Many times in relativity courses when arguments are given based on symmetry, the use of the fact that, all observers must agree on the events that occur, is used. However, this fact is presented as if it was something external to the theory or as an implicit postulate. But doesn't this directly follow from the principle of relativity? If an event occurs in one inertial frame, then it must occur in all other reference frames too, otherwise, one frame can be distinguished from another.
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asked Oct 29, 2022 at 22:37
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$\begingroup$ Events don't exist in nature. They are an abstraction of man, so of course they have to be postulated in the theory. $\endgroup$– FlatterMannCommented Oct 29, 2022 at 22:42
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$\begingroup$ @FlatterMann I don't get you, if two electrons repel each other then it is an event, I can't see how it's an abstraction. $\endgroup$– GedankenExperimentalistCommented Oct 29, 2022 at 22:49
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$\begingroup$ Electrons don't exist, either. They are simply energy, momentum, angular momentum and charge values that get exchanged in irreversible energy transfers. Atomism died in the 1920s, I am afraid and so did the 19th century point-like event structure. It's still useful for a lot of physics, of course, but one can't assume that it has any meaning on the fundamental level. General relativity is a classical theory, of course, so it simply postulates that the points of its light cones can be used as "events". $\endgroup$– FlatterMannCommented Oct 30, 2022 at 0:59
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It is in the nature of events that they either happen or they don't, and therefore all reliable observers must agree on this. Sometimes authors find it prudent to remind readers of this fact. That doesn't mean they're introducing it as a new postulate; it just means they're reminding the reader of something that was already implicit in the definition of an event.
