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Would it be fair to say that Observers have to be made from matter or, at least, some physicalist components? If Observers, including inertial reference frames, require some physically meaningful existence, then the 'observers' of special relativity, general relativity and Quantum Mechanics are ultimately part of the physical system of the universe (rather than being some sort of apparatus which is divorced from physical existence). The idea that inertial reference frames would require some physical definition (ie: a definition in terms of physical constructs) would mean that the inertial reference frames have to obey the laws of physics WHILST also being used to model physical situations. In that sense, ultimately, inertial reference frames would not really be invariant ways of measuring what happens within the universe. It is arguable that some physical situations would be so extreme that physically constructed inertial reference frames could not model them (and hence that ways of analysing physical situations without making use of inertial reference frames at all might be the only way forward, unless you are happy with the idea of using inertial reference frames which don't actually exist physically...)

These notions relate to Determinism and Superdeterminism (in that the construction of an inertial reference frame is itself predetermined by Nature by 'higher order' or 'higher energy' physical phenomena, if that inertial reference frame is taken to have some physical existence). ASavantDude (talk) 18:16, 10 July 2016 (UTC)Reply

This page needs linking-to, or even merging-with, the existing more-comprehensive one referring to Special Relativity Observer_(special_relativity). Similar with ones for General Relativity and Quantum Mechanics. See Observer. 159.134.104.55 (talk) 10:26, 24 May 2019 (UTC)Reply