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Groups of Worldview Transformations Implied by Isotropy of Space

Given any Euclidean ordered field, $Q$, and any 'reasonable' group, $G$, of (1+3)-dimensional spacetime symmetries, we show how to construct a model $M_{G}$ of kinematics for which the set $W$ of worldview transformations between inertial observers satisfies $W=G$. This holds in particular for all relevant subgroups of $Gal$, $cPoi$, and $cEucl$ (the groups of Galilean, Poincaré and Euclidean transformations, respectively, where $c\in Q$ is a model-specific parameter orresponding to the speed of light in the case of Poincaré transformations). In doing so, by an elementary geometrical proof, we demonstrate our main contribution: spatial isotropy is enough to entail that the set $W$ of worldview transformations satisfies either $W\subseteq Gal$, $W\subseteq cPoi$, or $W\subseteq cEucl$ for some $c>0$. So assuming spatial isotropy is enough to prove that there are only 3 possible cases: either the world is classical (the worldview transformations between inertial observers are Galilean transformations); the world is relativistic (the worldview transformations are Poincaré transformations); or the world is Euclidean (which gives a nonstandard kinematical interpretation to Euclidean geometry). This result considerably extends previous results in this field, which assume a priori the (strictly stronger) special principle of relativity, while also restricting the choice of $Q$ to the field of reals. As part of this work, we also prove the rather surprising result that, for any $G$ containing translations and rotations fixing the time-axis $t$, the requirement that $G$ be a subgroup of one of the groups $Gal$, $cPoi$ or $cEucl$ is logically equivalent to the somewhat simpler requirement that, for all $g\in G$: $g[t]$ is a line, and if $g[t]=t$ then $g$ is a trivial transformation (i.e. $g$ is a linear transformation that preserves Euclidean length and fixes the time-axis setwise).