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Before we discuss these matters further, let us consider for a moment what it is that ``where'' and ``when'' are part of. When we specify ``where'' by giving the location (or place) of an object in reference to other objects, or ``when'' by giving the time (or epoch) of an event (or happening), in reference to other events, we are not saying that the where or when are part of those objects or events. Rather, all of these things may be considered as taking place in an abstract arena. We shall follow modern terminology here by calling this arena spacetime. This terminology comes about by appending a time dimension to the familiar three dimensional world in which we live, which is often called space. The three dimensional space is just something that is experienced by any observer when they consider moving left or right, forwards or backwards, up or down.
Axiom 1(Spacetime)
By the homogeneity of space and time we mean that there is nothing different about either space or time from one place or epoch to another. The asserted isotropy of space adds the further rider that nothing about space distinguishes one direction from another. Continuity of spacetime is meant in the sense that there is no position in space, or instant of time, for which events can't happen. In particular we assert that there is no position in space, or instant in time, at which it is not possible to both emit and receive a flash of light.
Thus, the universe, as we know it, is simply spacetime populated with matter and radiation. Any region of spacetime that is devoid of matter we call a vacuum.
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