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The deformation-stability fundamental length and deviations from c

The existence of a fundamental length (or fundamental time) has been conjectured in many contexts. However, the "stability of physical theories principle" seems to be the one that provides, through the tools of algebraic deformation theory, an unambiguous derivation of the stable structures that Nature might have chosen for its algebraic framework. It is well-known that $1/c$ and $\hbar $ are the deformation parameters that stabilize the Galilean and the Poisson algebra. When the stability principle is applied to the Poincaré-Heisenberg algebra, two deformation parameters emerge which define two length (or time) scales. In addition there are, for each of them, a plus or minus sign possibility in the relevant commutators. One of the deformation length scales, related to non-commutativity of momenta, is probably related to the Planck length scale but the other might be much larger. In this paper this is used as a working hypothesis to compute deviations from $c$ in speed measurements of massless wave packets.