  In this letter, we study the implications of unitary completion of quantum
gravity on the low energy spectrums, through an infinite set of unitarity
bounds on the forward-limit scattering amplitudes. In three dimensions, we find
that light states with charge-to-mass ratio $z$ greater than $1$ can only be
consistent if there exists other light states, preferably neutral. Applied to
the compactification of the Standard Model, where the low energy couplings are
dominated by the electron with $|z|\sim 10^{22}$, this provides a novel
understanding of the need for light neutrinos.
