Precision NMR measurement of 7Li and limits on spatial anisotropy

⁷
Li NMR is observed with
⁷
LiCl in D
2
O using a Nicolet NT-360 NMR Spectrometer over a 3.5 month period. The linewidth is typically about 40 mHz. Fitting to a triple Lorentzian line shape gives the splitting Δf ≲ 2.2 mHz which is roughly 20 times better than previous experiments. This result puts a limit on mass anisotropy Δm /m and the nonuniversality |H
ij
− g
ij
| of the gravitational coupling H
ij
to electromagnetism and the gravitational coupling g
ij
, to particle mass as follows: δm/m < 2 × 10
⁻²⁴
, H
μν
− g
μν
| / U ≤ 10
⁻¹³
, | H
0μ
− g
0μ
| / U ≤ 10
⁻⁸
∼ 10
⁻⁹
, and | H
00
− H
00
| / U ≤ 10
⁻⁵
.