11 direction. Using some elementary trigonometry and the small angle approximation, we find that the force vectors differ by a vector tangent to the sphere which has magnitude
By using the small angle approximation, we have ignored all terms of order formula_17, so the tangential components are formula_18. Again, since there is no acceleration in the radial direction due to displacements in either of the azimuthal directions, the other azimuthal terms are zero: formula_19.
Combining this information, we find that the tidal tensor is diagonal with frame components
formula_20
This is the "Coulomb form" characteristic of spherically symmetric central force fields in Newtonian physics.
Hessian formulation.
In the more general case where the mass is not a single spherically symmetric central object, the tidal tensor can be derived from the gravitational potential formula_21, which obeys the Poisson equation:
where formula_23 is the mass density of any matter present, and where formula_24 is the Laplace operator. Note that this equation implies that in a vacuum solution, the potential is simply a harmonic function.
The "tidal tensor" is given by the "traceless part" 
of the Hessian
where we are using the standard "Cartesian chart" for E3, with the Euclidean metric tensor
Using standard results in vector calculus, this is readily converted to expressions valid in other coordinate charts, such as the "polar spherical chart"
Spherically symmetric field.
As an example, we can calculate the tidal tensor for a spherical body using the Hessian. Next, let us plug the gravitational potential formula_30 into the Hessian. We can convert the expression above to one valid in polar spherical coordinates, or we can convert the potential to Cartesian coordinates before plugging in. Adopting the second course, we have formula_31, which gives
After a rotation of our frame, which is adapted to the polar spherical coordinates, this expression agrees with our previous result. The easiest way to see this is to set formula_33 to zero so that the off-diagonal terms vanish and formula_34, and then invoke the spherical symmetry.
In General Relativity.
In general relativity, the tidal tensor is generalized by the Riemann curvature tensor. In the weak field limit, the tidal tensor is given by the components formula_35 of the curvature tensor.