Currently the charge-to-mass ratio of the electron is known to 10 orders of magnitude.
However, i'm curious if:
Are there any experiments trying to bound the anisotropy of this ratio for different directions of space? Over what time does the data needs to be averaged to get to our current ten-figure precision? Less than a day? Less than a second? Less than a month?
Does an spatial anisotropy (or an spatial inhomogeneity) of this ratio break Poincare symmetry, or can that be restored with replacing the mass scalar with a tensor object? Also, what theoretical ideas explicitly disallow a non-scalar fundamental mass? What about a non-scalar electromagnetic charge?
Does mass spectroscopic experimental data bound any anisotropic variations? Do we have any measurements of this ratio on small cavities where van der waals forces are dominant?