Common moment

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"Common moment" is a hypothetical moment in time that is measured as the same time for two or more events at different points in space by all observers in the universe. It assumes existence of a time that runs at the same rate for all observers in the universe or at least that can be "scaled" to such a common rate, in which this "common moment" can be determined. Such hypotetical time is called absolute time. In cosmology it is usually called "cosmic time".

According to relativity theory there can't be such time and each observer has its own time running at different rate than the times of at least some other observes in the universe. Therefore, strictly speaking, there are no "common moments" in nature since it is not possible to estblish uniquely the simultaneity of two events in two different points in space for some observers. Nature doesn't need "simultaneity" for anythng since nature doesn't operate at a distance only on contact between interacting agents, so the simultaneity, as not existing in nature, is a human rather than a physical idea, following from imprecise measurments of time. So is the "common moment".

However the differences between measurements of time may be smaller than the ability to detect them and so we may postulate an "approximate simultaneity" and "approximately common moment" for some pratical purposes.