  We show that the empirical signs of the fundamental {\it static}
Coulomb/Newton forces are dictated by the seemingly unrelated requirement that
the photons/gravitons in the respective underlying Maxwell/Einstein physics be
stable. This linkage, which is imposed by special relativity, is manifested
upon decomposing the corresponding fields and sources in a gauge-invariant way,
and without appeal to static limits. The signs of these free field excitation
energies determine those of the instantaneous forces between sources; opposite
Coulomb/Newton signs are direct consequences of the Maxwell/Einstein free
excitations' odd/even spins.
