Question: What is the name of the equations which are a set of partial differential equations produced by Einstein relating to the description of gravity as a geometric property of space and time which went on to replace Isaac Newton's rotating spheres argument which attempts to define that true rotational motion can be defined by observing the tension in the string joining two identical spheres?
Sentence (from "Symbol of a differential operator"): In the study of hyperbolic and parabolic partial differential equations, zeros of the principal symbol correspond to the characteristics of the partial differential equation.
Relevant evidence: No