 In 1965 Sachs had had a breakthrough while at the Aspen Physics Institute, Colorado.Sachs was able to derive a result for a unified field theory if quantum mechanics was considered to be a linear approximation for a field theory of inertia expressed in general relativity.Sachs argued that the work of Albert Einstein and Erwin Schrödinger in general relativity did not yet take account of the inertia of matter, which required consideration of the Mach principle.