  Neutrino lines from very long lived nuclei in simple crystals such as metals
have hypersharp natural width, motionally narrowed by lattice vibrations in
analogy to recoilless emission. A generalized hypersharp line fraction
including the recoilless part can be derived in a frequency modulation
approach. The nue lines of natural width in 3H to 3He 2-body beta-decay can
then be resonantly captured with geometrical cross section. The extreme
sharpness DeltaE/E~10-29 of the tritium nue line can probe the Planck length L
via its limits on the widths of states, DeltaE/E(L) =L(L/R)beta =10-20(beta ~1)
to 10-40 (beta= L/R(fm)). Stringent limits can be set on beta, thus, on models
of quantum gravity.
