variables. Step 2: Set the cosmological boundary conditions. Assume a homogeneous isotropic cosmology, with isotropic coordinates in the rest frame of the universe. A complete cosmological solution may or may not be needed. Call the results , , , . Step 3: Get new variables from , with , or if needed. Step 4: Substitute these forms into the field equations, keeping only such terms as are necessary to obtain a final consistent solution for . Substitute the perfect fluid stress tensor for the matter sources. Step 5: Solve for to . Assuming this tends to zero far from the system, one obtains the form where is the Newtonian gravitational potential and may be a complicated function including the gravitational "constant" . The Newtonian metric has the form , , . Work in units where the gravitational "constant" measured today far from gravitating matter is unity so set . Step 6: From linearized versions of the field equations solve for to and to . Step 7: Solve for to . This is the messiest step, involving all the nonlinearities in the field equations. The stress–energy tensor must also be expanded to sufficient order. Step 8: Convert to local quasi-Cartesian coordinates and to standard PPN gauge. Step 9: By comparing the result for with the equations presented in PPN with alpha-zeta parameters, read off the PPN parameter values. Comparisons between theories of gravity A table comparing PPN parameters for 23 theories of gravity can be found in Alternatives to general relativity#Parametric post-Newtonian parameters for a range of theories. Most metric theories of gravity can be lumped into categories. Scalar theories of gravitation include conformally flat theories and stratified theories with time-orthogonal space slices. In conformally flat theories such as Nordström's theory of gravitation the metric is given by and for this metric , which drastically disagrees with observations. In stratified theories such as Yilmaz theory of gravitation the metric is given by and for this metric , which also disagrees drastically with observations. Another class of theories is the quasilinear theories such as Whitehead's theory of gravitation. For these . The relative magnitudes of the harmonics of the Earth's tides depend on and , and measurements show that quasilinear theories disagree with observations of Earth's tides. Another class of metric theories is the bimetric theory. For all of these is non-zero. From the precession of the solar spin we know that