James L. Anderson analyzed the novelty of Einstein's theory of gravity as its
lack of "absolute objects." Michael Friedman's related work has been criticized
by Roger Jones and Robert Geroch for implausibly admitting as absolute the
timelike 4-velocity field of dust in cosmological models in Einstein's theory.
Using the Rosen-Sorkin Lagrange multiplier trick, I complete Anna Maidens's
argument that the problem is not solved by prohibiting variation of absolute
objects in an action principle. Recalling Anderson's proscription of
"irrelevant" variables, I generalize that proscription to locally irrelevant
variables that do no work in some places in some models. This move vindicates
Friedman's intuitions and removes the Jones-Geroch counterexample: some regions
of some models of gravity with dust are dust-free and so naturally lack a
timelike 4-velocity, so diffeomorphic equivalence to (1,0,0,0) is spoiled.
Torretti's example involving constant curvature spaces is shown to have an
absolute object on Anderson's analysis, viz., the conformal spatial metric
density. The previously neglected threat of an absolute object from an
orthonormal tetrad used for coupling spinors to gravity appears resolvable by
eliminating irrelevant fields. However, given Anderson's definition, GTR itself
has an absolute object (as Robert Geroch has observed recently): a change of
variables to a conformal metric density and a scalar density shows that the
latter is absolute.Comment: Minor editing, small content additions, added references. Forthcoming
in_Studies in History and Philosophy of Modern Physics_, June 200
