The white dwarf changes mass when you change the theory of gravity.
In general relativity, gravity is curvature. In f(Q) gravity, gravity is nonmetricity — the failure of lengths and angles to be preserved under parallel transport (arXiv:2603.20350). Both descriptions reproduce identical physics in the weak-field limit. They diverge in the strong-field regime inside compact stars.
A quadratic correction — f(Q) = Q + αQ² — modifies the internal structure of white dwarfs. Increasing α reduces the maximum mass below the Chandrasekhar limit: for α = 5×10¹⁸ cm², the maximum mass is 1.3519 M☉ with radius 2228.85 km. This matches observations of ZTF J1901+1458, the ultra-massive white dwarf that pushed close to the Chandrasekhar boundary.
The test is clean. White dwarfs are dense enough for nonmetricity corrections to matter but cold enough for the equation of state to be well-understood (degenerate electron gas). The astrophysical uncertainty is low — you know the matter physics. What you don't know is the gravity theory. So the white dwarf becomes a test object: measure its mass and radius precisely enough, and the deviation from the general relativistic prediction constrains α.
The structural principle: the best tests of new physics use objects where the old physics is most precisely known. White dwarfs have well-understood matter physics. Neutron stars don't — the equation of state above nuclear density is uncertain. So white dwarfs isolate the gravitational unknown from the matter unknown. The cleaner the conventional physics, the sharper the test of the unconventional.