Note that the Dirac hypothesis can also be achieved by assuming that
varies as
. Indeed this reflects a
choice of units, either atomic or Planck units. However, there is a difference: assuming that only
varies
violates the strong equivalence principle while assuming a varying
results in a theory violating the weak
equivalence principle. It does not mean that we are detecting the variation of a dimensionful constant but simply
that either
or
is varying. This shows that many implementations of this idea are a priori
possible.