Inertial frames Mach principle

Thus, it is a matter of record that a satisfactory incorporation of Mach s principle within general relativity can be attained when the constraint of closure is imposed. However, there is still the point of view that, because general relativity allows solutions that give an internally consistent discussion of an empty inertial spacetime—whereas it is operationally impossible to define an inertial frame in the absence of matter—then the theory (general relativity) must have a non-fundamental basis at the classical level. 

Although the conventional approach outlined above contains all the essential components of Mach s principle, it does not focus on what, in our view, is the essential point about the principle that it is impossible to define inertial frames in the absence of material. This fact is brought out most clearly in the following alternative approach. 

We have argued that the fundamental significance of Mach s principle arises from its implication of the impossibility of defining inertial frames in the absence of material or, as a generalization, we can say that it is impossible to... 

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