Reference Frame

But Einstein showed that it was precisely the same kind of difficulty, and that determinations of simultaneity involve reference to an arbitrary choice of reference-frame, just as much as determinations of velocity. ❋ DiSalle, Robert (2002)

More simply, an inertial reference-frame is one in which Newton's second law is satisfied, so that every acceleration corresponds to an impressed force. ❋ DiSalle, Robert (2002)

It is tacitly assumed by Lorentz's theory, and classical electrodynamics generally, that we have a reference-frame in which we can measure the velocity of light. ❋ DiSalle, Robert (2002)

Ernst Mach (1883) claimed that the law of inertia, and Newton's laws generally, implicitly appeal to the fixed stars as a spatial reference-frame, and to the rotation of the earth as a time-scale; at least, he held, such is the basis for any genuine empirical content that the laws have. ❋ DiSalle, Robert (2002)

However, when we consider an observer that is moving relative to event p, we will see that this definition of simultaneous is not invariant, but is reference-frame dependent. ❋ Unknown (2010)