. [96]
Let us recall that in all examples where tensor equations are discussed, the indices
and
(in the present case) are to be replaced by all manner of arrangements of the integers from 1 to 4 (since space-time has four dimensions). An equation such as
or as
represents therefore ten separate equations. As a matter of fact, sixteen equations would be obtained were we to substitute all possible arrangements; but as the tensors are symmetrical, only ten out of sixteen art independent, hence need be retained; the other six are mere repetitions We see, then, that in Einstein’s theory ten separate equations are necessary in order to define the distribution of the gravitational field in the interior and around matter, whereas in classical science one equation, i.e. Poisson’s equation (