It is interesting to note that different MOND theories offer (very) different answers to the generic question “acceleration
with respect to what?”. For instance, in the MOND-from-vacuum idea (see [304
] and Section 7.10), the total acceleration
is measured with respect to the quantum vacuum, which is well defined. In BIMOND (Section 7.8) it is the
relative acceleration between the two metrics, which is also well defined through the difference of Christoffel
symbols.