Abstract

Due to Einstein?s ?covariance principle?, Einstein?s requirement for weak gravity has not been universally accepted although many believe in both his requirement and principle. For example, Bondi, Pirani, & Robinson rejected Einstein?s requirement since they considered their unbounded metric as a weak wave. However, validity of this ?wave? actually has not been established since the related metric for no gravity clearly violates the principle of causality. Some theorists rejected Einstein?s requirement for being coordinate-dependent because they do not understand that it is based on the principle of causality. However, based on Einstein?s equivalence principle and related Einstein-Minkowski condition, such a coordinate-dependence is solved. Validity of this principle and condition are illustrated with the metric of Einstein?s rotating disk. Concurrently, Einstein?s requirement is proven valid. Moreover, invalidity of Einstein?s ?covariance principle? is illustrated with a counter example.